A PT6A-34AG operator asked me why his engine torque value changed on approach after he set the torque at a reduced amount for descent. When he reset the rpm from 2000 to 2200, he noticed that the torque reduced further to a value that was lower than needed.
This seemed to make it necessary for him to readjust his power lever to a more desirable torque. The operator worried that he might have a problem with the way the engine controls were rigged or possibly a problem with his torque indicator or torque system.
What this operator was experiencing were normal characteristics of any “free-turbine” design aircraft engine. The term free-turbine simply means the turbine driving the compressor isn’t mechanically connected to the turbine driving the propeller reduction gearbox / propeller.
The torque indicators in our industry are largely oil pressure gauges sometimes indicating in PSI, sometimes with a dial calibrated in foot-pounds of torque, depending on airframe manufacturer. The prop tachometer and the gas generator speed indicator both utilize signals from tach generators (same part numbers!) mounted, one on the prop
reduction gearbox and one on the accessory gearbox.
Actual power generation on the PT6A is coming from the gas generator / hot section and
determined by positioning of the power lever. What the pilot is doing with that power is tracked through prop RPM and torque indication. The free turbine design makes it possible for a pilot to vary torque readings at any given power lever position by adjusting the prop RPM and at any RPM by adjusting the Power Lever positioning.
Horse power can be calculated by use of a formula using prop RPM and torque. It goes like this:
Horse Power = Torque (foot pounds) X RPM ÷ 5252
Operators with PSI indicating torque gauges will need to convert the PSI to foot pounds
for use of this formula. The PT6A-6 through the -21 uses a conversion factor of 30.87 X
torque in PSI, the -27/-28/-34 uses 30.57 X torque in PSI. The -45 through -67 use 83.635 as a torque conversion factor. Example: -34AG indicates 40 PSI, multiply 30.57 X 40 = 1222.8
foot pounds of torque.
I didn’t get the particular torque value the operator set his gauge to but, we can do hypothetical and still understand what’s happening to him. Let’s say he reduced power through moving the power lever back to obtain 1000 foot pounds of torque at the 2000 RPM setting. He would have been at a shaft horse power of 380.8 (1000 ft. lbs TQ. X 2000 RPM = 2,000,000 ÷ 5252). Increasing the prop speed to 2200 RPM but changing no other settings would reduce the torque indication but not necessarily the horse power.
Typically on descent someone operating a small PT6A should see about 100 foot pounds of torque less by simply by shifting RPM from 2000 to 2200 RPM having an initial torque reading of 1000 foot pounds. How about his shaft horse power (propshaft)? 2200 RPM with 900 foot pounds TQ would calculate as follows: 900 X 2200 = 1980000 ÷ 5252 = 377 horse power, only a 3.8 horse power difference with a hundred foot pound change in torque reading!
Isn’t this the kind of stuff you want to read after a full day of flying! (Not!) What the pilot
needs to do is make adjustment in his procedure to accommodate the free turbine design by targeting an initial torque value at first power reduction of about 100 ft. lbs. higher than he wants. He’ll end up with very close to the torque he is after when he flattens the pitch for final approach.