Why does a pilot need to know the temperature of the air in which he is flying? There are several reasons:
• Avoidance of icing conditions
• Because temperature affects air density, which, in turn, affects:
• Engine power and aircraft performance
• Measurement of speed
• Measurement of altitude
Avoidance of Icing Conditions
The formation of ice on aircraft, particularly in cloud, can be very rapid. Icing can be very dangerous, because the following can occur:
• Loss of lift (ice will distort the aerofoil shape)
• Increase in drag (ice will distort the aerofoil shape)
• Increase in mass (can be as much as ten tons with thick icing on large aircraft)
• Freezing of control surfaces so that they cannot be moved
• Loss of engine power or total engine failure due to intake or carburettor icing
• Chunks of ice flying off the propellors and hitting the side of the fuselage
Many forms of weather are unpleasant or uncomfortable (turbulence, lightning, etc) but ice is particularly dangerous. There are two ways to minimize the risk. One is to avoid flying in cloud. The other is to climb or descend in order to avoid the temperature bands particularly associated with icing. These will vary from aircraft type to aircraft type but will be known for each particular type.
Engine Power and Aircraft Performance
To see why air temperature affects engine power, consider how any aviation engine (jet or piston) works. Air is drawn in through the intake and mixed with vapourized fuel which is burnt in a cylinder or a combustion chamber where its volume is greatly expanded. This expansion is partly caused by the addition of combustion products (i.e. fuel), but mainly caused by a large temperature rise. In order to achieve correct combustion, the fuel/air ratio has to be carefully controlled. If the air is dense, more fuel will be provided and more power will be available.
However, if the air is less dense, less fuel will be provided, in order to maintain the correct mix. Therefore less power will be produced. This can have a considerable effect, particularly on take-off performance. Pilots therefore need to be able to measure air temperature in order to calculate engine power and performance.
Measurement of Speed
We cannot measure airspeed directly, so we measure air pressure. However, pressure is dependent on both relative speed and air density. Air temperature affects air density and therefore affects the calculation of airspeed.
Measurement of Altitude
The rate of pressure change with altitude varies with temperature. Altimeter indication can be in error unless corrected for temperature differences from normal. This can have serious safety implications when flying near high ground in cloud.