Re: A320: effect of spoilers when airbraking?

Date:         01 Jul 98 02:42:33 
From:         Pete Mellor <pm@csr.city.ac.uk>
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On 26 Apr 1998, Ludwig Kammler wrote:
> for modelling the A320 on a flight sim, I need to know what effect the
> application of the speedbrakes has. I understand there are no dedicated
> speedbrakes, but some of the inboard spoilers are being actuated to
> function as one. does that create any pitching moment, loss of lift,
> etc. noticable to the pilot, or does the flight control system take
> appropriate action (elevators etc.) to cancel such effects?

Yes, the A320 and its successors do not have a dedicated set
of surfaces for airbrakes. Instead, the spoilers (five pairs
of surfaces on the upper surfaces of the wings) have three
functions:

- deployed asymmetrically, to assist the ailerons in achieving
  a roll (sometimes),

- deployed symmetrically at a small angle (no more than 5 degrees?)
  to increase drag and so act as airbrakes, and

- deployed symmetrically on air-to-ground transition at full
  extension (45 degrees?) to dump lift and keep the wheels on
  the deck after touchdown, i.e., ground spoilers.

All movements of the spoilers are commanded by the SECs (Spoiler
and Elevator Computers, of which there are three, with responsibility
for different pairs of spoilers) under normal flight laws (i.e., in
the absence of a failure of part of the flight control system or of
one or more of the three hydraulic circuits.

I would expect the deployment of the spoilers as air-brakes to have
a pitching moment (since they would decrease lift from the wings
slightly as well as increasing drag). I am quite sure that the
FCS would compensate for this automatically, since the use of
airbrakes is always completely automatic and under control of
the FCS, not directly by the pilots. (In an overspeed condition
on descent, the FCS would deploy the spoilers as airbrakes
automatically, without any input from the crew.)

Sorry to be a bit uncertain of the details, but I am writing from
home and don't have my A320 FCOM to hand.

Pete Mellor
-----------
Centre for Software Reliability, City University, London
E-mail: p.mellor@csr.city.ac.uk
-------------------------------