General Principles - Landing — Page 280, Lesson 346

We’ve just covered the landing distance available and the general idea of reverse thrust on a jet. Now I want to add an important limitation that applies specifically to jet engines, and then we’ll move on to how propeller aeroplanes do the same job in a completely different way. First, the jet engine limitation. You cannot hold reverse thrust right up until the aeroplane stops. Reverse thrust must be deactivated before the forward speed drops to a minimum value. Why? Because as the aeroplane slows down, that redirected airflow — the air that’s being thrown forward to create the braking force — can start to be re-ingested into the compressor. In other words, the engine starts sucking in its own exhaust. The jet engine then recycles its own gas flow. That does two bad things. It significantly increases the engine temperatures, and it also means debris on the runway can be sucked into the engine, potentially causing major damage. So because of that danger at low forward speeds, reverse thrust must be deactivated below about 50 knots. Now, put that together with what we said earlier about late activation. The combined effect of late reverse thrust activation and early reverse thrust deactivation means the time period for which reverse thrust can actually be used may be quite short. So on a jet, you get a narrow window of usefulness. Now let’s switch to propeller aeroplanes. Large turbo-propeller aeroplanes can also generate reverse thrust by redirecting the airflow forwards, but they do it in a very different way than a jet engine. For forward flight, the propeller blade is angled to displace air backwards, which produces forward thrust. To direct air forwards and create a rearward-acting force, the blade angle must change. That blade angle required for the propeller to generate reverse thrust is called reverse pitch. Here’s the key advantage. Because only a change of blade angle is required, a propeller aeroplane can switch from forward to reverse thrust far quicker than a jet aeroplane. That means a propeller aeroplane can use reverse thrust earlier in the landing roll than a jet can. And it can also maintain reverse thrust until the — well, the passage cuts off there, but you can already see the contrast: the jet has a narrow window, the propeller aeroplane has a much wider one because it’s just a blade-angle change, not a mechanical redirect of the whole gas flow. So the core contrast to remember: jet reverse thrust is powerful but limited by re-ingestion risk below about 50 knots, while propeller reverse thrust, using reverse pitch, is quicker to engage and can be held longer.