ADI precession in steep turn

[JammedStab]

According to the Boeing book:

"The ADI has cyclical precession in pitch during steep turns. Although the actual airplane pitch attitude remains constant in a perfect steep turn, the instrument indication of pitch attitude slowly varies throughout the turn. Do not rely upon it for pitch attitude other than for small corrections based on short period observations."

This is for a 737-200. I never did see this in the sim which is probably not surprising. And of course, I never have done a steep turn on the real machine.

Has anyone else actually seen this phenomenon on any large aircraft type?

[eckhard]

No, but I've seen it in a Chipmunk!

There will actually be a sequence of roll and pitch errors in a full 360 degree turn; steep or not. In the Chippie (and many other designs of that vintage) the A/H is suction powered and the base of the main rotor case is slightly bottom-heavy. This pendulous mass is acellerated by the centripetal force during the turn and therefore the bottom of the rotor housing 'swings out' very slightly. The force causing this apparent bank error is then precessed by the gyro to cause an actual pitch error. As the turn progresses, the combination of centripetal acelleration and precession causes a combination of pitch and roll errors that reach minima and maxima every 90 degrees of heading change. When the turn is completed and the wings are level, the pendulous vanes quickly remove any residual error.

I was once told that the top of the gyro was displaced very slightly from the vertical in order to minimise these effects but this was all about 37 years ago so the memory is fading somewhat!

The pendulous vanes themselves are also subject to acceleration errors of course.

Not sure how this applies to an electrically driven gyro as fitted to the B737-200.

Merry Christmas!

[Slasher]

Yep - saw it first hand during a few steepies at 60* AOB during a test flight
we did after a D check (we were gettin mighty bored!)

Just use your VSI to keep the nose where it should be - soon as it so much
as twitches make an immediate but small correction. Keeping a bit of
bug-poo on the windscreen relative to the outside horizon helps after
established in the turn, but the VSI is your first giveaway. Don't rely on the
FD-108 because it lags a bit when over 30* AOB. If I recall correctly it
should be turned off anyway. Displayed A/H attitude I think varied by about
a 1/2 degree when over 40* AOB level.

Lotsa fun if you get the chance!

eckhard a gyro will translate 90* to a real or apparent disturbance through
precession as you have mentioned. Same on 737 - no bank angle error just
pitch, but a constant error in this case through a 360 (as long as IAS remains
constant).

[Slasher]

Sorry that should've read 1-2 degrees when over 40* AOB level (Edit thingy's not working).