Jets leaving their undercarriage down after takeoff?

[Inbound Outbound]

G'day guys,

I have seen on few occasions jets such as B777 & A330s leave their undercarriage down well after takeoff and was curious on why?

p.s. I read another thread containing a similar question on this but the reply's wern't very informative.

Cheers,
IO

[NoseGear]

Two words....Hot Brakes...:ok::D

On shorter turnarounds in warmer weather, the brakes dont often have alot of time to cool, there is a great amount of heat energy to disapate and an hour or two wont often be enough. On the A330/340, our maximum brake temp for take off is 300degC, so if after rotation the brakes are above this, you can dangle the gear for cooling before retracting smoking hot brakes into the wheelwell. The reasons for cooling the gear prior to retraction is to minimise the risk of a wheelwell fire.

Hope this answers your query.

Nosey

[Capt Fathom]

Also....

If a brake is deactivated, the undercarriage is left down for a while after take-off to allow the wheels to spin down before retraction!

[Dick Smith]

Do they have published climb gradients for the gear left down after an engine failure?

[gary gearbox]

Gear retraction can also be delayed on a contaminated runway to enable slush etc to "fall off" the gear assembly.

Leaving it down though would be pretty rare. I have been on the 330 for only a year but have not seen it.

To leave the gear down, so the brake temps are lowered for the following turn around sounds a bit useless. They will get hot again when you brake at landing before the next turn around!

[framer]

They will get hot again when you brake at landing before the next turn around!
But won't they be getting hot from a "less hot" starting point and therefore be cooler at the start of the next take-off roll?
I only do short-haul stuff in baby boeings so don't really know what I'm talking about here.:eek: How long would the sector have to be for the brakes to cool from 300degrees to ambient temp in ISA conditions? Anything longer than this time would render the proceedure pointless.
Framer

[Inbound Outbound]

Thanks NoseGear and everyone for your reply's. Indeed positive stuff as this situation did quite interest me at times. I saw a EY A340-500 depart YSSY this afternoon with the gear extended throughout the climb so I thought it would be good to ask.

Cheers,
IO

[breakfastburrito]

Framer, the problem is there isn't much airflow in the sealed wheel wells, cooling is slow.

[gary gearbox]

Ok I have gone and got the books out...

The 330 will get a "brakes hot" caution on the ground or in the air if brake temp exceeds 300 degrees.

From the FCOM for a "brakes hot" caution inflight,

If perf permits
L/G................................down for cooling
Max Speed......................250 k

If perf permits, landing gear retraction should be delayed to improve
cooling.

I have seen a few take offs with temps very close to the 300 limit, yet little brake temp rise after T/O, thus need to leave the the gear down. Obviously it must happen though.

Framer, yes I guess you are right that they may get hotter from a cooler temp. Maybe a factor for shorter sectors.

[AlJassmi]

And the pilots will of course notify the tower controller ahead of time so they don't get stuck with faster following traffic, or have the firies asking if everything's ok...

[Capt Fathom]

Why would they do that AlJassmi.

Faster following traffic?

It's 250kts below 10,000 anyway. So it won't make any difference !!

[HAWK21M]

As mentioned.....
Hot brakes needing cooling or a Deactivated brake to permit stoppage of concerned wheel which would not brake due Autoretract braking.
regds
MEL.

[Keg]

Dick, no. Performance is adjusted to meet the degraded climb gradients. Normally this is either by limiting the performance limited weight (about 18 ton on the 767 from memory) if we're on a 'limiting runway' or increasing the thrust for a particular weight as if the aircraft was 18 ton heavier if the runway isn't limiting.

I've never heard of the wheels being left down longer after take off due to being too hot. If they were that hot then would you be taking off? If you're concerned about them building up heat on a subsequent arrival then throw them out a couple of minutes early on approach and they'll cool down pretty quickly. Given the temps in Sydney the last couple of days I'd be very surprised if brake cooling from a fast turn around was the reason.

Every time I've left them down in a 744 or a 767 it's due to having a brake capped and no after take off braking available.

[Metro man]

If a brake was U/S and the gear had to be left down for the wheel to stop spinning prior to retraction, you would have a weight penalty anyway due to the reduced stopping performance in the event of a rejected take off.

Brake fans, if fitted can usually get the temperatures down during taxi out. However a heavy aircraft on a hot day with a long taxi on a down slope can build up heat pretty quickly. Used to be something for the flight engineer to keep a carefull eye on.

[The Green Goblin]

Do they have published climb gradients for the gear left down after an engine failure?

I'm thinking that if you did loose a donk and you required more performance due to obstacle clearance requirements you would retrac the gear gain a bit of height and if a warning light did come on due to temps once you were at a safe height you could always extend the gear down!

[AlJassmi]

Why would they do that AlJassmi.

Faster following traffic?

It's 250kts below 10,000 anyway. So it won't make any difference !!

The aircraft i've observed with gear down after takeoff have significantly decreased acceleration performance compared with normal departure profile. If the tower controller is firing say a 747 behind an A340, 3nm wake turb is all that's req'd. A takeoff clearance can be issued when the first aircraft is pretty much at the upwind end of a longish runway leaving little time to assess. If the 340 is even slower than normal then you can have a loss of separation pretty quickly. Getting stood down because a pilot didn't feel like sharing details of non-normal operations would make me rather grumpy. If you know - please fill your friendly ATCs in :ok:

[ElPerro]

If they were that hot then would you be taking off? If you're concerned about them building up heat on a subsequent arrival then throw them out a couple of minutes early on approach and they'll cool down pretty quickly.

Well said Keg. That's exactly the point. If they are that hot you shouldn't be taking off as the brakes couldn't be relied on for an abort (would exceed brake energy limits). If you think the brakes are that hot then logically you believe your refusal speed etc is incorrect. If you think your Take Off performance will not be in accordance with the manual and you have no correction graphs etc. (or a correction on your laptop for your computer folks) then why are you using those take-off performance figures.

If you think that the brakes are so hot after taking off then surely your flight manual should deal with this. Unless of course you think you know something the Test Pilots and teams of Aeronautical Engineers with PhDs at the OEM didn't know.

[gary gearbox]

On the 330 the brake temp limit pre take off is for a different reason. It is to ensure, that in the event of a hyd fluid leak, the leaked fluid, if it comes in contact with the hot brakes, will not ignite in the wheel wheel.

[trimotor]

Will virtually never be necessary to leave the gear down on the B777. It has great brakes.

Unless the preceding landing was heavy weight, in very hot wx, perhaps F25 not 30, with nil/idle reverse, the ground cooling time is usually enough - approx 1:15 mins if the brakes were up to the caution zone (>4.9 units), and they seldom get that hot, even on the heavier models whose max landing weight is greater than the max take-off weight of the first 772.

A better question might be back towards Inbound Outbound: how long does it seem that the gear has been remaining down? (not trying to knock down your question, but just see if what appears to be non-normal to you might actually just be routine).

Leaving the gear down for cooling would be an unusual event, so to see it done more than very rarely would be unusual in itself. To add some perspective, if the 777 gear is raised when the indications meet the Flight Crew Training Manual pisitive climb indcations, the aircraft can be at least 800ft AGL before the gear is up anyway, at moderate weights and thrust settings.

[NoseGear]

That'll learn me to rush in and post without thinking!:\:uhoh:
We have an ECAM procedure, Brakes Hot, "If performance permits, landing gear retraction should be delayed to improve brake cooling." Ive only seen it once and I recall it was a binding brake that caused it, so its not obviously a "normal" procedure. The "If performance permits" is an interesting line, as I have combed the books and cant find a reference to an extended gear down segment....however, most modern jets can easily achieve the required climb gradients(A330 15.4%/A340 7.9%:{), even with an engine out, so a segment with gear down shouldnt cause any greater performance penalties. 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?
The "pre-cooling" of the brakes by early extention doesnt make much difference in my experience, and brake cooling fans are not available everywhere....not as straight forward as I intially thought this question!

[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!:\:E
As for dealing with it airborne...isnt that what we are talking about....?:E So I guess that answers your question in your second post:ok:;)