Figure 10-1.-Altmeter errors due to change in surface pressure.
hectopascals and the atmospheric pressure at New
Orleans is 1009 hectopascals, a difference of 10
hectopascals. Assume that an aircraft takes off from
Miami on a flight to New Orleans at an altitude of 500
feet. A decrease in the mean sea level pressure of 10
hectopascals from Miami to New Orleans would cause
the aircraft to gradually lose altitude, and although the
altimeter indicates 500 feet, the aircraft would be
actually flying at approximately 200 feet over New
Orleans. The correct altitude can be determined by
obtaining the correct altimeter from New Orleans and
resetting the altimeter to agree with the destination
adjustment.
NOTE: The following relationships generally hold
true up to approximately 15,000 feet:
34 hectopascals = 1 in. (Hg) = 1,000 feet of
elevation, Since 1 hectopascal is equal to about 30 feet
(below 10,000 feet altitude), a change of 10
hectopascals would result in an approximate error of
300 feet.
Altimeter Errors (Temperature)
Another type of altimeter error is due to nonstandard
temperatures. Even though the altimeter is properly set
for surface conditions, it will often be incorrect at higher
levels. If the air is warmer than the standard for the
flight altitude, the aircraft will be higher than the
altimeter indicates; if the air is colder than standard for
flight altitude, the aircraft will be lower than the
altimeter indicates. Figure 10-2 shows an example of
altimeter errors due to nonstandard air temperatures.
Figure 10-2.-Altimeter errors due to nonstandard air temperatures.
10-5
