Override mechanism versus shearout

[JammedStab]

Our FCOM states...

"The control wheels connect through an override mechanism which allows either wheel to move independently if the other wheel jams and significant manual force is applied to the free wheel. Roll control is then available through the ailerons on the wing corresponding to the free wheel. Approximately half of the flight spoilers are also available for roll control under these conditions.

Each side of the mechanical system also incorporates shearouts which may allow the jammed control wheel to be freed when a significant manual force is applied to the jammed wheel."

So apparently we have both override and shearout. What is the difference?

[FE Hoppy]

one is a clutch the other breaks the rod.

[NSEU]

The 747-400 shearout system involves sheer-able rivets at strategic points in the flight control system (aileron, rudder, elevator) and is non-recoverable. The override systems involve cams and springs.

On the 767, there was an override mechanism for the elevator (part of the torque tube which runs between the control columns). I can't find an equivalent in my 747-400 (engineering) training notes (unless it's part of the cable tensioning system on the captain's side).

I've read that the clutch type (override) mechanisms require significant force to activate (e.g. 30lbs). Is this a one time effort or do you have to apply 30 lbs continuously every time you operate the controls? :confused:

[172_driver]

The breakout mechanism down left corner. To me it appears the clutch will re-engage when force is removed. So it's required every time you need to make an elevator input. Must be a helluva job to fly that way.

Heard a story recently of an Iphone that got jammed between the control column and the rudder pedal body just in front. The breakout feature came in handy, pitch att. > 35 deg.

[JammedStab]

I do remember in the 727 sim simulating an aileron jam and having to fly using the breakout force. A large amount of force is required to deflect the ailerons to move them a bit and then after going back to neutral, you have to do it all over again.

It is not a pleasant experience for flying and would consider it an emergency and try to get to an airport where a landing in smooth air could be made.

[JammedStab]

The 747-400 shearout system involves sheer-able rivets at strategic points in the flight control system (aileron, rudder, elevator) and is non-recoverable. The override systems involve cams and springs.

On the 767, there was an override mechanism for the elevator (part of the torque tube which runs between the control columns). I can't find an equivalent in my 747-400 (engineering) training notes (unless it's part of the cable tensioning system on the captain's side).

I've read that the clutch type (override) mechanisms require significant force to activate (e.g. 30lbs). Is this a one time effort or do you have to apply 30 lbs continuously every time you operate the controls? :confused:

The 747 FCOM only mentions override for lateral control. It appears that this is for a jammed control wheel allowing the unjammed control wheel to control ailerons on its side and half the spoilers on both sides.

The shearout appears to be for cases of jammed flight control panels. With four elevator, 2 rudder and 4 aileron panels, if one becomes jammed, some or all of the others can still be operated once the rivets have been sheared for the affected panel.

Thanks for the help. Now I think I understand it.

[tommoutrie]

you've got an appropriate username too considering the question!

[Garnett_Lin]

Boeing really didn't make it clear about this part.
Years later I had the same question as you did and by reading the comments here my problem was solved. Thanks, guys.