Page 23 - Volume 15 Number 6

propeller due to its huge bite of air, so no governors would be required!
Even though the Primary and Fuel Topping Governors now share the same housing, the same speeder spring, and the same spinning flyweights, the PPG (Primary Prop Governor) portion gets the job done by controlling oil flow into and out of the propeller shaft whereas the FTG (Fuel Topping Governor) portion gets the job done by venting or not venting Py air from the Fuel Control Unit (FCU). When that air is vented and the FCU loses air pressure, fuel flow gets reduced, usually going all the way back to Minimum Fuel Flow, typically 80-100 pph, a bit less than the Low Idle fuel flow at Sea Level. Thus, I have always taught my students that although a single failure could wipe out both governors, there is no guarantee that it will affect both since one involves oil and the other involves air.
In my 49 years of King Air flying and instructing I have yet to hear of a PPG failing such that either the OSG nor the FTG have ever had to react. Don’t lose any sleep worrying about this “what if.”
Realize that the FTG’s operating speed changes from about 6% above the PPG’s setting to about 5% below when the power lever is moved to the Maximum Reverse
position. This means that, in the case of a B200 with the propeller levers fully forward (setting the PPG at 2,000 RPM) that the FTG would be “waiting” at 2,120 RPM (2,000 x 1.06) but that it would move down to 1,900 RPM (2000 x 0.95) when Max Reverse is selected. What is the propeller speed limitation in Reverse from the POH? Yes, 1,900 RPM! Hmmm, do we see a tie-in? Yes indeed.
Has it been explained to you what would happen if propeller speed could ever reach the PPG’s setting while the blade angle was in Reverse? Think this through: Whenever the PPG senses an overspeed condition, it vents oil from the propeller dome back into the engine’s nose case, allowing the springs and counterweights to make the blade angle increase, adding rotational resistance and bringing the speed back to where it should be. But if the blade angle starts out on the wrong side of flat pitch – at, say, minus 10 degrees – when the governor activates, the release of oil will again send the propeller to a bigger angle but (here it comes!) this bigger angle will be a lesser bite, less rotational resistance as the propeller moves toward flat pitch, so it would not stabilize its speed but instead overspeed even more in an unstable manner until finally a positive bite of the correct amount were established!
MAY 2021
KING AIR MAGAZINE • 21