General Principles - Landing — Page 280, Lesson 344

Let’s pick this up right where the wheels have settled on the runway. That’s the moment the lift spoilers, or lift dumpers, deploy. These are panels on the wing that pop up very quickly to disrupt the airflow over the wing and destroy lift. The point is, once the wheels are down, you want the wing to stop producing lift so the full weight of the aeroplane presses onto the wheels, giving you maximum braking grip. So the spoilers act fast to kill that lift. Now, reverse thrust. Think about level flight first. In level flight, the forward-acting force of thrust is essential to maintain enough speed for the wings to provide lift. But during landing, the aim is 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 pushing you forward when you’re trying to slow down. However, 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 is known as reverse thrust. Reverse thrust helps to reduce the aeroplane’s forward speed. And it’s especially important in conditions where braking force is reduced due to ice or water contamination on the runway — because in those conditions, the wheel brakes alone may not be enough, so you rely more heavily on reverse thrust. Now let’s look at jet engines specifically. Jet engines produce reverse thrust by using one of several methods, but all jet engines follow the same basic principle: redirect the jet efflux in a forwards direction. The jet efflux is the exhaust gas stream coming out of the back of the engine. Normally it goes backwards, pushing the aeroplane forward. In reverse thrust, you redirect that flow forwards, which pushes the aeroplane backwards — a braking effect. Many modern aeroplanes have a safety device whereby reverse thrust is not activated until a certain value of the aeroplane’s weight is pressing down on the main wheels, and until the wheels have reached a certain speed of rotation. So two conditions must be met: enough weight on the main wheels, and the wheels spinning fast enough. Once the reverse thrust system has detected this, 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. Now here’s the important operational point for you as a pilot. 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 fact reduces the effective time period during which reverse thrust can be used. Therefore, the effectiveness of reverse thrust on landing is reduced. In other words, reverse thrust is not instantaneous — there’s a delay while the engine physically reconfigures, and during that delay the aeroplane is still rolling forward. So you can’t count on reverse thrust to do all the work; you have to factor in that delay when you plan your landing distance.