Page 23 - October 2022

“A clever and common procedure is to conduct the PT6 feathering check while taxiing down a long taxiway on your way to the run-up pad or takeoff runway.”
to reach normal idle speed after the prop levers are pushed forward. This means you need to plan ahead so that you will have thrust available if you’ll need it to make a turn at the end of your taxiing “coast.” Do not lift the power levers to reenter Beta until the propeller RPM is back up to its normal idle value.
Now back to our pending question: What happens to ITT during this time? In fact, how about Torque, N1 and Fuel Flow?
Since the compressor (or gas generator) section is separate from the propeller and its power section, the fuel control unit should continue dutifully governing N1, meaning we should see no change in N1 when we feather. Yes, torque will increase due to the propeller blades slapping the air with the largest bite, but at Idle power its small rise will not be an issue. With the same compressor speed being maintained, the same fuel flow will be required. With the same compressor speed, the same amount of air flow is being moved through the engine.
Hence, ITT changes very, very, little. Almost all newcomers, when asked this very question, immediately presume that ITT will increase very noticeably since airflow will be reduced during feather. In a fixed-shaft turboprop like the TPE331, that would be correct, since whatever percentage speed loss the propeller experienced, the same would be true of the compressor! (In the King Air B100, with that powerplant, the same lever that does the feathering also manually shuts off the fuel flow ... first!)
OCTOBER 2022
KING AIR MAGAZINE • 21