Reverse Thrust: Vibrations, Shudders, Loud Noise normal?
 

Hi,

Following my previous thread about hard landings, I am pleased to say that the Ryanair landing was pretty good! I have overcome my fear of landings!

Now, I am not a confident flyer at times, despite flying once a fortnight. I do get quite nervous. And no, I am tee-total so I don't drink on flights!

When landing, what does "reverse thrust" actually do? On the 737-800NG aircraft on Wednesday, landing at Seville as soon as the wheels were on the ground the reverse came on? Then the noise built up, and there was some vibrations, slight shudders of lockers? And then the plane came to a stop pretty soon?

This was with Ryanair, whom I will say did a excellent flight on Wednesday to Seville from London Stansted (FR8361 EI-DAY) Hats off the the company for once again another excellent flight!

I just don't like the noises, vibrations, and slight rattling of lockers? Is this normal? And why do the engines make more noise as if we were about to take off?

Thank you for reading, and I look forward to your reply. I am very grateful for your time!

Regards,

Nick.

Designs vary from engine to engine but essentially "clam-shell" style doors open when reverse is activated shutting off the rearward thrust flow and re-directing it forwards. It doesn't actually "spin the engine backwards" (as I thought when I was little :O) but it uses the engine's thrust against the aircraft to help it decelerate. This does inevitably make the cabin fittings shake quite a lot.

The vibrations in the in 737NG aren't too bad....but some older types can be positively teeth shattering!

Thanks for your reply, I am not sure if it was a NG model or not. Just said "Boeing 737-800" EI-DAY?

But the cabin noises are perfectly normal?

I thought it spun the engines backwards as well! Yeah, the cap did move backwards!

Nick.

Rattling noises are perfectly normal. It's a 70 tonne airliner coming to a stop quick whilst moving over rock solid concrete. Things inside are bound to shake and rattle. The engines spool back up when reverse thrust is selected because the power needed is greater than idle, which is what they would have been in on touchdown, hence it sounds similar to the roar produced on takeoff.

I would have thought anyone who held a BA gold card would have flown enough to know about the effects of thrust reversers.

Come on, time to 'fess up - how many flights have you really taken :}

As others have said, noise and vibration is normal, in fact some of us "of a certain age" will remember the PA from the cabin crew alerting pax to that there would be a lot of noise after landing and that this was normal.

The engines spool back up when reverse thrust is selected because the power needed is greater than idle

Sometimes idle reverse is used and this is hardly noticeable.

Want noise and vibration, try being on an old 4 engine propjob doing runup.

Reverse thust is a good way to slow down, better than the old way of throwing out an anchor.

http://farm4.static.flickr.com/3021/...fda028.jpg?v=0

This is what you'll typically see when reverse thrust is applied, the back half of the engine cover slides back on touchdown as it does there are doors on the inside that block the incoming air and direct it out of the gap created by the engine cover sliding back.
So the more thrust you require to help slow down the more you need to increase power in order to get more air pushing out the gap, thats why the engine noise is loud and sometimes gets louder as more demand for "re-directed' air is needed.
I know some pendant will want to correct me as far as technical terms are applied but as a working stiff that flies airliners for the mere pittance that I recieve, thats the simplest way I can explain what's going on !! HTH

Yes all everything you mention is normal, and you have observed the point of it as well.

I've done more flights, than you've had breakfast. But until recently, I have never cared about what actually goes on.

And on the BA flights, I don't honestly remember if they used the reverse. I never paid attention, mainly due to sleeping or working.

But it is good to know the vibrations/rattles/cabin noise is normal, just wondered.

If anyone wants to know, I took the interest in flying after watching pretty much all of the Air Crash Investigation episodes on the Virgin Media cable TV (TV-Choice on-Demand)

Just out of curiosity, are "vibrations" during flight normal? And for take-off vibrations are expected in the similar way of landing?

The aircraft will normally shake a bit on take-off as well with often the cabin fittings making quite a lot of noise (and again this does seem to get worse as planes get older).

Some particular types have distinctive noises and vibrations as well...the HS 146's flaps being one well known example. The A320 if remember rightly sometimes makes a noise that sounds rather like an electric drill. Most turboprops give off a distinctive 'beating' sound from the propellers on take off which is caused by the revolution speed not quite remaining synchronised as the plane accelerates. So there are lots of noises and shakes...most of which are completely normal.

One which I couldn't identify once was on take-off in the 777...just prior to and after lift off there seemed to be a lot of vibration coming from the undercarriage...I thought that perhaps there was a rough patch on the runway at Gatwick...does anyone have any idea?

This is probably wrong, but could it be the tyres? Similar thing in my car on the M25. The noise was due to a tyre deflating whilst moving!

Nick

It happens on the 747 aswell, sometimes the undercarriage wheels are still moving and so are being slowed down fast before being stowed when gear is selected up.

The vibrations during rotation are often due to uneven tyre wear. The wheel is still spinning in the air before being stopped as the gear retracts, causing vibrations, this usually has stopped once the gear is fully retracted.

RyRNick,

Reverse thrust is simply a way of using aerodynamics to help slow the airplane, rather than so much brake. Airplanes have a lot of weight, and very small brakes. Any deceleration created by reverse thrust from the engines is less energy the brakes must absorb, and therefore lower brake wear and temperatures. Often the biggest limiting factor on how fast an airplane can be turned around at the gate is brake temperatures.

Reverse thrust is very simple. Just like your car engine, the jet engine requires a lot more power to do the work than the work it actually does. What this means is that the net thrust produced by the engine is less than the total power it produces; a lot of the power the engine produces is taken up by moving internal components and by keeping itself running. This includes overcoming internal drag. Your car is the same way; the engine produces a lot more power than what it actually puts out; most of it is required to overcome the internal friction and mechanical resistance of the engine itself, from turning oil pumps to other accessories, to overcoming spring tension in valve trains, to the friction in bearings, etc. The jet engine is the same way; it has accessories, and uses much of it's own thrust to turn it's compressor.

Take away the useful thrust being put out by the jet engine, and what you have left is a lot of drag. When reverse thrust is applied, the actual redirection of the exhuast gasses isn't what's producing the deceleration effect, though it helps contribute. It's the drag that's left over when the thrust portion is taken away. Blocker doors of varying types, or aerodynamic "cascades" alter the direction of the airflow being put out by the engine; the thrust or exhaust gasses are often directed sideways or sometimes slightly forward, instead of out the back of the engine, by blocker doors. Taking away the thrust leaves only the drag...this means the brakes need to do less work to produce the same slowing of the airplane on landing.

The exhaust gasses which went beneath the wing and aft during normal operation, are suddenly made to sound much louder as they are directed by the cascades or blocker doors or vanes in an outward direction. In a sense, it's as though you were suddenly placed beyind the engine; you're now in line with the exhaust gasses instead of having them diverted below and behind you in normal flight. It seems louder. The engine may shake and vibrate somewhat on it's pylon. It's designed to do that, as is the pylon mount.

All landings are calculated without the help of reverse thrust. Reverse thrust is therefor a bonus in helping the airplane slow down, achieving less wear and tear on the brakes, and quicker ground turns when going back out for another leg with passengers or freight.

As others indicated, the vibration you feel as the landing gear is coming up after takeoff, is simply wheel vibration due to balance. Generally the main gear will incorporate a feature in the brakes to slow the wheel and stop it from spinning as the gear retracts. Until that time, however, any imbalance in the tire will result in a shaking or vibration which will decrease in frequency as the wheel slows. The imbalance is created through landings when the tire is spun up from a complete standstill to the landing speed of the aircraft. You may have noticed a puff of smoke when an aircraft lands; that smoke is loss of some tire material as the wheel touches down and is quickly spun up. This loss of material naturally causes a slight imbalance, and this in turn causes the vibration you feel.

Wheel assemblies are typically balanced when being built up...much in the same way that a care tire is balanced before being put on your car.

When the airplane comes off the ground, the vibration will be felt until the tire assembly stops rotating. Often the main gear will stop rotating fairly quickly, due to some back pressure which is induced in the brake lines during the retraction process. The nosegear, however, may continue to spin for a short while in some airplanes, while others incorporate a mechanical snubber in the gear well to bring it to a stop.

FAQ
 

Reverse thrust :

I've met more than one person who thought that reversing the engines was done literally - that for a few seconds the inlet air was now drawn in at the rear and the exhaust and thrust went out of the front. While this might amuse the regulars here, it is what a lay person could feel was the obvious way to do it.

Likewise reverse pitch on a propeller plane being thought to be achieved by turning the engine in the opposite direction, issues such as variable pitch being generally not understood. I attempted to draw a comparison with a car's reverse gear, only to be told that this did indeed "turn the engine backwards" when selected.

General noises :

One of the most amusing I recall was in a BAe146, Euromanx, London City to Isle of Man. In the cruise, suddenly a quite noticeable "Kerboinggggg" twanging-spring noise from the cabin wall, like the sort of silly noise The Goons used to specialise in. I guess there was some spring-activated device inside there which had suddenly operated/let go. Any 146 engineers hazard a guess at what it was ? !

This link here will show you the other version of reverse thrust that pops 2 doors over the back end of the engine to redirect the airflow forward, incidentally the likes of most military fighters such as the Tornado use this design, if you see a picture of one, you will tend to see a lot of soot staining on the vertical fin, this is because of this air flow being redirected forward.

NationMaster - Encyclopedia: Thrust reversal

http://upload.wikimedia.org/wikipedi...kker70.arp.jpg

RYRnick. The Boeing 737-800 is a Next Generation (NG) in fact the breakdown goes as follows:
737-100, 737-200. Original
737-300, 737-400, 737-500, Classic (CL)
737-600, 737-700, 737-800, 737-900, Next Generation (NG)

I would have thought that a experienced passenger like yourself, who has more hours of flight that most of us pilots here you would know the difference.

I personally don't believe, you are 28, a ex BA Gold card holder, even out of nappies yet. My suggestion is grow up and don't criticise about things that you have no knowledge of.

well done ford cortina you take a honest question from a passenger (the point of this section) and turn it into insults, that's quite a talent you got there :D

A perfect example of why someone felt the need to start this thread.

http://www.pprune.org/passengers-slf...ml#post4380255