Air Temperature Measurement — Page 24, Lesson 33

Let's start with the big question: why does a pilot need to know the temperature of the air he's flying through? I want to give you four solid reasons, because this is the foundation for everything else in this chapter. First, avoidance of icing conditions. Ice formation on an aircraft, especially in cloud, can be extremely rapid. And icing is genuinely dangerous, not just uncomfortable. Let me walk you through what can happen. Ice distorts the aerofoil shape, which means you lose lift. The same distortion increases drag. Ice adds mass — and this can be as much as ten tons with thick icing on a large aircraft. Control surfaces can freeze solid so they can't be moved. You can get loss of engine power or even total engine failure from intake or carburettor icing. And chunks of ice can fly off the propellers and hit the side of the fuselage. So the pilot needs to know the air temperature to recognize when he's in icing conditions. There are two ways to minimize the risk: one is to avoid flying in cloud altogether, and the other is to climb or descend to avoid the temperature bands particularly associated with icing. Those bands vary from aircraft type to aircraft type, but they're known for each particular type. Second, engine power and aircraft performance. Let's think about how any aviation engine works, whether jet or piston. 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. That expansion is partly caused by the addition of combustion products — that is, the fuel — but mainly caused by a large temperature rise. To achieve correct combustion, the fuel/air ratio has to be carefully controlled. Here's the key relationship: if the air is dense, more fuel is provided and more power is available. But if the air is less dense, less fuel is provided to maintain the correct mix, so less power is produced. This has a considerable effect, particularly on take-off performance. So the pilot needs to measure air temperature to calculate engine power and performance. Third, measurement of speed. We cannot measure airspeed directly — we measure air pressure instead. But pressure depends on both relative speed and air density. And since air temperature affects air density, it therefore affects the calculation of airspeed. So temperature feeds into your speed readout. Fourth, measurement of altitude. The rate of pressure change with altitude varies with temperature. That means your altimeter indication can be in error unless it's corrected for temperature differences from normal. This has serious safety implications when you're flying near high ground in cloud. So there's your complete picture: temperature affects icing avoidance, engine power and performance, speed measurement, and altitude measurement. Those four reasons are why air temperature measurement matters to a pilot.