U.S. Department

of Transportation


Federal Aviation



Air Traffic


A Communication from the  

Vice President,
  System Operations Services


Issue # 2004-7

*** SPECIAL ***

November 2004                                                                                             

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In this Issue:

Snow Removal a Collaborative Effort

Winter Operations and Runway Incursions

Good News and Bad News

Braking Action Advisories and PIREPs

Just a Reminder

Forewarned is Forearmed


Forecasting the Icing Hazard

Points to Remember

You Really Ought to Know

Snow Removal A Collaborative Effort
Due to climatic changes, snow removal is no longer just a northern issue.  Developing snow removal procedures and a familiarity program between controllers and municipal employees are vital to safety and runway capacity.  Snow and ice may be great for winter sports but, when it comes to aviation, it can be deadly. 

There are times during blizzard conditions when air traffic operations are discontinued.  However, during light to moderate snow or after a winter storm, activity on the taxiways and runways can be greatly increased.  Vehicular traffic can outnumber aircraft traffic.  Every affected municipality should have some form of snow removal policy and/or procedures in place detailing the priorities for snow removal that are based upon issues such as the wind, traffic volume, airport lighting, and weather forecasts.  Using this information, facilities can develop good operating procedures and memory joggers for and between ground and local control that can assist in reducing runway incidents and incursions.  Advising the municipalities of air traffic procedures, such as deicing time, restrictions, flow control, and daily ground traffic, should assist them in developing procedures that can accommodate air traffic's request.  Through collaborative efforts, positive results should be experienced. 

Prior to the winter season, a meeting between the tower and the municipality is suggested to review existing and new procedures.  Controllers should review snow removal procedures and be advised of any additional procedures not in place the previous year. 

It is important to keep in mind that during the snow removal operation on the ground, the view from the vehicle is totally different from that from the tower.  Blowing snow from either the wind or the vehicle itself can reduce visibility.  Snow can obscure airport signage.  Whiteout conditions can lead to disorientation by those operating on the airport surface.  Frequency congestion and nonstandard phraseology can feed apprehension and confusion to all airport users.  New employees of the municipality at the airport could benefit from a review of winter procedures. 

Allowing municipal employees the opportunity to view the airport from the tower and to witness the workings of a control tower can be very beneficial.  This firsthand knowledge displays to them that the controllers in the towers are collaborative partners and are there to assist them should the situation arise. 

Working together as a collaborative team can improve safety, reduce delays, and serve the flying public more efficiently. 

Winter Operations and Runway Incursions
/ER*FT/ Several factors that occur during the winter months deserve our attention in our quest to further reduce runway incursions. 

Personnel at stations that provide an airport advisory service, as well as those in towers, should keep the following factors in mind:

         Snow removal vehicle operation on runways and other movement areas.

         Aircraft taxiing slower because of surface conditions. 

         Aircraft needing and using more time to exit or cross runways because of surface conditions. 

         Various forms of precipitation reducing controller and pilot visibility. 

         Plowed snow drifts causing blind spots for taxiing aircraft. 

         Glare caused by bright sunlight reflecting off of snow (and ice) on the ground. 

These are just a few of the important items we must consider as we approach winter operations. 

I Have Some Good News and Some Bad News
/*FTER/ Do not keep operational information to yourself.  Whether it be good news or bad news, share the wealth (or the grief) with everyone who has an operational need.  To be useful, up-to-date information on both weather and field conditions must be in the hands of those who need it. 

If as a controller, you see that you are going to have to restrict traffic, relay that information up and down the line as soon as you can.  Do not surprise the adjacent sector at handoff time.  Of course, the other side of this coin is that when restrictions are no longer needed, do your part to get them lifted. 

As a briefer, make sure you give complete information to each pilot based on his/her requests.  Items such as freezing level and, of course, reports and forecasts of icing take on an added meaning this time of the year.  The type of delivery may vary with each individual pilot, but the final product does not.  Each contact with a pilot should result in enough information being conveyed so that the pilot can make an intelligent decision. 

A Seasonal Reminder About Braking Action Advisories and Pilot Weather Reports (PIREP)
/*FTER/ Runway braking action reports are furnished by the pilot or airport management.  These reports require categorization using the terms "good," "fair," "poor," "nil," or a combination of these terms. 

When braking action advisories are in effect and the braking action report affects only a portion of a runway, describe the braking action for that portion of the runway and issue it in descriptive terms to each arriving and departing aircraft. 

When the report includes the terms "poor" or "nil," or whenever conditions are conducive to deteriorating or rapidly changing runway conditions, terminal facilities are required to broadcast on the Automatic Terminal Information System the statement, "Braking action advisories are in effect."

Braking action PIREPs should be solicited when braking action advisories are in effect or when requested.  PIREPs should be solicited far enough in advance to allow the pilot time to adequately evaluate the situation and render a meaningful braking action report. 

Remember, it is not only our responsibility to solicit these reports when required, but also to issue this information in time for it to be useful to the pilot.  Procedures concerning this subject are in Federal Aviation Administration Order (FAAO) 7110.65, paragraphs 3-3-3, 3-3-4, and 3-3-5, and FAAO 7110.10, paragraphs 4-6-6 and 14-1-21. 

Just a Reminder
/*FTER/ Portions of title 14, Code of Federal Regulations, parts 91 and 135, prohibit visual flight rules flights into areas of known light icing under some conditions.  In addition, some military aircraft are extremely sensitive to airframe icing of any degree.  Therefore, it is important that all icing reports from pilots be processed in accordance with established procedures.  Soliciting and relaying PIREPs of light icing is also required. 

Forewarned is Forearmed
/*FTER/ The thought that we have been trying to convey throughout this document is the importance of communications.  While the focus of this issue is winter operations, there really is not anything special about winter operations.  Certainly, snow and ice on the ground are unlikely in the warmer months, but hazardous weather conditions of one kind or another are not unique. 

Weather-related information such as PIREPs, significant meteorological information (SIGMET), meteorological impact statements, central weather advisories, and other advisories always require special attention and handling.  This information will always be an important factor that pilots must consider.  No matter what time of year, timely action is important. 

Do not be fooled into thinking that winter weather replaces hazards that exist during the rest of the year.  Icing, turbulence, low-level wind shear, restricted visibility, and even thunderstorms can, and do, exist during the winter season. 

/*FTER/ Aircraft icing is one of the hazards we have been talking about that can be with us all year round, but gets extra attention during the winter.  Ice, including frost, can be a hazard because of the way it affects airframes and power plants.  Ice destroys the smooth flow of air, decreasing the ability of the airfoil to create lift.  Accumulations of ice on the outside of aircraft impair wing lift and propeller thrust.  Ice can reduce engine performance to dangerous levels.  In the most severe cases, icing can cause engine failure.  The double danger is that while lift and power are being reduced, that same icing is increasing the weight of the aircraft a deadly, vicious circle. 

There are several forecasts that contain warnings of icing.  However, PIREPs are the only source of actual icing reports.  PIREPs regarding icing are more than just nice-to-know information. 

Because of their importance, procedures for soliciting PIREPs are contained in FAAO 7110.65, paragraph 2-6-3, and FAAO 7110.10, paragraphs 9-2-5 and 9-2-9.  The briefing and broadcast paragraphs of these directives also contain PIREP handling procedures. 

The arrival of the winter season is a good time for all operational personnel to review PIREP procedures, especially those concerning icing. 

Forecasting the Icing Hazard
What do meteorologists at the National Aviation Weather Advisory Unit (NAWAU) look at when trying to determine if an icing hazard exists? How do they determine where the hazard will be during the valid time of the upcoming area aviation forecast?

In a nutshell, NAWAU meteorologists try to determine where there will be enough moisture to form clouds above the freezing level.  If they look at the moisture too far above the freezing level, they find they are tracking ice crystals instead of liquid water droplets. 

This brings up an important question.  Why is there liquid water above the freezing level? Liquid cloud droplets in an environment of rising air can rise a substantial distance above the freezing level, becoming colder and colder, without freezing as long as they remain undisturbed.  What is meant by "undisturbed"? If an airplane would happen to fly through these "supercooled" cloud droplets, the droplets would most likely freeze on impact with the aircraft.  The smaller drops would freeze instantly into rime ice.  If the clouds happened to be made up of larger drops, it might take a few seconds for the drops to freeze into a glaze of clear ice. 

Generally speaking, the stronger the upward motion of the droplets within the cloud, the greater the vertical distance droplets will rise before changing into ice crystals.  However, meteorologists studying clouds have learned that 95 percent of the cloud droplets at the -16 degrees Celsius (C) level have changed into ice crystals already, and at the -25 degrees C level, 99.9 percent of the droplets have changed to ice crystals.  Therefore, we should expect occurrences of aircraft icing to be scarce at temperatures around -16 degrees C and essentially nil at -25 degrees C or colder.

In addition to forecasting the horizontal and vertical extent of cloud formations above the freezing level, NAWAU meteorologists must determine which cloud areas will most likely contain significant amounts of supercooled water droplets during a specific 12- to18-hour period.  Needless to say, this is not an easy task. 

Points to Remember
/*TERF/ Base your advice to pilots concerning icing on forecasts and PIREPs.  Forecasts delineate general areas of icing potential; PIREPs pinpoint actual encounters.  In using PIREPs, remember that conflicting reports of type or intensity may be due to different types of aircraft.  By piecing together several reports, you can frequently get a more comprehensive picture of icing potential. 

An area forecast always contains a section on icing.  It specifies freezing levels, expected changes in freezing level, and altitudes where icing is most likely.  SIGMET and airmen's meteorological information are also excellent sources of icing information. 

Always pass on any icing reports to the forecaster or the appropriate person, and do not hesitate to ask for his/her help when needed.  He/she is in an excellent position to integrate pilot reports and the latest analyses and observations into a current picture of expected icing. 

Remember the following points:

         In stratiform clouds, rime icing may be very extensive horizontally.  An altitude change of the aircraft to either a flight level with above-freezing temperatures or a flight level colder than -5 degrees C likely will alleviate icing conditions.  An altitude change also may take the flight out of clouds. 

         In cumuliform clouds, clear ice usually is encountered with brief, heavy accumulations from 0 degrees C to -10 degrees C, and lesser amounts at lower temperatures.  Any flight path change to get out of the clouds and into visual conditions is in order. 

         In freezing rain due to frontal overrunning, a climb into the warmer air aloft is in order.  Above-freezing temperatures may be found at a lower level in some cases and then terrain must be considered. 

         Many small aircraft do not have deicing equipment, especially those based in warm climates.  Never assume that an aircraft has deicing equipment. 

         During icing or potential icing conditions, PIREPs are especially valuable to the entire aviation community and should be collected and transmitted into the system. 

         Ground icing, frost, and carburetor icing are generally considered operational problems.  However, if you are aware of ground icing or frost potential you can alert the pilot.

You Really Ought to Know
/*FTER/ As aviation professionals, we must all be knowledgeable about the basic conditions which are most likely to produce winter flying problems.  It does not take much time for a problem in the cockpit to become a problem in the air traffic facility.

Aircraft icing can occur either in the air or on the ground.  A common condition for icing is when an aircraft taxis through slush or water at or near freezing level.  It can also occur when aircraft fly through precipitation and the air temperature is near or below freezing level.  The most severe icing occurs with a free air temperature between
0 and -10 degrees C.  However, icing is not uncommon at much colder temperatures, all the way down to -40 degrees C.

Cumuliform clouds are more likely to produce serious ice formation than other clouds, particularly if freezing rain is present.  However, at altitudes above the freezing level, any layer of air with a narrow temperature dew point spread is a potential icing zone.  Remember, ice can form by sublimation, water going directly from its gaseous state to the solid state.  In this case, going directly from water vapor (always present in the atmosphere) to solid ice.  Types of aircraft icing include clear, rime, and mixed. 

Clear Ice 

Clear ice forms when, after initial impact, the remaining liquid portion of the water drop flows out over the aircraft surface, gradually freezing as a smooth sheet of solid ice.  This type of icing forms when drops are large as in rain or in cumuliform clouds.

Clear ice is hard, heavy, and tenacious.  Its removal by deicing equipment is especially difficult. 

Rime Ice 

Rime ice forms when water drops are small, such as those in stratiform clouds or light drizzle.  The liquid portion remaining after initial impact freezes rapidly before the drop has time to spread over the aircraft surface.  The small frozen droplets trap air between them giving the ice a white appearance. 

Rime ice is lighter in weight than clear ice and its weight is of little significance.  However, its irregular shape and rough surface make it very effective in decreasing aerodynamic efficiency of airfoils, thus reducing lift and increasing drag.  Rime ice is brittle and more easily removed than clear ice. 

Mixed Clear and Rime Ice 

Mixed clear and rime ice forms when water drops vary in size or when liquid drops are intermingled with snow or ice particles.  It can form rapidly and ice particles become imbedded in clear ice, building a very rough accumulation, sometimes in a mushroom-like shape on leading edges.

Icing-related problems that air traffic personnel should be alert to include intermittent, and sometimes total, loss of communications.  This problem can be created when aircraft antennae become ice-coated and sometimes fail.

Another situation to be alert for is false flight instrument indications that may be caused by pitot tube icing.  If an aircraft climb rate seems abnormally high, you may want the aircraft to verify the Mode C readout.  (ATP-120)