Re: Thrust reversers [Was: Peruvian 757 crash -- possible cause reported]

Date:         23 Nov 96 20:09:55 
From:         Reid Fairburn <cr_king@cr_king.seanet.com>
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At 03:36 AM 11/23/96, Adam wrote:
>BMADDISO@bcsc02.gov.bc.ca (Brian Maddison) writes:
>
>=> The Fall 1988 issue of Airliners magazine has a landing photo (p30) of
>=> an Air Malta 737-2K2C with reversers deployed and the nosewheel still
>=> about 3 to 4ft in the air. Unless the airplane is in the middle of a
>=> real gnarly bounce, it looks like only the mains need to to be down.
>
>I would like to followup on that particular issue. The question is: when did
>the necessity of wheels being on the ground in order to allow activating the
>reversers become a common thing? Did 707s/DC-8s etc. have that, too? I have a
>picture, taken in mid-1970s, of a landing Iliushin 62, still in the air and
>before the runway, but with the reversers clearly deployed. I wonder whether
>it was also possible on Western planes built in the same era (late 1950s -
>early-mid 1960s).

======Adam:    This is a long and involved story!

There have been several incidents and accidents involving the 737-100/200
airplanes in overrun violations.  At one time a few years ago, a study of
some 31 accidents/incidents showed that most of them involved long and fast
touchdowns on slippery runways.  In addition, late or no spoiler & thrust
reverser deployments were factors...these were not due to system malfunctions.

The original design (1968) involved a sensor on the nose gear and on the
main gear.  The logic provided the isolation valve in the open position for
takeoffs and landings...however, the airplane was required to have the nose
on the ground before the valve would open.  This was to prevent inadvertant
contact between the thrust reverser buckets and the runway at nose high
attitudes on the ground.  In situations where the airplane was landed fast,
the pilots try to use aerodynamic braking to slow down...leaving the nose
off the runway for a longer period.  This is not a good idea and has proved
to be detrimental to stopping the airplane...as thrust reversers will not
deploy and brakes are less effective.

This was later changed by deleting the nose gear sensor and leaving only the
main gear requirement.  (1973 and on airplanes).

In 1979, after an accident which involved incomplete thrust reverser stowage
prior to takeoff from a touch and go, an option was available to maintain
full hydraulic power to the reversers until they were completely
stowed...once they had been opened.  Not many operators implemented this
change option.

In 1978/79 there was a complete FAA review of the 737
reverser/spoiler/brakes for logic and effectiveness.  The airplane passed
with flying colors and a suggested review of pilot procedures for landing on
slick runways was made with several bulletins and briefings taking place.

In 1989, the nose gear sensor was added back into the system, with the logic
being either the main or the nose gear on the ground..(.In case of a real
fast landing?)...and next came the enabling of the reversers through the
radar altimeter system like it is on the 737-300.


Now, there are airplanes out there in all kinds of configurations I would
suspect.  Most of them should have sensors on the main and nose gears with
the "OR" logic invoked.  Some may have the radar altimeter tie in.  However,
the following logic is fairly normal, I think:

        Spoilers:  Auto flight-spoiler deployment when the wheels spin up
(60 kts +) or the right main gear is compressed, thrust levers are at idle,
and speedbrake lever is armed.  Ground-spoilers will come up when the
speedbrake lever comes aft more than 29 degrees and the right main gear
strut is compressed.

        Trust Reversers:  Right Main gear compressed, or nose gear
compressed, or if available, the radar altitude option.


        Landing:   Land in the specified touchdown zone, land at the proper
speed, and lower the nose to the runway smoothly.  Apply thrust reversers
and spoilers to stop the airplane.  Do not use aerodynamic braking and do
not land fast or long...if this appears to be the option, then go around and
set up again.  If the airplane has autobrakes...would recommend using them
as the runway conditions require.


Most of the problems have been caused by long landings...up to halfway or
more down the runway, fast landings at speeds up to in excess of 200 kts,
and on slippery runways to begin with.  Not a good showing for our pilot
skills.  A lot of these have happened at night on the "drag in"
non-precision approaches, in rain or other visibility limited
conditions...with the pilot seeing the runway late and not being absolutely
sure of where he was in relation to the ends.

Isn't it funny the way things all add up to make the pilots job a challenge?

All the data above is just thrown out for information and should not be
implemented without consulting your airline procedures and desires.  The
goal is enlightenment not direction.  Hope it helps someone ... some dark night.

Reid Fairburn
Creative Kingdom, Inc.
cr_king@cr_king.seanet.com
206-946-4815