Air Temperature Measurement — Page 24, Lesson 33

I want to walk you through why air temperature matters so much in the cockpit, because this is one of those measurements that quietly affects almost everything else we do. Let me start with the big question: why does a pilot need to know the temperature of the air he's flying through? There are four main reasons, and I want you to hold onto all of them. First, to avoid icing conditions. Second, because temperature affects air density, and air density in turn affects three things: engine power and aircraft performance, the measurement of speed, and the measurement of altitude. Let's take the first one, icing. Ice can form on an aircraft very rapidly, especially in cloud, and it's genuinely dangerous. Here's what can happen. Ice distorts the aerofoil shape, and that distortion causes a loss of lift. The same distortion also causes an increase in drag. Ice adds mass — and this can be as much as ten tons with thick icing on a large aircraft. Ice can freeze the control surfaces so they physically cannot be moved. Ice can cause loss of engine power or even total engine failure, either through intake icing or carburettor icing. And chunks of ice can fly off the propellors and hit the side of the fuselage. Now, a lot of weather is just unpleasant — turbulence, lightning, those sorts of things. But ice is particularly dangerous, and there are two ways to minimise the risk. One is simply to avoid flying in cloud altogether. The other is to climb or descend to avoid the temperature bands particularly associated with icing. Those temperature bands will vary from aircraft type to aircraft type, but for each particular type, they'll be known. Now let's look at engine power and performance, because this is where the physics starts to bite. Think about how any aviation engine works — jet or piston, it doesn't matter. Air is drawn in through the intake and mixed with vapourized fuel. That mixture is burnt in a cylinder or a combustion chamber, where its volume is greatly expanded. Now, that expansion is partly caused by the addition of combustion products — that is, the fuel itself — but it's mainly caused by a large temperature rise. Here's the key point. 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. But if the air is less dense, less fuel will be provided, in order to maintain the correct mix — and therefore less power will be produced. This can have a considerable effect, particularly on take-off performance. So pilots need to measure air temperature in order to calculate engine power and performance. Next, the measurement of speed. We cannot measure airspeed directly, so we measure air pressure instead. But pressure depends on both relative speed and air density. Since air temperature affects air density, it therefore affects the calculation of airspeed. So temperature sneaks into your speed reading even though you never see it on the dial. And finally, the 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. And this can have serious safety implications when you're flying near high ground in cloud. So there's your foundation: temperature drives icing avoidance, engine power, airspeed calculation, and altitude accuracy. In the next part we'll look at how we actually measure that temperature in flight.