• An altimeter is a type of barometer which measures vertical distance to the surface below, using pressure, radio, radar, laser, or capacitive technology
  • Measures the absolute pressure of the ambient air and displays it in terms of feet or meters above a selected pressure level
  • A squawk, also called an altimeter setting, must be issued to pilots in order for them to maintain their assigned altitude or flight level
  • These altimeter settings are measured in the pilot's local area, and distributed via Air Traffic Control
  • For VFR flight operations there are no regulations but the pilot should exercise extra diligence in flight planning and in operating in these conditions is essential
Instrument Flying Handbook. Figure 3-3, Sensitive Altimeter Components
Figure 1: Instrument Flying Handbook, Sensitive Altimeter Components

How it works:

  • The altimeter functions like a barometer, detecting pressure differences from a standard (29.92 in-Hg) using the static system
  • Consists of evacuated, corrugated bronze aneroid capsules which connect to needles on the face of the instrument
  • The altimeter can be calibrated for non-standard pressure by changing the pressure setting in the Kollsman window
    • Rotating the knob changes both the barometric scale and the altimeter pointers in such a way that a change in the barometric scale of 1" Hg changes the pointer indication by 1,000' which indicates the standard pressure lapse rate
    • The scale is set based on the local AWOS/ASOS/ATIS reading
  • Function:
    • As an aircraft increases in altitude the aneroid capsules which are calibrated to 29.92 expand due to decreasing ambient air pressure which moves the needles through gears and linkages to show an increase in altitude
    • Conversely, as the aircraft descends, increasing pressure on the capsules causes them to contract while gears and linkages display a decrease in altitude
  • When set to the local altimeter setting, the altimeter shows height above sea level (true altitude), but not above such land features as hills, mountains, or valleys (absolute altitude)
  • If set to the local field elevation prior to takeoff, such as the case with gliders, the altimeter will show height above the ground (absolute altitude), but not above sea level (sea level)
  • Reading the Altimeter
  • Instrument Flying Handbook. Figure 3-5, Drum-Type Altimeter
    Figure 2: Instrument Flying Handbook,
    Drum-Type Altimeter
    • A striped segment is visible below 10,000', while above, a mask begins to cover it until 15,000', where it is fully covered
    • Stripes start to cover at 10,000' and completely cover at 15,000'
  • QNE: En-route (MSL)
  • QFE: Height above field elevation (AGL)
  • QNH: Local altimeter setting

Types of Altimeters:

  • Drum Type:
    • These instruments have only one pointer that makes one revolution for every 1,000'
    • Each number represents 100' and each mark represents 20'
    • A drum, marked in thousands of feet, is geared to the mechanism that drives the pointer
    • To read this type of altimeter, first look at the drum to get the thousands of feet, and then at the pointer to get the feet and hundreds of feet
  • Sensitive Altimeter:
  • Instrument Flying Handbook. Figure 3-4, Three Pointer Altimeter
    Figure 3: Instrument Flying Handbook,
    Three Pointer Altimeter
    • One with an adjustable barometric scale allowing the pilot to set the reference pressure from which the altitude is measured
    • This scale is visible in a small window called the Kollsman window
    • A knob on the instrument adjusts the scale
    • The range of the scale is from 28.00" to 31.00" inches of mercury (Hg), or 948 to 1,050 millibars
  • Absolute Altimeter: Also referred to as a radio altimeter, indicating the altitude over objects below based on radio signal pulses

Types of Altitude:

  • Absolute altitude: the height of an aircraft above the terrain over which it is flying, usually expressed in AGL (above ground level) (HAA, HAT, TCH)
  • Indicated altitude: after setting the altimeter at the current altimeter setting, this is the uncorrected altitude read directly from that altimeter
  • True altitude: actual aircraft's height above sea level, usually expressed as MSL (mean sea level); all elevations on aeronautical charts are expressed in terms of true altitude
  • Pressure altitude: after adjusting the altimeter's setting to 29.92 mb or 1,013.2 mb, this is the altimeter reading that corresponds to the altitude in the standard atmosphere where the pressure is the same as you are
    • Calculate pressure altitude
    • LAGS: (inversely proportional)
      • If the given altimeter setting is less than 29.92, then you ADD to your altitude
      • If the given altimeter setting is greater than 29.92, then you SUBTRACT from your altitude
    • See more here in the atmosphere section
  • Density altitude: the pressure altitude corrected for temperature variations

Altimeter Errors

  • The accuracy of an aircraft's altimeter depends on a number of factors:
    • Nonstandard temperatures of the atmosphere
    • Nonstandard atmospheric pressure
    • Aircraft static pressure systems (position error); and
    • Instrument error
  • Most aircraft do not have the capability of setting an altimeter above 31.00 in-Hg or below 28.00 in-Hg
    • 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
  • 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.”
  • 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. It is this “difference” that causes the error in indicated altitude. When the air is warmer than standard, you are higher than your altimeter indicates. Subsequently, when the air is colder than standard you are lower than indicated. It is the magnitude of this “difference” that determines 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 country, 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
  • 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
  • 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


  • A sensitive altimeter is designed to indicate standard changes from standard conditions, but most flying involves errors caused by non-standard conditions and the pilot must be able to modify the indications to correct for these errors
  • Most pressure altimeters are subject to mechanical, elastic, temperature, and installation errors
    • 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
    • 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

Pressure Altitude Change
Figure 4: Instrument Flying Handbook, The loss of altitude experienced when
flying into an area where the air is colder (more dense) than standard

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
  • Higher altitudes may need to be selected when flying at low terrain clearances
  • 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 5: Instrument Flying Handbook,
ICAO Cold Temperature Error Table

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

Preflight Check:


  • The altimeter is a required instrument as per Federal Aviation Regulation 91.205 for both day and night visual and instrument flight rules (VFR/IFR)
  • According to FAR 91.411, No person may operate an airplane, or helicopter, in controlled airspace under IFR unless:
    • Within the preceding 24 calendar months, each altimeter has been tested and inspected and found to comply with appendices E and F of part 43 of this chapter;
    • Except for the use of system drain and alternate static pressure valves, following any opening and closing of the static pressure system, that system has been tested and inspected and found to comply with paragraph (a), appendix E, of part 43 of this chapter; and
    • Following installation or maintenance on the automatic pressure altitude reporting system of the ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter
  • These tests must be conducted by:
    • The manufacturer of the airplane, or helicopter, on which the tests and inspections are to be performed;
    • A certificated repair station properly equipped to perform those functions and holding:
      • An instrument rating, Class I;
      • A limited instrument rating appropriate to the make and model of appliance to be tested;
      • A limited rating appropriate to the test to be performed;
      • An airframe rating appropriate to the airplane, or helicopter, to be tested; or
    • A certificated mechanic with an airframe rating (static pressure system tests and inspections only)


  • 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

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

Nonstandard Pressure on an Altimeter

  • Maintaining a current altimeter setting is critical because the atmosphere pressure is not constant
  • That is, in one location the pressure might be higher than the pressure just a short distance away
  • 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 3-8) 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"

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 6: 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)

Operations Below 18,000' MSL

  • When barometric pressure is 31.00 in-Hg or less set your altimeter to:
    1. The current reported altimeter setting within 100 NM of the aircraft
    2. The current reported setting of the appropriate available station
    3. Departure elevation before departure

  • When barometric pressure exceeds 31.00 in-Hg:
    • 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
    • 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
    • When the altimeter cannot be set to the higher pressure setting, the aircraft actual altitude will be higher than the altimeter indicates
    • 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
      • 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
      • 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
    • If you set your altimeter according to ATIS/AWOS and it is not perfect for field elevation, remember your location on the field
    • Filed elevation: the highest elevation of any usable landing surface
    • Gliders may set the airport elevation so they can know AGL

    Lowest Usable Flight Level
    Figure 2: Lowest Usable Flight Level
  • Low barometric pressure (below 28.00 in-Hg):
    • Flight operations by aircraft unable to set the actual altimeter setting are not recommended
      • The true altitude of the aircraft is lower than the indicated altitude if the pilot is unable to set the actual altimeter setting

Operations at or above 18,000' MSL:

  • Set the altimeter to the standard 29.92 as you enter the "flight levels"
  • The lowest usable flight level is determined by the atmospheric pressure in the area of operation [Figure 2]

  • 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:

Altimeter Adjustment Factor
Figure 3: Altimeter Adjustment Factor
Instrument Flying Handbook. Figure 3-7, ICAO Cold Temperature Error Table
Figure 2: Instrument Flying Handbook,
ICAO Cold Temperature Error Table

Inertial Reference Unit (IRU), Inertial Navigation System (INS), and Attitude Heading Reference System (AHRS)

  • IRUs are self-contained systems comprised of gyros and accelerometers that provide aircraft attitude (pitch, roll, and heading), position, and velocity information in response to signals resulting from inertial effects on system components
    • Once aligned with a known position, IRUs continuously calculate position and velocity. IRU position accuracy decays with time. This degradation is known as "drift"
  • INSs combine the components of an IRU with an internal navigation computer
    • By programming a series of waypoints, these systems will navigate along a predetermined track
  • AHRSs are electronic devices that provide attitude information to aircraft systems such as weather radar and autopilot, but do not directly compute position information
  • Aircraft equipped with slaved compass systems may be susceptible to heading errors caused by exposure to magnetic field disturbances (flux fields) found in materials that are commonly located on the surface or buried under taxiways and ramps
    • These materials generate a magnetic flux field that can be sensed by the aircraft’s compass system flux detector or "gate", which can cause the aircraft’s system to align with the material's magnetic field rather than the earth's natural magnetic field
    • The system’s erroneous heading may not self-correct
    • Prior to take off pilots should be aware that a heading misalignment may have occurred during taxi
    • Pilots are encouraged to follow the manufacturer’s or other appropriate procedures to correct possible heading misalignment before take off is commenced