General Principles - Landing — Page 280, Lesson 346

I want to walk you through the landing phase now, and we're going to focus on a critical safety point about reverse thrust — and then how propeller aeroplanes do it differently. Let me start with the jet engine, because there's a hard limit you must know. Jet engine reverse thrust cannot be maintained right up to the moment the aeroplane comes to a full stop. You have to deactivate reverse thrust before the forward speed drops to a minimum value. Why? As the aeroplane slows down, that redirected airflow — the air being thrown forward to create the braking force — can start to be re-ingested into the compressor. So the engine literally starts recycling its own gas flow. That does two bad things. First, it significantly increases the engine temperatures. Second, debris sitting on the runway can get sucked into the engine, and that can cause major damage. Because of that danger at low forward speeds, the rule is that reverse thrust must be deactivated below about 50 knots. Now, put that together with the fact that reverse thrust also can't be activated too late in the roll — you need forward speed for it to be effective. So the combined effect of late activation and early deactivation means the time period during which you can actually use reverse thrust may be quite short. That's a key operational point: reverse thrust is a limited tool, not something you hold all the way to the stop. Now let's switch to propeller aeroplanes, specifically large turbo-propeller aeroplanes. They can also generate reverse thrust by redirecting the airflow forwards, but they do it in a very different way than a jet engine. Here's the mechanism. In forward flight, the propeller blade is angled so that it displaces air backwards, which produces forward thrust. To make the propeller direct air forwards and create a rearward-acting force — that's the braking force — the blade angle must change. The specific blade angle required for the propeller to generate reverse thrust is called reverse pitch. That's the term you need to remember: reverse pitch. Here's the big advantage. Because it's only a change of blade angle, a propeller aeroplane can switch from forward to reverse thrust far quicker than a jet aeroplane can. And that means a propeller aeroplane can use reverse thrust earlier in the landing roll than a jet aeroplane can. So the timing window is much more favourable for the turboprop. Now, there's one more point that the text is leading into, and I want to flag it because it's the natural contrast. A propeller aeroplane can also maintain reverse thrust until the — and that's where the passage cuts off. But the direction is clear: the turboprop can hold reverse thrust later into the roll, right up to a much lower speed, because it doesn't have that re-ingestion problem the jet has. The jet has to back off at 50 knots; the propeller aeroplane doesn't face that same compressor re-ingestion danger. So let me pull the whole picture together for you. Reverse thrust is a braking aid used during the landing roll. For a jet, you're limited at both ends — you can't engage it too early and you must disengage it below about 50 knots to protect the engine from re-ingesting its own hot gas and sucking in runway debris. For a large turboprop, you achieve the same braking effect by changing the blade angle to reverse pitch, and because that change is fast, you can use reverse thrust earlier in the roll — and you can keep it on longer, because there's no compressor re-ingestion hazard. That's the fundamental contrast between the two powerplant types in the landing roll.