Topic of the Day: Effects of Temp on Altitude
Resource: PHAK
Effect of Nonstandard Pressure and Temperature
It is easy to maintain a consistent height above ground if
the barometric pressure and temperature remain constant,
but this is rarely the case. The pressure and temperature can
change between takeoff and landing even on a local flight.
If these changes are not taken into consideration, flight
becomes dangerous.
If altimeters could not be adjusted for nonstandard pressure, a
hazardous situation could occur. For example, if an aircraft is
flown from a high pressure area to a low pressure area without
adjusting the altimeter, a constant altitude will be displayed,
but the actual height of the aircraft above the ground would
be lower then the indicated altitude. There is an old aviation
axiom: “GOING FROM A HIGH TO A LOW, LOOK OUT
BELOW.” Conversely, if an aircraft is flown from a low
pressure area to a high pressure area without an adjustment
of the altimeter, the actual altitude of the aircraft is higher
than the indicated altitude. Once in flight, it is important to
frequently obtain current altimeter settings en route to ensure
terrain and obstruction clearance.
Many altimeters do not have an accurate means of being
adjusted for barometric pressures in excess of 31.00
“Hg. When the altimeter cannot be set to the higher
pressure setting, the aircraft actual altitude is higher than
the altimeter indicates. When low barometric pressure
conditions occur (below 28.00), flight operations by
aircraft unable to set the actual altimeter setting are
not recommended.
Adjustments to compensate for nonstandard pressure do
not compensate for nonstandard temperature. Since cold
air is denser than warm air, when operating in temperatures
that are colder than standard, the altitude is lower than the
altimeter indication. [Figure 8-3] It is the magnitude of this
“difference” that determines the magnitude of the error. It is
the difference due to colder temperatures that concerns the
pilot. When flying into a cooler air mass while maintaining a
constant indicated altitude, true altitude is lower. If terrain or
obstacle clearance is a factor in selecting a cruising altitude,
particularly in mountainous terrain, remember to anticipate
that a colder-than-standard temperature places the aircraft
lower than the altimeter indicates. Therefore, a higher
indicated altitude may be required to provide adequate terrain
clearance. A variation of the memory aid used for pressure can be employed: “FROM HOT TO COLD, LOOK OUT
BELOW.” When the air is warmer than standard, the aircraft
is higher than the altimeter indicates. Altitude corrections for
temperature can be computed on the navigation computer.
Extremely cold temperatures also affect altimeter indications.
Figure 8-4, which was derived from ICAO formulas, indicates how much error can exist when the temperature is extremely cold.
