Re: Braking

Date:         17 May 97 15:15:49 
From:         "Robert Pata" <72023.436@compuserve.com>
References:   1 2
Followups:    1
Next article
View raw article
  or MIME structure


Commercial jetliners are design to stop with only the use of wheel brakes.
The use of spoilers and thrust reversers are additional aids available to
the pilot.

Spoilers do three things during the landing rollout. First they transfer
the weight of the aircraft onto the wheels. This increases friction  which
not only helps slow the aircraft, but  also helps it track better.
Second, spoilers help ensure positive contact of the air/ground safety
switches. These are important
switches because on many airplanes, devices such as anti-skid systems and
thrust reversers are not available until these swithces make contact. And
lastly, the extended spoiler panels produce aerodynamic drag. Spoilers are
only effective at the high speed end of the landing rollout. Most spoilers
are set for automatic extension upon touchdown. In the Boeing 757, if
spoilers need to be manually deployed, this results in a 700 ft. penalty.

Thrust reversers are most effective at the high speed end of the rollout.
The use of thrust reversers at the low speed end is not recommended because
of possible engine damage that may occur due to
re-ingestion of its own exhaust gases as well as ingestion of debris off
the runway.

At the airline that I fly for, standard procedure requires that automatic
spoilers be use on every landing,
and  thrust reversers down to 80 knots, with wheel brakes below 80 knots.
This helps reduce heat and wear of the brakes, while utilizing a
deceleration technique that is suitable on most runways.

Robert Pata
Boeing 757 Captain