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Altimeter Settings

Introduction:

  • Maintaining a current altimeter setting is critical because the atmosphere pressure is never constant
    • That is, in one location the pressure might be higher than the pressure just a short distance away
  • There are several factors which can impact the accuracy of an aircraft's altimeter which will inducealtimeter errors:
    • Nonstandard temperatures of the atmosphere
    • Nonstandard atmospheric pressure
    • Aircraft static pressure systems (position error)
    • Instrument error
  • Taking these errors into consideration, a set of altimeter procedures standarize altimeter settings in extreme conditions

Altimeter Accuracy:

  • 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° Fahrenheit)
  • The temperature gradient from sea level is minus 2° Celsius (3.6° Fahrenheit) per 1,000'
  • Pilots should apply corrections for static pressure systems and/or instruments, if appreciable errors exist
  • 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

Altimeter Errors:

  • Nonstandard Temperatures of the Atmosphere:

    • Temperatures can effect the accuracy of altimeters
    • 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 these “differences” and their magnitude that determine the error in indicated altitude
    • The crucial values to consider are standard temperature versus the ambient (at altitude) temperature and the elevation above the altitude setting reporting source
    • 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
    • ICAO Cold Temperature Error Table

      • The cold temperature induced altimeter error may be significant when considering obstacle clearances when temperatures are well below standard
      • Pilots may wish to increase their minimum terrain clearance altitudes with a corresponding increase in ceiling from the normal minimum when flying in extreme cold temperature conditions
      • Most flight management systems (FMS) with air data computers implement a capability to compensate for cold temperature errors. Pilots flying with these systems should ensure they are aware of the conditions under which the system will automatically compensate
      • If compensation is applied by the FMS or manually, ATC must be informed that the aircraft is not flying the assigned altitude
        • Otherwise, vertical separation from other aircraft may be reduced, creating a potentially hazardous situation
      • To use the table, find the reported temperature in the left column, and then read across the top row to the height above the airport/reporting station
      • Subtract the airport elevation from the altitude of the final approach fix (FAF)
      • The intersection of the column and row is the amount of possible error
        • Example: The reported temperature is -10° Celsius and the FAF is 500' above the airport elevation
        • The reported current altimeter setting may place the aircraft as much as 50' below the altitude indicated by the altimeter
      • When using the cold temperature error table, the altitude error is proportional to both the height above the reporting station elevation and the temperature at the reporting station
      • For IFR approach procedures, the reporting station elevation is assumed to be airport elevation
      • It is important to understand that corrections are based upon the temperature at the reporting station, not the temperature observed at the aircraft's current altitude and height above the reporting station and not the charted IFR altitude
      • To see how corrections are applied, note the following example:
        • Airport Elevation: 496'
        • Airport Temperature: - 50°C
      • A charted IFR approach to the airport provides the following data:
        • Minimum Procedure Turn Altitude: 1,800'
        • Minimum FAF Crossing Altitude: 1,200'
        • Straight-in Minimum Descent Altitude: 800'
        • Circling MDA: 1,000'
      • The Minimum Procedure Turn Altitude of 1,800' will be used as an example to demonstrate determination of the appropriate temperature correction
      • Typically, altitude values are rounded up to the nearest 100' level
      • The charted procedure turn altitude of 1,800' minus the airport elevation of 500' equals 1,300'
      • The altitude difference of 1,300' falls between the correction chart elevations of 1,000' and 1,500'
      • At the station temperature of -50°C, the correction falls between 300' and 450'
      • Dividing the difference in compensation values by the difference in altitude above the airport gives the error value per foot
      • In this case, 150' divided by 500' = 0.33' for each additional foot of altitude above 1,000'
      • This provides a correction of 300' for the first 1,000' and an additional value of 0.33 times 300', or 99', which is rounded to 100'. 300' + 100' = total temperature correction of 400'
      • For the given conditions, correcting the charted value of 1,800' above MSL (equal to a height above the reporting station of 1,300') requires the addition of 400'
      • Thus, when flying at an indicated altitude of 2,200', the aircraft is actually flying a true altitude of 1,800'
      • Minimum Procedure Turn Altitude 1,800' charted = 2,200' corrected
      • Minimum FAF Crossing Altitude 1,200' charted = 1,500' corrected
      • Straight-in MDA 800' charted = 900' corrected
      • Circling MDA 1,000' charted = 1,200' corrected
    Instrument Flying Handbook. Figure 3-7, ICAO Cold Temperature Error Table
    Figure 1: Instrument Flying Handbook,
    ICAO Cold Temperature Error Table
  • Cold Weather Altimeter Errors:

    • A correctly calibrated pressure altimeter indicates true altitude above mean sea level (MSL) when operating within the International Standard Atmosphere (ISA) parameters of pressure and temperature
    • Nonstandard pressure conditions are corrected by applying the correct local area altimeter setting
    • Temperature errors from ISA result in true altitude being higher than indicated altitude whenever the temperature is warmer than ISA and true altitude being lower than indicated altitude whenever the temperature is colder than ISA
    • True altitude variance under conditions of colder than ISA temperatures poses the risk of inadequate obstacle clearance
    • Under extremely cold conditions, pilots may need to add an appropriate temperature correction determined from the chart in Figure 3-7 to charted IFR altitudes to ensure terrain and obstacle clearance with the following restrictions:
      • Altitudes specifically assigned by Air Traffic Control (ATC), such as "maintain 5,000" shall not be corrected. Assigned altitudes may be rejected if the pilot decides that low temperatures pose a risk of inadequate terrain or obstacle clearance
      • If temperature corrections are applied to charted IFR altitudes (such as procedure turn altitudes, final approach fix crossing altitudes, etc.), the pilot must advise ATC of the applied correction
  • Nonstandard Atmospheric Pressure:

    Instrument Flying Handbook. Figure 3-8, Effects of Nonstandard Pressure on an Altimeter of an Aircraft Flown into Air of Lower than Standard Pressure (Air is Less Dense)
    Figure 2: Instrument Flying Handbook,
    Effects of Nonstandard Pressure
    on an Altimeter of an Aircraft Flown into Air of Lower than
    Standard Pressure (Air is Less Dense)
    • Take an aircraft whose altimeter setting is set to 29.92" of local pressure
    • As the aircraft moves to an area of lower pressure (Point A to B in [Figure 2]) and the pilot fails to readjust the altimeter setting (essentially calibrating it to local pressure), then as the pressure decreases, the true altitude will be lower
    • Adjusting the altimeter settings compensates for this
    • When the altimeter shows an indicated altitude of 5,000', the true altitude at Point A (the height above mean sea level) is only 3,500' at Point B
    • The fact that the altitude indication is not always true lends itself to the memory aid, "When flying from hot to cold or from a high to a low, look out below"
    • High Barometric Pressure:

      • 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
      • In this case, the aircraft actual altitude will be higher than the altimeter indicates
      • When the barometric pressure exceeds 31.00 inches, air traffic controllers will issue the actual altimeter setting, and:
        • En Route/Arrivals: Advise pilots to remain set on 31.00 inches until reaching the final approach segment
        • Departures: Advise pilots to set 31.00 inches prior to reaching any mandatory/crossing altitude or 1,500 feet, whichever is lower
        • The altimeter error caused by the high pressure will be in the opposite direction to the error caused by the cold temperature
    • Low Barometric Pressure:

      • When abnormally low barometric pressure conditions occur (below 28.00), flight operations by aircraft unable to set the actual altimeter setting are not recommended
      • This is different from high barometric pressure because the true altitude of the aircraft is lower than the indicated altitude if the pilot is unable to set the actual altimeter setting
  • Aircraft Static Pressure Systems (position error):

  • Instrument Error:

    • Most pressure altimeters are subject to mechanical, elastic, temperature, and installation errors. Although manufacturing and installation specifications, 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:
      • Set the current reported altimeter setting on the altimeter setting scale
      • Altimeter should now read field elevation if you are located on the same reference level used to establish the altimeter setting
      • 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
    • 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”
    • TBL 7-2-3, derived from ICAO formulas, indicates how much error can exist when the temperature is extremely cold. To use the table, find the reported temperature in the left column, then 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
    • 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 7−2−3 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
    • The possible result of the above example should be obvious, particularly if operating at the minimum altitude or when conducting an instrument approach. When operating in extreme cold temperatures, pilots may wish to compensate for the reduction in terrain clearance by adding a cold temperature correction
      • 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
    • Mechanical Errors:

      • Errors with the instrument itself
      • If the indication checked during preflight is off by more than 75' from the surveyed elevation, the instrument should be referred to a certificated instrument repair station for recalibration
      • Differences between ambient temperature and/or pressure causes an erroneous indication on the altimeter
    • Inherent Altimeter Errors:

      • When the aircraft is flying in air that is warmer than standard, the air is less dense and the pressure levels are farther apart
        • When the aircraft is flying at an indicated altitude of 5,000', the pressure level for that altitude is higher than it would be in air at standard temperature, and the aircraft is higher than it would be if the air were cooler
      • If the air is colder than standard, it is denser and the pressure levels are closer together
        • When the aircraft is flying at an indicated altitude of 5,000', its true altitude is lower than it would be if the air were warmer

Altimeter Procedures

  • The cruising altitude or flight level of aircraft must be maintained by reference to an altimeter
  • Altimeter setting prcoedures vary based on altitude and atmospheric pressures:
    • Below 18,000 feet MSL:

      • 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 controllers 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
      • When the barometric pressure exceeds 31.00 inches Hg

        • The following procedures will be placed in effect by NOTAM defining the geographic area affected:
          • 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' 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
      • During preflight, barometric altimeters must be checked for normal operation to the extent possible
      • 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
      • For aircraft operating VFR, there are no additional restrictions, however, extra diligence in flight planning and in operating in these conditions is essential
      • 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
      • For aircraft operating IFR and unable to set the current altimeter setting, the following restrictions apply:
        • To determine the suitability of departure alternate airports, destination airports, and destination alternate airports, increase ceiling requirements by 100 feet and visibility requirements by 1/4 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
        • For 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 500-11/4
          • On approach, 31.00 inches will remain set. Decision height (DH) 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
          • 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
        • 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
    • At or above 18,000 feet MSL:

      Lowest Usable Flight Level
      Figure 1: Lowest Usable Flight Level
      • To 29.92 inches of mercury (standard setting)
      • The lowest usable flight level is determined by the atmospheric pressure in the area of operation as shown in [Figure 1]
      • Where the minimum altitude, as prescribed in 14 CFR Section 91.159 and 14 CFR Section 91.177, 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 [Figure 2]
        • EXAMPLE: The minimum safe altitude of a route is 19,000' 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)
Lowest Flight Level Correction Factor
Figure 2: Lowest Flight Level Correction Factor
Pressure Altitude Change
Figure 3: Instrument Flying Handbook, The loss of altitude experienced when
flying into an area where the air is colder (more dense) than standard

Altimeter Enhancements (Encoding):

  • It is not sufficient in the airspace system for only the pilot to have an indication of the aircraft's altitude; the air traffic controller on the ground must also know the altitude of the aircraft
  • To provide this information, the aircraft is typically equipped with an encoding altimeter
  • When the ATC transponder is set to Mode C, the encoding altimeter supplies the transponder with a series of pulses identifying the flight level (in increments of 100') at which the aircraft is flying
  • This series of pulses is transmitted to the ground radar where they appear on the controller's scope as an alphanumeric display around the return for the aircraft
  • The transponder allows the ground controller to identify the aircraft and determine the pressure altitude at which it is flying
  • A computer inside the encoding altimeter measures the pressure referenced from 29.92" Hg and delivers this data to the transponder
  • When the pilot adjusts the barometric scale to the local altimeter setting, the data sent to the transponder is not affected
  • This is to ensure that all Mode C aircraft are transmitting data referenced to a common pressure level
  • ATC equipment adjusts the displayed altitudes to compensate for local pressure differences allowing display of targets at correct altitudes
  • 14 CFR part 91 requires the altitude transmitted by the transponder to be within 125' of the altitude indicated on the instrument used to maintain flight altitude

Regulation

  • Each person operating an aircraft shall maintain their assigned altitude
  • Within the preceding 24 calendar months, each static pressure system, each altimeter instrument, and each automatic pressure altitude reporting system has been tested and inspected
  • Except for use of system drain and alternate static, following any opening or close of the static pressure system it has been tested and inspected
  • Following installation or maintenance on the automatic pressure altitude reporting system of an ATC transponder where data error could be introduced, it has been tested and inspected
  • Tests must be conducted by:
    • Manufacturer
    • Certificated repair station
    • Certificated mechanic with an airframe rating
  • No person may operate in controlled airspace under IFR at any altitude above the maximum altitude the system has been tested for

NOTE:
All equipment is considered tested and inspected as of their date of manufacturer

  • To convert minimum altitude prescribed under 91.119 and 91.177 to the minimum flight level, the pilot shall take the flight level equivalent to the minimum altitude in feet and add the appropriate number of feet specified according to the following table:

References: