Jets leaving their undercarriage down after takeoff?

[ElPerro]

On the A330/340, our maximum brake temp for take off is 300degC, so if after rotation the brakes are above this

The caution is also inhibited from 80kts to lift-off

Maybe I'm missing something here but how would the brakes heat up between brakes release and rotate?

In my mind the brakes are either hot or not prior to brakes release. If they are hot you shouldn't be taking off unless you have some way of confirming you won't exceed brake energy limits if you were to abort immediately prior to rotate / refusal (whichever is lower).

The caution is also inhibited from 80kts to lift-off, so you wouldn't see it during the high speed take off roll, and if it was going to affect your RTOW performance, wouldn't it be better to have the ECAM warning, irrespective of speed/Flight phase, in order to reject early?

Wouldn't you think that aborting 2 knots before rotate places the aircraft at higher risk than lifting off and dealing with it airborne. Especially given the fact that you'll rely on the brakes to stop you on the runway.

[404 Titan]

ElPerro

It’s not uncommon on the A330/A340 to see a 100 °C temperature rise in the brakes from brakes release to airborne. Some is in the taxi out but most is from the take-off roll, and it has nothing to do with brakes binding. I large amount of heat is produced when you spin a wheel at 150 kts.

[ElPerro]

Right. So it's standard, and the performance calculations of the aircraft account for it. So why leave the wheels out?

[slamer.]

Depends on the problem and the A/C.

If it's a disco-brake the wheels must be allowed to spin down for a set time or risk excessive gyroscopic/precessional forces damaging the gear on retraction. The perf penalty will be diff if the brake is either capped or disconnected with the tool. If the brake is only capped the penalty is huge.

I have a vague recollection of the 747 either leaving the GD or slow retraction with an ADP U/S, this also required a large 2nd segment perf penalty. maybe someone a bit more current can refresh me..

If you reject a takeoff you need your brakes to be at their best. The problem is they can take some time to show a significant temp rise. I remember forever checking the old steel-brakes on the B747 before T/O after a long/heavy taxi. Carbon brakes are a diff story they like being hot but dont forget to consider tyre sidewall temps on long/heavy taxis. If it all gets too hot the thermal plugs will deflate the tyre but this may also take some time.

[Inbound Outbound]

Indeed an interesting topic. trimotor, It's very rare to see something like this though I have honestly only seen it on three occasions. The airlines were two Cathay's (last year) and a Etihad A345 (last week). They all were on climb from Sydney Intl and were approximately 5 nm away from the airport with the gear still down. I foolishly thought for a second that the crew forgot to retract the gear . But that obviously wasn't the case.

IO

[Tee Emm]

During simulator training on aborted take off's, the scene was based upon a balanced field length. One pilot consistently stopped within the runway length from a V1 cut (loose term but you know what I mean) while his crash-mate consistently over-ran by 200 metres or so. Yet both were aborting at the same speed and same rapid actions.

A closer look at the instructor's panel which showed brake pressure at each pedal, showed why the difference. The pilot over-running started each take off with his feet high on the pedals and throughout his take off run was occasionally and unknowingly, applying dabs of brake pedal pressure as the take off run progressed and while he was maintaining the centre line. In turn this increased brake temperature during the take off run.

The pressure varied but overall his take off run was longer due to dragging the brakes and thus V1 was invalid. Due to the lack of light it was impossible for the sim instructor to see if the pedals were being depressed slightly but the read-out on brake pedal pressure proved it. Once he was aware of the problem, his aborts were kept within the runway length.

[NoseGear]

El Perro....there are three scenarios that spring to mind, one I already mentioned with regard to binding brakes, the others Tee Emm and 404Titan explained, all will cause an increase in brake temps during the roll. I did say the 300deg is a limitation, so if they are above 300deg, we cant start the take off, there is no allowance for what I might think hot is, and during an abort, you can bet every last dollar they will go above that temp, but as long as they are below 300deg at the beginning of the take off roll, you will have your required RTOW performance, right up to rejecting at V1. Im not talking about rejecting 2kts prior to rotate, but on reflection I can see the logic at leaving the caution ihibited above 80kts....takes me awhile to get there sometimes!
As for

dealing with it airborne

...isnt that what we are talking about....? So I guess that answers your question in your second post