Trouble for Throttle Jockeys

How to avoid wrecking your expensive turboprop engine

April 1, 2012


April 2012
Turbine Pilot Contents

Many times I think I hear a silent sigh of relief from pilots during their first instructional flights in turboprops. They’re relieved because their backgrounds are in large, piston-powered, turbocharged engines and they’ve become accustomed to being gentle with power adjustments. They think that turbine engines are immune from the sorts of internal engine overpressures, mechanical stresses, and thermal shocks that can plague unwary operators of large piston engines. Ham-fisted, big-barrel piston drivers know from bitter experience that sawing away at the throttle(s) translates directly into early top-end overhauls; missing the manufacturers’ recommended time between overhaul (TBO) by hundreds and hundreds of hours; and thousands of dollars’ worth of unscheduled, unanticipated maintenance bills.

True, turbine engines run smoother because of their design. Instead of the piston engine’s clattering mass of connecting rods, wrist pins, camshafts, and valves, a turbine engine uses a series of circular fan, compressor, and turbine sections that rotate on central shafts. The flow through a turbine engine is in one end, and out the other, with a circular combustion chamber to ignite fuel and create thrust. This all means less wear and tear—and no exhaust valves, valve seats, valve guides, and exhaust components to generate damaging hot spots and wear concentrations. So the step-up turbine driver sees all this as some sort of nirvana: a world where more power isn’t penalized by short, financially crippling maintenance woes. The turbine engine’s 3,000-hour-plus TBOs reinforce the notion that these engines are somehow bulletproof.

How wrong can you be?

The takeoff trap

Pratt & Whitney’s PT6 series of turboprop engines has earned a justifiably proud reputation for durability. Thousands upon thousands of pilots and passengers have drawn great comfort from looking out at a cowling with the Pratt logo and its “Dependable Engines” tag line. But there are ways you can ruin a PT6 power train in short order. Someone new to a PT6 can easily exceed its torque limitations during takeoff. The POH publishes torque limits based on ambient conditions. Exceeding them can cause excessive torque—the twisting forces imparted by the engine to the propeller gearbox.

Here’s how it usually happens. A neophyte or complacent pilot applies power for takeoff, using the torquemeter(s) for guidance. He/she pushes the power levers up—right to the torque gauge red radial line. The thought is that with max power set, all that remains is to wait for V1 and rotation speed, and all’s well. Wrong-o!

As airspeed builds, ram air forced into the engines causes torque to rise. If you don’t catch this rise in time, you can blow past torque limits before you leave the runway. In the Cessna Caravan I regularly fly, I avoid this by setting power below torque limits in the early part of the takeoff run. First I advance the power until the propeller rpm reaches its takeoff value. This is an assurance that the gearbox, prop governor, and propeller are working the way they should. This also helps prevent the propeller rpm from surging past its redline during the remainder of the takeoff.

By the time the airplane accelerates to 50 knots, I always see a rise of about 80 foot-pounds in the torque value. By the time VR comes, torque rises to the proper value without moving the power lever any farther. So by compensating for the ram-induced torque rise early in the takeoff, I spare the gearbox from damaging internal stress.

Torquemeter The torquemeter (top gauge) shows the twisting force exerted on a turboprop engine's gearbox. And if it's at redline at the beginning of the takeoff run—as depicted here—trouble awaits. As speed builds, ram air will boost torque values and the result may be an excursion well past redline. At lower altitudes and high power settings the principal danger is an overtorque. In the thin air at high altitudes your concern should shift to the interturbine temperature (ITT, as shown on the lower gauge). Less dense air means less cooling, so too much power can translate into an ITT redline excursion as internal engine temperatures rise past limits.


So back to basic turbine operations—what if you blew past torque limits? You could always pull back on the power and save the day. Who would know? Any onboard engine trend monitoring (ETM) software, that’s who. In Garmin G1000-equipped airplanes, you’ll see a very unwelcome “ETM exceed” annunciation as soon as you bust limits. This will be seen by maintenance technicians during your next shop visit. Other avionics setups use similar trend monitoring protocols.

Depending on how much and how long you exceeded torque limits, you could be in for some big-league repairs. This could even include a hot-section inspection or a gearbox teardown to check for damage.

But I have full-authority digital engine controls (FADEC) on my Williams turbofan, you boast. Well, lucky you. You probably think you can slam the power levers all the way forward without a care in the world, secure in the knowledge that FADEC will automatically curtail power when torque—and temperature—limits are reached, and save your ego. Maybe, maybe not.

What if the FADEC isn’t working, or fails when it’s most needed? It happens. That thought alone should keep you from carelessly ramming the power forward. And bear in mind that with many airplane models, a pretakeoff FADEC control system fault annunciation is a no-go item. You’re grounded until maintenance fixes the problem and releases the airplane for service.

As long as you treat your engine right, follow checklist procedures, and run all the takeoff charts as part of the preflight process, you’re golden. You did check the POH for the torque limits for your field elevation and ambient temperature before you walked to the airplane, right?

Mark Evans is a 14,000-hour ATP and a CFI with instrument and multiengine ratings.