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Altimeter

Introduction:

Altimeter Purpose:

  • Altimeters exist as a means of maintaining altitude
  • Air Traffic Control issues altimeter (pressure) settings to pilots of a local area to ensure everyone is using the same reference datum
  • This is especially important during Instrument Flight Rules where separation responsibilities fall upon ATC
  • Unless under control of ATC, altitudes are not assigned to pilot's however, to ensure safety and instrumentation accuracy, local altimeter settings are still very important
    • Instances where VFR pilots need to be accurate with their altitude include flying VFR cruising altitudes, flying traffic patterns, and maintaining minimum heights above protected areas

Altimeter Function:

  • The altimeter functions like a barometer, detecting pressure differences from a standard (29.92 in-Hg, aka, absolute pressure) using the static system
  • 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)

Types of Altimeters:

  • Different altimeters measure altitude through various methods such as radio, radar, laser, or capacitive technology
  • Drum Type:

    • Instrument Flying Handbook. Figure 3-5, Drum-Type Altimeter
      Instrument Flying Handbook,
      Drum-Type Altimeter
    • 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
      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:

  • Altitude is a measure of how far you are above a reference point
  • 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
    • This is most important to takeoff and climb performance of an aircraft
    • Calculate density altitude
    • See more here in the atmosphere section
  • GPS Altitude:

    • GPS Altitude is a measure take from, and therefore in reference to, the GPS Satellite Constellation
    • Since pressure readings passed to pilots are taken on the ground, atmospherics will guarantee a disparity between GPS altitude and pressure altitude as altitude increases
    • Although GPS altitude can be highly accurate, it is usually not referenced while in flight
    • This ensures all aircraft are using the same reference, and any error from pressure settings is generally shared across all aircraft

Altimeter Construction:

  • Consists of evacuated, corrugated bronze aneroid capsules which connect to needles on the face of the instrument
  • 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'
  • Instrument Flying Handbook. Figure 3-3, Sensitive Altimeter Components
    Instrument Flying Handbook, Sensitive Altimeter Components

Altimeter Errors

  • The accuracy of an aircraft's altimeter depends on a number of factors:
  • Non-Standard Atmospheric Temperature:

    • 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 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
    • 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
      • 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
        Instrument Flying Handbook,
        ICAO Cold Temperature Error Table
      • Instrument Flying Handbook. Figure 3-7, ICAO Cold Temperature Error Table
        Instrument Flying Handbook,
        ICAO Cold Temperature Error Table
  • Non-Standard Atmospheric Pressure:

    • 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
    • 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)
      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)
    • 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)
      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)
  • Lowest Usable Flight Level
    Lowest Usable Flight Level
    Lowest Usable Flight Level
    Lowest Usable Flight Level
  • Position 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
  • 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
  • Instrument Errors:

    • 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
    • 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
    • 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
    • Static Blockage
      Static Blockage

      Static Port Blockage:

      • Trapped static pressure causes the altimeter to freeze at the altitude where the blockage occurred
  • 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
Lowest Flight Level Correction Factor
Lowest Flight Level Correction Factor
Lowest Flight Level Correction Factor
Lowest Flight Level Correction Factor
Pressure Altitude Change
Instrument Flying Handbook, The loss of altitude experienced when
flying into an area where the air is colder (more dense) than standard
Pressure Altitude Change
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

Preflight Action:

  • Should indicate within 75' of field elevation when set with the proper barometric pressure (not regulatory) according to Aeronautical Information Manual 7-2-3
  • Operating with a known altimeter error, especially in IFR conditions, may not be illegal but certainly falls under careless and reckless operation
  • 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
  • Obtaining an altimeter setting:
    • Altimeter settings can be obtained from ATC, automated broadcasts, weather reports, as well as various other applications and programs

Inspections & Tests:

  • Inspections & tests will differ based on the flight rules with which you operate
  • Visual Flight Rules:

    • No specific inspections or tests are required
  • Instrument Flight Rules:

    • Altimeter inspections and tests are governed by Federal Aviation Regulation 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 FAR part 43;
      • 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:

      1. The manufacturer of the airplane, or helicopter, on which the tests and inspections are to be performed;
      2. 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
      3. A certificated mechanic with an airframe rating (static pressure system tests and inspections only)
    • Altimeter and altitude reporting equipment approved under Technical Standard Orders are considered to be tested and inspected as of the date of their manufacture
    • No person may operate an airplane, or helicopter, in controlled airspace under IFR at an altitude above the maximum altitude at which all altimeters and the automatic altitude reporting system of that airplane, or helicopter, have been tested

Altimeter Regulations:

  • 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
  • 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)
  • 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 [Figure]

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

Nonstandard Pressure Effects on an Altimeter:

  • 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)
    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)
  • 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"
  • 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

Altimeter Procedures:

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

    • When barometric pressure is 31.00 in-Hg or less set your altimeter to:

      1. 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 barometric pressure exceeds 31.00 in-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
      • 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
    • Lowest Usable Flight Level
      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:

    • Pilots set the altimeter to the standard 29.92 when entering the "flight levels"
    • Since VFR pilots can fly at 17,500' MSL, atmospheric pressures below 29.92 inHg (think "when flying high to low, lookout below") will bring those pilots flying at 18,000' MSL lower, decreasing the required 500' separation from possible VFR traffic
    • As a result, ATC establishes a lowest usable flight level depending upon the atmospheric pressure in an area of operation [Figure 2]
    • Altimeter Adjustment Factor
      Altimeter Adjustment Factor
    • The lowest usable flight level is determined by the atmospheric pressure in the area of operation as shown in [Figure 3]
    • 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 4]
      • 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)

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

Altimeter Terminology:

  • QNE: En-route (MSL)
  • QFE: Height above field elevation (AGL)
  • QNH: Local altimeter setting

Conclusion:

  • 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
    • Standard temperature at sea level is 15° 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
  • A NOTAM will be issued defining the geographic area affected
  • 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
  • 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
  • Always keep in mind the effects of parallax error
  • 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
  • Changing altimeter settings is critical during frontal passes

References: