Ever wondered why engine noise gets louder on touchdown at an airport?

Aircraft thrust reversers are systems designed to redirect the exhaust airflow of gas turbine engines forward upon landing, providing additional deceleration during the landing rollout. Essentially, they rotate the direction of the engine thrust without having to rotate the entire engine 180 degrees, helping to slow the aircraft after it touches down on the runway. As a passenger, you will most likely notice a noise that sounds like an increase in thrust, but is actually the exhaust being redirected forwards.

Thrust reversers enhance an aircraft’s braking efficiency, particularly during landing on relatively short runways or in adverse weather conditions. By deploying the reversers on touchdown, you can reduce the reliance on the aircraft’s wheel brakes at higher speeds after touchdown. In addition to reversers and wheel brakes, ground spoilers are deployed to ‘dump lift’, and effectively secure the aircraft on the runway. Whilst it’s not required to use reverse thrust in some circumstances, most large passenger aircraft will deploy it on landing. 

What do thrust reversers look like?

Thrust reversers can take a variety of forms, but some are far more common. Let’s take a look at the main designs of reverse thrust systems found on commercial and private aircraft.

1. Clamshell reversers – officially called ‘Target-Type Reversers’ these are simple, hinged doors or clamshell-like structures that cover the rear section of the engine exhaust nozzle. When deployed, they redirect the exhaust gasses forward, generating the reverse thrust. This design is most commonly seen on early large passenger jets and business aircraft.

2. Cascade Thrust Reversers – also known as ‘bucket’ reversers, these use a series of blocker doors that extend into the exhaust stream, diverting the airflow forwards. These are commonly found on high bypass turbofan engines, so they are the design you will most likely see on newer generation widebody and narrowbody passenger jets.

3. Pivot Door Thrust Reversers – these reversers pivot the entire engine or nacelle assembly to direct the exhaust forward. These are most commonly seen on older Airbus aircraft, including the Airbus A330ceo and older models of the A320 family. 

4. Cold Stream Thrust Reversers – this is essentially any reverser that only directs the cold stream of air that is bypassed in modern turbofan engines. Some military engine designs have included reversers that redirect both the hot and cold airflow for extra braking, but most modern passenger aircraft such as the Airbus A350 and Boeing 787 use cold stream reversers.

Do all engines need thrust reversers?

The Airbus A380 features a thrust reverse system that is unique amongst four engine aircraft, with Cascade type reversers fitted only to the inboard engines. This is because two reversers alone provide an adequate amount of reverse thrust. Commercial aviation is driven by costs, and additional reversers would simply add to the construction and maintenance cost of the aircraft. 

Do turboprop aircraft have reverse thrust?

Whilst turboprop aircraft do not have traditional thrust reversers like those found in gas turbine engines, they can use a different method called ‘beta range’ or ‘beta mode’.

This involves changing the angle of the propeller blades to alter the direction of the exhaust airflow. When the aircraft is on the ground and the propellers are in the beta range, the blades are set to a negative angle. This causes the airflow through the propeller disc to be directed partially forward, creating reverse thrust. Unlike jet reversers, beta range can also be used to maneuver the aircraft on the ground.

Can reversers be used in the air?

Broadly speaking, no, and commercial aircraft are incapable of deploying their thrust reversers in flight as a safety precaution. The deployment of the left-hand thrust reverser in the air led to the crash of Lauda Air flight 004 in 1991. The loss of lift and thrust caused the aircraft to stall and enter a diving left turn from which it did not recover. Following this tragedy, a system that uses limit switches, proximity sensors, or proximity switches was developed that prevents the reversers being usable until weight is detected on the aircraft wheels.

One notable exception to this is the Boeing C-17 Globemaster, which can deploy all four of its reversers in the air, increasing its rate of descent without increasing its airspeed during a ‘tactical approach’ manoeuvre. You can see this in action in this video.  

Why can’t jets use reverse thrust to… reverse?

At one point in history is was common for some aircraft to reverse on the ground using thrust reversers. This was a common practice on the Douglas DC9, but fell out of favor over the years for a variety of reasons. 

Most airports have strict safety regulations that prohibit the use of thrust reversers for pushback. Reversers are really not designed for precise movements. They generate powerful forces that can create uncontrolled and potentially dangerous movements if misused. The jet blast could lead to potential collisions, damage to ground surfaces and equipment, or injuries to the ground crew.

Reversers are a tried and tested tool that contribute to stopping your aircraft quickly and safely after landing. Next time you see a landing aircraft, look out for the reversers deploying on touchdown. 

Cover photo: John Absolon, JetPhotos.