Trial by newspaper

Charles Oppermann wrote:

I'll be curious to know how the Autobrakes usage (if in fact that is true)
affects the outcome. On one hand, the Autobrakes can prevent wheel lockup
and keep the aircraft under control. On the otherhand, manual braking
should be able to result in shorter ground rolls, generally.

I disagree that this is true generally. Everything I've read about
anti-lock braking systems suggests that they will outperform humans
under all but a few special conditions. The special conditions are the
cases where locking the wheels is beneficial to a short stop. These
conditions are basically deep snow or other soft material such as sand.
In these cases, the material builds up in front of the lock tires and
increases the resistance of the tire moving through the material (forms
a bow wave essential). However on dry or wet pavement, ice, light snow,
light layer of sand or loose dirt, etc., the ABS wins.

I have seen a few tests (auto and motorcycle, not airplane) where very
highly skilled racers have been able to outbrake and ABS vehicle on dry
pavement. However, the margin of winning was very small, and only a few
REALLY skilled drivers/riders could beat the ABS with any regularity.
And through in a patch of loose dirt or oil, etc., and, at least with
the motorcycles, the ABS would allow control to be maintained.


Matt

I disagree that this is true generally. Everything I've read about
anti-lock braking systems suggests that they will outperform humans under
all but a few special conditions. The special conditions are the cases
where locking the wheels is beneficial to a short stop.

This is good info, thanks. Here's a nugget from the NTSB report on the SWA
accident at Burbank:

"At the request of the Safety Board.s Airplane Performance Group, Boeing ran
stopping distance simulations for this accident wherein maximum, medium, and
minimum 737 autobrake applications, as well as maximum manual brake
applications, were simulated for wet runway conditions after the 182-knot
touchdown. These data indicate that the accident airplane would have
required about 5,000 feet of runway length after touchdown to stop using
maximum autobrakes and about 4,700 feet of runway length after touchdown to
stop using maximum manual brakes."

Still, that's not to say that manual braking would always result in shaving
off 300 feet of the ground roll. I guess it depends on the exact conditions
and pilot experience and technique.

My current opinion is that stomping on the brakes would have been worse than
allowing the Autobrake system, but that's just a WAG.

Charles Oppermann
http://spaces.msn.com/members/chuckop/

Charles Oppermann wrote:
I disagree that this is true generally. Everything I've read about
anti-lock braking systems suggests that they will outperform humans under
all but a few special conditions. The special conditions are the cases
where locking the wheels is beneficial to a short stop.


This is good info, thanks. Here's a nugget from the NTSB report on the SWA
accident at Burbank:

"At the request of the Safety Board.s Airplane Performance Group, Boeing ran
stopping distance simulations for this accident wherein maximum, medium, and
minimum 737 autobrake applications, as well as maximum manual brake
applications, were simulated for wet runway conditions after the 182-knot
touchdown. These data indicate that the accident airplane would have
required about 5,000 feet of runway length after touchdown to stop using
maximum autobrakes and about 4,700 feet of runway length after touchdown to
stop using maximum manual brakes."

These were simulations, though, and not tests. It would be curious to
see actual test data to see if the simulations are relatively accurate.


Still, that's not to say that manual braking would always result in shaving
off 300 feet of the ground roll. I guess it depends on the exact conditions
and pilot experience and technique.

Yes, every situation is somewhat unique.


My current opinion is that stomping on the brakes would have been worse than
allowing the Autobrake system, but that's just a WAG.

Impossible to know for sure. This is one case where it actually might
have been better if the airplane had left the runway earlier and got
into some grass or softer area. That might well have allowed a shorter
stop than did staying on the runway. Then again, if they'd have hit
something more solid by doing this, they might have saved the child's
life, but lost lives on the aircraft. No way to know.


Matt