proper trim technique

[flybubba]

Any suggestions for proper rudder/aileron trim technique in modern jet transport aircraft? As an aside, I have been told that the ball is not calibrated accurately. A yaw string would sure be nice.

[Intruder]

If you want a yaw string, find an F-14, AV-8, or glider.

If the ball isn't accurate, squawk it!

In general: In level flight, center the ball with the rudder. Trim the ailerons. The yoke should be level.

[ifleeplanes]

Select Hdg, then trim to centre the ailerons, a small offset of the ball is OK then reselect LNAV

[flybubba]

This last post sounds like you are trimming with the autopilot on? This raises another question.

[BizJetJock]

Holding wings level with the ailerons and trimming for no change of heading, or using the autopilot in heading mode and trimming for zero bank will accurately trim the rudder. You then need to disengage the autopilot (if used) to find out if the ailerons are unloaded or if they need trimmed. If after all this you have a beuatifully trimmed aircraft but the ailerons aren't neutral you either have a significant fuel imbalance or the airframe isn't straight - the latter is commoner than you might think!!
Using the autopilot in heading mode only works if you have a series yaw damper (permanently engaged), like most transport aircraft. If you have a parallel yaw damper, i.e one you have to disengage for t/o and landing, then all you are doing with the autopilot on is loading up the servo until eventually it will give up and disconnect. On the other hand, in theory this type should always fly the aircraft in balance so you only know if the aircraft is out of trim when hand flying it. Sadly the aeroplane often hasn't read the theory well enough and doesn't do this....:=

[ifleeplanes]

If the aileron trim is used with the autopilot engaged the trim is not reflected in the control wheel position, the autopilot overpowers the trim and holds the control wheel where it is required for track control.. Any aileron trim applied when the autopilot is engaged can result in an out of trim condition and an abrupt roll when the autopilot is disconnected. On the B737NG anyway!

[flybubba]

Bizjetjock,
thanks, good info. In the next post from ifleeplanes: as a side note, in the MD-80, you can trim the ailerons with the ap engaged. The control yoke can be centered. You have just traded aileron position (thru control tab) for trim tab position. So obviously this is different from the 737 which I would expect it to be since the 80 is flown with control tabs.

Regards

[2R]

Sorry to answer a question with a question ,but
Q.What is the purpose of trim
A.To releive pilot control forces and reduce fatique.
Does not really matter what aircraft you fly .The trims primary purpose is to relieve control pressures and make flying easier for the pilot.The secondary effects of trim are many and provide a wide range of amusement depending on what you are flying.