Ntsb/aar-79-01 - AirDisaster.Com

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For the conditions set forth in the airworthiness standards,
engine failure on a dry runway, the accelerate-stop distance for
Flight 603 was calculated to be 9,450 ft, with 5,980 ft required for
acceleration to V1 and 3,470 ft required to stop the aircraft. These
distances were derived from actual rejected takeoff data developed
during aircraft certification flight tests. When compared to the total
runway length, this distance would provide 835 ft of stopping margin on
runway 6R under dry conditions. The calculated accelerate-stop distance
for a rejected takeoff with a failed engine on a wet runway was 10,300 ft,
4,320 ft of which would have been required for stopping. Therefore, the
estimated stopping distance on a wet runway is 850 ft more than the
stopping distance required for the dry runway. Essentially, this means
that a DC-10 aircraft could have stopped on the wet runway with one
failed engine, with normal tires and maximum braking. Allowing for fan
reversers, a 600-ft stopping distance margin would be provided.
14 CFR 25 states further that means other than wheel brakes
may be used to determine the accelerate-stop distance if that means:
(1) is safe and reliable, (2) is used so that consistent results can be
expected under normal operating conditions, and (3) is such that exceptional
skill is not required to control the airplane. The engine fan and turbine
thrust reversers provide an operational safety margin, because they
reduce the dry runway stopping distances determined during certification
testing. However, currently FAA disallows reverse thrust credit in
determining accelerate-stop distances, because thrust reverser systems
have not fully met these criteria. Therefore, many operators of wide
body aircraft have disconnected the turbine reversers, a portion of the
reverser system, because they have not been reliable and maintenance
difficulties have been encountered. The accident aircraft did not have
operable turbine reversers to augment the engine fan reversers nor were
they required to be operable under current regulations.
The effect of three turbine reversers on the wet stopping
distance was calculated. Calculations indicated that these reversers
would reduce the fan-reverser-only stopping distance by 600 ft. Applying
actual braking coefficients and crew reaction times, turbine reversers
could have reduced the actual runway overrun speed in this accident from
68 kns to about 20 kns. This lower overrun speed would have allowed the
aircraft to stop 100 ft beyond the end of the runway, which would have
drastically reduced the severity of the accident.
Re Safety Board recognizes that calculations based on the
NASA DBV data are estimates only. However, these estimates have allowed
reasonable assessments to be made of the relative value each parameter
contributes to stopping performance. No alternative analytical techniques
or actual flight test data were available to the Board to otherwise
estimate the aircraft's wet runway stopping performance from actual
runway friction measurements.
I