Re: A320: effect of spoilers when airbraking?

Date:         26 Jul 98 23:57:10 
From:         John Vincent Lombardi <uniphone@home.com>
References:   1
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>Pete Mellor wrote:

>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:

Four functions actually: All five spoilers (on each wing) are used for
ground spoilers. The four outboard spoilers act for roll augmentation.
The three middle panels are used as speed brakes. Additionally, on most
A320's (not A319's) the two outermost panels are used for gust load
alleviation in combination with the aileron.

>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,

The speed brakes on the A320 do cause a slight pitch change when deployed
or retracted. My suspicion is that the rate of deployment and retraction
is controlled so as to minimize any resultant excursions. Retraction at
high speeds (>315/.75) or with autopilot on, can take up to 25 seconds.
Surprisingly, or all its automation, a pronounced "ballooning" which
accompanies flaps extension to position 2 was not worked out of the
system. Why, I don't know.

>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.)

This is not correct. There are no inflight situations that can cause the
speed brakes to extend automatically (although there are several criteria
that can cause automatic retraction). Although the degree and rate of
extension are controlled by the three SEC's, they are deployed in flight
only in response to pilot selection. High speed protection will not cause
the speed brakes to extend. The aircraft will pitch to slow.
Interestingly, this can cause "annoying" altitude excursions in mountain
wave activity. The aircraft will pitch up without regard to assigned
altitude or certificated ceiling at Vmo+6 or Mmo+.01. Speed brakes are
then applied manually to limit the degree of excursion.

Although it has been reported that the speed brakes retract automatically
in response to go around power, this is not the case. Angle of attack
protection must be active to cause the inhibition. Given sufficient
margins, one could fly around all day with max power and speed brakes
full.

Thanks for helping me study for my annual proficiency check!

John Lombardi