727 Autopilot Question

[SandDune]

Do any tech types or old timers on the 72 have any idea what could cause the autopilot to oscillate in the pitch mode. It occurrs during all phases of flight, but is intermittent. It seems to be precipitated by turbulence. It isn't divergent and the oscillations do not cause any change in altitude-it just won't dampen out. It does seem to be less when the ac is heavier. When the autopilot is disconnected there is no appreciable trim change. It definitely is the elevators and not the stab as it is a quick, small frequency oscillation and the yokes move. Everyone has had a stab at this and no one has been able to solve it yet. Any ideas?

[threestable]

Phenomenon known as 'pitch bumps, documented in 727maint manual as a service bulletin I seem to recall. Delving deep now but probable causes either: elevator lvdts (pcu change maybe), stab trim pot, pitch channel comp, a-p pedestal controller, and a couple that escape my memory. You trying to fix it, or just sick of feeling queasy?! Was a mech at LHR for PanAm/Ual and encountered problem a few times. Lvdts most probable, then pedestal ctrllr.
Or then again it might be something completely different!!!!
Happy hunting...............;)

Threestable.

[avioniker]

What autopilot do you have. -50 or -150?
The older autopilot has (had) a rate gyro in the Pitch Channel Computer that'll cause exactly that problem. The newer uses the attitude input from the vertical gyro or whatever attitude system is installed.
All autopilots require elevator position input from position transmitters in the top of the tail. They must be synchronized to within 50mvac of each other, have a less than 50mvac (the closer to zero the better)null at neutral and their respective nulls must be on the same side of zero. That is the most common cause of a short cycle porpoise or pitch oscilation. Ask FEDEX about their experience in the early 90's when one of their engineers decided that the fleet needed to have the elevator position nulls reset and didn't follow the three rules.
The air data computer will also cause pitch oscillations.
I once had an aircraft with an ADI loading down the circuit and causing elevator flutter.
The vertical speed wheel in the control panel is a possible but unlikely cause.
Since you seem to have a problem with turbulence you have a problem with something that reacts to pitch attitude changes and the response isn't appropriate to the input.
Two important rules a lot of people forget: 1. Flutter or oscillation is caused by position input (the system doesn't know where the elevator or the aircraft attitude is) and 2. Hardover or excessive movement is caused by a rate input. (the system doesn't know how fast the elevator is moving)
The very lasts thing I'd look at is the power control cylinder.

[wrenchbender]

Throw these parts at it; (1)Pitch Computer, then if problem continues, (2) Air Data Computer.