An engine built to push an aircraft forwards can be asked, for a short spell after touchdown, to push it backwards instead. This lesson covers how the trick is rigged: the components and principle, the hot-stream and cold-stream ways of doing it, why reversing only the fan is enough on a big turbofan, and the layers of protection that make an in-flight deployment nearly impossible.
Principle and components
The principle of a reverse-thrust system is redirection: doors or vanes turn part of the engine's flow so it leaves forwards and outwards instead of rearwards, and the reaction that was thrust becomes brake. The redirection is never complete, the flow leaves diagonally rather than straight ahead, so reverse delivers a useful fraction of forward thrust, not all of it. The components:
Reverse-thrust select levers — small piggyback levers on the thrust levers, liftable only at idle, through which reverse is commanded and reverse power set.
Power source — pneumatic or hydraulic muscle that moves the reverser hardware.
Actuators — the rams and screwjacks that drive the doors between stowed and deployed.
Doors — the moving surfaces themselves: buckets behind the nozzle, or blocker doors and translating cowls on a turbofan.
Annunciations — the cockpit's reverser lights and messages, from unlocked through in-transit to deployed.
The two families
Check you understood
Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.
1. Warm-up
The main components of a reverse-thrust system are:
2. Applying it
The protection features against inadvertent thrust-reverser deployment in flight include:
3. Pushing it
Shortly after a normal touchdown with reverse selected, one engine's reverser indication stays amber while the other turns green. Reverse power is being commanded on both. The correct reading is:
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What you are looking at: the two reverser families. Above, a hot-stream reverser: the bucket doors swing together behind the nozzle and the jet that meets them leaves by the two forward-and-outward arrows. Below, a cold-stream reverser on a turbofan, its fan the labelled line at the left: the blocker doors close the bypass duct above and below the core alike, the fan air escapes forwards through the cascade vanes in both cowls, and the core's own flow, as the right-hand arrow shows, continues rearwards undisturbed.
Hot-stream reverser — works on the core's exhaust itself: clamshell or bucket doors swing across the jet pipe or behind the nozzle and deflect the hot jet forwards. The fit for turbojets and low-bypass engines, where the hot jet is most of the thrust.
Cold-stream reverser (turbofan only) — works on the bypass air: blocker doors close the bypass duct while a translating cowl exposes cascade vanes, rings of curved vanes that turn the fan air forwards and outwards. The hot core flow is left alone.
Reversing only the cold stream is enough on a high-bypass engine because of the first lesson's arithmetic: at bypass ratios of 5:1 and beyond, the fan's stream carries the great majority of the thrust, so reversing it reverses most of what the engine makes. The core continues to produce a little forward thrust throughout, which merely subtracts slightly from the net reverse: the balance is still firmly a brake, bought without any machinery in the engine's hottest gas.
What reverse buys, and what it costs
Advantages — a decelerating force that does not depend on wheel-brake friction, which is exactly what a wet, icy or contaminated runway takes away; shorter landing rolls and rejected take-offs; and brakes and tyres spared their heat.
Disadvantages — noise; at low speed the forward-thrown air kicks up debris ahead of the intakes for the engine to swallow, and the engine can re-ingest its own exhaust or disturbed air, which is why reverse is normally cancelled by a specified low speed; the redirection is only partially effective; and an asymmetric deployment yaws the aircraft.
Keeping it stowed in flight
An uncommanded deployment at altitude is a catastrophic event, and the protection against inadvertent deployment on transport aeroplanes is therefore layered: the reverse levers are mechanically blocked unless the thrust levers are at idle; air/ground logic (weight-on-wheels and radio altimeter) refuses the command in flight on most types; the actuation power itself is isolated from the reverser until deployment is commanded; independent primary and secondary locks hold each reverser stowed, engineered so that no single failure frees it; and an auto-restow function drives a reverser back if it ever creeps unlocked.
The controls and indications close the loop: reverse is commanded by lifting the piggyback levers over their idle interlock, reverse power is set with the same levers, and the crew reads the system's state from the annunciations, typically an amber REV or unlocked indication while the doors are in transit, turning green when the reverser is fully deployed and reverse power may be applied, with warnings if a reverser unlocks uncommanded.
What the exam asks
The component list and the two families are recall; the fan-only logic, the advantage/disadvantage trade and the protection layers carry the understanding marks. The overview, following the learning objectives:
Name the reverse-thrust system's components and their functions