We’re starting a brand-new chapter: Gas Turbines — Reverse Thrust — Page 329, Lesson 415

We’re starting a brand-new chapter: Gas Turbines — Reverse Thrust. Let’s get into it. First, the big picture. Modern aircraft braking systems are extremely efficient — they incorporate anti-skid units and other sophisticated devices. But here’s the catch: bad runway conditions can reduce the ability of even the most refined braking systems to the point where they become a liability. Think of a wet or icy runway — the tyres just can’t generate the friction they need. That’s where reverse thrust comes in. The addition of a reverse thrust capability has improved the situation so much that landing a modern aircraft on a wet and/or icy runway in crosswind conditions need now hold no terrors for the capable pilot. The difference in stopping distance between an aircraft with and without reverse thrust is quite marked. Here’s the key operating detail: reverse thrust is selected immediately the weight of the aircraft is firmly on the mainwheels. And when it’s coupled with ground spoilers, it can reduce the landing distance dramatically — and importantly, without producing friction at the wheels. So you’re getting deceleration without relying on tyre grip. Now, there are three basic thrust reversal systems presently in use. They are: clamshell doors, bucket doors — also called external doors — and blocker doors. They are typically operated by hydraulic or pneumatic actuators or motors driving screwshafts, and they reverse the direction of the gas flow, thereby reversing the thrust. So the principle is simple: you’re redirecting the exhaust gas flow forward instead of aft, which produces a decelerating force. Let’s look at the first system in detail: clamshell doors. The name “clamshell” is applied because of the shape of the reverse thrust doors, which resembles that of a clamshell — two hinged halves that open like a shell. The reverser doors are usually pneumatically operated, and they use high pressure compressor air — that’s P3 air — as the power source. So P3 is the bleed air tapped from the compressor. Pneumatic rams move the doors from their stowed position — that’s the forward thrust position — to their deployed position — the reverse thrust position. Here’s the clever part. In their stowed position, the clamshell doors cover cascade vanes. When the doors move to the deployed state, those cascade vanes are revealed. And whilst deployed, the clamshell doors close the normal exhaust gas exit, so the gas escapes through the cascade vanes instead of straight out the back. Let me show you the layout of these systems. That figure shows the throttle and reverse thrust lever, and it highlights the five safeguards built into the selection of reverse thrust — we’ll come to those shortly. So to summarise where we are: reverse thrust is a deceleration aid that works without wheel friction, selected when the weight is on the mainwheels, and there are three systems — clamshell, bucket, and blocker. We’ve covered clamshell doors in detail: pneumatically operated by P3 air, with cascade vanes revealed when deployed, and the exhaust redirected through them. Now, let’s move on to the next system — the external door, or bucket, reverser.