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.
:)
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.
:)