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Blip
23rd Apr 2002, 14:25
Just wondering why it is that when landing on a slippery runway, it is considered correct practice to select a higher autobrake setting than if it was dry (Autobrake 3 instead of Autobrake 1).

I would have thought that it would be better to reduce the wheel braking to a minimum and rely more on the reverse thrust and spoilers. By minimising the braking action of the wheels, you maximise their cornering capability, ie they're ability to resist lateral forces due to crosswinds, therefore minimising the chances of sliding sideways toward the edge of the runway.

Having to commence a recovery manoeuvre if this situation arises involves reducing reverse thrust to idle, and releasing the brakes. Obviously you'd rather avoid this situation if you can help it.

Consider this example.

B737 at Max Landing Weight:

Dry runway distance required = 1630m (factored 1.67)
Wet runway distance required = 1880m (factored 1.67x1.15)
Contaminated runway with = 2600m (factored 1.15)
poor braking action.

Autobrake ground roll distances (Vapp = 145 kts):

Autobrake Max = 900m
Autobrake 3 = 1300m
Autobrake 1 = 2300m

Add say 500m to these distances (300m for the aiming point and 200m for the flare) to get actual landing distance.

Comparing the contaminated runway distance with the autobrake 1 distance, it seems to me that that is about the best deceleration rate you can expect regardless of how hard you jump on the brakes.

As the braking effect of the spoilers and reverse thrust reduce with decreasing speed, as does the downforce from the spoilers on the wheels, there is an increasing tendency for the antiskid to modulate the brakes due to wheel lock up. The lower the autobrake setting selected, the lower this speed will be.

And until this speed is reached, the deceleration rate will be constant. (Autobrake setting is a deceleration setting, not a wheel braking setting.)

We are told that it is important to make firm contact with the runway on touchdown to maximise wheel spin up and prevent aquaplanning. Great. But then if the wheels
start to skid a little during anti-skid modulation, all that good work is wasted.

Many runways at major airports are 3000m - 4000m long. In this case you could even land and pull up with no wheel braking at all!

So to sum up, if you land on a runway in summer and consistantly use Autobrake 1 with 1500m to spare, why not use the same autobrake setting in winter and minimise the chances of sliding to the edge of the runway.

Am interested to know what others may think about all this.

Thanks.
:)

HOMER SIMPSONS LOVECHILD
23rd Apr 2002, 20:07
I too wondered about the wisdom of breaking "harder" in the wet given that as you rightly say you are selecting a rate of deceleration.
I concluded that if the anti -skid system is operating as per the brochure the less time you spend at high speed with all that lift on the wings the better.On balance I'd rather be doing 40kts on a flooded runway than 100 kts and the sooner I can safely do that after landing the better-sod the brake life!. Just my humble opinion.

Intruder
23rd Apr 2002, 21:28
HSL has it right.

You want to slow down as quickly as possible, so you don't run into problems at the far end of the runway, where paint and rubber deposits may decrease braking ability even more.

As long as the anti-skid is working, get it slowed. Use reverse as well as autobrakes.

GlueBall
24th Apr 2002, 20:59
On very wet/contaminated runways it's more about technique and preference whether to use autobrakes. Manual braking provides better "hands-on" directional control. Because the autobrakes don't "know" whether the airplane is on centerline, or off centerline, and by how much; nor does the autobrake logic correct for drift. Check out the following incident report about a 767-300 departing a very wet pavement.
Sliding Off The Pavement (http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001211X11043&key=1) :eek:

SyncMaster
24th Apr 2002, 22:31
On some aircrafts there is a considerable delay between touch down and brake application in case that less than Max is selected on Auto Brakes.
During that time the decceleration is less than it could be if the brakes were applied immediatelly which is the case in Max setting on the Auto Brake.
Also as a consideration, in case that the directional control difficulties require that all the available tyre to ground friction be used for control od the aircraft path the brakes must be released.
This means that the actual landing lenghts will be longer. So if you have a combination of a cross wind and low friction on the runway the required landing lenght should be increased or the cross wind limit be reduced.

Blip
28th Apr 2002, 12:18
Thanks for the link Glue Ball.

From what I can gather the B767 slid off the SIDE of the runway with autobrakes 4. This is exactly what I'm talking about.

This need to reduce speed ASAP just doesn't make sense to me especially when there may be so much runway to use. (See the example I gave above).


I understand that the last 600 or so metres would be affected by rubber deposits as you have stated Intruder, but I'm quite sure that that would not come in to play on most long runways, unless you deliberately wanted to roll through to the end.

Regarding the spoilers, I was under the impression that the downforce created by their deployment on touchdown not only countered the lift created by the rest of the wing but actually produced a positive downforce on the wheels.


Refering back to that example of Glue Ball's, is that an example of aquaplanning causing the water under the tyres to boil, creating a cushion of steam? I seem to remember reading about that somewhere. Pretty amazing!!

GlueBall
28th Apr 2002, 15:16
QF 1 at BKK Accident Report
http://www.atsb.gov.au/aviation/acci/ojh/index.cfm

Checkboard
29th Apr 2002, 08:51
The idea for max autobrake is to reduce the speed below the dynamic hydroplaning speed as quickly as possible. On water contaminated runways, the water is usually not of uniform depth along the runway, so the aircraft will run through good and bad sections, you need to maximise the breaking on the good sections. Remember also that autobrake gives a decelration rate, not a brake pressure, so using the reverse thrust will relieve the load on the brakes at any setting.

Interesting article, GlueBall, the "steam cleaned or sandblasted" comment would tend to indicate reverted rubber hydroplaning, but there is no comment on the tyre condition.

While the "the airplane's autoland system continued to attempt to correct the ground track of the airplane towards the centerline of the runway." the reverse thrust was still in operation. As the aircraft began to point towards the runway centre line, the reversers would actually be pulling the aircraft off the runway - the recommended procedure is to reduce reverse thrust to idle in this case, remaining distance permitting (at least it was in my airline).