From: rdd@rascal.ics.utexas.edu (Robert Dorsett)
Date: 22 Oct 93 01:05:22 PDT
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From: Robert Dorsett <rdd@rascal.ics.utexas.edu>
Reply-To: rdd@rascal.ics.utexas.edu
To: airliners@chicago.com,ata-watchers
Subject: A320 braking methods
The following is an overview intended to clarify some of my thoughts, and
outline in one place the major issues at hand.
There are two braking mechanisms in common use. In order of importance:
Wheel brakes + Anti-skid + ground spoilers
Thrust reversers
Wheel brakes are how the airplane is stopped. Many people think the
thrust reversers play a large role: it's marginal, not affecting roll-out
distance by more than 10-20%. Most airplanes have automatic braking
systems, which apply pressure after touch-down.
Anti-skid significantly improves on braking distance in most runway
conditions, wet or dry.
Ground spoilers are used to dump lift: by destroying airflow over the
wings, they force the entire weight of the airplane on the wheels, thus
ensuring maximum friction with the available surface. Most airplanes
have automatic spoilers: after touchdown, all panels will deploy.
Thrust reversers are designed for failure: it is MUCH LESS desirable to
have them deploy in flight, than have them fail on the ground. I am
not aware of ANY airplane which has automatic thrust reversers.
Landing distances are based on landing in 60% of the available runway
length (plus touch-down zone distance, ~1300'), This figure provides
the minimum field length of a landing. To this figure is applied
correction factors for runway condition, winds, and whether anti-skid
is available, all indexed by weight and airport elevation.
Thrust reversers may NOT be used in anticipating landing distances.
Specific to the A320:
1. Wheel brakes are modulated by a dual channel "Brake and Steering Control
Unit." The A320's brakes and nosewheel steering may be considered "steer by
wire." The primary actuation mechanism is through pedals located near
the pilots' feet.
The BCSU is a digital computer which modulates hydraulic valves to apply
braking pressure to the carbon disc brakes. One of two hydraulic systems,
green and yellow, may be used. The BCS uses inputs from the two ADIRS's
(Air Data/Inertial Reference Sytem), and four wheel tachometers, to arrive
at an integrated speed. If the ADIRS's are not valid, then the speed is
limited to the maximum of the four main landing gear wheel speeds.
If this value passes muster, then the gain is amplified, and the valves are
modulated. The system automagically senses failure of the primary braking
hydraulic system (green) and then switches to yellow.
2. The A320 has an automatic braking system. This system can command
a high, medium, and low rate of braking. If the "low" mode is selected,
braking commences gradually about 8 seconds after ground spoiler deployment,
and the deceleration limit is set at 1.7 m/s^2. If the "medium" mode is
selected, then the braking commences immediately after ground spoiler
deployment, and the deceleration limit is set at 3.0 m/s^2. The "hi"
mode provides maximum braking, and is normally armed only for take-off.
I would speculate that this system would be OFF in a situation like Warsaw.
3. The A320 has an anti-skid system. This system maintains the brakes
at the limit threshold of an impending skid. If the system detects that
a wheel speed has dropped beneath 87% of what the system calculates the
actual airplane speed is, the brake is released. THe maximum rate of
deceleration is 1.7 m/s^2. The anti-skid system must be manually selected
"on." The anti-skid system is a "modulation" applied to a braking command,
either manual or automatic. A working anti-skid system can cut brake
distances by up to 40%.
4. The A320 has ground spoilers. These consist of the flight spoilers,
plus four inboard ground spoiler panels. The purpose is to destroy lift
above the wing.
An autoamtic ground spoiler system may be armed. When the speed brake
lever is pulled into the armed position and the thrust reversers are at
idle, or when reverse thrust is selected on either engine, the surfaces
will extend to 45 degrees, IF the airplane is on the ground and airspeeds
are greater than 66 knots. It seems this limitation also applies to
manual selection of the spoilers.
If the airplane is in the air, spoilers are retracted at high AOA, or
in full landing configuration. This could be relevant if the plane was
floating, or didn't sense it was on the ground.
5. Lastly, the A320 has thrust reversers. As stated before, thrust reversers
are more of a guarantee than a primary braking mechanism: provided the
airplane landed properly, on speed, thrust reverser failure would not be
a factor. The thrust reversers are only available with both engine control
units operating, both air/ground sensors showing the airplane in the
GROUND configuration, and the thrust lever in the "reverse" detent.
If all three criteria apply, the reverser doors are unlocked, and hydraulic
pressure is used to move the doors. While the doors are in transit, the
FADEC will command the engine to IDLE.
Speculation:
With all this in mind, I'm tending to think "failure to select
automatic spoilers" at this point. Nervous pilot, foul weather, high
workload, easy mistake. If it's assumed that they're set, and they're
not, valuable time is lost: the spoiler lever is on the left hand side of
the center pedestal (captain being qualified, and well aft, out of the
normal scan (unlike Boeing or A300/A310 spoiler/flap levers, which are in
the same area as the thrust levers).
Also, it seems to be yet another damned Airbus push/pull interface, and the
lever probably doesn't move when full spoilers are commanded. So if the
spoilers don't get deployed quickly, the airplane floats, the A/G sensor
may not click, the thrust reversers remain locked closed, the brakes don't
work at maximum effectiveness (if they're enabled at all in the air with
the gear lever in the down position), and the anti-skid is irrelevant. It
takes time to recover from something like this.
--
Robert Dorsett
Internet: rdd@rascal.ics.utexas.edu
UUCP: ...cs.utexas.edu!rascal.ics.utexas.edu!rdd
