If I may say so...
It makes no sense to me selecting a high autobrake setting when landing on contaminated runways, especially when there is any crosswind involved (which is virtually all the time if the wind is not calm).
By selecting MAX autobrake, you are certain to activate the antiskid system which indicates that the tyres are right on the edge of their ability to apply force on the runway surface. By increasing the braking force of the tyres to a maximum, you have reduced the cornering capability to virtuallly zero!
Imagine you are in a car, hurtling down a two lane road (one lane each way) at a speed way above that which would cause aquaplaning, say 130 km/h (never mind 130 kts!). If you come across a section of the road which is contaminated with 13 mm of standing water (maximum allowed by Boeing) or slush or snow etc, I can imagine the thought would cross your mind " I really shouldn't be going so fast!". But to reduce speed, do you really think it is wise to jump on the brakes as hard as you can and put all your faith in the anti-skid system. No way!. As soon as you do that, you're going to loose all cornering capability, and slide off the side of the road and hit a tree, or drift into oncoming traffic.
The best thing you can do (to maintain directional control) is to take your foot of the brake and throttle, put it in neutral, and let drag slow you down. Once your'e below the aquaplaning speed, perhaps then you might consider applying light pressure on the brakes, but certainly not enough to risk activating the antiskid modulation.
If I'm landing a B737 on a dry runway with a touchdown speed of 135 kt, with autobrakes 1 set, the ground roll to a stop will be around 1950 metres (as per the performance manual). Initially with reverse thrust and spoilers extended, the deceleration is virtually wholy achieved without the use of wheel brakes! Only in the later part of the ground roll when the reverse thrust and spoiler become less affective do the wheel brakes start contributing to maintain a constant deceleration.
The point I would like to make here is that quite often there is enough runway length to select Autobrake 1 and pull up with a comfortable margin. Even if 1 doesn't do it for you, Autobrake 2 would. If this is the case, why would you select Max autobrake to land on the same runway when it is contaminated and risk sliding off the side of the runway?
Remember that if you do start to slide to one side of the runway, you will have to release the wheel brakes, reduce reverse thrust to idle, and then try to regain control and get back on to the centreline before applying braking again. How much runway is that going to cost you?
Wouldn't it be far better to keep the wheel braking to the minimum extent possible (thus maximising cornering capability and directional control), and decelerate at a rate that will still have you pulling up by the end of the runway, plus a reasonable margin as previously calculated by reference to the performance manual before comencing the approach?
(edited for the inevitable typo errors)
Last edited by Blip; 15th December 2005 at 09:16.