A Guide to Reverse Thrust

Hello everyone, been a long time since I last posted so here’s another hopefully shorter topic, this time on thrust reversers and the different types!

What are Thrust Reversers?

Thrust Reversers are a name commonly applied to the mechanisms on an engine which directs the flow of thrust anywhere else than normally intended with the purpose to slow down or stop the aircraft. (e.g. flow now goes off to the side instead of the rear as normally happens)

A diagram showing the different types of reversers.

Why not just run the engine in reverse?

Look if it were that easy it would have been done already… In order to understand why you first need to understand how a jet engine works.

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As with all internal combustion engines, the rule of suck, squeeze, bang, blow applies. The air is first sucked in through the front by the N1 fan (the largest one right at the front) then goes through the compressor where it is squeezed and slowed down, before fuel is introduced, creating the bang. This is then sped up again and sent through the back to produce extra thrust along with the bypass air from the N1 fan.

In order to reverse this, you’d have to basically turn the whole engine around. If only the N1 fan is reversed, you’d first have to stop it, using valuable energy. Then, you’ll have to speed it up again. There’s one problem however, remember how the suck phase is done by the N1 fan? Now with it in reverse, you would be getting lesser air in the chamber, leading to lower power. There is one way to solve this however- add a gearbox! (see below)

Now lets say you were able to reverse the flow in the combustion chamber properly. The problem now would be getting all the materials in front of the chamber to deal with the heat. This creates extra weight and also problems with costs.

A current solution would be to take inspiration from the turboprops with variable pitch blades. Rolls-Royce’s future Ultrafan would introduce these blades, further saving weight as the mechanism in the nacelle can now be removed.

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Also note the “Thrust class 25-45klbf”- even though the technology is there it will be limited to smaller engines, for now.

Types of Thrust Reversers

The first type is the “swinging bucket” type reverser. These have a huge bucket split into 2 at the rear of the engine which swings down when engaged, directing ALL air to the sides/slightly forwards. These are special as they direct bypass air as well as the core airflow.

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This is also called “target door type” as it targets the air away.

The next type is the “clamshell” reverser. This, as used on the Vickers VC10 (next PoTP maybe?) opens the door and pushes it into the bypass flow, blocking off the path to the exit, forcing it out of the nacelle.

The last type is the most commonly used now, the sliding cowl type. This uses a combination of blocker doors and the entire rear section of the nacelle sliding back to provide an exit for the air.

The blocker doors, without which the cowl sliding open would be useless suffice for more wind noise. These force the air out through the open space in the nacelle.

In conjunction, you get this.

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Also of note, the bucket style hybrid in use by Trent 700 powered A330s, as well as CFM56 powered A320s.

The reversers on an Air France A320 with the CFM56 engines.

What’s the point of these, the plane has brakes for a reason!

Thrust reversers do not give a flying f*** about the conditions that they are activated in. They will work as long as there is an atmosphere. Let’s go back to 1969, where you’re in a RC-135, with a wet runway and tyres that hydroplane…

The P&W J57 turbojets that this Rivet Ball was equipped with did not have any thrust reversers to stop the plane once the plane started hydroplaning. This led to a runway overrun with, luckily, all 18 crew members escaping safely.

Want a more recent example?

On the 23rd of September 1999 Qantas flight 1 was on final approach to Bangkok’s Don Mueang International Airport. There was rain so heavy that the runway lights could only be seen after each wiper stroke. Despite this, the crew elected to continue with the landing, with the first officer flying. As the plane proceeded to touchdown late, past the long touchdown point, the captain took things into his own hands and ordered a go-around while telling the first officer to push the thrust levers to TO/GA. However, visibility then cleared so he had a change of mind, so he brought the engines back to idle but missed one, leaving it at TO/GA thrust. This caused the autobrake to disengage as logic assumed that this meant the pilots wanted to take off so braking would hamper this. Manual braking was only started around 1600m down the 3500m runway. The standing water on the runway then caused the aquaplaning and the brakes to lose effectiveness.

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It was decided that the decision not to abort the takeoff combined with the lack of thrust reverser use caused the runway overrun.

I hope that this article has taught you more about how important thrust reversers and how they work! Constructive comments are appreciated as usual, they help improve my writing as well as tailoring them to everyone’s liking.

To finish off, here’s a pic of a RSAF KC-135R that I took yesterday!

Have a great day, see you in the skies!

Credits:

Apologies if I missed out any links, PM me and they will be added in.

EDIT: also I’m sorry that I said this would be short and it kinda isn’t…

21 Likes

Good topic and guide! Learnt a lot today! Thanks!

1 Like

Ah, you had to use Qantas as an example 😉
Very well written, it’s great you provided evidence.

3 Likes

About to say the sme - using Qantas as the example

Great topic BTW!!!

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Well I did search up other examples, but only your beloved airline had concrete evidence that the incident was caused by the lack of thrust reversers.

Not that I have anything against them, they are after all the only airline so far that has given me hot chocolate and marshmallows…

4 Likes

Yeah, I didn’t have anything against you using it. They caused a crash either way.

3 Likes

Very nice topic. I loved it, thanks for sharing it!

1 Like

Great topic ! Very well explained and put together.

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