Tech question of the week.
 

Why do air trapped in a brake cause it to become hotter than normal ?.

A winner will be drawn from all correct answers, and announced next week.

My suggestion: 1. air is a worse conductor of heat energy than the brake fluid which is supposed to be in its place. 2. Air is relatively compressible compared to brake fluid, compression results in temp. increase with a corresponding temp. increase of the brakes.
Any good? I bet the cheque's in the post already!

Techie,
I assume you mean air in the brake hydraulic line. In that case, when brakes applied the air would be compressed. When hyd brake press released the air would tend to expand and voila - a dragging brake. Also heat from the brake pack would tend to make the air expand in the line. In an older system with lockout deboosters - a la B707 - said air certainly cannot escape, not too sure about modern systems.
How'd I do, coach???

Thanx for the memories Mustafagander.Lockout deboosters eh.In the green band and all that.
How about Q bellows for another blast from the past.
Sorry ,I digress.Air in the brakes?
Dunno.

On the older a/c using Master cylinder (light a/c) pressure is a direct function of the force applied using our foot. With air trap in the line it may well expand with temperature especially during brake application. This energy would then be absorb by the brake fluid and subsequently by the trap air. Because the brake system is a close loop, the heat absorb by the air may well cause it to expand and increase the fluid press within the close loop causing the caliper to perhaps lightly grip on the wheel rotor when foot pressure has been remove. This grip may be to light to be felt in the cockpit but may well be sufficient to generate heat during the taxi roll to bay after landing preventing the heat from the landing to be decipated in the normal rate resulting in hotter brake.

Well...er....how did I do?

O.K I'll have a go.
I assume we are talking about a sophisticated system with anti-skid?
If this is the case,because of the compressibility of the air trapped in the line when a "release" signal is received from the anti-skid system only a partial release is achieved. i.e. a "dragging" brake.
Well?

Hello,anybody home?
Hey Techman,don't keep us in suspenders!

Since when has air trapped in a brake system caused hot brakes.?

Justa,
I'd estimate that this first occured shortly after the first time juice brakes were changed in a hurry. It's possible any time brake lines are opened and not properly bled afterwards. Remember air is compressible, hydraulic fluid is not.

You'd also get very poor braking!

The air which is being compressed gets hot as a result of the compression, as gander says, Hyd fluid is not compressible (for all intents and purposes), but air is.
Regards

Yes. That is correct.

Often air in brake systems produces a dragging brake which can make the unit hot. It can also degrade the operation of the unit forcing its partner on the same axle to work harded particuarly with anti skid thus increasing that units temp. Its always advisable to treat a hot brake unit as suspect likewise its partner on the same axle.

I can't believe you guys answered the question. It was posted in 1998 and obviously moved from another forum.

I suppose this is all about the car type brakes on puddle jumpers? I'm watching these dirty great Boeings, fitted with heat shields to keep the heat from the carbon brakes melting the beads on the tyres, as they do their landing roll with brakes glowing in the dark. The question of the week for next week is: How's a bit of air in the brake cylinder going to make them any hotter than white hot?

Blackie,
As you should know :E air is not as good a heat conductor as fluids, therefor the heat dissipation through the fluid would be less if said fluid turned out to be air :8 :8 :8
alright a bit academic i know, but I am bored
Brgds
Doc

Next question.
Why are spoilers always hinged at approx 70% MAC