Brake temps hot on downwind side.

[9 points]

Why is it that on a manual landing with auto brake (A320), no input from the pilot to confuse things (assume letting the auto brake just about stop the aircraft on the runway) with a crosswind the downwind side tends to get hotter?

[giggitygiggity]

Because there's less cooling on that brake unit as it is in the lee of the aircraft. When it's stopped, the upwind side will be subject to greater airflow than the downwind side and either cool quicker, or not get as hot in the first place. Or am I missing something?

[flapassym]

Could it have anything to do with the weathercocking effect? Aeroplane tries to turn upwind so downwind brake has more work to do.

Of course this ignores the nose wheel steering keeping it straight but just a thought.

[Centaurus]

I don't know about the A320.

In the 1980's the runway at Nauru was 5600 feet long with no over-run areas. Depending very much on pilot technique (some were lead-footed) the brakes on the 737-200 could get quite hot after landing.

After parking on the small tarmac on Nauru and with parking brake released we would note the wind direction and with the APU running one aircon pack, we would turn off the pack that was downwind. The air exiting the two air conditioning packs was very hot and bathed the wheels.

The downwind pack, if left on, would have its efflux flowing around the nearest main wheel. This had the effect of bathing the brakes on that wheel with hot air.
So we figured we should turn that pack off and use the other pack which had its hot air outlet further away from a main wheel. Sounds complicated and I am not explaining it very well.
After 15 minutes or so, there was a distinct difference in brake temperature between the two sets of main wheels as they cooled down.

Later I read somewhere in a Boeing document that this procedure was recommended by Boeing as part of brake cooling on the ground.

[Cough]

9 Points - Did all of the brake units show even wear on walk around? - Those with less meat often get hotter than others for obvious reasons...

[Fursty Ferret]

I work on the theory that a hefty crosswind means more weight on downwind wheels, with more energy absorbed during moderate braking. Hadn't even thought about the weathercock effect and heavier braking on one side to keep it straight.

[Uplinker]

I think flapassym is right: Asymmetric braking against weathercocking is the major reason for hotter brakes on the downwind side.

[stilton]

Doesn't make sense, the autobrakes don't 'know' there's a crosswind, equal pressure will be applied on both sides.


Manual application on the other hand will certainly see hotter temps on the downwind side, even if only for the short amount of manual braking applied after AB disengagement.


I've seen that on many occasions.

[Uplinker]

No, but the aircraft does know if it is keeping straight and can 'tell' the autobrake? I wonder if autobrake uses differential braking to keep straight on the runway - otherwise it might veer off one way or the other. So if there was a strong crosswind, different brake temperatures would result.

I am just off to fly but will have a look for a reference later.

[Goldenrivett]

the autobrakes don't 'know' there's a crosswind, equal pressure will be applied on both sides.
True if the weight was the same on both sets of wheels.
In a crosswind, (with no aileron applied) the into wind wing generates a bit more lift than the downwind wing and more weight is transferred to the downwind wheels. The upwind wheels may have the brake pressure modulated to avoid tyre slip and the down wind wheels do most of the work.

If blogs unwittingly uses aileron to help "steer" the aircraft then the effect is even more pronounced.

See page 10 of http://www.airbus.com/fileadmin/media_gallery/files/safety_library_items/AirbusSafetyLib_-FLT_OPS-LAND-SEQ05.pdf

[9 points]

Thanks to all. May I just pull these ideas together, without offending anyone...


Giggity giggity, Cenaurus – That’s a different question. I’m interested in what is happening during the landing roll only but thanks for the input.

Flapassym, Uplinker – I don’t think so but I’m no expert. My question is specifically about auto brake.
How would the autobrake sense the need for asymmetric braking? I’ve never heard of the auto brake doing this.

=================

Cough – Brake wear. I never thought of that. Can you explain further?

Stilton – thanks.

Fursty ferret, Goldenrivett – Asymmetric weight.
Yeah I think this may be a factor but I’d like to know for sure. I look forward to hearing more.
Just to be clear though, the auto brake is not trying to steer the aircraft as far as I know.

[Uplinker]

the auto brake is not trying to steer the aircraft as far as I know.

I looked through FCOM yesterday, and could not find any reference to its doing so.

[Pontius]

With absolutely no thought being given to this phenomenon before, nor having read any expert explanation, how about:

Given the auto brakes are trying to give you a fixed level of retardation/deceleration, they must surely add more braking effort to a faster wheel than a slower wheel and, therefore, get hotter. As the aircraft weather-cocks into wind the downwind wheels will be on the outside of the turn into wind and will accelerate, relative to the upwind wheels. Now those wheels are going faster, the auto brakes have to apply more braking; ergo......

[Centaurus]

On the subject of brake use generally. It might be useful to be aware that some pilots in the simulator (and by association in the real aeroplane) have been known to inadvertently apply light braking during the take off roll, but without being conscious of it happening.

During the landing run, some simulators automatically display a page on the instructor panel that shows the brake pressure on each set of wheels being applied by the pilot at any instant. This same page automatically pops up if the pilot is (for example) unknowingly touches the brakes during the take off run.

During the simulator training of two cadets on a 737 simulator, the sequence included practice at rejected take off procedure at 100 knots. The simulated runway was reduced in length such that if the rejected procedure was conducted correctly the aircraft would come to a stop with 100 metres to spare.
While one cadet performed the rejected take off perfectly and stopping with 100 metres to spare , the other cadet would invariably overrun the end of the runway while still performing the actions swiftly and accurately. This had the instructor mystified until he by chance happened to be looking at the instructor screen while the cadet was in the middle of the take off run.

There was no specific reason for the instructor to look at his screen at that moment during the take off run - it just seemed a good idea at the time. What he saw surprised him. The brake pressure readings were popping up intermittently on the screen during the take off run. This could only mean the cadet was unknowingly touching the top of the brake pedals during the take off roll.

The extra drag caused by intermittent brake pressure had extended the take off roll. When at 100 knots IAS, the rejected take off procedure was started, the aircraft was already further down the runway that it should have been with a normal take off run. This explained the instances of the over-run.

The cadet was then reminded to keep his feet low on the pedals during the take off roll and it was explained to him he had his feet too high on the pedals thus causing inadvertent brake pressure. The cadet refused to believe this was happening and denied he had his feet too high up on the rudder pedals. This was a prime example of ethnic culture or loss of face at work.

In the end the instructor asked the PM to get out of his seat and observe what was happening on the brake pressure page during his colleagues next take off run. When it duly happened yet again, the instructor asked the PM to tell the offender in his own language what was happening re partial brake pressure on the take off roll. That satisfied `Face` and after that, the box was ticked off and there were no more problems.

In the simulator it is difficult for an instructor sitting at the back to see the exact position of a student's feet on the rudder pedals especially during a night take off scene. Hence it is useful for him to occasionally glance at his own screen to see if inadvertent brake pressure is being applied to the rudder pedals during take off. V1 could become invalid if the brakes are being dragged even slightly. And of course the brake temperatures could rise,

[Cough]

9 Points - Just something I've observed. When you have a temperature mismatch between the brake units on the same wheel, its usually the case that the hotter one is an old unit with more wear than the newer one. Guessing it works something like (for the new brake unit) More mass = more material to absorb the temperature increase = lower peak temperature.

[Goldenrivett]

the auto brake is not trying to steer the aircraft as far as I know
Correct.

Autobrake simply applies equal pressure to both sides in order to achieve a set deceleration rate. A slipping wheel will be detected by the anti skid unit and its individual brake pressure will be reduced until the wheel spins up to speed again.

If the weight load of the aircraft is not evenly balanced due to the crosswind effect, then the upwind wheels will have less load on them, tend to start to skid earlier which will cause the antiskid units to reduce brake pressure on those wheels.

Thus the downwind brakes work harder and show it as an increase in temperature.

[RAT 5]

9 Points:

1. How do you use TR's?
2. On an AB do the auto brakes reduce as REV Thr is increased?
3. Is this phenomenon equal across the fleet?
4. Do you have brake temp gauges per wheel or per brake unit?
5. If per unit is there a difference, on the hotter side, between the front units and the rear units?
6. Do the brake units have depth pins on them? have you noticed a correlation between the depth reading and the difference in hotness of one side v the other?

Further; I'm dubious about the upwind wing creating more lift than the other. I understand the theory, especially on swept wing a/c, but...the spoilers come up PDQ to dump the lift and increase the brake effect. With in seconds both wings are well below stall speed.

[agg_karan]

Having flown an Airbus 310 before, this phenomenon of downwind brake temp going up was observed quite significantly.

The answer in my opinion is simply the control wheel or aileron deflection inputted by the pilot in landing roll. Assume a left cross wind, manual landing with automatic braking, the usual pilot tendency is to apply aileron into the wind (mostly overdo it) thereby inducing more a/c weight on the left set of wheels. Obviously this is an aerodynamic increase of weight on left set of wheels with same Hyd pressure on both sides. How much the airplane decelerates and how drifting is controlled by brake modulation is beyond the scope of this discussion. The left set of wheels have higher resistance to ground and hence temp rises faster.

Assume the same landing with an autopilot and from what I have observed the autopilot might use side slip/crab etc method but the input on landing roll really never showed a difference in brake temp on any side later on as much as in a manual landing.

To sum it up - more aileron on landing roll more difference on BRK temp.

[stilton]

Interesting account Centaurus.


There's still a few Pilots around that believe you should always have your feet 'up' on the rudder pedals at all times, even during the take off roll, steering with their heels in effect and of course easily / inadvertently applying the brakes.

[9 points]

Pontius – the aircraft is prevented from weathercocking by the pilot.

Centaurus – my question specifically refers to auto brake only.

Cough – yeah could be. Would love to get this confirmed.

Goldenrivett – I’d say this would probably be the main reason, in my opinion, but Cough’s idea could also be it. Would love to get an experts opinion.

Rat 5 –
1. How do you use TR's?
As per SOP, usually fully on touchdown until 70kts then to idle until taxiing off the runway.
2. On an AB do the auto brakes reduce as REV Thr is increased?
The auto brakes on the airbus work to maintain a selected rate of deceleration based on the IRU groundspeed I believe, so the brake pressure would normally vary as needed to achieve the selected rate.
3. Is this phenomenon equal across the fleet?
I have not got enough data, this is not a scientific analysis it’s just an informal query.
4. Do you have brake temp gauges per wheel or per brake unit?
Per wheel.
5. If per unit is there a difference, on the hotter side, between the front units and the rear units?
See 4.
6. Do the brake units have depth pins on them? have you noticed a correlation between the depth reading and the difference in hotness of one side v the other?
Yes and I have not looked for this but will do in future.
*You’re dubious….
So can you propose an alternative theory?

Agg karan – on the fly by wire airbus we release the stick on touch down so there is no aileron input.

Stilton – not relevant to this discussion but if you have a low speed high thrust engine failure on a 45 meter runway and don’t start with your feet up you will not keep the airbus on the runway. This was demonstrated to me by 2 check Captains in 2 different airlines, one was an ex Airbus employed instructor. I flew A320/330/340 foot down for 18 years but am now a passionate foot up man. Just saying.

TngoAlphad _ mate you must be good when you’re rested if you are spotting the obvious like this when you are fatigued. Thanks.

So we have two theories:
1. More weight on the downwind side.
2. Brake wear issues.

As I think about it I think the brake wear issue might not hold up because it would be fairly evenly distributed between upwind and downwind but what I have observed is a downwind hotter trend so I'm tending towards theory 1.

[RAT 5]

Theory 1 would presuppose the upwind wing is producing effective lift. On landing the spoilers are delayed pretty damn quickly = PDQ. The lift is dumped. The brakes are ON. IMHO neither wing will be producing any significant lift. Then the TR's are deployed and the brakes reduce. All this happens in seconds.
Hence my scepticism about this theory.
To confirm, yes/no you'd need lift sensors on the wings. Perhaps AB, or Boeing, has some wind tunnel data to share with us. I'm sure they researched the lift dump phenomena during the design of spoilers and brakes and the stopping calculations including that of RTO. Remember TR's are not included in that. Hence they would want to design a very effective lift dump system to maximise the braking and reduce their size & weight.

[Chu Chu]

Apart form any lift effects, the wind pressure on the fuselage and vertical stabilizer (acting at a center some distance above the runway) would create a torque and a weight shift onto the downwind gear. But I doubt this is very significant, and rudder input to prevent weather-cocking would also also offset some of the torque about the longitudinal axis. And unless there was an anti-lock brake release on the upwind gear (unlikely on a dry runway?) it's not clear how it could affect brake temps at all.

[Goldenrivett]

On landing the spoilers are delayed pretty damn quickly = PDQ. The lift is dumped. The brakes are ON. IMHO neither wing will be producing any significant lift.
Please see https://www.youtube.com/watch?v=7P9OAng32F0
from 5:24 and note the oleo extension visible on both main gears.
At 5:33 the ground spoilers are fully deployed. Note the oleo extension visible on the LHS but RHS seems to be fully compressed.

MED autobrake is attempting to achieve the selected deceleration rate 2 secs after spoiler deployment.
On which side do you think the wheels are carrying more weight?

I'm sure they researched the lift dump phenomena during the design of spoilers and brakes and the stopping calculations including that of RTO.
This crosswind effect only transfers some weight from one side to the other. The total weight on the tyres remains unchanged.

And unless there was an anti-lock brake release on the upwind gear (unlikely on a dry runway?)
If the weight is sufficiently reduced on one side and the applied brake pressure too great to prevent tyre slip then the anti-skid unit will reduce braking effort on that side.

[stilton]

9 points.


Never flown an Airbus but I'm not seeing your point, are you stating this is AB procedure, to always have your feet up on the pedals even during take off, thus steering with your heels and quite possibly applying brake pressure ?


Anyone else do this, on any type ?



And what does having an engine failure at low speed have to do with it, there will be an initial swing which you correct with rudder and nosewheel steering then moving your feet up to the brakes to stop.


But since you will immediately close the throttles the swing will be brief.


Heels on the floor for take off.

[RAT 5]

Heels on the floor for take off.

Remember he Air Crash Investigation into the Russian B727 look alike crash with the national hockey team onboard. One contributory factor was the unfamiliar PNF having feet on brakes during takeoff.

Is this not basic Cessna/Cherokee stuff in flight school?

[9 points]

Rat 5 – Indeed. I am just guessing. Would love to hear from Airbus or Boeing on this.

Chu chu – agreed.

Goldenrivett – interesting.

TangoAlphad – sorry I was rushing and literature is not my strong point. No sarcasm intended. Genuinely grateful for your insight.

THE OTHER THREAD – HEELS UP OR DOWN:
Stilton – Perhaps this should be a new thread. Briefly, there will be significant swing because it takes time for the pilot to recognise and react. At low speed the aerodynamic rudder response is minimal. It’s all down to nose wheel and differential braking, basically. The nose wheel won’t save you so it comes down to differential braking. The Airbus FCTM now says:
“The flight crew must have their feet in a position so that full rudder deflection combined with full
braking, even differential, can be applied instinctively and without delay.”
You can’t do that with your heels on the floor my friend. Have a look at the Airbus rudder pedals next time you fly it.
If you want to follow this up further, PLEASE start a new thread on it.
Rat 5 – Again, please start a new thread. Briefly, some (perhaps most) aircraft are designed to be flown heels down, some are designed so that you can do it either way. It is not a law of physics that designers must comply with the heels down philosophy no more than it is a law of physics that you must use a control wheel instead of a stick. PLEASE, start a new thread. I won’t comment on this point again on this thread.

Back to the subject of this thread.
Thanks to all for your input. I’m still left unsure of what is happening. Are there any Airbus or Boeing people out there who would like to comment?

[stilton]

Well you might not want to comment further 9 points but I will.


I've done low speed aborts with engine failure in the sim and never was a problem, NEVER had any instructor or fellow Pilot advocate your 'technique'


Never flown an Airbus that's true but I'd like to hear from any other AB Pilot that advocates this, in fact I don't think that's their intention from the excerpt you've included.


After all they don't actually state 'place your feet up on the rudder pedals for take off'



I think it's just your interpretation and It doesn't seem right to me, it sure isn't on any Boeing.

[9 points]

Stilton. I don't appreciate being called a liar and note that you have never flown an Airbus. It seems to me that you are too keen to generate this topic, perhaps you lack a social life? Why don't you go bother your ex wife.

[vilas]

9 points, RAT5
Both of you are saying half truth. Nothing is forbidden or recommended. If there is any diktat about feet position on take off it is only from some airline. I have read a discussion on this at an airbus TRTO where both techniques were used and taught. SO feet up is as much valid as feet down. I have used feet up while flying B747 classic where the rudders were not connected with the nose wheel and with it's size, inertia and wet run way it can be a handful. I did execute a low speed reject in B747 and with my feet up I didn't have to give it a second thought. But there is a technique while using rudder on take off if you keep the feet up. In the A320 with rudder connection feet in any position is not a problem. I didn't see in the FCTM anything about feet I position. Perhaps you can give the page No. But what it does say is "To select both reversers irrespective of which engine has failed" obviously more for the effect akin to differential braking than stopping. And to use steering hand wheels when taxi is used and not before because nose wheel will skid and aggravate the problem. There has been a thread on this before and no point wasting time starting again promoting personal preferences. .

[RAT 5]

Regarding feet position. I'm assuming that >90% of current jet a/c have an RTO function on autobrakes. Thus, in an RTO, should they work as advertised, your feet are for steering and not stopping. Should, OMG, the RTO function not work, it is sensed PDQ by the alert pilot, and in the flash of a big toe nail your feet should be able to transition from heels down to heels up.
There will always be the argument in the case of a Stop limiting runway that any delay could be critical: but then you'd have to have a multiple failure of the system that caused you to RTO and an autobrake failure. Statistically unlikely.
The same argument can go for landing. Where do you put your feet then?
In my sim experience I see too many crews disarm RTO autobrake function during an RTO. That is less likely to happen heels down. In a strong x-wind slippery runway I would rather have my feet concentrating on steering that stopping. If you inadvertently disarm the autobrakes you now have a greater problem with pedal steering & stopping.

[Canute]

I think the idea that the downwind brake is hotter because of weight being reduced on the upwind leading to brakes being released to stop locking is unlikely. The wheels are nowhere near skidding on a low or medium brake, even with reduced weight.


It is indeed taught to have your wheels up on the brakes in an A320. I certainly was. I ignored it.

[vilas]

I agree that with Auto brake working there is no need to keep the feet up. In 747 classic without RTO and free rudders was tricky. In airbus you won't have AB working below 72kts but stopping is not an issue at that speed. Slippery runway crosswind engine fail feet up differential braking helps in keeping it straight as well stopping. Feet up you have to use rudders with heel or lower half of the foot for directional control otherwise inadvertent brake application will send brake temperatures up or disconnect AB. Landing is different because AB is not a necessity and limiting landing distance you are in a position to quickly apply both or differential braking as required. Airbus rudder pedals are flat perhaps made to keep your feet up.

[stilton]

9 points , I thought you 'weren't going to respond' anymore ?!


You might want to go back and re-read my reply, no one called you a liar, I just questioned your interpretation of recommended feet placement.


You might want to look at your social skills a little yourself.

[9 points]

Canute – yes I agree, I don’t think the upwind brakes release. However, they may do less work without releasing simply because of having less weight on them. I’m talking about auto brake LO, not MED. So even without releasing they may still work harder > get hotter.


Feet up or down >
1. The current FCOM and FCTM use a more digital referencing system and page numbers can be a bit complex but if you look in the FCTM under normal ops, pre-start, miscellaneous you will find it. It is a relatively new addition to the FCTM, was put in within the last year or so I think.
2. If you have not had or do not have the opportunity to try an engine failure in the A320 at 40 kts on a wet narrow runway with a tailwind and TOGA then you should consider refraining from condemning the foot up option until you do have the opportunity or until you can get an official ruling from Airbus.

[vilas]

9 points
All I find in FCOM is
The rudder pedals must then be adjusted to ensure the pilot can achieve both full rudder pedal displacement and full braking simultaneously on the same side.
Although I have been a protagonist of feet up I cannot say it means that. All it means is you should be capable of applying full rudder and differential braking. Why differential braking is added I can tell you that. There was one runway excursion because of sitting too far back the pilot ran out of leg length to apply differential braking. So it is as I said nothing is forbidden or recommended. In A320 I didn't have to particularly teach to keep feet up. Although it is quicker to use differential braking feet up most manage it either way. In a low speed reject right seat take off creates more problems than feet down.

[RAT 5]

What I am curious about, unless there is some intrinsic design item, is why some discussions on Tech Log relate to type. Often, IMHO, the topic is a general aviation one and I was not aware that one manufacturer had a greater claim to an inside knowledge of aviation techniques, physics, aerodynamics etc., than another.
Does this not fit the general category, or is there someone at AB who knows better than all the other manufacturers and has designed their a/c accordingly?
It was like when a new CP arrived and suddenly some of the old tried & tested SOP's changed with no explanation nor apparent good reason. Usually a personal preference.

[misd-agin]

Below 80-85 kts, depending upon manufacturer, the auto brakes don't work.


If you lose an engine at low speed, or one engine is slow accelerating, the rudder pedal isn't the way to prevent going off the runway. Idle power to stop the asymmetrical thrust is the answer. LHR 27R has two long skid marks left by departing a/c that had asymmetrical thrust. The nose wheel tires, depending upon speed, asymmetrical thrust level, runway friction, can't handle the side load.


Feet up for 34 years after flying the SDS3-30 in crosswind landings, but thrust reduction is the primary method to control a thrust asymmetry event on takeoff.

[vilas]

It is actually off topic but low speed reject there are simultaneous actions. Feet applying as much rudder including differential braking to keep it straight(I have seen people going off to other side). Hands pulling thrust levers back all the way to reverse. Stopping is not an issue.

[stilton]

'thrust reduction is the primary method to control a thrust asymmetry event on takeoff. '


Exactly :ok:

[Uplinker]

Just been revising for my next SIM and found this: FCOM PER-LDG-GEN 'Use of the autobrake system'..........furthermore, it ensures a straight roll-out............

So, as I suspected, the autobrake does keep the aircraft straight on the runway, and therefore, with a crosswind, the downwind brake will do more work to prevent weather cocking, and hence get hotter.

Thinking about it, during landing I never have to do much to keep straight in a crosswind after the autobrake cuts in, but I do have to work the rudder/NWS to keep straight during a crosswind take-off.

.

[Escape Path]

Interesting. Reading that last paragraph I seem to have the same recollection, re: rudder input

But how does the aircraft know what "straight" is? Is it via rollout mode or via heading/track data sent to brakes?

[applecrumble]

I would think it is highly unlikely to receive any input of heading or roll out.
It will be the fact that as you touch down (with no input from the pilot) let's say there is a slight nose right movement.
The left wheel on the outside on the turn will be moving faster and therefore more brake will be applied to that wheel. (The wheel tacometer is measuring wheel speed and comparing against reference speed ADIRS speed). The right wheel is moving slightly slower as it is on the inside of the turn and so less brake will be applied thus helping steering.

[Uplinker]

But how does the aircraft know what "straight" is? Is it via rollout mode or via heading/track data sent to brakes?

Well not rollout, because this (braking) feature is available even without an ILS.

So maybe the IRS, although I cannot find whether the autobrake is unavailable if all IRSs have failed.

Or possibly wheel speeds, as applecrumble suggests, although if the runway was contaminated and the anti-skid was operating, the directional control might then go pear shaped?

[Chu Chu]

Using wheel speeds sounds a little scary. If the aircraft got off the runway heading (maybe because of momentary wheel slip, slight differences in tire wear or inflation, or just not enough autobrake authority to fully prevent weathercocking) the autobrakes would then fight to keep it on the wrong heading.

[Goldenrivett]

Thinking about it, during landing I never have to do much to keep straight in a crosswind after the autobrake cuts in, but I do have to work the rudder/NWS to keep straight during a crosswind take-off.
Hi Uplinker,

The effect you have noticed is due to more weight on the wheels because of the ground spoilers. During take off, the wing develops more lift for a given speed than the landing wing (with ground spoilers deployed) and the tyres tend to slip more in a cross wind. As they slip, you will have to "fly" more upwind with a bigger heading change.

The note about "ensuring a straight roll out" is due to equal braking effort on all wheels until one or more tyres start to slip.

So maybe the IRS, although I cannot find whether the autobrake is unavailable if all IRSs have failed.
Autobrake and antiskid need a ground speed reference to work (from IRS).

[Escape Path]

That's probably a last resort call.

It's all probably just a moot point anyway, as the brakes get hot unevenly since they are most of the time at a different wear.

Interesting discussion though

[applecrumble]

I had a look at this in the cruise yesterday and I think I was talking more about the anti skid.
Thinking about it, I think it's nothing more than just the fact that it applies even braking.
So it will be more straight than if the pilot manually brakes as it's hard for him to get exactly even pressures on both sides.