History of Flight

Barrow, Alaska

On March 6, 2002, a Cessna 208B, N208TF, operated by Tatonduk Flying Service of Fairbanks, Alaska, departed Deadhorse Airport, Deadhorse, Alaska, heading to a remote airstrip (Puviak). The accident aircraft encountered substantial damage while landing. The pilot had 2,000 flying hours in the make and model of the airplane.

The pilot said that the visibility was good during the flight from Deadhorse, but once they approached the airstrip they dropped "into the fog." During a telephone discussion with the investigator-in-charge, the pilot stated, "He passed over Puviak without seeing the airstrip."

At around 500 feet, the pilot and four passengers could see the ground and the pilot made a left turn toward the airstrip when the airplane began to shake. According to investigators, the pilot applied full power and leveled the wings, but the airplane impacted the ground seconds later.

Parks, Arizona

On November 8, 2002, a Cessna 208B N514DB, operated by Brown County Financial Services, LLC, departed Las Vegas, Nevada on a personal, cross-country Instrument Flight Rules (IFR) flight heading to Midland, Texas. The pilot had logged a total flight time of 1,880 hours, including 125 hours of actual instrument time and 76.9 hours in this airplane make and model.

At 0844, the pilot contacted the Fort Worth, Texas Automated Flight Service Station to file an instrument flight rules flight plan. The pilot informed the briefer that he planned to leave Las Vegas at 0900 and cruise at 12,000 feet and was told about an airman's meteorological advisory (AIRMET) for icing in effect for the pilot's route of flight. The report included moderate mixed and rime icing from the freezing level up to 24,000 feet. Flight service also told the pilot that the freezing level was forecasted at 12,500 feet in Arizona and that it was dropping down to 10,500 feet further east.

At approximately 1015, the pilot reported "getting...mixed...right...now" and requested a climb to 17,000 from 15,000 feet and he was cleared to do so. Air traffic radar revealed the airplane had climbed to 15,200 feet before entering a rapid descent. At 1019, the air traffic controller attempted to contact N514DB, to no avail.

A retired airline pilot who was close to the accident site witnessed the airplane appear from the clouds with the nose pointed straight down in a spin. Another witness flying in the area stated he heard the pilot of the accident airplane ask air traffic for clearance to 17,000 feet due to "getting mixed ice." A third witness stated that he heard the accident aircraft pilot report that they were "getting ice." The commercial pilot, a private pilot-rated passenger, and two other passengers were fatally injured.

Moscow, Russia

On November 19, 2005, a Cessna 208, Aruba registration P4-OIN and operated by Evolga AVV, departed from Voronezh Airport, Russia at 2117 local time with a planned destination of Domodedovo Airport, Russia. Two pilots and six passengers were onboard for a personal flight.

Weather conditions in the area included a snowstorm, poor visibility, icing conditions, and gusty winds. About 35 minutes prior to scheduled landing time, and while cruising at 9,800 feet, air traffic control (ATC) advised the flightcrew of a forecast for moderate icing conditions in the clouds.

The aircraft was equipped with a cockpit voice recorder (CVR) and flight data recorder (FDR). According to CVR data, the pilots discussed that they were not experiencing icing conditions at that time. Soon after, however, they mentioned they were transitioning from severe to light icing.

Following a descent to 4,900 feet, the airplane was in level flight at 118 knots before its pitch attitude increased and the airspeed decreased to 102 knots. Without the stall warning activating, the airplane stalled and the autopilot disengaged, by design. The airplane descended, reaching a maximum airspeed of 226 knots before impacting the ground. The site was approximately 22 miles southeast of the destination airport in a forest. The aircraft was destroyed, and all eight occupants were fatally injured.

The Cessna 208 aircraft began experiencing icing-related accidents as early as 1987. The FAA addressed this issue in 1991 through an Air Carrier Operations Bulletin (ACOB). The bulletin identified a possible negative trend from Cessna 208 aircraft operators indicating the aircraft's pneumatic boots could lose effectiveness after the third cycle in icing conditions.

The 1994 Roselawn accident of an Avions de Transport Regional (ATR) model 72 showed that super-cooled large drop (SLD) can cause severe icing (see the lessons learned module on SLD). In 1996 and 1998, the FAA issued over 40 Airworthiness Directives on Part 23 and 25 airplanes to minimize the potential hazards associated with operating the airplane in severe icing conditions. Severe SLD icing conditions were defined as ice accumulating aft of protected surfaces due to flight in freezing drizzle and freezing rain. These conditions were not included in the Part 25, Appendix C icing conditions to which all airplanes are certified.

A series of icing accidents on another transport category airplane led to changes in icing certification standards and helped lead to a solution that dramatically increased the safety of the Cessna 208 in icing. In 1997, the model Embraer 120 Brasilia (EMB-120) experienced a fatal accident (NTSB DCA97MA017) in Monroe, Michigan (an accident module contained within this library) which the airplane stalled during a hold in icing conditions. This is a transport category turboprop with a maximum weight of 29,000 pounds. Airworthiness Directive 97-26-06 was issued on the EMB-120 to install an ice detector. It also revised the Airplane Flight Manual Limitations section to require pneumatic deice boot activation at first sign of ice accumulation and added a minimum airspeed for holds in icing conditions. The airplane manufacturer and the FAA believed these corrective actions would reduce the risk of a loss of control event in icing.

However, the EMB-120 experienced another loss-of-control icing accident in 2001 (NTSB DCA01MA031). This event was non-fatal but the airplane required 8,000 feet to recover from a stall at 18,000 feet. The associated figure shows the flight data recorder (FDR) information. The FDR and cockpit voice recorder (CVR) showed that the autopilot was engaged and the airspeed decayed after ice accumulated and the boots activated. The two pilots did not notice the airspeed decrease. The NTSB's analysis showed that the aerodynamic stall occurred prior to stall warning activation.

In the 1999-2000 timeframe, the FAA, along with NASA and the Goodrich Corporation, conducted research to determine the critical amount of ice that can collect on pneumatic deicing boots in normal operation in typical Part 25, turboprop airplanes, along with the aerodynamic penalty. In 2005, the FAA conducted additional research to determine the amount of ice accumulation at airspeeds typical of Part 23 airplanes, including the Cessna 208. 

The figures below show ice that can accrete on deicing boots in normal operation in the conditions defined in Part 25, Appendix C, to which airplanes are certified. 

The following videos show pneumatic deicing boot operation in an icing tunnel on a model representative of the Cessna 208 outboard wing.

They show inter-cycle ice (ice that remains between boot activation) and residual ice at a cruise airspeed in icing conditions as defined in Part 25, Appendix C. The colder temperature (6º) video shows ice accreting near the stagnation area of the wing, but the stronger ice adhesion at cold temperatures results in poor shedding. The warmer temperature (26º) video is a total temperature near freezing. The waterdrops can be seen running back and refreezing farther aft, where there is no ice protection, including the last one inch of the boot.

Pneumatic Deicing Boot Testing Video: 26 °F

 

 

 

Pneumatic Deicing Boot Testing Video: 6 °F

Subsequent flight testing in 2002 with similar ice shapes on the EMB-120 showed that ice accretion on boots in normal operation can significantly increase the stall speed and thus requires that the stall warning activate at a lower angle of attack.

As the following animation shows, ice accretion results in a wing stall at a lower angle of attack.

Clean vs Iced Wing Animation

This research and the EMB-120 events led the FAA and Cessna to conduct flight testing with simulated inter-cycle ice on the pneumatic deicing boots. Soon after, the Cessna 208 Moscow accident occurred. Part 23 airplanes are rarely equipped with an FDR or CVR, which can make it more difficult to determine the cause of fatal accidents. The Cessna 208 Moscow accident was a milestone event, since the aircraft was equipped with both an FDR and CVR, which revealed that this accident was similar to documented events on Part 25 airplanes: 

• The autopilot was engaged in altitude hold or vertical speed mode
• The pilot recognized the icing conditions and activated the ice protection systems
• The airspeed decayed as ice accumulated on the airplane
• The flightcrew did not recognize this airspeed decay
• The flightcrew did not recognize stall buffet
• The airplane stalled with the stall warning activating after the stall, or not at all

The following simulation, for a Part 23 airplane, reproduces the scenario of all icing accidents in the last 30 years.

Ice Induced Stall Pilot Training Video

 

Two months after the Cessna 208 Moscow event, a Saab 340-B stalled during a climb in icing over Santa Maria, California (NTSB LAX06IA076). The FDR showed a similar scenario to the Cessna 208 and EMB-120 icing events.

Corrective actions on the Cessna 208, EMB-120, and Saab 340 were similar; a low airspeed awareness system was developed for the Cessna 208 and the stall warning systems were modified on the EMB-120 and Saab 340 by adding a schedule for icing.

Government and Industry Initiatives

For nearly two decades, government and industry conducted research and worked to address NTSB recommendations to reduce inflight icing accidents. Major areas addressed included aircraft certification, pilot training, and aircraft operation.

Aircraft Certification

In the 1980's, when the Cessna 208, EMB-120, and Saab 340 were certified, the Part 23 and Part 25 regulations required the capability of the airplane to operate safely in icing conditions. The icing conditions for certification were specified in Appendix C of Part 25 (for both Part 23 and Part 25 airplanes). However, there was no guidance on what "capable of operating safely" meant. The industry standard at the time was to evaluate airplane performance and flight characteristics, with no quantitative pass/fail criteria; and with simulated ice shapes only on unprotected surfaces, such as the wing root and tip. Natural icing flight tests were accomplished with no specific targets for ice accretion on protected surfaces such as pneumatic deicing boots.

Part 23 and 25 were amended in 1993 and 2007, respectively, to define "capable of operating safely" as compliance to the same airplane performance and flight characteristics requirements that exist for non-icing flight.

The FAA improved airplane icing certification guidance after the investigations and research described previously with the following two changes:

• Airplane performance and flight characteristics, particularly stall warning margin, were to be evaluated with critical ice accretions (such as inter-cycle, residual ice, and runback ice on pneumatic deicing boots) that can collect on protected surfaces in normal system operation
• Stall warning in icing had to be the same as in non-icing. This means that buffet cannot be used as a stall warning in icing conditions on an airplane that is equipped with a stall warning system

Pilot Training

In 1995, following several Cessna 208 accidents, Cessna developed and offered a Safety Awareness Program to address:

• Cold weather ground operations
• Icing equipment
• Exiting icing conditions
• Using maximum engine power to counter degrading airspeeds

The next year, the company issued a supplement to their existing documentation, Known Icing Equipment, which addressed aircraft performance degradation. Over the following decade Cessna made further updates to the supplement.

In parallel with the work the FAA was doing, the NTSB conducted their own investigation of the Cessna 208. In December 2004, the NTSB issued Safety Recommendation A-04-64 through -67. These stated that the NTSB believed the FAA should:

• Require all pilots and operators to undergo annual, seasonal training for ground deicing and flight into icing conditions
• Work with operators to develop operational strategies and guidance materials to minimize icing accidents and ensure strategies are incorporated into operator manuals and training programs
• Require all pilots/operators to conduct a visual and tactile examination to ensure the aircraft is free from ice
• Evaluate its current surveillance procedures

By 2006, the FAA, working with industry and Cessna, developed and implemented targeted pilot education that included elements regarding tactile inspections of the wing preflight; specialized preflight procedures; and mandatory, recurrent and seasonal icing training.

Aircraft Operation

Following several accident investigations, the NTSB recommended that the FAA prohibit all operators of Cessna 208-series airplanes from conducting flight into icing conditions determined to be more than trace or light icing. In addition, three major concerns received significant attention regarding Cessna 208 operation in icing conditions; minimum airspeeds, stall warnings, and the use of autopilot.

Minimum Airspeed

Findings from the NTSB's icing assessment showed that the FAA's minimum operating airspeed in icing conditions of 105 knots does not provide an adequate margin of safety for pilots who encounter icing in flight. As a result, the FAA ruled that Cessna 208 pilots should exit icing conditions immediately if unable to maintain a minimum operating airspeed of 120 knots during flight in icing conditions and should sacrifice altitude to maintain a minimum of 105 knots.

Stall Warnings

Ice accumulation on the airframe may cause a 20-knot increase in stall speed. However, as was the case with numerous Cessna 208 accidents, stall warning systems did not assist the pilots in recognizing that their aircraft was entering an inadvertent stall. A stall occurs when the angle of attack increases beyond a certain point, causing lift to decrease. Even though the Cessna 208 was equipped with a stall warning system, the NTSB determined that pilots should not rely on precautional alarms when in icing conditions. Instead, pilots should treat buffet or aural stall warnings as a signal of imminent stall. Through an AD, the FAA issued an Airplane Flight Manual (AFM) warning that stated the stall warning system was unreliable in icing conditions. In the Russian Cessna 208 accident, the pilot lost control only three knots slower than the published minimum operating icing airspeed of 105 knots and no stall warning activated. To address the stall warning issue, the FAA issued AD 2007-10-15 to install a low airspeed awareness system.

Autopilot

Safety should not be predicated on requiring the flightcrew to constantly monitor the aircraft's state when the autopilot is engaged, particularly in a high workload environment. The autopilot maintains altitude at the expense of airspeed and this decay in airspeed can go unnoticed by the flightcrew until the airplane has stalled.

Since the use of autopilot can mask signs of deteriorating performance, the NTSB determined that pilots should hand-fly the airplane in icing conditions to enable them to sense the aerodynamic effects of icing and help retain control of the airplane. Because of these accidents, the NTSB stated that, "Degradation to flight performance standards is insidious and not apparent to the flight crew while the autopilot is engaged." The pilot can easily miss the buffet if it is used as a stall warning as buffet would only occur after the stall warning, stall, or the autopilot disconnecting, by design.

In 2006, the FAA followed with an AD that required all operators of Cessna 208 series airplanes to disengage the autopilot and fly the airplane manually when operating in all icing conditions. Additionally, the pilot was to extend flaps below 110 knots. By flying the airplane manually (without the autopilot engaged), pilots would likely:

• Notice the increased control wheel force needed to maintain altitude
• Become aware of the airplane's altered performance characteristics
• Increase airspeed or otherwise alter their flight situation to avoid loss of control

In 2007, the FAA issued an AD that required a low airspeed awareness system, which alerted the pilot when airspeed decreased below 110 knots in icing conditions. The AD allowed the pilots to use the autopilot above 110 knots in icing conditions (except severe icing) via a revised Airplane Flight Manual (AFM) limitation.

Cessna 208 Accident History

Cessna Aircraft Company began production of the Model 208 Caravan in 1985 for use in multipurpose commercial operations. This single-engine, turboprop aircraft quickly evolved into a rugged, multi-task land and sea airplane used worldwide. It is known as an economical, high-volume utility airplane that is used in nearly 70 countries and has a fleet flight time of more than 8.5 million hours. Most versions accommodate a pilot and nine passengers or 3,000 pounds of cargo.

Following aircraft certification, several Cessna 208 icing accidents occurred, involving not only entry-level, new-hire pilots, but also pilots with years of experience. The FAA, the manufacturer, and industry groups conducted studies to seek solutions for icing problems with the Cessna 208. The FAA, with recommendations from the NTSB, took efforts to mitigate icing risk and preserve the Caravan's "known ice" certification. In addition, Cessna conducted aircraft hardware studies and evaluated pilot training as it related to airframe ice. The Regional Air Cargo Carriers Association (RACCA) assisted in directing its members and industry in reviewing the studies and they took an active role in assisting the FAA in the process.

The following timeline represents three groups of actions built around Cessna 208 accidents: industry initiatives, NTSB recommendations, and FAA directives. Between 1998 and 2006 there were nearly 30 accidents that involved Cessna 208 flights in icing conditions. During that timeframe, the manufacturer, airline trade groups, safety organizations, and other industry partners studied the accidents to determine solutions. The NTSB provided a series of recommendations. The FAA conducted an interim assessment in 2002 and issued several ADs and ACs throughout this accident period that culminated in AD 2006-06-06 that effectively eliminated additional Cessna 208 icing accidents until the time of module publication in 2020.

Although the Cessna 208 was certified to operate in known icing conditions, investigators incorrectly attributed early accidents to pilot error. Investigators believed that these accidents, such as Barrow, AK and Parks, AZ, were due to pilot's failure to maintain adequate airspeed, resulting in an inadvertent stall. None of the accident reports considered that incorrect aircraft data involving correct airspeed in icing conditions might be a potential cause.

In 1996, the FAA issued an AD applicable to many small commuter airplanes. It instructed pilots on visual cues to turn on their anti-ice systems, procedures for exiting from severe icing, and turning off the autopilot. This third action was because use of the autopilot masked a pilot's ability to recognize the effects of ice contamination. However, Cessna 208 accidents continued to occur, and investigators continued to believe that pilot error was still the primary cause.

Due to the Comair Embraer accident in Monroe, Michigan in 1997, the NTSB, in 1998, recommended the FAA require all operators of turbo-propeller-driven air carrier airplanes, including the Cessna 208, to require pilots to disengage the autopilot and fly the airplane manually when they activated the anti-ice systems.

Through the NTSB 2003-2004 assessment of 26 icing-related accidents between 1987-2003, most accident flights were found to have occurred within parameters of the FAA icing certification envelopes. However, due to inconsistencies in manufacturer, industry, and government training doctrine, it was still believed that pilot error and lack of training were key factors to the high number of accidents. The NTSB recommended the FAA standardize training and require pilots to attend annual Cessna 208 icing training.

It was not until the Moscow, Russia accident in 2005 that recorded aircraft data led to a review of past icing accidents. While unusual for the Cessna 208, the accident airplane in Moscow was equipped with both a Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR). Investigators were thus able to determine that the aircraft was flying at a speed considered acceptable by the Airplane Flight Manual (AFM) for flight in icing conditions. Data revealed aircraft airspeed to be 118 knots when the pitch attitude increased. The airspeed then decreased to about 102 knots and the aircraft stalled without an alert. There were similarities between this accident and an earlier Comair Embraer accident, which had been attributed to inadequate airspeed in icing conditions. As a result, it was recognized that published in-ice airspeeds may be the actual cause of prior inflight icing Cessna 208 accidents.

After the Moscow accident, the NTSB issued safety recommendations. As a result, RACCA, Cessna, and the FAA implemented solutions that included hardware upgrades, new procedures and limitations, and targeted pilot education. Additionally, due to a series of NTSB recommendations to Transport Canada, the FAA in turn issued three ADs from 2005-2007 that:

1. Established minimum airspeed of 120 knots in icing conditions
2. Warned that the aural stall warning system does not function properly in icing conditions
3. Required the installation of an improved stall warning system, additional pneumatic boots, and a pilot-assist handle for preflight
4. Required installation of placards stipulating minimum airspeeds of 120 knots except 110 flaps up when exiting icing conditions, disengagement of autopilot in icing conditions, and exiting moderate or greater icing conditions.
5. Allowed the return of autopilot use in non-severe icing conditions

Conclusion

Airspeed decay with the autopilot engaged, combined with a stall warning system that was not designed for icing (not required for certification as aerodynamic buffet was acceptable), were factors in almost every inflight icing event during the Cessna 208's first 30 years of operation.

Corrective actions on the Cessna 208, EMB-120, and Saab 340 were similar:

• Inclusion of a low airspeed awareness system for the Cessna 208
• Modification of the stall warning systems on the EMB-120 and Saab 340 by adding a schedule for icing

Since incorporation of these two corrective actions, there were no fatal icing accidents on these airplanes, which are certified to fly in icing, through the time period of this module's initial publication in 2020.

There was one fatal icing accident, an Embraer 500 / Phenom 100 on December 8, 2014 in Gaithersburg, MD. (NTSB DCA15MA029). This airplane had been certified in 2008 and incorporated a stall warning system with an icing schedule. However, the stall warning was not activated because the pilot did not turn on the ice protection system in accordance with the AFM Limitations section. The NTSB concluded, "Had the pilot turned on the airplane de-ice system, he likely would have received the aural stall warning about 20 seconds earlier and well before the stall break." Additionally, there was a fatal Cessna 208 icing accident in Canada in 2015; but the accident airplane was not certified for icing and was not equipped with the low airspeed awareness system as mandated by the airworthiness directive.

The new ice certification regulations and guidance material have dramatically improved the safety of flight in icing conditions for certain Part 23 airplanes. The following table lists the Part 23 airplanes that have been certified for icing since 2000; all of them certified to the new standards. 

Part 23 airplanes that have been certified for icing since 2000:

Models Certified at Amendment 23-43	Year Certified	No. of U.S. Registered Airplanes
Hawker Beech 390	2001	206
Extra 400	2002	12
Emivest SJ30-2	2007	5
Diamond DA-42 / DA-62	2007	223* icing certification optional
Cessna 510	2007	301
Embraer 500	2008	187
Eclipse 500	2008	271* icing certification optional
Cirrus SR22	2009	4191* icing certification optional
Embraer 505	2009	309
Cessna 525C	2010	221
Quest Kodiak 100	2012	144
PZL M28	2013	17
Cessna T240	2013	135* icing certification optional
Honda HA-420	2016	113
Cirrus SF50	2016	80
Pilatus PC-24	2017	6

 

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At the time of the Barrow, AK and Parks, AZ accidents, the NTSB attributed these accidents to pilot error that included flying in icing conditions, leading to an inadvertent stall. It was not until the Moscow accident, with the benefit of CVR and FDR, that the accident board determined there were three key findings impacting a series of Cessna 208 airplanes that consisted of minimum airspeed, stall warning, and autopilot use.

In 2004, the NTSB conducted an icing assessment study of accidents and incidents involving Cessna 208 series airplanes. In addition to the study, the NTSB found that accidents in Winnipeg and Moscow showed that the minimum operating airspeed in icing conditions of 105 knots did not provide an adequate margin of safety for pilots who encounter icing in flight. In the Moscow accident, the airplane departed controlled flight only three knots slower than the published minimum operating icing airspeed.

In addition to the Moscow accident being an example of the minimum airspeed finding, the NTSB also determined no stall warning was provided to the pilots prior to the stall. The sound of the stall warning horn was not heard on the CVR until after the disconnect of the autopilot and onset of the roll excursion.

Investigators determined if the pilots involved in the Moscow accident had been flying the airplane manually (without the autopilot engaged), they likely would have noticed the increased control wheel force needed to maintain altitude, become aware of the airplane's altered performance characteristics, and increased their airspeed or otherwise altered their flight situation to avoid the loss of control. Thus, the NTSB recommended to manually fly the Cessna 208 in icing conditions so pilots could sense the aerodynamic effects of icing and enhance their ability to retain control of the airplane. At the time of the Barrow, AK and Parks, AZ accidents, autopilot was not identified as a specific finding.

View Barrow, AK ANC02FA020 Accident Report with Findings.

View Parks, AZ DEN03FA012 Accident Report with Findings.

No accident report for the Moscow accident is available.

View 2006 NTSB Recommendations for additional information.

 

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The NTSB issued the following icing-related recommendations to the FAA:

• A-98-96: "Require the manufacturers and operators of all airplanes that are certificated to operate in icing conditions to install stall warning/protection systems that provide a cockpit warning (aural warning and/or stick shaker) before the onset of stall when the airplane is operating in icing conditions."
• A-98-97: "Require all operators of turbo-propeller-driven air carrier airplanes to require pilots to disengage the autopilot and fly the airplane manually when they activate the anti-ice systems."
• A-03-53: "Convene a panel of aircraft design, aviation operations, and aviation human factors specialists, including representatives from the National Aeronautics and Space Administration, to determine whether a requirement for the installation of low-airspeed alert systems in airplanes engaged in commercial operations under 14 Code of Federal Regulations Parts 121 and 135 would be feasible, and submit a report of the panel's findings."

The NTSB initiated an in-depth assessment in late 2003 of 26 icing-related occurrences involving Cessna 208 series aircraft from 1987 to 2003. As a result of this assessment, on December 15, 2004 the NTSB issued the following Cessna 208 icing-specific recommendations to the FAA:

• A-04-64: "Require all pilots and operators of Cessna 208 series airplanes equipped for flight into known icing conditions to undergo seasonal training for ground deicing and flight into icing conditions on an annual basis. This seasonal training should be timed to precede the operator's cold weather operations and should specifically address (1) the limitations of the Cessna 208 in icing situations; (2) the Cessna 208 deice and anti-icing systems and controls and their use; (3) pilot actions during cold weather ground operations, with emphasis on the need for careful visual and tactile examination of wing and horizontal stabilizer upper surfaces during the preflight inspection to ensure that they are free of ice before takeoff; (4) pilot actions during cold weather flight operations, with emphasis on the timely recognition of potentially dangerous accumulations of ice and the importance of having an appropriate strategy for escaping the icing conditions and acting on that strategy promptly; (5) the hazards of performance degradation caused by ice that remains after activation of the deice boots; and (6) Cessna 208 Pilot Operating Handbook icing-related limitations, warnings, and notes."
• A-04-65: "Require Cessna Aircraft Company, working with Cessna 208 operators, to develop effective operational strategies and related guidance materials to minimize the chance of Cessna 208 icing events; the FAA should then verify that these strategies and guidance materials are incorporated into Cessna 208 operator manuals and training programs in a timely manner."
• A-04-66: "Require all pilots and operators of Cessna 208 series airplanes to conduct a visual and tactile examination of the wing and horizontal stabilizer leading edges and upper surfaces to ensure that those surfaces are free of ice and/or snow contamination before any flight from a location at which the temperatures are conducive to frost or ground icing."
• A-04-67: "Evaluate its current procedures for surveillance of operators of Cessna 208 series airplanes equipped for flight into known icing conditions to determine whether the surveillance effectively ensures that these operators are in compliance with Federal deicing requirements and, if necessary, modify the surveillance procedures to ensure such compliance."

As a result of the November 19, 2005 Moscow accident, and a Cessna 208 fatal accident soon after takeoff in Winnipeg, Canada on October 6, 2005, the NTSB issued the following Cessna 208 specific recommendations to the FAA:

• A-06-01: "Require all operators of Cessna 208 series airplanes to maintain a minimum operating airspeed of 120 knots during flight in icing conditions, even if a descent is required to do so."
• A-06-02: "Prohibit all operators of Cessna 208 series airplanes from conducting flight into any icing conditions determined to be more than light icing."
• A-06-03: "Require all operators of Cessna 208 series airplanes to disengage the autopilot and fly the airplane manually when operating in icing conditions."

On January 31, 2006, the Transportation Safety Board of Canada issued two recommendations each to Canadian operators of the Cessna 208 and to the FAA that duplicate NTSB safety recommendations A-06-01 and A-06-02.

 

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Prior to 1973, there were no requirements to test Part 23 airplanes in icing conditions. Part 23 airplanes were approved for flight in light icing conditions, and moderate icing for a limited time, if they were properly equipped. Many of these airplanes remain in the fleet today. Bureau of Flight Standards Release 434

Section 23.1419, at Amendment 23-14 in 1973, required the capability to operate safely in the icing conditions defined in Appendix C of part 25.

Advisory Circular (AC) 23.1419-1 was issued in 1986 to define one means of compliance to Section 23.1419. However, the stall warning was not specifically addressed and aerodynamic stall warning (clear and unambiguous buffet) was accepted for flight in icing even if the airplane was equipped with a stall warning system.

Section 23.1419, at Amendment 23-43, in 1993, defined "capable of operating safely" in icing as compliance to most of the Part 23, Subpart B performance and flight characteristics in icing.

AC 23.1419-2B, issued in 2002, required stall warning to comply with § 23.207(a), (b), and (c) with critical ice accretions, and the means of stall warning in icing conditions should be the same as in non-icing conditions. It also required applicants to address inter-cycle ice and residual ice on pneumatic deicing boots when evaluating critical ice accretions. This led to dry air ice-shape testing with critical ice shapes on the entire wing span.

Ice accretions on pneumatic deicing boots in normal system operation can reduce the angle of attack (faster airspeed) at which the wing will stall. In new designs, airplane manufacturers have adjusted the stall warning to activate at a higher speed in icing conditions.

Airplanes certified for icing at Amendment 23-43 were also subject to a means of compliance issue paper, which detailed how to determine critical ice accretion.

The FAA rewrote Part 23, effective August 30, 2017, using non-prescriptive language in Amendment 23-64. The requirements of §23.1419 were written into §23.2165 and §23.2540. §23.2165 addresses airplane performance and flight characteristics and mandates the stall warning requirements in AC 23.1419-2D. §23.2540 requires a low airspeed awareness system in icing conditions when the autopilot is engaged. Means of compliance are in ASTM F3120, "Standard Specification for Ice Protection for General Aviation Aircraft." 

14 CFR §25 Appendix C, Icing Design Envelopes

AC 20-73A, Aircraft Ice Protection 

 

 

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It was widely understood that if an airplane was certified to fly in known icing conditions, it was safe to operate when the aircraft is properly equipped with ice protection systems, pilots are trained to fly in these conditions, and the aircraft is operated in accordance with these instructions.

 

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• Published minimum aircraft speeds in icing conditions were incorrect
• Initial accident investigation findings did not determine the correct cause of the accidents
• The early stall warning system was not effective in icing conditions
• The use of autopilot in icing conditions masked stall warning buffet and declining airspeed

 

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Published airspeeds for flight in icing conditions are correct.

• It was determined that certificated minimum icing airspeeds for the Cessna 208 were too slow to avoid inadvertent stalls.

The stall warning system works properly in all flight conditions throughout the aircraft envelope.

• The Cessna 208 stall warning system did not activate prior to stall in icing conditions and required a replacement system.

 

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Ardmore, Oklahoma, March 02, 1995

A Cessna 208-B crashed near Ardmore, Oklahoma on March 2, 1995 due to icing. The NTSB determined the probable cause of this accident to be the pilot's continued flight into adverse weather conditions. Factors were that icing conditions prevailed at the destination airport, and the pilot was unable to maintain visual conditions due to windshield ice, which completely obscured his vision.

Structural ice from freezing drizzle rapidly accrued on the airplane while the pilot was performing a procedure turn during an instrument approach. He applied maximum power but was unable to stop the descent. Because he maintained 110 knots, after breaking out of the clouds, he was able to perform a forced landing. However, he was unable to arrest his descent, and the airplane impacted terrain in a five-degree nose-down attitude. The instrument-rated commercial pilot suffered only minor injuries.

NTSB accident report: FTW95FA129

Grand Island, Nebraska, January 20, 1998

A Cessna 208-B, registered as N738FX, had a hard landing at Grand Island Airport, Nebraska. The NTSB determined the probable cause of this accident was ice build-up on the airplane's wing and empennage which led to an inadvertent stall and hard landing. Factors contributing to this accident were the pilot's inadvertent flight into icing conditions.

Moderate ice began forming on the airplane in cruise flight at 4,000 feet. The pilot requested and received clearance to climb to a higher altitude; however, the plane could not climb higher than 6,700 feet. The pilot maintained an airspeed between 125 and 140 knots while on approach for runway 31 at Grand Island. Once the airplane was over the runway, the pilot decreased the power from cruise and the airplane began to descend. The pilot reapplied power, but this did not arrest the sink rate and the airplane touched down hard on the runway.

NTSB accident report: CHI98LA084

Monroe, Michigan, January 9. 1997

In 1997 a Comair 3272, an EMB-120, stalled during a hold in icing conditions. This airplane is a transport category turboprop; but with a maximum weight of 29,000 pounds it is about the same size as a Part 23 commuter category airplane. This accident resulted in numerous NTSB safety recommendations and an FAA Airworthiness Directives on the EMB-120 to install an ice detector and require boot activation at the first sign of ice accretion.

NTSB accident report: DCA97MA017

West Palm Beach, Florida, March 2001

Following issuance of numerous ADs after the Monroe, Michigan accident, another EMB-120 experienced a stall in 2001. This incident, Comair 5054, had no fatalities, but the airplane took 7,000 feet to recover from a stall at 17,000 feet. The FDR and CVR showed the autopilot was engaged and the airspeed decayed after ice was detected and the boots were activated. Analysis by the NTSB showed that an aerodynamic stall occurred prior to the stall warning.

NTSB accident report: DCA01MA031

Comair 5054 Flight Data Recorder Data

 

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In support of the Cessna 208 icing-related accidents, there were numerous safety initiatives, directives, and notices instituted to include:

• In 1991, the FAA issued Air Carrier Operations Bulletin NO. 2-91-2 (ACOB) that indicated the identification of a possible negative trend when the Cessna 208 aircraft was operated in icing conditions.
• In 1995, Cessna implemented a Safety Awareness Program for Cessna 208 operators. It emphasized the importance of pilots immediately getting out of icing conditions because performance progressively degrades if ice remains after repeated activation of the deicing boots. The program also instructed that pilots might need to use maximum continuous engine power to get out of icing conditions and counter the loss of airspeed.
• AD 96-09-15 added an AFM Limitation to provide the flight crew with recognition cues for, and procedures for exiting from, severe icing conditions, and to limit or prohibit the use of various flight control devices. A similar AD was issued to numerous pneumatic boot equipped airplanes.
• In 1996, Cessna issued a Known Icing Equipment supplement regarding aircraft performance degradation.
• In response to the Embraer EMB-120 icing accident in Monroe, Michigan, Comair 3272, the NTSB issued recommendations addressing industry-wide concerns for all turbo-propeller aircraft when encountering icing conditions. It addressed training, guidance updates, needed research and analysis, minimum maneuvering airspeed information, and more. NTSB Safety Recommendation, November 30, 1998 - A-98-88 through 106
• In 2001-2002, the FAA conducted an evaluation regarding training and qualification programs and surveillance. They issued a system safety analysis Alaska Interim report
• In January 2003, the FAA issued Notice N8400.39, "Minimum Maneuvering Airspeeds and Flight in Icing Conditions."
• The NTSB issued the first of two lengthy Safety Recommendations that specifically addressed Cessna 208 operations in icing conditions and outlined deficiencies with certification standards, operational procedures, and deice/anti-ice design issues. NTSB Safety Recommendation, December 15, 2004 - A-04-64 through -67
• AD 2005-07-01 Required AFM Limitations to ensure pilots inspect the upper wing surface in ground icing conditions prior to takeoff and provided 105 knots minimum airspeeds in icing conditions.
• AD 2006-01-11 Required installation of a pilot-assist handle to facilitate pre-flight contamination inspection of the upper wing surface and of the pneumatic deicing boots on the landing gear and cargo pod.
• Investigators Urge Restrictions on Cessna 208 Icing Operations was published in February 2006 by the Flight Safety Foundation due to fatal loss-of-control accidents.
• In January 2006 the NTSB issued safety recommendations to Transport Canada: 465 that stated:
  • Do not dispatch airplane into forecasted icing conditions if exceed light, and prohibit operations in this icing
  • Require Cessna maintain 120 knots and exit as soon as performance degrades
• The NTSB issued the second of two lengthy Safety Recommendations that specifically addressed the Cessna 208 in icing conditions and concerns about deficiencies in its cold weather procedures. The document noted that it was imperative that the FAA determine airspeeds that provide the performance, controllability, maneuverability, stability requirements, and adequate stall warning thresholds per current icing certification requirements. This is because the immediate establishment of conservative airspeed margins is critical for continued safe flight in icing conditions. NTSB Safety Recommendation, January 17, 2006 - A-06-01 through -03
• AD 2006-06-06 provides information to prevent loss of control of the airplane while inflight during icing conditions. The AD prohibited continued flight after encountering moderate or greater icing conditions and provided the following cues for pilots to recognize moderate icing:
  • Indicated airspeed in level cruise flight at constant power decreases by 20 knots
  • Engine torque required to maintain airspeed increases by 400 foot pounds
  • Airspeed of 120 knots cannot be maintained in level flight
  • An accretion of 1/4-inch of ice is observed on the wing strut

In addition, the AD required AFM Limitations to disconnect the autopilot in all icing conditions, added an AFM warning that the stall warning system was unreliable in icing conditions, added a requirement to extend flaps below 110 knots.

• Advisory Circular 23.1419-2D was published in 2007 and added guidance for ice on protected surfaces and stall warning with critical ice accretions. A means of compliance issue paper also applied to all new Part 23 icing certification projects. That paper supplements AC 23.1419-2D in the areas of critical ice accretions, stall warning, and stall recovery with the autopilot engaged.
• AD 2007-10-15 Requires installation of a low-airspeed awareness system for icing conditions and removal of the autopilot prohibition in non-severe icing conditions. It also requires AFM Limitations for mandatory recurrent icing training and added a WAT (weight/altitude/temperature) limitation chart for icing.
• In 2007, the Cessna 208 was the first model to incorporate recurrent icing training via AFM Limitations and also was mandated by AD 2007-10-15.
• Advisory Circular 91-74A, The FAA published "Pilot Guide: Flight in Icing Conditions" in 2007. It included a summary of icing certification requirements, which highlighted that in older certifications they used buffet during icing certification in lieu of a stall warning system, and did not account for ice on protected surfaces. Thus, the FAA issued Advisory Circular 91-74B Pilot Guide: Flight in Icing Conditions in 2015.
• The FAA issued a Special Airworthiness Information Bulletin (SAIB CE-11-18), effective January 24, 2011, to inform pilots of normal, utility, acrobatic, and commuter category (Part 23) airplanes certificated before 2000 of the potential hazards associated with stall warning characteristics in icing conditions. It also added a warning about unrecognized airspeed decays with the autopilot engaged in icing conditions.
• Amendment 23-64 was issued on December 30, 2016 and introduced high level, performance-based language into Part 23 with corresponding standards developed by the American Society for Testing and Materials (ASTM) F44 committee. It contained the following significant changes to the icing certification rules:
  • Section 23.2165 retained the requirement to comply with Subpart B requirements in Part 25, Appendix C icing conditions
  • Section 23.2165 added a requirement that the stall warning in icing must be the same as in non-icing conditions
  • Section 23.2165 added a requirement to detect and safely exit icing conditions if the airplane is not certified for those conditions beyond Part 25, Appendix C (SLD), as well as associated AFM Limitation requirements (prohibition of intentional flight in freezing drizzle and freezing rain)
  • Section 23.2540 added a requirement for low airspeed awareness in icing when the autopilot is engaged. This requirement targeted certain STCs on aircraft certified prior to Amendment 23-43. For new airplanes, the stall warning required in 23.2165 is a valid means of compliance

 

 

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The FAA issued the following Airworthiness Directives related to Cessna 208 icing:

AD 96-09-15 Added AFM Limitation that provided the flight crew with recognition cues for, and procedures for exiting from, severe icing conditions, and to limit or prohibit the use of various flight control devices. A similar AD was issued to numerous pneumatic boot equipped airplanes.

AD 2005-07-01 Required AFM Limitations for pilots to conduct tactile inspection of the upper wing surface during icing conditions prior to takeoff. It also provided 120 knots minimum airspeeds in icing conditions.

AD 2006-01-11 Required installation of a pilot-assist handle to facilitate pre-flight contamination inspection of the upper wing surface and pneumatic deicing boots on the landing gear and cargo pod.

AD 2006-06-06 Warning that aural stall warning system does not function properly in all icing conditions and should not be relied upon. Additionally, required placards regarding:

• Flight in moderate or greater icing prohibited
• 120 knots minimum in icing flaps up except 110 knots if climbing to exit icing
• Disconnect autopilot at first indication of ice accretion

AD 2007-10-15 Required installation of a low airspeed awareness system for icing conditions and removed the prohibition of using the autopilot in non-severe icing conditions. An AFM Limitation required that pilots receive recurrent icing training and added a WAT (weight/altitude/temperature) limitation chart for icing to the AFM. The FAA issued similar ADs to modify the stall warning system in icing conditions on other airplanes.

 

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Airplane Life Cycle

• Operational
• Maintenance / Repair / Alteration

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Accident Threat Categories

• Icing
• Loss of Control

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Groupings

• Business/Commercial
• Personal

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Accident Common Themes

• Organizational Lapses
• Flawed Assumptions

 

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Organizational Lapses

In a period of approximately two decades, more than 25 inflight icing-related accidents occurred in the Cessna 208. Many of the accidents investigations concluded that pilot error was the cause due to flightcrews entering icing conditions, resulting ice accumulation and an imperceptible decline in airspeed that resulted in a stall. While the government and industry sought certification, operation, and training solutions, it was not until the Monroe, Michigan EMB-120RT accident in 1997 that new information came to light.

Prior to this accident, the following misperceptions existed:

• Ice on leading edges that have ice protection does not have to be accounted for in certification
• Buffet can be used for stall warning in icing conditions if the stall warning system is designed only for non-icing conditions
• The stall warning system works in icing conditions since the sensor is heated
• Minimum airspeeds in the AFM Limitations section are sufficient to prevent pilots from approaching stall in icing conditions

Certification standards at the time the Cessna 208 was certified were not required to account for the aerodynamic effects of ice on leading edges, factors other than buffet, or reduction in airspeeds caused by increased drag, resulting in insufficient operational safety margins. During the EMB investigation, key information was obtained from both the FDR and CVR that offered insight into the use of autopilot, reduced airspeed, and stall situations. Additionally, the Moscow Cessna 208 aircraft that crashed in November 2005 was the first Caravan to have these recorders at the time of this accident. Information gleaned from these devices mirrored similar data found in the earlier EMB accident. These similarities helped industry, the manufacturer, and the FAA make advances in research and regulatory changes that resulted in a dramatic reduction in the risk of these types of icing accidents.

 

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In addition to the three Cessna 208 inflight icing accidents discussed in this module, these similar Caravan accidents occurred between 1985 and 2005:

• ANC88FA022 - December 19, 1987 in Bethel, AK - aircraft weight and balance exceeded; inadequate surveillance, wing ice, ice/frost removal not performed (one of the 21 U.S. accidents studied by the NTSB)
• NYC90FA060 - January 29, 1990, Schuyler Falls, NY - poor weather evaluation of snow and fog; ice/frost removal not performed; airspeed not maintained; wing/skin contamination; inadvertent stall (one of the 21 U.S. accidents studied by the NTSB)
• NYC90FA061 - January 29, 1990, Williston, VT - improper preflight planning; procedures not followed; snow with wing contamination; climb not attained; airspeed not maintained; inadvertent stall (one of the 21 U.S. accidents studied by the NTSB)
• DEN90FA068 - February 27, 1990, Denver CO - snow, drizzle, mist and icing conditions; wing ice; inadvertent stall; loss of control (one of the 21 U.S. accidents studied by the NTSB)
• 208-0310 - December 4, 1994, Oslo, Norway - no report available
• FTW95FA094 - January 18, 1995, Lubbock, TX - freezing rain; inadequate preflight surveillance; ice/frost removal not preformed; procedures not followed; uncontrolled decent; forced landing (one of the 21 U.S. accidents studied by the NTSB)
• FTW95FA129 - March 2, 1995, Ardmore, OK - icing conditions with drizzle/mist; continued flight into known adverse weather; airframe ice; level off not possible; windshield ice; visual lookout not possible; inadequate equipment, emergency landing; flare not possible (one of the 21 U.S. accidents studied by the NTSB)
• ANC97MA161 - April 10, 1997, Wainwright, AK - weight and balance exceeded; low ceiling with fog and icing conditions; intentional VFR into IMC; altitude not maintained (one of the 21 U.S. accidents studied by the NTSB
• ANC98MA008 - November 8, 1997, Barrow, AK - asymmetrical fueling and improper refueling; weight and balance disregarded; aircraft control reduced; improper ice/frost removal; self-induced pressure; inadequate surveillance; inadvertent stall; inflight collision; uncontrolled descent (one of the 21 U.S. accidents studied by the NTSB
• CHI98LA084 - January 20, 1998, Grand Island, NE - icing conditions; inadvertent flight into adverse weather; empennage and wing ice; inadvertent stall; loss of control; hard landing; inadvertent swerve and roll (one of the 21 U.S. accidents studied by the NTSB)
• MIA98FA091 - March 5, 1998, Clarksville, TN - encounter with weather; icing conditions; control not maintained; inadvertent stall (one of the 21 U.S. accidents studied by the NTSB)
• CHI98FA119 - April 7, 1998, Bismarck, ND - icing conditions; inadequate airspeed, lack of experience, inadvertent stall, uncontrolled descent (one of the 21 U.S. accidents studied by the NTSB)
• ANC00LA017 - December 6, 1999, Bethel, AK - icing conditions; airframe ice; inadequate preflight surveillance; loss of control; emergency landing (one of the 21 U.S. accidents studied by the NTSB)
• DEN01FA094 - May 5, 2001, Steamboat Springs, CO - icing conditions; warning lights activated; diverted attention; lack of pilot experience; inadvertent stall; uncontrolled descent (one of the 21 U.S. accidents studied by the NTSB)
• 208B0580 - April 28, 2001, Roque Perez, Argentina - icing conditions; loss of control
• DCA02MA003 - October 10, 2001, Dillingham, AK - icing conditions; inadequate ice/frost removal; inadequate preflight; loss of control; uncontrolled descent (one of the 21 U.S. accidents studied by the NTSB)
• IAD02LA021 - December 20, 2001, Lewiston, ME - snow conditions; wing contamination; inadequate preflight; climb rate not attained; aborted takeoff (one of the 21 U.S. accidents studied by the NTSB)
• CHI02FA093 - March 15, 2002, Alma, WI - icing conditions; ice/frost removal not performed; intentional flight into adverse weather; aircraft performance deteriorated; altitude/clearance not possible (one of the 21 U.S. accidents studied by the NTSB)
• FTW03FA089 - January 24, 2003, San Angelo, TX - icing conditions; anti-ice/deice system not activated; inflight encounter with weather; airspeed not maintained; inadvertent stall; uncontrolled descent (one of the 21 U.S. accidents studied by the NTSB)
• DEN04MA015 - October 29, 2003, Cody, WY - icing conditions; inappropriate medication/drugs used; aircraft control not maintained; alternate destination not performed; aircraft control not maintained; inadvertent stall; uncontrolled descent (one of the 21 U.S. accidents studied by the NTSB)
• NYC04SA023 - icing conditions; loss of control; hard landing (one of the 21 U.S. accidents studied by the NTSB)
• SEA05FA025 - December 12, 2004, Bellevue, ID - icing conditions; aircraft control not maintained; in adequate airspeed
• A05C0187 - Canadian registration C-FEXS, October 6, 2005, Winnipeg, Canada, operated by Morning Star Air Express as cargo flight 8060 - icing conditions; known flight into instrument meteorological conditions; loss of control
• NYC06RA032 - November 19, 2005, Moscow, Russia - icing conditions; level at 5,000 feet; steep descent; loss of control

Additional related inflight icing accidents included:

• DCA97MA017 - January 9, 1997, Monroe, Michigan, an Embraer EMB-120 - icing conditions; airframe ice; inadequate certification/approval; operator information unclear; inadequate certification/approval; inadequate in-flight decision; airspeed not maintained; stall; uncontrolled descent
• DCA01MA031 - March 19, 2001, West Palm Beach, FL, an Embraer EMB-120 - severe icing conditions; airspeed not maintained; inadvertent stall; loss of control; structural damage
• LAX06IA076 - January 2, 2006, Santa Maria, CA, a Saab 340B - supercooled large drop (SLD) icing conditions; inadequate airspeed; loss of control; climb in icing using prohibited autopilot mode

 

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Technical Related Lessons

Aircraft certification for flight in icing conditions requires sufficient operational safety margins for stall warning, stall protection, and critical ice accretion on both protected and unprotected surfaces. (Threat Category: Icing)

• Aircraft certification in icing conditions includes verification that an aircraft's stall warnings occur prior to a stall, which allows pilots time to react; that the stall protection systems operate in icing conditions; and, that ice accretion on protected surfaces, such as pneumatic deicing boots, as well as on unprotected surfaces, are considered in the overall evaluation of the aircraft.
• Certification standards at the time the Cessna 208 was certified did not require a stall warning system to account for the reduction in stall angle of attack with critical ice accretions in normal operation, within the Part 25, Appendix C icing environment. Instead, airplane buffet was accepted for stall warning in icing conditions. However, Cessna 208 pilots were unable to detect a stall warning buffet with the autopilot engaged, which resulted in an inadvertent stall and subsequent loss of control.

Relatively slight amounts of inflight ice accretion on the wing leading edges of certain Part 23 airplanes can have catastrophic effects on controllability. (Threat Category: Icing)

• Small airplanes, such as the Cessna 208 and similar configurations, have exhibited a greater vulnerability to the effects of contamination compared to those of large transport jet airplanes. This is especially true for the effects of wing leading edge contamination and its impact on stall speed. Icing conditions typically exist at lower altitudes where small airplanes commonly operate and large airplanes can avoid. In addition, the scale relativity of small versus large airfoils results in greater adverse impact to controllability.

Common Theme Related Lessons

Catastrophic incidents and accidents that reveal high fleet risk with poorly understood causes require urgent, effective, and tangible actions to reduce the likelihood of future accidents. It is imperative to employ an interim fix that either limits exposure (e.g. prohibit flight in icing conditions) or terminates flights until a permanent solution can be implemented. (Common Theme: Organizational Lapses)

• Regulators needed to continuously review and assess, through accident investigation, surveillance, inspections, and processes, whether observed occurrences, recommended action, enforcement, or a change in policy or directive had a positive outcome and reduce the risk for further accidents in inflight icing conditions.
• Beginning in 1985 through the first decade of operation, Cessna 208 aircraft experienced six accidents while operating in inflight icing conditions. In 1991 the FAA issued with an Air Carrier Operations Bulletin (ACOB) noting a negative trend and in 1996 issued an AD directed at pilot training and minimizing operations in severe icing.
• Prior to the 2005 Moscow accident, there was no FDR or CVR data for Cessna 208 icing accidents, since these recorders were not installed. The FAA used lessons learned on a similar airplane with icing accidents (Comair Embraer), to request Cessna to conduct Cessna 208 flight tests to determine the effect of inter-cycle ice on climb performance and stall warning.
• The FAA issued an Airworthiness Concern Sheet in 2004 to Cessna 208 pilots. The feedback from the pilots led to FAA research in 2005 that showed inter-cycle ice on pneumatic deicing boots increases as airspeed and ambient temperature decreases.
• In 2005, with the issuance of AD 2005-07-01, and after another series of accidents, the FAA provided expanded AFM Limitations regarding minimum icing airspeeds. However, two fatal accidents in the winter of 2005 showed that relying on AFM language to prevent airspeed decay to the stall was not sufficient. A system to warn the pilot of low airspeed in icing conditions was warranted. (Such a system was eventually mandated in 2007 by AD 2007-10-15).
• In 2004, the NTSB recommended an FAA review of Cessna 208 pilot training as a result of an increased number of icing-related accidents. The FAA initially responded by only issuing Advisory Circulars pertaining to seasonal training and guidance material and reviewing Cessna's Winter Weather Training Program. After two additional fatal accidents in the winter of 2005, the Regional Air Cargo Carriers Association (RACCA) and Cessna asked the FAA to require icing training for Cessna 208 pilots. The FAA implemented the resulting recurrent training for icing in 2007 via an AFM Limitation and mandated the training by AD 2007-10-15.
• Since a 1999 public conference to address NTSB safety recommendations in icing, the FAA and industry position had been to leave the autopilot engaged in icing conditions for which airplanes are certified to operate. It was not until the 2005 Moscow accident in which an FDR and CVR showed that, with the autopilot engaged, a 1-2 knot airspeed decay in icing can go undetected in the Cessna 208. As a result, the FAA issued AD-2006-06-06, which required pilots to disconnect the autopilot in all icing conditions in order to enhance the tactile feedback available by manually flying the airplane. This prohibition was modified in 2007 to allow autopilot use in icing conditions for which the airplane is certified so long as the airspeed remains above the low awareness system threshold.
• Since the last AD in 2007, there have been no reported Cessna 208 fatal inflight icing accidents worldwide through early 2020 (the time of this initial publication).

 

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