737 Spoilers large movement whilst parked
 

One for a 737 Pilot.

I was boarding a 738 in Australia recently at the rear stairs and I heard a loud banging sound, like banging pans together. Looked over and for about 10 seconds all spoiler surfaces were banging up and down quite rapidly (fully deployed, right up then banging down and so on..).

Why would this be the case?

What was the wind? But I doubt that was a cause. Did you see the ailerons deflecting as well? If so it was probably a flight control check with rapid reverse movement, and perhaps more than just a simple left/right but repeated. Why? Could be engineering.
If no aileron it could have been a spoiler check and again, for some reason, being repeated rapidly.

Wind was about 15kn.

Didn't check the ailerons. I assumed they were probably checking some engineering related event as they did it about 10 times.

Once we checked together with a technician the spoilers on a B733 and it was pretty impressive both the velocity and the noise of them gettin' up and down so I think that was the case... ;)

Keep in mind that spoilers are powered to be capable of rapid deployment even against a slipstream (relative wind) of ~120 knots after landing, or over 200 knots when used as speed brakes partially deployed in flight. So parked, with no wind/slipstream resistance to hold them back, they will slam up pretty smartly when tested.

Could have been a strong tail-wind getting under the control surfaces.


Back in my Glider flying days if you parked a glider in a tail wind, all the control surfaces would slam over. We used to hold the control column to one side with the seat belts, and jam a few car tyres against the rudder.

Hydraulically operated flightcontrols not slamming around by wind, actuator itself is a restrictor. They will pop up fairly quickly when commanded, but if you try to lift them by hand you feel quite strong resistance just due to actuating mechanism/actuator.

I'm surprised you can move them at all by hand.

Yes. Could be problem as they are propably locked on down position. Point is, if they are unlocked and unpressurized and you try to move them by hand during maintenace or whatever reason they will not move freely as long as hyd actuator is connected. :)

Ground spoilers (Lift dumpers) are generally locked down by an over centre mechanism, whereas Flight spoilers, be they speed brakes or roll spoilers, are not locked and can be moved by hand if there is no hydraulic pressure. They will only move slowly as you are moving fluid through valves and restrictors, etc.
In flight, if there is no hydraulic pressure to a flight spoiler, the surface may lift a little due to the airflow over the wing.

Hi Dixi, thanks for the reply..... But.... If you can move them by hand against the system hydraulic oil the first time, won't it be easier any subsequent movement, when the oil has been displaced.
Also if they have been moved several times, you have effectively pumped out all the oil in the system...?
.

Not dixi... hydraulic oil is not going anywhere from system. It is just going back and forward depending direction of actuator movement. These actuators are two way hydraulic rams which extends and retracts by hyd press.
When residual pressure decays from system, moving spoiler panel by hand became more easier but it will not going total sloppy as long as all lines and linkages stays connected. Because oil is still pumped around by ram piston.

Generally speaking.

It's a long time since I was last in a hydraulic workshop, but I recall that even without fluid in the bore, a piston is normally a sufficiently tight fit that it's not at all easy to move.

Of course it moves a lot more readily with 3000 psi of pressure acting on it. :O

Dixi, Flight Spoilers definitely have hydraulic pressure to them in flight, that's what makes them move.

The most obvious, and easiest to explain, answer to the OP's question would be; the pilots were playing around with the flight controls. :cool:


..surprised none had mentioned it before.

Did you not read post 3?

Exup
"Dixi, Flight Spoilers definitely have hydraulic pressure to them in flight, that's what makes them move."

Not if you have lost an hydraulic system.
IIRC, On the A300 we added 5 knots to V Ref to allow for floating spoilers.

Dixi, my mistake reread your post, you are correct, case of rtfq as most instructors tell you.

Might be 'obvious', and it might be 'easiest to explain' but it's probably bollocks. During boarding hydraulic pumps are off by default. If they were on, it would only be for a specific engineering action.

Depends on SOP, I have tried both in different airlines.