General Principles - Landing — Page 280, Lesson 344

We've just covered the lift spoilers and lift dumpers — those panels that pop up on the wing once the wheels are on the runway, disrupting the airflow and destroying lift. Now I want to move on to the next major deceleration device: reverse thrust. Let me set the scene. In level flight, the forward-acting force of thrust is essential — it maintains sufficient speed for the wings to produce lift. But during landing, our aim is the exact opposite: we want to bring the aeroplane to a stop. So the thrust force must be reduced to zero. Any residual thrust would actually be detrimental to the landing performance — it would be fighting against our deceleration. Now here's the clever part. Large propeller and jet aeroplane engines have the capability of redirecting the force of thrust in order to generate a braking effect on the aeroplane. That's what we call reverse thrust. Reverse thrust helps to reduce the aeroplane's forward speed. And it's especially important in conditions where the braking force is reduced — for example, when the runway is contaminated with ice or water. In those slippery conditions, the wheel brakes aren't as effective, so reverse thrust becomes a critical tool. Let's look at how jet engines actually achieve this. Jet engines produce reverse thrust by using one of several methods, but they all follow the same basic principle: redirect the jet efflux in a forwards direction. The jet efflux is the exhaust gas stream blasting out of the back of the engine. Normally it goes backwards, pushing the aeroplane forwards. For reverse thrust, we redirect that flow forwards, which pushes the aeroplane backwards — a braking effect. Now, there's an important safety device on many modern aeroplanes. Reverse thrust is not activated until two conditions are met. First, a certain value of the aeroplane's weight must be pressing down on the main wheels — that confirms the aeroplane is firmly on the ground. Second, the wheels must have reached a certain speed of rotation — that confirms the aeroplane is actually rolling. Once the reverse thrust system has detected both of these, reverse thrust is activated, and the engine will reconfigure itself so that the exhaust gas flow can be redirected forwards. You can see the extent of that reconfiguration in Figure 6.3. But here's the catch — and this is a key point for pilots to recognize. The process of getting the engines reconfigured to generate full reverse thrust takes time. So the aeroplane will have travelled a small distance from the touchdown point before reverse thrust actually takes effect. That delay reduces the effective time period during which reverse thrust can be used. And therefore, the effectiveness of reverse thrust on landing is reduced. It's not instant — there's a lag between touchdown and full reverse thrust, and that lag costs us distance and time. So to tie it together: reverse thrust is a braking force generated by redirecting the jet efflux forwards, it's safety-locked until the main wheels are loaded and rotating, and its effectiveness is limited by the reconfiguration time after touchdown. That's the complete picture of reverse thrust for jet engines.