The pressure altimeter is subject to certain errors that fall in five general categories.
A mechanical error is caused by misalignment of gears and levers that transmit the aneroid cell expansion and contraction to the pointers of the altimeter. This error is not constant and must be checked before each flight by the setting procedure.
A scale error is caused by irregular expansion of the aneroid cells and is recorded on a scale correction card maintained for each altimeter in the instrument maintenance shop.
Installation or Position Error
An installation, or position, error is caused by the airflow around the static ports. This error varies with the type of aircraft, airspeed, and altitude. The magnitude and direction of this error can be determined by referring to the performance data section in the aircraft technical order. An altimeter correction card is installed in some aircraft that combines the installation or position and scale errors. The card indicates the amount of correction required at different altitudes and airspeeds. Installation, or position, error may be considerable at high speeds and altitudes. Apply the corrections as outlined in the technical order or on the altimeter correction card.
A reversal error is caused by inducing false static pressure in the static system. It normally occurs during abrupt or large pitch changes. This error appears on the altimeter as a momentary indication in the opposite direction.
A hysteresis error is a lag in altitude indication caused by the elastic properties of the material within the altimeter. This occurs after an aircraft has maintained a constant altitude for an extended period of time and then makes a large, rapid altitude change. After a rapid descent, altimeter indications are higher than actual. This error is negligible during climbs and descents at slow rate or after maintaining a new altitude for a short period of time.
Setting the Altimeter
The barometric scale is used to set a reference plane into the altimeter. Rotating the barometric pressure set knob increases or decreases the scale reading and the indicated altitude. Each .01 change on the barometric scale is equal to 10 feet of altitude. The majority of altimeters have mechanical stops at or just beyond the barometric scale limits (28.10 to 31.00). Attempting to adjust outside this range may cause damage to the instrument. Altimeters not equipped with mechanical stops near the barometric scale limits can be set with a 10,000 foot error. Therefore, when setting the altimeter, ensure the 10,000 foot pointer is reading correctly. Check the altimeter for accuracy before every flight.
To check and set the altimeter:
- Set the current altimeter setting on the barometric scale.
- Check the altimeter at a known elevation and note the error in feet.
- Set the reported altimeter setting on the barometric scale and compare the indicated altitude to the elevation of a known cockpit.
Nonstandard Atmospheric Effects
The altimeter setting is a correction for nonstandard surface pressure only. Atmospheric pressure is measured at each station and the value obtained is corrected to sea level according to the surveyed field elevation. Thus, the altimeter setting is the computed sea level pressure and should be considered valid only in close proximity to the station and the surface. It does not reflect nonstandard temperature nor distortion of atmospheric pressure at higher altitudes.
Types of Altitude
Indicated altitude is the value of altitude that is displayed on the pressure altimeter.
Calibrated altitude is indicated altitude corrected for installation or position error.
Pressure Altitude (PA)
The height above the standard datum plane (29.92 “Hg and 15 °C) is PA. [Figure 3-13]
Density Altitude (DA)
Density is mass per unit volume. The density of the air varies with temperature and with height. Warm air expands and is less dense than cold air. Normally, the higher the PA, the less dense the air becomes. The density of the air can be expressed in terms of the standard atmosphere. DA is the PA corrected for temperature in the density altitude window of the DR computer. This calculation converts the density of the air to the standard atmospheric altitude having the same density. DA is used in performance data and true airspeed (TAS) calculations.
True Altitude (TA)
TA is the actual vertical distance above mean sea level (MSL), measured in feet. It can be determined by two methods:
- Set the local altimeter setting on the barometric scale of the pressure altimeter to obtain the indicated true altitude (ITA). The ITA can then be resolved to TA by use of the DR computer. [Figure 3-17]
- Measure altitude over water with an absolute altimeter.
The height above the terrain is called absolute altitude. It is computed by subtracting terrain elevation from TA, or it can be read directly from a radar altimeter.
The two altitudes most commonly accomplished on the computer are TA and DA. Nearly all DR computers have a window by which DA can be determined; however, be certain that the window is labeled density altitude.
True Altitude (TA) Determination
In the space marked FOR ALTITUDE COMPUTATIONS are two scales: a centigrade scale in the window and a PA scale on the upper disk. When a PA is placed opposite the temperature at that height, all values on the outer (miles) scale are equal to the corresponding values on the inner (minutes) scale, increased or decreased by 2 percent for each 5.5 °C that the actual temperature differs from the standard temperature at that PA, as set in the window. Although the PA is set in the window, the ITA is used on the inner (minutes) scale for finding the TA, corrected for difference in temperature lapse rate.
PA = 8,500 feet
ITA = 8,000 feet
Air Temperature = –16 °C
Place PA (8,500 feet) opposite the temperature (–16) on the FOR ALTITUDE COMPUTATIONS scale. Opposite the ITA (8,000 feet) on the inner scale, read the TA (7,600 feet) on the outer scale. [Figure 3-l7]
Density Altitude (DA) Determination
DA determination on the computer is accomplished by using the window just above FOR AIRSPEED AND DENSITY ALTITUDE COMPUTATIONS and the small window just above that marked DENSITY ALTITUDE.
PA = 9,000 feet
Air temperature = 10 °C
Place pressure altitude (9,000 feet) opposite air temperature (10) in window marked FOR AIRSPEED AND DENSITY ALTITUDE, read DA (10,400 feet). [Figure 3-18]