LH526
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What Is That Gear Bang At Liftoff?

Thu Feb 27, 2003 6:05 am

Hi

I experienced the following several times while fliyng as pax on B747 (Both on -200 and -400, so no matter if OG or NG)
At rotate, shortly before or after liftoff, a loud "bang" comes from near the gear bay area that sounds like the gear depressing in their suspension. What is that sound? Maybe some engineers can shed some light on it!

Thanks
Mario
LH526
Trittst im Morgenrot daher, seh ich dich im Strahlenmeer ...
 
XFSUgimpLB41X
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RE: What Is That Gear Bang At Liftoff?

Thu Feb 27, 2003 8:53 am

It's the gear doors closing.
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LH526
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RE: What Is That Gear Bang At Liftoff?

Thu Feb 27, 2003 9:45 am

gear doors closing?
I don't think so. The sound comes the second split the main gear leaves the ground I guess, so wayyy before the gear is retracted.

Mario
LH526
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EssentialPowr
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RE: What Is That Gear Bang At Liftoff?

Thu Feb 27, 2003 10:10 am

Happens on the 737 as well- no gear doors.

Sounds like the extension of the struts...one can't hear the nose gear strut either, unless you're in the cockpit. The snubbers are loud as well.
 
Boeing Nut
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RE: What Is That Gear Bang At Liftoff?

Thu Feb 27, 2003 10:29 am

I know there are "bangs" when you hit the expansion joints on some runways, but that doesn't explain the ones after liftoff.
I'm not a real aeronautical engineer, I just play one on Airliners.net.
 
L-188
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RE: What Is That Gear Bang At Liftoff?

Thu Feb 27, 2003 10:46 am

Usually it is the sound of the oleo's hitting the full extension point at takeoff.

But gear doors closing also make a pretty loud bang too.
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Mr.BA
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RE: What Is That Gear Bang At Liftoff?

Thu Feb 27, 2003 10:50 am

My enducated guess... maybe it's the gears 'dropping' back to it's tilting position after the plane leaves the ground? So it's like... 'straight' dropping back to it's 'hanging' or tilting position.

Boeing747 万岁!
 
EssentialPowr
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RE: What Is That Gear Bang At Liftoff?

Thu Feb 27, 2003 11:10 am

L-188-
Read what people have posted....

"Usually it is the sound of the oleo's hitting the full extension point at takeoff."

A/c landing gear assemblies that have doors, or boggies, are dampened...
 
Guest

RE: What Is That Gear Bang At Liftoff?

Thu Feb 27, 2003 2:59 pm

I would go along with "sound of the oleo hitting the full extension" as the reason of the loud "bang" which occurs at lift-off of the 747...
xxx
According to loadmasters - who fly in cargo airplanes as additional crewmembers - they tell me it is quite noticeable in empty 747 cargo planes - in passenger airplanes, the noise is reduced due to carpets - etc...
xxx
I doubt gear doors would be that noisy, the body gear doors are hinged and quite slow to close, or... to open...
xxx
(s) Skipper  Smile
 
Leej
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RE: What Is That Gear Bang At Liftoff?

Thu Feb 27, 2003 9:28 pm

LH526
I posted a similar question about a year or so ago - I'll have a look for it. I know exactly what you mean - it happens as the gear leaves the tarmac. As Skipper says, it's the gear dropping to full extension once the weight is off.
Cheers!
 
L-188
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RE: What Is That Gear Bang At Liftoff?

Fri Feb 28, 2003 6:39 am

I did read the other posts

Now read what I wrote Essential Power.

Usually it is the sound of the oleo's hitting the full extension point at takeoff.

But gear doors closing also make a pretty loud bang too.


Line #1. The sound of the Oleo's extending. I agree with the majority on that one.


Line #2. Not with the Majority but it has been my experience that you can hear those doors closing on 727's and MD-80's. Along with the change in wind noise because the doors aren't haning out in the breeze.
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EssentialPowr
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RE: What Is That Gear Bang At Liftoff?

Fri Feb 28, 2003 8:37 am

You repeated what I wrote; I said it was the struts. I made no ref on closing gear doors, b/c:

1. 737s, for ex, don't even have them (on the mains, that the pax could hear)

2. Gear doors don't close "At rotate, shortly before or after liftoff..." but a measureable time after rotation.

If a poin has been made by someone, there is no need to repeat it...so you should read other's posts.
 
Monocleman
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RE: What Is That Gear Bang At Liftoff?

Fri Feb 28, 2003 10:02 am

I guess I'm pretty out of it, but "oleo's"? Is that just another word for shock absorbers and struts?
 
KAUSpilot
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RE: What Is That Gear Bang At Liftoff?

Fri Feb 28, 2003 10:19 am

Yes, oleo or "oleo-aero" as I've sometimes heard it called, refers to the suspension system for the aircraft's landing gear. The system uses a combination of oil and air to provide shock absorption; when there is no pressure being exerted on the suspension by the ground due to the weight of the airplane, the strut extends to maximum length. It is quite likely an aircraft with a relatively heavy undercarriage would produce a noise when the struts hit the stops after liftoff.
 
L-188
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RE: What Is That Gear Bang At Liftoff?

Fri Feb 28, 2003 11:49 am

I didn't mention anything about 737's did I? And neither did the original poster. I just stated that you can hear the doors closing on those two types.

Electra's also have gear doors but I have never heard them closing on those airframes. That is probably because there are way the hell out in the Nacelle.

Go and watch mechanics swing the landing gear on a Learjet sometimes. Trust me. The landing gear doors do close up a with a pretty good bang.


Again I agreed with the majority, and offered a second possibility.
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FredT
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RE: What Is That Gear Bang At Liftoff?

Fri Feb 28, 2003 5:53 pm

I have to say I’m not buying the oleo extension explanation. First off, the struts have dampers. The purpose of dampers is to prevent rapid compression or extension of the struts. Second, suspension systems tend to have bump rubbers at the end of their travel to prevent the stress of metal to metal contact when the struts hit either the full extension or the full compression stops. These would reduce any potential ‘bang’ to a jolt.

And even if you had landing gear struts without dampers, the aircraft does not come off the runway instantaneously. The gear struts would extend at the same rate as the rate at which aircraft lifts off the runway. Is that fast enough to cause a bang at the moment when the wheels leave the tarmac? Tilting bogies would also soften the process.

On the other hand, I have been running landing gears up and down and heard (and felt!) very noticeable bangs - from the landing gear doors. In fact, with one aircraft you couldn’t run the gear at full speed or it’d jump off the jacks. This has been fighters, but with the big gear doors on commercial aircraft the velocity of the edge of the door farthest from the hinges will be quite high, if you want the gear doors to close at any noticeable rate.

I’m not sure here, been a while since I got close and personal with a 737 - but don’t they still have gear doors? They just don’t cover the wheels themselves.

I’ll bow down to conclusive technical evidence, but I am not convinced yet.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Guest

RE: What Is That Gear Bang At Liftoff?

Fri Feb 28, 2003 6:33 pm

Dear Fred -
A couple of years ago, we did a "gear swing" on one of our 747s at the airport, a maintenance procedure after a "D" check... I had "invited" myself with a videocam to tape a gear retraction for ground training audio visual... Since the airplane stood high, to keep the main gear off the ground, to permit retraction, the gear itself was far from being noisy during repeated retraction and extention cycles... I did not notice any particular "bang" noise with the opening and closing of the various doors, actually these doors cycle rather smoothly. The hydraulic systems 1 and 4 were normally pressurized - what can I say then? - I suspect the oleo struts, really to be the source of noise - if any - remember also, the gear bogies are subject to an abrupt "tilt" in the liftoff process - to retract the gear, and that, I suspect could be the source of a loud noise, by itself... is this the answer...?
xxx
(s) Skipper  Smile
 
L-188
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RE: What Is That Gear Bang At Liftoff?

Fri Feb 28, 2003 8:38 pm

I agree the Oleo's are the source too.

But I was just making the point that other componets can make other banging noises too.
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Leej
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RE: What Is That Gear Bang At Liftoff?

Fri Feb 28, 2003 11:16 pm

From what I understood, it isn't the oleo (at least on the 747's which I have noticed it on) - it's the wing mounted gear wheels springing back to the uncompressed stage. Look at some pics of the gear just before touchdown, the rear wheels are down first, then the front follows - the actuator is compressed to smooth out the landing as the weight is placed on the wheels.
So, on take off, as the plane rotates and lifts off , the noise you hear in the rear of the cabin is the gear dropping / tilting back to it's uncompressed stage. Does that make sense?
The discussion I believe is not about the gear doors closing, as the 'thump thump' always occurs at the moment just after takeoff.
I have also noticed it on the 767 - but not any of the single bogie planes. I'm on a 737 tomorrow morning, I'm going to listen for it though anyway.
Ciao!
 
FDXmech
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RE: What Is That Gear Bang At Liftoff?

Sat Mar 01, 2003 1:33 am

I also believe this is the strut going to full oleo extend.

Yes the strut is damped from going from compressed to full oleo extend, if it wasn't, the bang at lift-off would be much louder with accompanied damage.

>>>And even if you had landing gear struts without dampers, the aircraft does not come off the runway instantaneously. The gear struts would extend at the same rate as the rate at which aircraft lifts off the runway. Is that fast enough to cause a bang at the moment when the wheels leave the tarmac?<<<

I would think the strut extends at a slower rate than lift-off due to dampening. Any metal to metal (solid contact) contact with the large weight of the strut piston under pneumatic pressure will cause a bang.

I've never seen rubber bumpers on any of the struts I've taken apart and repacked.

>>>I’m not sure here, been a while since I got close and personal with a 737 - but don’t they still have gear doors? They just don’t cover the wheels themselves<<<

The main gear doors are attached to the gear assembly and cover the struts, but there are no gear doors that covers the wheels. They remain uncovered.
The nose doors are mechanically opened and closed with movement of the nose gear and are not hydraulically actuated.

Not all bogies are tilted though. Aircraft such as the DC10 & A300-600 are level for retraction.

Let's see what LeeJ says.
You're only as good as your last departure.
 
EssentialPowr
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RE: What Is That Gear Bang At Liftoff?

Sat Mar 01, 2003 6:24 am

FredT, as the strut reaches full extension in a no load condition it will make a nice thump as it reaches the limit...

No, L188, you mentioned that it could be the oleos after I had already stated it was possibly the struts. You should read what people post, and if you agree, state that. ¿comprende?
 
L-188
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RE: What Is That Gear Bang At Liftoff?

Sat Mar 01, 2003 5:07 pm

You just aren't big on consensus opinions, are you Essentialpowr.





Now will you consent that there can be more then one bang during the liftoff and retract cycle on a modern aircraft.
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Leej
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RE: What Is That Gear Bang At Liftoff?

Mon Mar 03, 2003 10:47 pm

Well, as I said, here I am back at my desk after a quick visit to the land of the black stuff this weekend.
Boeing 737-500 - I sat right on the wing, exit seats both ways. Waited until that moment, and could detect only a slight 'thump' on lift off. So, I guess that that must be the oleo going to full extend.
On the way back, 737-400, again exit seat on the wing, no thump or bang detected, however I had by then had a few pints of Guinness!!!!
 
EssentialPowr
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RE: What Is That Gear Bang At Liftoff?

Tue Mar 04, 2003 1:47 am

L188,
???? Do you read others posts, ever?

I never stated that this bang was solely from the struts, I only postulated that and cited the 737 as an example. If you think that it's the flapper valve in the lav, I don't care...but the I mentioned the strut in the 3rd response, and that appears to be the consensus.

If you agree (you said oleos), state that, and establish the fact that you are reading others posts...as opposed to mindlessly arguing.
 
cdfmxtech
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RE: What Is That Gear Bang At Liftoff?

Tue Mar 04, 2003 2:08 am

B737
A snubber valve is installed on the inner cylinder just below the upper bearing. The valve consists of an angular sectioned bronze ring that moves up and down when the shock strut operates to act as a one-way restrictor. When the shock strut is compressing, the snubber valve allows an unrestricted flow of oil from the upper chamber into the annular space between inner and outer cylinders. When the shock strut extends, the snubber valve moves up to close off holes in the upper bearing; this restricts flow from the annular space and so absorbs shock strut rebound.

B757
Called a "Rebound Snubber"

B767
Called a "Recoil valve"

I think it's safe to assume that the sound you are hearing is not the strut extending....if indeed it is as loud as you say it is.
 
EssentialPowr
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RE: What Is That Gear Bang At Liftoff?

Tue Mar 04, 2003 12:38 pm

And so Cdfmxtech,
the source of the noise is?????

A strut assembly which is designed to support thousands of pounds will certainly make a resounding "thump" when reaching the extension limits under zero load....thinkabout it...
 
cdfmxtech
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RE: What Is That Gear Bang At Liftoff?

Tue Mar 04, 2003 2:24 pm

That may very well be so, but I'll tell u...I flew on some very old b747s (Tower Air) that were at the time verly long btwn gear changes.
Never heard it.
If he did hear it, and it was from the strut extending, I would think that it is not normal. He didn't say slight thump like one guy stated. He said loud bang.
 
LH526
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RE: What Is That Gear Bang At Liftoff?

Wed Mar 05, 2003 8:43 am

... it's a bang that sounds like the gear hitting a small rock or bumper on the runway at lift-off.

I talked to some Lufthansa engineers and they said it has to do with the struts and extension thingys.

LH526
Mario
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QANTAS747-438
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RE: What Is That Gear Bang At Liftoff?

Wed Mar 05, 2003 2:23 pm

Yeah, Lh526, I know exactly what sound you're talking about! You're going down the runway, the nose lifts up, and as the plane tilts upward, the wing looks like it's about to scrape pavement, and then some crumpled, buffered rumple sounds come. I usually hear 2 of the sounds right after each other. I have always wondered that too, but now I'll have to agree in the landing gear extending.

I don't think it's the doors closing because that's usually a rumbling sound preceeded by a motorized type of sound of the doors physically opening.

Interesting post!
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.
 
ndege
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RE: What Is That Gear Bang At Liftoff?

Fri Mar 07, 2003 3:46 pm

Perhaps it's a gremlin latching on to the wing at the last possible moment. When all else fails, blame the gremlins!

BT

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