Hi Gent's
tecchie response coming up! One of the most common causes of Pod-Nod on the RB211-22B(L1011) and the early RB211-524's C2/D4 is an oscillation of fuel flow. Causing the engine speed to 'pulse'. The change in the engine rotor speed as a result of this harmonic pulsing was for the engine to swing from side to side, I've seen them move by 6-10' in the test cell, probably more on wing. Exactly the same way the engine in your car rocks as you blip the throttle open and closed.
The most recent cause of this phenomena was a weeping fuel seal in the pump and governor (PAG) fuel pump. As it was weeping the change in fuel pressure caused the demanded fuel flow to increase and decrease, thus causing the pulsing/oscillation. Needless to say this unit is made by LUCAS, say no more!
Hope this helps you guy's.
Brgd's
A Tecchie
