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I have read in a book that if the engine RPM increases while descending, the probable causes are:

  1. Engine transfer bearing leaking excessively
  2. Excessive friction in blade bearings

I am not able to understand these reasons.

Can someone explain "engine transfer bearing" and what these two reasons mean?

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    $\begingroup$ Welcome to aviation.SE! Your question appears to be related to a Pilot's Operating Handbook or Line Maintenance Manual, in which case more details about the make&model would be welcome. $\endgroup$ – AEhere supports Monica Apr 29 '18 at 11:15
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    $\begingroup$ The title talks about propellers, but you tagged this as jet-engine. Could you clarify which type of engine this is about? Maybe you want turboprop? $\endgroup$ – a CVn Apr 29 '18 at 15:54
  • $\begingroup$ Could you please expand point 2. ? $\endgroup$ – BambOo Apr 29 '18 at 16:15
  • $\begingroup$ Your title suggests that you may be troubleshooting a problem on a specific airframe. If so, more details would be helpful. $\endgroup$ – J Walters Apr 30 '18 at 0:29
  • $\begingroup$ Not talking about specific airframe/ engine, as I am a student and studying about propellers! $\endgroup$ – Amir waseem May 1 '18 at 7:09
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The troubleshooting section from the Hartzell Propeller Owner's Manual No.411 states:

B. Engine Speed Varies with Flight Attitude (or Airspeed)
   (1) Small variances in engine speed are normal and are no
       cause for concern.
   (2) Increase in engine speed while descending or increasing
       airspeed:
       (a) Non-feathering ( )(A,B)1 series propeller:
           1 Governor is not increasing oil volume in the
             propeller.
           2 Engine transfer bearing leaking excessively
           3 Excessive friction in the blade bearings or pitch
             changing mechanism

Reasons 1-3 relate to the propellers inability to adjust its blade angle to maintain a constant RPM when descending and airspeed is increased on non-feathering propellers.

The transfer bearing allows governor controlled oil into the crankshaft interior then to the propeller hub. If the bearing is leaking excessively the oil may be leaking back into the crankcase and there may not be enough oil pressure going into the hub to increase the propeller blade pitch and slow the engine down.

If there is excessive friction in the blade bearings the propeller blades may not move freely within the hub even when adequate oil pressure is supplied by governor and oil system.

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