Reverse Thrust Noise

[A300Man]

Why do some aircraft engines howl on landing when reverse thrust is applied for braking, whilst on other occasions, the same type of aircraft, carrying the same load, will land and roll to a stop, spoilers and reverse thrust applied, but with NO hint of an increase in engine noise???

I am a passenger who flies regularly on several types of aircraft. I have noted the above phenomenon on mainly the A300 and the Boeing 757, which fly the routes that I regularly use most.

In particular the A300 - landing full with sometimes no noise, and screaming at other times. On most occasions, the aircraft appears to exit the runway at the same point, so doesn't appear to be braking harder to get off earlier.

[iainpoll]

I would be interested to hear a technical answer, although I suspect that the noise level will,to some extent vary according to where you are sitting in the cabin.

[jet_breeze]

Obviously different landings need different amounts of reverse thrust to slow and stop the plane. Depending on conditions such as weight , speed there is a lot of things that can affect how much is needed.

[lomapaseo]

I suspect that you are hearing the effect of bleeds open during reverse as the power levers are retarded while still in reverse. This would result in a step change in noise. I can only recall this on the B757, but it's possible on other machines as well depending on reverser technique.

[Youwererobbed]

The amount of reverse makes little or no difference in normal landing conditions to the stopping distance. This is decided by the autobrake (or foot pressure on the brakes) which will give a predetermined decelaration. In most cases all reverse does is help reduce brake wear and make the flight crew feel they are being proactive in helping stop the aircraft.
Some crews may elect to use more than reverse idle to help keep brake temps down on short turnarounds, especially if the aircraft has no brake fans.

[Intruder]

The amount of reverse thrust applied depends on local regulations (e.g., idle reverse only in LUX), airline policy (e.g., idle reverse only at JetBlue), landing weight/speed, and runway conditions (e.g., length, wet, tailwind...).

While most modern autobrake systems apply brakes to attain a constant deceleration, not all airplanes/airlines use autobrakes for all landings or for the entire rollout. The Captain and Pilot Flying (Handling Pilot) will decide if, how much, and for how long autobrakes and reverse thrust are used. The "how long" decision is the most dynamic.

For our 744s, policy is autobrakes and reverse thrust for ALL landings. Autobrakes 2 is normal (3 for Cat II/III operations or wet runway). At least idle reverse is mandated, any more is at the discretion of the Pilots. Generally, full reverse is used for at least part of the rollout.

[PAXboy]

Intruder: You say that, on your 744s ... Generally, full reverse is used for at least part of the rollout. I recall (in this thread or another) that only partial thrust is used. I know that there are more correct definitions and N1 RPM and all that jazz but, for the sake of us down the back ...

After the TR levers have been engaged, can you shift the throttle/power levers forward to any point you choose? Does the system (particularly an Airbus) auto limit what percentage of power is applied, whilst TR is engaged?

In other words, what does 'full reverse' mean in terms of % of engine thrust. I am aware that only a certain part of the thrust is deflected forward in any event.

Thanks.

[Faire d'income]

The amount of reverse makes little or no difference in normal landing conditions to the stopping distance.

This is not correct. What otherwise would be the point of reverse thrust? In normal circumstances the idea is to use reverse thrust to decelerate at high speeds and then at lower speeds ( say 80 knots ) use the brakes. This obviously saves on brake wear but as rate of deceleration determines how quickly you will stop, it also influences stopping distance. I.E if you really want to stop short you use max braking and max reverse!

On a given aircraft with constant weights, same runway, it still requires different amounts of energy ( via brakes and reverse ) to make the same turnoff point. For example the headwind varies. If any hint of a tailwind then significantly more retardation required.

Also the touchdown point is rarely the same. If you land a little deep ( further along the runway ) more retardation needed. Likewise if you land with a little firmer than normal more energy is dissipated into the ground and less retardation required.

Lastly you just might be slow in opening the reversers because of a long day or just you dont like them and never open them fully. Different strokes for different folks.

[Intruder]

Paxboy:

By "full reverse" I mean whatever the EECs (Electronic Engine Controls) will give when I pull the reverse thrust levers up to the stops.

With GE CF6-80 engines and EECs in the normal mode, the EECs determine the max N1 available for current conditions. I can push the thrust levers full forward to the stops on takeoff, go-around, or windshear, or pull the reverse levers full up to the stops after landing, without fear of overboosting the engines. IIRC, the actual max reverse thrust is on the order of 80-85% max T/O thrust (around 95% N1), and net reverse thrust is about 40%.

[Youwererobbed]

Faire d'income the stopping distance is uneffected by reverse on a normal landing. This is decided by the autobrake setting used (although this is usually disconnected at some stage to fine tune the turnoff) which will give a predetermined rate of decelaration. All that happens by applying extra reverse is that the brakes do less work as some decelaration is achieved through the use of reverse.

This is not quite the same as using the max stopping power availiable to the aircraft in an emergency on a limiting RW in which case max foot braking and max reverse are the order of the day.

Landing performance doesn't usually take in to account the use of reverse, MEL reverse inop doesn't usually include perf corrections for the landing roll.

The touchdown point should always be the same, but as you say we all have bad days and get it wrong and land a bit deep. However it will be of absolutely no help at all to pull extra reverse with the autobrake in, all it does is make the guy pulling reverse feel he is helping the situation (although I'm guilty of this whilst desparately deciding if the other chap has realised how deep he has landed). If it's a case of max braking and full reverse then we should have gone round.

[Maximum]

wow, some of you certainly know how to confuse people.....
the question A300 man asked was:Why do some aircraft engines howl on landing when reverse thrust is applied for braking, whilst on other occasions, the same type of aircraft, carrying the same load, will land and roll to a stop, spoilers and reverse thrust applied, but with NO hint of an increase in engine noise???

The answer, A300 man, is that in the quiet case, reverse thrust has been selected, so you see the reverser sleeves move back on the engine pods, but the engines have been kept at idle.

In the noisy case, reverse thrust has been selected, and the engines have been spooled up above idle, so reverse thrust in the true sense is being used, and creates noise.

Some airlines demand that at least 75% reverse thrust should always be used on landing to save on brake wear. Other airlines think that the noise can upset nervous passengers, and therefore use only idle reverse to keep the noise down.

Some airlines leave it up to the crew.

However, idle reverse is generally always selected on landing so it's available if needed.

[BAe 146-100]

Hi,

What aircraft has the loudest full reverse thrust noise? Good reply Maximum :ok:.

Thanks
BAe 146-100

[jmc_757-200]

Hi,
The loudest thrust reverse i've heard was inside a FR 737-200.

Thanks

jmc_757-200

[Pilot Pete]

Just to add;

some airlines advocate idle reverse thrust as normal because brakes are cheaper than engines (especially steel brakes as opposed to carbon), others advocate full reverse as normal (on the same aircraft type, I know because I've flown the same type with more than one operator!) unless local noise abatement etc over-rules.

Also just out of interest, we don't use autobrake level 1 with my current employer on the 757/767 with carbon brakes due to them 'cycling' on and off which is detrimental to carbon brakes which prefer to be applied and heated up to work best.

I must say I agree that the noise difference is probably down to the amount of reverse taken. As I have said, with my current employer we take full reverse as normal, but, operating into the same airfield at night with a noise sensitive area and a Jeppessen note saying 'idle reverse only' (with the time restrictions) we will still set the autobrake to turn off at the (same) desired turn-off and may well be at the same weight, landing at the same point on the runway but you will hear the difference between full reverse and reverse idle.

Regards

PP

[jmc_757-200]

Hi,
Pilot Pete this is off topic but which airline do you fly for?

Thanks

jmc_757-200

[Pilot Pete]

Brits, since MelB painted all your fleet at my expense and flying rubber dog poo out of EMA every night didn't suit................

Hope life is treating you all well in the 'Global' world.;)

PP

[Faire d'income]

You are making three assumptions:

1 Autobrake is fitted,

2 It is serviceable,

3 You choose to use it.

On non limiting, good weather days I never used it. :ok:

[A300Man]

Maximum
Thanks for your reply, which to an uneducated pax like me, is the only answer that I truly understood. Thanks.

On the other hand, the loudest "noise" that I can recall was on an old BY 737-200, several years back, after landing at AGP. I had flown on the BY 73's several times and there was always loud noise - in fact, the purser would always do a PA to advise pax not to be alarmed by the "increase in engine noise, which is just an indication that reverse thrust is being applied to assist braking on the runway......"

However, the AGP landing in question was phenomenal - noise like you would never believe, but no explanations were given at the time as to why.

[Pilot Pete]

A300Man,

Someone more familiar with the 737-200 will be able to give you a better explanation than me, but I remember correctly 737-200s had(have) low bypass engines, ie small in diameter when viewed from the front. This means that more of the air drawn into the engine is routed through the core of the engine, where it is mixted with fuel and burnt, then chucked out the back at high speed. This makes these engines significantly louder than a modern turbo-fan engine which has a higher bypass ratio (= less air is fed through the core and accelerated and more is pushed outside the core by the fan(the big set of blades at the front of an engine on a more modern jet)) these means less noise as well due to the lower speed of the bypassed air. I suspect, but stand to be corrected, that when a -200 is put into reverse the cabin occupants hear a significant increase in noise due to more of the airflow being directed towards the cabin, as on a more modern jet, but it is relative - you start off with a noisier engine directing forward thrust parallel to the body and then direct it towards the body and you get a larger increase in cabin noise.

Hope that helps, and is indeed correct, having never flown one I feel a bit of a fraud!:suspect:

PP

[Maximum]

A300 man, on the -200 the reverser system has "buckets" which actually deploy aft of the engine (about a foot) - these are literally metal deflectors which (just like it says on the tin) deflect the airflow from the engine forwards. As you can imagine, this produces one hell of a noise. Also, as pilot pete says, the -200 has low-bypass engines, which are noisier anyway.

On more modern engines (-300, -400, -500 etc), the flow is directed forward by a matrix system after the reverser "sleeve" (aft part of the engine pod) has slid back about two feet. A much quieter system. And quieter high-bypass engines too.

Two other factors which may be relevant. If you're sitting close by or just aft of the engines, the noise you hear will be significantly higher. Also, if full reverse thrust was selected (ie, the maximum allowable thrust setting), for example to make a certain turn-off, then that too can cause a serious increase in noise. The final 5% to 10% of allowable thrust has an exponential effect on the noise level.

Hope that explains it.

[rwm]

To: Youwererobbed,

Curious about the type of machine you operate. I know that the airport I used to work from if you didn't do the numbers right, and were heavy, you would probably end up in the weeds. We used to get 727s come in, and they all used full revers thrust, or they probably would have ended up in the weeds. So yes reverse is efective, and very useful at not only limiting brake wear, but also stopping.

[M.Mouse]

As a point of interest a well known British Airline introduced a policy of idle reverse unless conditions dictated otherwise.

It was discovered shortly thereafter that brake wear DECREASED using less reverse on the 747-400.

[northwing]

If you look at the thrust reversers on a B737-200 you will see that the buckets operate about an axis canted outboard by around 45 degrees. There is often a sooty mark on the upper rear fuselage as a result. I was once told that this was because they started with them operating about the horizontal axis but found that this gave a ground cushion effect under the wing which reduced the load on the wheels and thus reduced the braking force (=mu x weight). This meant that the aircraft took longer to stop with brakes plus reverse thrust than it did with brakes only. Incidentally, there may be other considerations in how much of each to use. I was recently on a B767 doing a demo flight to some people thinking of buying some for a specialist conversion job. The excited pilot announced that he was going to use full brakes and full reverse to show how well the jet could stop. It stopped in a comendably short distance - and the pax were well amused by the tray of glasses which slid on to the floor in the galley with a resounding crash.

[Intruder]

M.Mouse:

The decrease in brake wear you cite is only applicable if the Autobrakes 1 setting is used. Rull reverse gives more deceleration than programmed for the autobrakes, causing the brakes to release and re-apply, therefore causing increased wear.

If Autobrakes 2 setting is used, reverse works properly/conventionally.