Page 22 - Volume 15 Number 6

ASK THE EXPERT
Understanding the Fuel Topping Governor
by Tom Clements
Last month’s article described the King Air’s reversing propellers and made the statement: A pilot cannot force the propeller to reverse; he can only allow it to do so. Reversing is accomplished by repositioning the propeller’s Low Pitch Stop (LPS). Therefore, unless the propeller’s blade angle is being determined by the LPS, then the blades will not follow the LPS to smaller and even negative angles.
Only when a combination of low power and low – were separate devices mounted on the three pads
airspeed (less windmilling air blast) force the propeller governor to flatten the propeller’s bite of air enough that the LPS is finally reached, does the LPS play its role of preventing further flattening. Now the RPM of this temporarily fixed-pitch propeller will follow the airspeed: Speed up and the RPM increases; slow down and, of course, it decreases. The conclusion is that our “finding” the LPS occurs sooner – at a higher airspeed – when we ask for the maximum propeller speed that the governor can provide. That explains why King Airs contain an annunciator light – RVS NOT RDY – that will not extinguish when the landing gear handle is down until we push both propeller levers fully forward to set the propeller governor to its maximum speed.
The Fuel Topping Governor plays an important role here.
In the early King Air 90-series, powered with PT6A-6 or PT6A-20 engines, the three propeller governors – Primary, Overspeed (often called Secondary) and Fuel Topping (sometimes called the Power Turbine Governor)
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on the Reduction Gearbox housing at the 12, 9 and 3 o’clock positions respectively, as viewed from the rear of the engine. With the advent of the PT6A-28 and after (literally all PT6 King air engines except for the -6 and -20), a major simplification and improvement of the propeller reversing linkage occurred and a part of this saw the Primary and Fuel Topping governors being combined into one device, sitting on the 12 o’clock pad, which correctly – although rarely – is called the Constant Speed Unit (CSU).
If this unit falls off the engine because, say, no one remembered to install nuts on the mounting studs, you would lose both Primary and Fuel Topping Governor functions. However, it would not be significant because this unit also includes the pump that boosts engine oil pressure up to the approximately 300-400 psi level needed for the propeller pitch change mechanism to operate. Without that oil pressure, the counterweights and feathering springs would drive the blades to the feathered position immediately. Of course, were that to happen, it would be impossible to overspeed the
MAY 2021