A320 autobrake differential

[timmyEGCC]

Hi all,

I notice after a crosswind landing with Autobrake (Low or Med) the downwind side brakes always seem to warm up more than the other side. Any one knows why? FCOM/FCTM don't seem to have the answer...

Are the SECs applying differential braking to assist rudder input after touch down?

Thanks in advance

[rudderrudderrat]

Hi timmyEGCC,

See Page 10. Effect on braking.http://www.airbus.com/fileadmin/medi...LAND-SEQ05.pdf

The effect of different amounts of weight on left and right bogies produces different braking forces on their wheels, reflected in the brake temperatures.

[timmyEGCC]

Thanks rudderrat,

That document explains how more weight is on the 'low wing' side gear when applying 'sidestick into wind' after landing, but the phenomenon I'm experiencing definitely happens even when I don't apply any aileron input after landing.

[rudderrudderrat]

Hi timmyEGCC,

but the phenomenon I'm experiencing definitely happens even when I don't apply any aileron input after landing.

When landing in a cross wind with no aileron applied, do you think the loading on the L & R bogies is equal?
Initially, the downwind wing will be producing significantly less lift, hence a bigger load on the wheels that side, and able to generate a greater braking force.

[HazelNuts39]

The Airbus note talks about the the braking force developed under maximum braking when the brake pressure of the individual wheels is controlled by the anti-skid system.

That doesn't explain timmy's observation when using autobrakes Low or Med (on a dry runway?), except in the case that the upwind wheels are controlled by anti-skid while the downwind wheels are not.

[WhyByFlier]

Is there any chance you are disconnecting or overriding the brakes too soon whilst still applying rudder and braking unevenly as a result of a heavy foot on the downwind side?

[FLEXPWR]

Watch closely, the same happens most of the time with manual braking.

[timmyEGCC]

Thanks guys for all your input

I've been observing this for my entire time on the 320 (2.5years) pretty well after every landing with any pronounced crosswind (>10kt ish, DRY or WET), whether I use MED or LOW autobrake doesn't seem to make any difference to the fact. I believe I do see it even when I kick out Autobrake prematurely (to roll through to the end etc).

So here's the fact compiled from all your comments

Downwind wing, after touchdown, produces less lift, therefore, Downwind side gear has more WEIGHT on it

Now, this "sort of" makes sense to my dim brain, as more weight means more loads on the wheel bearings therefore more heat being produced...

My original guess was that autobrake was modulating and differentiating left and right gear brake pressure after touchdown to compensate for the wind, but I guess that was too wild a guess!

[CL-44]

Timmy egcc, good observation,I've noticed the same brake temp.differences for years and while I don't know for certain, I was always under the impression that the IRU's with anti skid worked together to track runway centerline.Really good for gusty x- winds and very low vis auto land.

[rudderrudderrat]

Hi timmyEGCC,

as more weight means more loads on the wheel bearings therefore more heat being produced...

Er... not so.
The wheel bearings don't generate the heat, the brakes do.

The brake energy absorbed is proportional to the Force * Distance traveled, so if they apply a constant braking force (LO say) the brakes generate heat rapidly during the early part of the landing roll. But this is when the biggest difference between the weight on L & R bogies is felt during a crosswind landing with no aileron applied.

As HazelNuts39 points out, this effect will only happen if the anti skid is releasing the (cooler) wheel brakes. Since the total weight on the wheels is the same (with and without a cross wind), but the aircraft wants to roll in a crosswind (with no aileron applied), then the down wind bogie supports more weight than the upwind bogie. If the required braking force is sufficient to trigger anti skid release on the upwind bogies initially, then you'll notice the difference in temperatures. The overall deceleration will be the same, just the downwind brakes will be working harder.

[timmyEGCC]

Thanks once again rudderrudderrat,

Can you, then, explain why we see the temperature difference even when anti-skid has not been active?

Yes, I agree that the majority of heat around the wheels is due to brake applications. I mentioned wheel bearings because, from your explanation I cannot see how autobrake alone can create ANY brake temperature differentials in a crosswind landing... as long as Autobrake is providing EVEN brake pressure to the left and right gear shouldn't the two brake temperatures be similar?

Thanks

[rudderrudderrat]

Hi

Can you, then, explain why we see the temperature difference even when anti-skid has not been active?

I can only think of different brake pressures applied manually, or different mass of brake material (new v old) able to absorb the heat, or binding brakes.

as long as Autobrake is providing EVEN brake pressure to the left and right gear shouldn't the two brake temperatures be similar?

Yes - provided the antiskid system didn't reduce the brake pressure to one side.

[Screwballs]

If one bogey was 50% worn and the other was brand new would that affect temperatures?

I have a theory that the left bogey gets heavier use because most people drive manual gearboxes... the left foot is used to applying more pressure to activate the clutch than the right which works the accelerator and brake. So as mere humans when we apply our toes equally in our heads, our actual feet/muscles apply slightly differing pressures resulting in unequal temperatures. This difference would not be much but enough to give a temperature difference of a few tens of degrees. And it would be easily counteracted by a tiny amount of rudder input to keep the roll out straight.

Easy to check, next time you're driving, try braking with your left foot and see how delicate it is.

As I said... it's all in my head.