Posted Tuesday, July 12, 2011 @ 07:20 AM  

When the airbrakes are locked how far should the over-centre mechanism go over centre, if at all? The mechanism is to be found under the rear rudder pedals. The airbrake torque tubes going through the feet cut-outs behind the pedals.

My problem started when once the undercarriage warning sounded when the u/c was retracted. The cause was that the front airbrake level had slipped off the microswitch, which appeared to be acting as a stop for the lever. Having put an extra half turn or turn onto the control eye to relieve the load on the microswitch I found that the airbrakes will under circumstances open in flight. Not unlock but float up about 2 inches.

The cause I have figured is that the over-centre goes too far over, thereby releasing some of the load on the airbrakes (proven by removing the cap strips and watching the motion)

My conclusion - either the over-centre is incorrectly set up or there is a stop missing in my system (unless the microswitch is it).
Any info welcome.

Posted Monday, July 18, 2011 @ 04:51 PM  

After further investigation I have solved the problem.
The over-centre should be held just over centre by a stop (small wooden block between the floor and bulkhead under the seat) at the bottom of the front lever.
In my case the lever appears at some point to have been moved slightly inboard, using washers, to clear a cockpit lining. This means the lever slides past the stop.

Posted Sunday, August 7, 2011 @ 11:10 AM  

It may be helpful for others to check the stops on their airbrakes. I can tell you that the airbrakes opening, even 1 - 2" at 135+ mph on a Vne test is quite alarming!

On the picture you can see the wooden block that holds the airbrake lever. In my case the front view shows contact only on the outer edge (its shiney). The top view shows (even after some correction) that the block barely extends to the centre of the lever (arrowed). It can be seen that the microswitch still only just makes contact.

At some point in the past someone had put 2 extra washers under the lever to move it inboard. This, combined with a slightly loose bolt and some wear in the lever bush meant that the lever could slip past the stop block.

My mod - remove the washers and add a small nylon block to the original wooden one to prevent the lever slipping past.

I am assuming that the 2 big washers under the lever pivot tube actually belong there otherwise the tube sinve the tube seems fairly flush on the outside of the fuselage. Can anyone tell me for sure as I am questioning the aircraft was built with such a misalignment.