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Querer · Ïðàâàê

Hi Folks,

I already found a post dealing about some trimming issues, but somehow I did not get the point. Yesterday I did a flight from Sochi to Vienna and during descent I suddenly lost control of the plane. First thing: I did the initial descent in AP mode, with the wheel on the rigth of the black AP box used for keeping -8m/s constant. Second: I realized already during flight, that with decreasing fuel this wheel had to be turned further and further forward (white arrow almost on top). Below 20000ft, I was no longer able to maintain -8m/s because I was not able to turn the wheel any further. So, it came to my mind that there was a thing with trim and stabilizer not controlled by the same buttons/wheels and I tried the "ordinary" trim button on my stick to increase descent rate. What happened? Exactly, the plane began to stutter, rudders moving up and down extremly fast and the descent rate incresed to -30m/s... This was to much stress for the plane, it broke apart and crashed. So I wondered what I did wrong? Is there anything specific about trim and stabilizer to take care already at takeoff or during cruise?

regards
Chris

EDIT: I am actually talking about the Tu-134...

WalterLeo · ÊÂÑ

Querer

Hi:

its a problem which have the autopilots of AN 24, Tu-124? and Tu-134 in common:

The manuals dont give clear advice, only the AN 24 manual states: Be good to you AP, he will hand you over the plane in a trimmed state, do the same to him!
But assuming, that the stabilator after T.O. was reduced from 1,5 deg to 0 degrees, even then the problem occures, which you have described: The trim- indicator of the AP moves slowly to an extreme value, the plane starts wobbling and finally crashes.
The cure is easy: If you see the manual trim going to an extreme, switch of the pitch-chanel of the AP, put the AP-trim into middle position (click in the middle of the scale), trim manually (indicator on the sidepanel) switch on the pitch chanel again and put the AP on (eg. altitude hold). Same do, if you change the flightregime (e.g.: from climb to cruise and from cruise to descent and before starting an approach), but put the plane first into the climb or descent manualy you want and afterwards switch on the AP-pitch chanel.
Put it short: always fly with the AP-trim indicator in the middle and dont use the AP wheel for changing flightregimes.
If you know the Tinmouse B 737 model with the Richard Probst Panel, there the old Sperry (SP 50??) AP should be treated the same way, but doesnt have a trimwheel.

P.S.:
PT Tu154M is much better in this respect at least. Óëûáêà

Regards

Walter

Querer · Ïðàâàê

Thanks Walter for this explanation, this was the problem I did not remember anymore. Well, hope the next flight will be save Ñìóùåíèå

WalterLeo · ÊÂÑ

If you want you can use in the Tu-134 the trimwheel of the AP for changing a flightprofile, if its in the middle: Trim with the AP, let stabilize the flight then switch of the pitch chanel and further on as described above. The important thing is, that you put the trimwheel of the AP in the middle, if you have established the desired flightprofile.

regards

Walter