What are the thuds you hear on take-off?

[JohnnyRocket]

Just as the nose lifts and the rest of the plane lifts off the ground... you often hear two thuds.

I assume these are the rear wheels... but WHY does it sound like a thud?

They're already locked in place right? So why does it sound like thats happening again?

[Rainboe]

The mainwheels are on an extendable shaft within the undercarriage leg which acts as a suspension. As the weight of the aeroplane lifts off this, it extends pretty sharply with a thud as it reaches its limit. It is stressed to absorb a load of several times the aircraft weight for landing, so it is very powerful. When it is unloaded and extends, it must be quite a force. It is not the sound of the landing gear retracting into the bay.

[skiddyiom]

Rainboe,

While you are correct in the manner in which the U/C extends on rotation, I have to say that when it locks up you will hear a thud or thump on many aircraft. This is the mechanical uplock engaging and holding the assembly in place.

Cheers

Skiddy

[Rainboe]

Yes, but the question was about the thud as the aeroplane lifts off the runway. There's loads of other thuds inflight, including the downlock engaging before landing, and finally the tremendous thud as our hero lands! There's roars as the nosewheel brakes on the fixed pad, the flaps move etc., but will someone explain why those dratted Airbooses always have dogs barking in the rear hold before flight?

[Dr Illitout]

The bangs might be the oleo's bottoming out. Also when the pilot selects gear up the wheel brakes are applied to stop the wheels spinning in the bays. (Causes an annoying vibration). The pressure used is a reduced pressure but it still might cause the wheels to stop with a bang. I'd still go for the oleo's bottoming out though!.
The dogs barking in the rear hold is the sound of the Power transfer unit (PTU) working.
( A hydraulic pump driving a hydraulic motor so that in the event of one hyd system failing one of the other systems can power it!)

Rgds Dr.I.

[FlightDetent]

I've been told it is the hydraulic PTU test in progress as the second engine gets started (narrowbody). However from the cabin sidem it seemed to take much longer...

[moleslayer]

If the crew taxi out single-engine (permitted by Airbus, but not some companies) the PTU will run continuously. It will stop when the other engine is started just before departure.

Of course, animals are permitted in the cargo holds, so it may just be a couple of woofers.





:D :D moley.

[MAN777]

Well here at Manchester ýts the cobbled runway that causes the thuds !!!

[Byrna]

Hi Moleslayer... I don't think my Siamese cat Skittles would appreciate that joke... :))

John

[ifleeplanes]

The toilet seat dropping down!

[Georgeablelovehowindia]

If it's an aircraft with a bogie maingear e.g. 757/767/747, the truck levelling system forces the bogie into the correct position for retraction. The 767, for example, has the bogie pointing slightly down. The thump can be felt in the flightdeck. The landing gear will not retract unless the truck leveller microswitches confirm that the bogie is correctly positioned, amongst other things.

[numloxx]

Hi all,

This is my first post here, so Hello all.
The company I work for (Ground Staff) fly 146's, and when the gear is lowered the aircraft literally gets jolted left and right as the left and right gears lock into place one after the other.
There is a gap of 1/10 or more between the left and right gears locking, but quite noticeable if you know about it.

Cheers,

[scrubed]

Interesting how you guys can all postulate various theories when no aircraft type has been mentioned......

Oh well, may as well join in then. I reckon it's one of these:

- xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

- xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

- the over-centre lock being overcome


Choose the best answer.

EDIT: Your Jet Blast post reduced to a more acceptable one

[yeoman]

It's my eyelids clanging shut in anticipation of yet another night of drilling holes in the skies to/from The Canaries.:E

Dear Roster Department. Love you. Really.:{

[Tempsford]

The thud is the Captains wallet as it falls out of his back pocket onto the flight deck floor. The other thud is the sound of the F/O fainting and hitting the flight deck floor as he picks up the Captains wallet and espies the Captains wage slip.....


take cover!........

Temps

[sir.pratt]

you won't hear the oleos extending, as they do so reasonably gradually as the aircraft takes off. neither will you hear the bogie tilt gear, as they are hydraulically locked, and by the time the gear is right off the ground, the wheels are all the way back.

the 2 distinct sounds you can hear are: 1. the gear locking into place in the uplocks (under the floor) and 2. the doors slamming shut behind them - especially on a 747/767/757, where the main (wing and body for 747) doors are hydraulically opened and closed, independent of the gear. the 767 ones especially close with a bang. next time you're spotting, watch the 76 doors open, then the gear raise, then the doors close behind.

[scrubed]

Thanks Pratt... I think that's what I already said....... :rolleyes:

[Rainboe]

I think we're answering questions that have not been asked. Just to review, it was:Just as the nose lifts and the rest of the plane lifts off the ground... you often hear two thuds.

This is before the gear is raised. It is the sound of the oleos extending I believe, but it seems with 10 pilots answering, there are 11 different opinions!

[sir.pratt]

with a couple of 000 psi in the oleo, it is well extended as soon as the a/c is off the ground - there is NO delay.

just had a thought - it could be the auto (or manual) braking on the mains to stop the wheels spinning as they are retracted into the gear bays. i'll ask the guys - see what they think, but my best guess is the gear uplocking. the breaking of the downlocks is not a 'loud' occurance.

one other thing could be the hydraulics knocking as the system demand comes on-line.

SP

[expedite_climb]

Tempsford, you joke - I've seen money falling out of the skippers pocket on rotation before! Very amusing!

[Rainboe]

Let's all sit in a circle and hold hands and read the question again!
Just as the nose lifts and the rest of the plane lifts off the ground... you often hear two thuds.

It is not the automatic brakes coming on (that is a function of the gear actually raising), it is not gear downlocks disengaging/uplocks engaging, it is the bang of the oleos extending to their stops and the gear becomes unweighted. Of course it might be birds hitting the fuselage or lightning strikes or tyres bursting or stewardesses dropping their knitting, but I think it is safe to say it is incontrovertibly oleos extending to their stops!

Perhaps end of thread now?

[Dan Winterland]

What ae those thumps? Hedgehogs on the runway!

[Rainboe]

This thread is leaving me quite flat, let alone the hedgehogs! The usual prune 11 different answers from 10 people to a sensible question with one obvious answer!

[msexyjet]

generally at lgw on the airbus 319 its the bin in the toilet falling out of the cupboard?

[chuckT]

the answer is found in the african forum...
http://www.pprune.org/forums/showthread.php?s=&threadid=176698&perpage=15&pagenumber=1