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individuals who sought to mask an engine that was running too hot by lowering the resistance of the temp stick to lower the reading on the gauge in the cockpit. This gave the appearance that the engine was making power without temping out, when in fact it was running hotter than a poker.
Temp sticks these days are no longer adjustable. They are pre-set and available in several different “classes.” The technician examines the engine data plate and then consults a chart in the engine maintenance manual. The chart identifies which class of temp stick will work in that particular PT6.
Temp stick failure is not a common problem. The pre-set temp sticks are less prone to failure than the old adjustable versions. That said, it is still something to remember when confronted with a sudden temperature spike in an engine. The first thing that inevitably comes to mind is hot section repairs and lots of dollar signs. So, if it turns out to be a faulty temp stick, you can breathe a sigh of relief.
Troubleshooting and trend monitoring
Successful troubleshooting is impossible without correct information. This is true for any squawk on an airplane, but it is particularly crucial in diagnosing engine problems. Trend monitoring, accurately done, enables your technician to identify the problem quickly. This can save many hours of labor and thousands of dollars in parts. Keeping track of engine temperatures, N1, fuel flow and torque on a regular basis is vital to getting optimum performance from your engines. Trend monitoring is a godsend to your mechanic.
Let’s say that you’ve been trend monitoring for some time. Your N1s are close, fuel flows are pretty close and the torque is equal on both sides, but you see the temperature jump up on one side. If none of the other parameters have changed, then your temp stick is the first place to look.
This is a closeup of the temp stick sensor end at the engine inlet; the metal mesh protects the inlet from FOD.
Without trend monitoring data, I can’t zero in on the temp stick right away. I’d have to start by calibrating the engine gauges and verifying the internal probes in the engine. Next, I’d run the engine with the temp stick in, noting engine temp. Then I’d remove the temp stick, run it again to the exact same torque value and compare the temp readings. I’m looking for a difference in temperature. The stick-in temp should be lower than the stick-out temp.
If there is no temp difference, the stick is not doing its job. That is the sign of an “open” (i.e., faulty) stick, but I would still ohm-out the stick to be certain. To order the correct replacement I would check the engine data plate and compare that information to a chart in the PT6 manual which determines the correct class of stick for that engine.
If, after the stick-in and stick-out runs, I do get a drop in temperature, I still must study the engine data plate and compare it with the chart to verify that (a) the stick is the correct class for that engine and (b) the stick is offering the correct amount of resistance. If the stick checks out OK, then I know this hot-running engine has another problem. A borescope inspection would be the next step.
To swap or not to swap?
A frantic customer called me after seeing a sudden spike in engine temperature on one side while returning from a flight to Mexico. His engines were past TBO and he worried the hot section was going bad.
I quizzed him on the engine’s performance: Was the N1 slower? Did the fuel flow jump up? This pilot was diligent with trend monitoring and could confidently answer no to all my questions on engine parameters. The only change was the temperature on one side; in that circumstance, a bad temp stick is the likely culprit.
22 • KING AIR MAGAZINE
DECEMBER 2024