ENR 1.7 Altimeter Setting Procedures

  1. General
    1. The accuracy of aircraft altimeters is subject to the following factors:
      1. Nonstandard temperature of the atmosphere.
      2. Nonstandard atmospheric pressure.
      3. Aircraft static pressure systems (position error).
      4. Instrument error.
    2. EXTREME CAUTION SHOULD BE EXERCISED WHEN FLYING IN PROXIMITY TO OBSTRUCTIONS OR TERRAIN IN LOW TEMPERATURES AND PRESSURES. This is especially true in extremely cold temperatures that cause a large differential between the Standard Day temperature and actual temperature. This circumstance can cause serious errors that result in the aircraft being significantly lower than the indicated altitude.

      NOTE-

      Standard temperature at sea level is 15 degrees Celsius (59 degrees Fahrenheit). The temperature gradient from sea level is minus 2 degrees Celsius (3.6 degrees Fahrenheit) per 1,000 feet. Pilots should apply corrections for static pressure systems and/or instruments, if appreciable errors exist.

    3. The adoption of a standard altimeter setting at the higher altitudes eliminates station barometer errors, some altimeter instrument errors, and errors caused by altimeter settings derived from different geographical sources.
  2. Procedures
    1. The cruising altitude or flight level of aircraft must be maintained by reference to an altimeter which must be set, when operating:
      1. Below 18,000 feet MSL.
        1. When the barometric pressure is 31.00 inches Hg. or less: to the current reported altimeter setting of a station along the route and within 100 NM of the aircraft, or if there is no station within this area, the current reported altimeter setting of an appropriate available station. When an aircraft is en route on an instrument flight plan, air traffic controllers will furnish this information to the pilot at least once while the aircraft is in the controller's area of jurisdiction. In the case of an aircraft not equipped with a radio, set to the elevation of the departure airport or use an appropriate altimeter setting available prior to departure.
        2. When the barometric pressure exceeds 31.00 inches Hg.: the following procedures will be placed in effect by NOTAM defining the geographic area affected:
          1. For all aircraft. Set 31.00 inches for en route operations below 18,000 feet MSL. Maintain this setting until beyond the affected area or until reaching final approach segment. At the beginning of the final approach segment, the current altimeter setting will be set, if possible. If not possible, 31.00 inches will remain set throughout the approach. Aircraft on departure or missed approach will set 31.00 inches prior to reaching any mandatory/crossing altitude or 1,500 feet AGL, whichever is lower. (Air traffic control will issue actual altimeter settings and advise pilots to set 31.00 inches in their altimeters for en route operations below 18,000 feet MSL in affected areas.)
          2. During preflight, barometric altimeters must be checked for normal operation to the extent possible.
          3. For aircraft with the capability of setting the current altimeter setting and operating into airports with the capability of measuring the current altimeter setting, no additional restrictions apply.
          4. For aircraft operating VFR, there are no additional restrictions; however, extra diligence in flight planning and in operating in these conditions is essential.
          5. Airports unable to accurately measure barometric pressures above 31.00 inches of Hg. will report the barometric pressure as “missing” or “in excess of 31.00 inches of Hg.” Flight operations to and from those airports are restricted to VFR weather conditions.
          6. For aircraft operating IFR and unable to set the current altimeter setting, the following restrictions apply:
            1. To determine the suitability of departure alternate airports, destination airports, and destination alternate airports, increase ceiling requirements by 100 feet and visibility requirements by  1/ statute mile for each 1/ 10 of an inch of Hg., or any portion thereof, over 31.00 inches. These adjusted values are then applied in accordance with the requirements of the applicable operating regulations and operations specifications.

              EXAMPLE-

              Destination altimeter is 31.28 inches, ILS DH 250 feet (200- 1 / 2 ). When flight planning, add 300- 3 / 4  to the weather requirements which would become 500-1  1 / 4 .

            2. On approach, 31.00 inches will remain set. Decision height or minimum descent altitude must be deemed to have been reached when the published altitude is displayed on the altimeter.

              NOTE-

              Although visibility is normally the limiting factor on an approach, pilots should be aware that when reaching DH the aircraft will be higher than indicated. Using the example above the aircraft would be approximately 300 feet higher.

            3. These restrictions do not apply to authorized Category II and III ILS operations nor do they apply to certificate holders using approved QFE altimetry systems.
        3. The FAA Regional Flight Standards Division Manager of the affected area is authorized to approve temporary waivers to permit emergency resupply or emergency medical service operation.
      2. At or above 18,000 feet MSL : to 29.92′ Hg (standard setting). The lowest usable flight level is determined by the atmospheric pressure in the area of operation, as shown in TBL ENR 1.7-1.

        TBL ENR 1.7- 1
        Lowest Usable Flight Level

        Altimeter Setting
        (Current Reported)

        Lowest Usable
        Flight Level

        29.92 or higher

        180

        29.91 to 29.42

        185

        29.41 to 28.92

        190

        28.91 to 28.42

        195

        28.41 to 27.92

        200

      3. Where the minimum altitude, as prescribed in 14 CFR Sections 91.159 and 91.119, is above 18,000 feet MSL the lowest usable flight level must be the flight level equivalent of the minimum altitude plus the number of feet specified in TBL ENR 1.7-2.

        TBL ENR 1.7- 2
        Lowest Flight Level Correction Factor

        Altimeter Setting

        Correction Factor

        29.92 or higher

        none

        29.91 to 29.42

        500 feet

        29.41 to 28.92

        1000 feet

        28.91 to 28.42

        1500 feet

        28.41 to 27.92

        2000 feet

        27.91 to 27.42

        2500 feet

        EXAMPLE-

        The minimum safe altitude of a route is 19,000 feet MSL and the altimeter setting is reported between 29.92 and 29.42 inches of mercury. The lowest usable flight level will be 195, which is the flight level equivalent of 19,500 feet MSL (minimum altitude plus 500 feet).

      4. Aircraft operating in an offshore CONTROL AREA should use altimeter setting procedures as described above, unless directed otherwise by ATC.

        NOTE-

        Aircraft exiting the oceanic CTA/FIR destined for the U.S. or transitioning through U.S. offshore control areas should use the current reported altimeter of a station nearest to the route being flown. When entering an oceanic CTA/FIR from U.S. offshore control areas, pilots should change to the standard altimeter setting 29.92.

  3. Altimeter Errors
    1. Most pressure altimeters are subject to mechanical, elastic, temperature, and installation errors. (Detailed information regarding the use of pressure altimeters is found in the Instrument Flying Handbook, Chapter IV.) Although manufacturing and installation specification, as well as the periodic test and inspections required by regulations (14 CFR Part 43, Appendix E), act to reduce these errors--any scale error may be observed in the following manner:
      1. Set the current reported altimeter setting on the altimeter setting scale.
      2. Altimeter should now read field elevation if you are located on the same reference level used to establish the altimeter setting.
      3. Note the variation between the known field elevation and the altimeter indication. If this variation is in the order of plus or minus 75 feet, the accuracy of the altimeter is questionable and the problem should be referred to an appropriately rated repair station for evaluation and possible correction.
    2. Once in flight, it is very important to obtain frequently current altimeter settings en route. If you do not reset your altimeter when flying from an area of high pressure into an area of low pressure, your aircraft will be closer to the surface than your altimeter indicates. An inch error in the altimeter setting equals 1,000 feet of altitude. To quote an old saying: “GOING FROM A HIGH TO A LOW, LOOK OUT BELOW.”
    3. Temperature also has an effect on the accuracy of altimeters and your altitude. The crucial values to consider are standard temperature versus the ambient (at altitude) temperature and the elevation above the altitude setting reporting source. It is these “differences” that cause the error in indicated altitude. When the column of air is warmer than standard, you are higher than your altimeter indicates. Subsequently, when the column of air is colder than standard, you are lower than indicated. It is the magnitude of these “differences” that determine the magnitude of the error. When flying into a cooler air mass while maintaining a constant indicated altitude, you are losing true altitude. However, flying into a cooler air mass does not necessarily mean you will be lower than indicated if the difference is still on the plus side. For example, while flying at 10,000 feet (where STANDARD temperature is -5 degrees Celsius (C)), the outside air temperature cools from +5 degrees C to 0 degrees C, the temperature error will nevertheless cause the aircraft to be HIGHER than indicated. It is the extreme “cold” difference that normally would be of concern to the pilot. Also, when flying in cold conditions over mountainous terrain, the pilot should exercise caution in flight planning both in regard to route and altitude to ensure adequate en route and terminal area terrain clearance.

      NOTE-

      Non-standard temperatures can result in a change to effective vertical paths and actual descent rates while using aircraft Baro -VNAV equipment for vertical guidance on final approach segments. A higher than standard temperature will result in a steeper gradient and increased actual descent rate. Indications of these differences are often not directly related to vertical speed indications. Conversely, a lower than standard temperature will result in a shallower descent gradient and reduced actual descent rate. Pilots should consider potential consequences of these effects on approach minimums, power settings, sight picture, visual cues, etc., especially for high-altitude or terrain-challenged locations and during low-visibility conditions.

    4. TBL ENR 1.7-3, derived from ICAO formulas, indicates how much error can exist when operating in cold temperatures. To use the table, find the reported temperature in the left column, read across the top row to locate the height above the airport/reporting station (i.e., subtract the airport/ reporting elevation from the intended flight altitude). The intersection of the column and row is how much lower the aircraft may actually be as a result of the possible cold temperature induced error.
    5. Pilots are responsible to compensate for cold temperature altimetry errors when operating into an airport with any published cold temperature restriction and a reported airport temperature at or below the published temperature restriction. Pilots must ensure compensating aircraft are correcting on the proper segment or segments of the approach. Manually correct if compensating aircraft system is inoperable. Pilots manually correcting, are responsible to calculate and apply a cold temperature altitude correction derived from TBL ENR 1.7-3 to the affected approach segment or segments. Pilots must advise the cold temperature altitude correction to Air Traffic Control (ATC). Pilots are not required to advise ATC of a cold temperature altitude correction inside of the final approach fix.

      TBL ENR 1.7- 3
      ICAO Cold Temperature Error Table

       

      Reported Temp °C

      Height Above Airport in Feet

       

       

      200

      300

      400

      500

      600

      700

      800

      900

      1000

      1500

      2000

      3000

      4000

      5000

       

       

      +10

      10

      10

      10

      10

      20

      20

      20

      20

      20

      30

      40

      60

      80

      90

       

       

      0

      20

      20

      30

      30

      40

      40

      50

      50

      60

      90

      120

      170

      230

      280

       

       

      -10

      20

      30

      40

      50

      60

      70

      80

      90

      100

      150

      200

      290

      390

      490

       

       

      -20

      30

      50

      60

      70

      90

      100

      120

      130

      140

      210

      280

      420

      570

      710

       

       

      -30

      40

      60

      80

      100

      120

      140

      150

      170

      190

      280

      380

      570

      760

      950

       

       

      -40

      50

      80

      100

      120

      150

      170

      190

      220

      240

      360

      480

      720

      970

      1210

       

       

      -50

      60

      90

      120

      150

      180

      210

      240

      270

      300

      450

      590

      890

      1190

      1500

       

       

       

       

       

       

       

       

       

       

       

       

       

       

       

       

       

       

      EXAMPLE-

      Temperature-10 degrees Celsius, and the aircraft altitude is 1,000 feet above the airport elevation. The chart shows that the reported current altimeter setting may place the aircraft as much as 100 feet below the altitude indicated by the altimeter.

  4. High Barometric Pressure
    1. Cold, dry air masses may produce barometric pressures in excess of 31.00 inches of Mercury, and many altimeters do not have an accurate means of being adjusted for settings of these levels. As noted in paragraph 3.2, when the altimeter cannot be set to the higher pressure setting, the aircraft actual altitude will be higher than the altimeter indicates.
    2. When the barometric pressure exceeds 31.00 inches, air traffic controllers will issue the actual altimeter setting, and:
      1. En Route/Arrivals. Advise pilots to remain set on 31.00 inches until reaching the final approach segment.
      2. Departures. Advise pilots to set 31.00 inches prior to reaching any mandatory/crossing altitude or 1,500 feet, whichever is lower.
    3. The altimeter error caused by the high pressure will be in the opposite direction to the error caused by the cold temperature.
  5. Low Barometric Pressure
    1. When abnormally low barometric pressure conditions occur (below 28.00), flight operations by aircraft unable to set the actual altimeter setting are not recommended.

      NOTE-

      The true altitude of the aircraft is lower than the indicated altitude if the pilot is unable to set the actual altimeter setting.