MJBow2:
Your remarks are substantially correct, but your theory about the effect of control by the Nf governor following failure of both the primary and overspeed governors could be misleading to some readers.
If both propeller governors on an engine failed, oil supply to the propeller will be unrestricted, and the propeller will then move towards a very fine blade angle, leading to a gross overspeed. If sufficient power is being delivered to the propeller to achieve an overspeed as the Nf governor sees it (that being selected propeller speed +6%), fuel supply will then be reduced until the propeller is no longer >6% above selected propeller speed. Following a few momentary oscillations, the propeller speed will stabilize on the threshold of activating the Nf governor, and the net effect will be a zero thrust condition - caused by the FCU only supplying sufficient fuel to maintain the propeller at selected Np +6%.
If, following failure of both governors, not enough power is being supplied to the propeller to cause it to reach selected Np +6% (e.g. power lever at idle position), then the propeller blade angle will decrease until it reaches the normal angle for ground idle, at which point the beta reverse valve will intervene to regulate oil supply as the actual blade angle reaches idle blade angle.
It is critical that pilots flying PT6A series engines understand that in the event of a failure of the primary propeller governor, the propeller lever must always be moved to the full forward (maximum RPM, or takeoff speed) setting. If the propeller lever is at the maximum RPM setting, then the overspeed governor will take control of the propeller following failure of the primary governor, before the propeller speed reaches the threshold at which the Nf governor is set to intervene. If the propeller lever is moved back ONE TINY BIT from the maximum RPM position following a failure of the primary governor, then the Nf governor will intervene prior to the overspeed governor. The overspeed governor setting is a fixed value, whereas the Nf governor setting is variable - it is equal to selected propeller speed +6%.
In summary - if you encounter a propeller problem at any time during the takeoff phase of flight - make sure all the levers are forward. You won't have time to troubleshoot, and if you have any levers pulled back - most especially the propeller lever, in the case of a primary governor problem - you could easily lose all the propulsion from the powerplant with the problem. If you have the levers forward, you just drive away, with the propeller speed of the engine with the failed primary governor being controlled by the overspeed governor - just the way Pratt & Whitney designed it.
Last edited by V1... Ooops; 5th Feb 2005 at 09:03.