May 20, 1981. .L1011 Tristar - Operating Information Memo. .Subject: Wing Engine Pod Motion
Recently there have been a few pilot reports of small lateral and/or vertical airframe motion felt in the cockpit and/or cabin of 524 powered aircraft. Observation of the wing engine pods appeared to be moving in a somewhat elliptical path with the major axis in a lateral direction and at a frequency of 2 hz. The amplitude of the motion was reported at about 2 inches at the cowl inlet with 80% of the motion in the lateral direction and 20% vertically.
Testing and analysis has determined that this phenomenon is the result of thrust oscillations attributable primarily to the variable inlet guide vane system and possibly the engine fuel system (FFR, PAC). The oscillations, although felt through the airframe may not be detected by reference to engine instruments because of intrument damping characteristics.
The pod movement does not affect structural integrity or flight safety as it is a non-divergent limit cycle oscillation and the loads are very low - less than those normally associated with very light turbulence. Thrust oscillations of one engine usually results in observed motion of both wing pods.
If this phenomenon occurs in flight, and it has only been encountered in extremely smooth air, the offending engine can easily be found by individually retarding each power lever to flight idle and then restoring power. The oscillations will disappear when the power lever of the affected engine is moved to flight idle - and when restoring power, if the power is reset either slightly higher or lower than the earlier setting, the oscillation will not recur. The wing/pylon response to thrust oscillations is dependent somewhat on airspeed and fuel loading, thus wing pod motion has been reported for a range of power settings ranging from hold conditions to lightweight climbs usually around 250 knots indicated airspeed.
A Lockheed Information letter and a Rolls-Royce Notice to Operators Number 198, are being issued to advise Engineering and Maintenance personnel or recommended corrective actions in the event the phenomenon be encountered.. .End.
Rolls-Royce Notice to Operators. .Engine Thrust Oscillation:
......Rolls-Royce are informed by LCC (???) that the natural frequency of the L1011 wing/pylon suspension is in the order of 2.0 to 2.5 Hz, varying with wing tank loading. The coincidence of the frequency of engine control systems instability with the natural frequency of suspension, results in the amplitude of movement that has been observed. The resultant pod movement has on occasions also been referred to as a pod nod, rocking, circular, or figure of eight movement.
LCC confirms that such movement is not a safety item.
Reports to date show that the problem:. .a) Occurs in the power range 1.2 to 1.4 EPR and can be eliminated by moving the throttle to a setting above or below the power range.. .b) May, but not always, be accompanied by noticeable cyclic fluctuations of engine flight deck parameters.. .c) Is not always repeatable on ground run.. .d) Has occured with engines incorporating the P4/PF and P4/PS1 (not clearly readable) variable inlet guide vane (VIGV) control system.. .e) Has not been encountered with the B747 aircraft installation.
Rolls-Royce has recently taken full advantage of the opportunity to investigate this problem in flight with a L1011 aircraft at Palmdale and also with a service engine on a Derby test bed. Testing involved comprehensive instrumentation, particularly so on the test bed, to monitor the stability of the engine control system. In each investigation, instability of the VIGV control system was detected and was thereupon identified as originating from the VIGV controller.
From the initial experience gained to date in investigating this problem, Rolls-Royce consider the following order of approach to be appropriate when trouble shooting:. .a) Check air signal lines for leaks (VIGV and FF? loops).. .b) Change VIGV controller.. .c) Change separate bleed valve control unit (SBVCU).. .d) Change fuel flow regulator (FFR).. .END.
That's it folks. I skipped a few notes to make it shorter a bit, in the hope that it still makes sense.. .Cheers!!!