Thrust reversers

[thing]

I think just about every airline flight I've ever had up to recently has used thrust reversal on landing. I recently flew to Singapore on a B777 ER and either the reversers are extremely quiet (unlikely)or they didn't use them. It seemed to land very slowly and stop very quickly too. Any pilot type people care to elaborate?

[Hampshire Hog]

Thing,

I'm only a humble PPL, so if any ATPL wants to correct my following reply - feel free.

The noise from thrust reversers varies between aircraft types, when heard in the cabin - when used fully. However, they are not always used fully. Most pilots use them to some extent - to save the brakes - but if the runway is long and they want to exit at a taxiway further away from the threshold they may be used only at idle. In that case, chances are you wouldn't hear them.

The noisiest is when they want to quick exit from an early taxiway or the runway is very short (try Gibralter!). Then they may use full reverse thrust - very noisy, lots of vibration and noticable slowing down.

Use of reverse thrust may be noisy, but it is never dangerous (unless asymetric, in which case a marked yaw may develop;) )

If you want an interesting one, try flying in a turboprop that has reverse thrust on the variable pitch propellors!

Note, some types don't have reverse thrust at all - e.g. Embraer 145, BAE 146.

Hope this helps.

HH

[thing]

Thanks for the reply HH. I spent a lot of flying time in the back of C-130's in a previous job and when the reverse props came on after landing I always thought the bloody wings were going to fall off......probably the noisiest and bone shaking way to stop there is.

I doubt whether they had chance to deploy the reversers on the 777 as it really did come down to taxiing speed in seconds, without much effort seemingley. I think it may have due to the light fuel load, there can't have been much left after 13 hours. In fact now I think about it, I'm sure an old mate of mine used to fly them, I'll give him a bell and let you know what he said.

[Globaliser]

We really need a technical person to confirm this, but ISTR an issue that came in when carbon fibre brakes began to be used. Apparently the life of these brakes was best when they were used less often but harder - and so a practice developed of airlines specifying the use of idle reverse only unless conditions dictated otherwise. Normally, there were no safety implications in this, but IIRC this practice was a factor in the QF overrun accident at BKK, because idle reverse only was selected when more reverse thrust was really needed in the circumstances.

If idle reverse was an SOP for your flight, that may explain why there was little noise. You also have to remember that the newer generations of engine are ever more quiet than their predecessors, which can also mask the amount of thrust (forward or reverse) actually being used. Sometimes the only way of knowing for sure whether the reversers have been deployed on landing is to stick your face in the window and have a look.

[Final 3 Greens]

IIRC reversers are not generally used to reduce the landing roll (distance) but rather to reduce brake wear.

Also, reverse is more effective at higher airspeeds and is not used at less than a certain speed (80kts?) to reduce the risk of ingesting foreign objects.

Therefore you probably experienced reverse idle which is pretty quiet, as Globalier says.

[TopBunk]

Most modern autobrake systems will deliver a programmed rate of retardation.

What do I mean by this?

Well, you select an autobrake setting 1/2/3/4/max (min/med if A320) and this will deliver a rate of deceleration off x kts/sec etc.

You choose a setting to deliver the performance you require based on an approach (ground)speed, lets say for example 150 kts and A/B 2 will stop you in 2300 metres for your planned exit point.

The reverse helps here, but the programmed decel rate remains the same, so reverse reduces the amount of braking effort of the autobrake system.

I'm sure that all airlines will select at least idle reverse (at least momentarily) and will use up to full reverse if required, although many airports stipulate the use of minimum reverse idle commensurate with aircraft safety. The selection of reverse idle will result in modern high-bypass engines in very little noise at all.

[Monarch Man]

Most modern airports that are served by turbo-fan aircraft with a runway in excess of 2600 mtrs (8000ft) require very little in the way of reverse thrust.
As has been previously stated, one of the main considerations is brake wear vs engine wear.
On the aircraft I fly B757/767, it is normal for us to only select reverse idle which is very quiet inside the cabin. :ok:

[toppledgyro]

Quite correct - use of reverse thrust reduces the amount of wheel braking required for a given rate of retardation. This has 2 major advantages; firstly, brake wear is reduced considerably - brake parts are expensive whereas rev thrust is virtually 'free'. Secondly, and very important for low cost carriers on short turn-rounds, use of rev thrust reduces brake cooling time. This means that the length of the turn-round does not have to be extended whilst waiting for the brakes to cool sufficiently for the crew to get airborne again confident in the knowledge that they could provide 100% efficiency should they be needed to stop the aircraft in the event of a rejected take off.

tg
737 Driver

[gas-chamber]

If on this occasion it landed slowly and stopped quickly, it could have been a combination of very light landing weight and strong headwind on the runway, so that touchdown groundspeed was low. Combined maybe with a requirement to roll through to a high speed turnoff it is possible the crew did no more than 'crack' the reversers to the idle position without actually applying any noticeable thrust. In some cases, this would be good airmanship to get off the runway quickly. Mostly, though reverse is used as described in the above thread - to assist and reduce brake wear and keep brake and tire temperatures down. Reverse is not calculated into stopping performance when the test program is run to certify an airplane but its use is seen by pilots as an insurance policy and normal SOP's do require it to be used. Noise abatement rules at certain airports at certain hours ask that idle reverse only is used. They do not have the right to mandate it, as the pilot will still have the authority to use it if needed on safety grounds. Mind you, at a place like Singapore you would want a good reason otherwise the authorites could give you a hard time.

[Intruder]

brake parts are expensive whereas rev thrust is virtually 'free'
Reverse thrust burns a bunch of fuel, so it's not as "free" as it used to be. However, especially in airplanes with older design steel brakes (747 Classic) instrad of carbon brakes (747-400, 777, etc), reverse thrust adds a significant safety and brake wear margin.

At least idle reverse is used by virtually all air carrier airplanes when fitted. Use of more than idle is up to local policies.

[MrBernoulli]

Idle reverse is doing pretty much cock-all. It is selected so that the reverser doors, translating sleeves or whatever system is being used, are all in the right place so that IF reverse is called for, then there is minimum of time getting it.

[Intruder]

Idle reverse is doing pretty much cock-all.
Nope. It also negates the residual forward thrust so the brakes are not fighting against that as well. Idle thrust is enough to accelerate some airplanes to a pretty good speed on the ground, so it is significant.

[Bangkokeasy]

BE was relaxing on Saturday and taking a last chance to play golf between the runways at Don Muang, while surrounded by planes taking off and landing.

The varied selection of reverst thrust on landing is pretty obvious when you are 100metres away from the plane and lining up a putt. I blame the pilot of an Emirates 777 for my bogey on the 3rd green. By contrast, the TG 747 that landed when I was lining up a similar putt on the 4th, hardly moved the levers at all, meaning a par was achieved. Unfortunately, on holes 6-12, you are subjected to the takeoff noise, which meant the score went to pot.

One point I would make is it is noticeable that the pilots who are very familiar with Don Muang, such as those with TG and SQ, land earlier and use less reverse thrust, while others were landing later and slamming on the anchors.

Most planes land on the military runway at Don Muang, which means they have to cross between holes 11 and 12 to get to the terminal. For those who have experienced it, looking at the expressions of crew and passengers alike, as you wave with your 5 iron to them pasing in front of you, is priceless. Unfortunately, after next weekend, it will be a thing of the past, as operations will transfer to the new airport, where, sadly, no such golf course exists.

[skiesfull]

Singapore is not "noise-sensitive", i.e. idle reverse is not required by the authorities,
If you need reverse then use it wherever, but as far as the bean-counters are concerned, brakes are cheaper to replace than engines. Unnecessary use of reverse does annoy local inhabitants and it is up to the pilots to be as quiet as possible in such circumstances that allow idle reverse.

[Captain Rat]

Skiesfull reply is the most accurate. Carbon fibre brakes work best when they have one steady application, not lots of smaller ones. While taxying, crews are trained, if the taxy speed is getting a bit high, to apply the brakes in one steady application, not 'dab' them. When landing, a lot of ailines SOP is to select reverse, and as posted this puts the reversers in the corrct position if they are needed in anger, and then use the brakes to slow the aircraft. Obviously this is quiter, but the main reason is that the cost of the wear on the brakes, (overhaul etc) is less than the cost of fuel for using full reverse, and the wear on the engine (cyling up to a highr speed and down again). One airline I know will actually make a tech log entry to say full reverse was used. This is for the engine monitoring department to keep track of it.