Undercarriage question

[AeroSpark]

Hi, I was wondering out of pure curiosity what happens to the wheels after the gear is retracted. Are the brakes applied or are the wheels just left to spin down on their own?
Thanks

[de facto]

Braking is applied to the main gear and antiskid disactivated.
Nosewheel uses spin brakes pads.

[Tinwacker]

..on a B737 that is.

On all big aeroplanes that I can think off, after gear selected UP, auto braking occurs with a reduced brake pressure to stop the main gear wheels rotating and vibrating before finally entering the wheel bays.

[dixi188]

Two reasons for large jets having auto brakes for main wheels during retraction.

1. Very high centrifugal forces fighting retraction mechanism with wheels revolving at up to 180 kts. peripheral speed.

2. If there is a piece of tyre tread coming adrift you don't want it flailing around the landing gear bay causing damage.

If there is a problem with anti skid or one or more brakes are de-activated (ref. MEL) then the gear will be left down for about one minute after take off to allow the wheels to stop turning before retraction.

[de facto]

Tinwacker

Join Date: Oct 1998
Location: CGK to HKG
Posts: 231
Quote:
..on a B737 that is.
On all big aeroplanes that I can think off, after gear selected UP, auto braking occurs with a reduced brake pressure to stop the main gear wheels rotating and vibrating before finally entering the wheel bays.


Same same 737:p

[grounded27]

A surge of pressure is generally sent to the MLG wheel brakes through gear up sequence but not held and friction devices are generally installed in the NLG W/W.

[bcgallacher]

Dixie - brake deactivation on 747 using the required tool still gives spin braking
, no need to leave gear down. Leaving gear down gives large penalties. If no deactivation tool available brake hydraulics can be capped - this will require delayed gear retraction.

[dixi188]

bcgallacher.

Don't know about the 747. Most of my experience is BAC1-11, A300, BAe 146, etc.

On A300 there is no performance penalty, other than stopping capability, as in the event of engine failure you retract the gear anyway, on the basis that if you have engine fail and tyre tread failure at the same time you should have stayed in bed.

How does the brake deactivation tool work on the 747?

[bcgallacher]

At least one tool should be kept on board the aircraft - it is a pressure reducing valve that is sandwiched between the brake unit and the hydraulic connector block with longer bolts. It reduces the pressure to that wheel assembly to a figure that will give spin braking but not enough to lock a wheel during normal braking - a figure of about 300psi comes to mind. Last time I installed one was in Doha with an ambient temp of 40deg plus on top of that the captain switched the left packs on and the heat exchanger exhaust made it considerably hotter the result being that I collapsed unconscious after finishing the job.