Years ago, a King Air 200 was in a shop for phase inspections and the owner was waiting for it to be released. It was the first phase inspection since he had purchased the aircraft. Unfortunately, the shop found both engines would not make book power – they temped out before reaching max torque – and, accordingly, the shop would not release the aircraft.
The owner directed the shop to JETCAL the instruments and they did so. The results confirmed the engine gauges were reading correctly, so the JETCAL testing supported the shop’s position that the engines were not airworthy. At this time, these engines were about 100 hours from overhaul. The shop was eager to arrange the overhauls or discuss replacement options. The owner, however, was not convinced.
He called me and we discussed the situation. I gave him a possible scenario that could make the engines appear to temp out when, in fact, they were still good to go. He obtained a ferry permit and brought his 200 to my shop. Sure enough, my hunch was correct: Both engines had a bad temp stick. I ordered and installed a new temp stick on each engine, and both engines made book power within the allowable temperature range.
The owner was pleased to say the least. A little later he put the engines on the M.O.R.E. (Maintenance On Reliable Engines) program and ran them another 5,000+ hours before selling that King Air and buying a 350.
Engine data plate
Not all PT6s are created equal. As I’ve said many times over the years, every engine is a tad different from the next. I’m fat. You’re skinny. The next guy is somewhere in between. It’s the same with engines. Every brand-new PT6 is brought to acceptance level in the test cell where it gets a good bit of customized tweaking.
One of the last things checked before a new engine makes its way into the world is the measurement of the resistance needed to bring the temperature reading into line. That measurement, stated in degrees of temperature drop (they call it “ITT trim”), is stamped on the engine’s data plate and the corresponding temp stick is installed.
In the future, if the temp stick fails on that engine, the correct replacement can be ascertained by checking the engine data plate.
Temp stick 101
For the PT6 engines in most King Airs, the temp stick is located over the engine inlet (T1) and hitched up to the T5 harness. It can go by many names (trim stick, T1 probe, T5 stick) but there is only one temp stick and it is sensing the ambient inlet temperature of the engine. It takes that reading, reduces it proportionately and transfers it to the engine gauges.
Other aircraft engines have a similar feature. They call them “pilot pleasers” because somewhere along the line an engine designer thought pilots would be nervous if they knew the real temperature reading inside their engines. My understanding is various methods were developed to make the engine gauges read lower than the actual temperature while still preserving accuracy. In simplistic terms: Your PT6 temp stick is a resistor and the electrical signal going from the engine to the cockpit goes through this stick, which has been adjusted specifically for that engine. The stick reduces the electrical signal on the way to your temp gauge. In reality it is a pretty sophisticated resistor, considering the change in OAT as you go from ground level to altitude. (FYI – if you’re a “sparky” reading this, the stick is wired in parallel, not in series.)
The most important thing for you to know is this: The required resistance of the temp stick for that engine was determined “at birth” and stamped on the data plate. The temp stick takes the engine’s real running temperature and recalculates it into what you see on your engine gauges.
Adjustable or fixed
Way back in the day, temp sticks were adjustable. There was only one type of stick. When installing a new stick on an engine, the mechanic consulted the engine data plate and adjusted the new temp stick accordingly. The stick was then sealed and installed.
Although straightforward and handy, these adjustable temp sticks were tempting to a few unscrupulous 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.
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. When he got his King Air to my shop, I duplicated the discrepancy on the ground run.
First, I calibrated the temp system and it was fine. Next I ohmed-out the temp stick and sure enough, it was open. When I checked the data plate, I happened to check both sides and found they both had the same ohm reading. Rarely do both engines have the same class temp stick, but it gave me the unique opportunity to illustrate the problem to the pilot. I put the bad temp stick on the other engine and the problem went to that side. Then I reinstalled the good stick where it was originally. We ordered a new one for the problem side, and everything was fine once the new temp stick was installed.
Typically, swapping temp sticks from side to side is a bad idea because the temp sticks do not match from engine to engine. Swapping temp sticks can really make a mess of things. I once unraveled an engine temp problem complicated by earlier troubleshooting where they swapped the temp sticks. This created problems with both engines.
I tested both sticks with an ohm meter. One was good and the other was bad (open). The class on the good stick married up with the left engine data plate so that’s where we put it. The class on the bad stick was not the correct class for the right engine! That told me they had engine temp problems even before the stick went bad on the right side. The wrong stick throws off the engine temp reading. When the correct class temp stick was installed on the right side, all their engine temp problems finally went away.
On the ground versus in the air
Remember the King Air 200 with two bad sticks? Years later, I found another trim stick squawk on that very same King Air, which I discovered completely by chance. They had a problem with the cabin door. It had developed a leak that wasn’t resolving and was bothersome to the passengers. When a short trip cropped up, I went along to troubleshoot the door in the air. I figured out the door seal problem right away, but I was surprised to see the left engine was running hotter than it should. I’d been maintaining this King Air for over 12 years – I knew it inside and out – and I had never seen this in any ground running.
I asked the pilot about it. He didn’t seem too concerned. He chalked it up to age and time past overhaul. By this time, both engines had been on the M.O.R.E. program for several thousand hours. In spite of the pilot’s blasé attitude, I couldn’t let it go.
To complicate things, this pilot was not into trend monitoring, so there was no history of engine parameters to look at. I asked him to perform a number of tests during his next few flights and report back to me. Although he didn’t really do what I asked, I managed to eke enough information out of him to indicate we might be looking at another temp stick problem – but this time it was one that didn’t show up on ground running.
The next time that aircraft was in my hangar, I did an ice test on the L/H temp stick to fool it into thinking it was at altitude. The stick opened right up! So, I checked the data plate, ordered the corresponding class of temp stick, installed it on the left engine and suddenly the L/H engine temp was back in line.
It’s something for your technician to think about when everything points to a bad temp stick but the ohm meter doesn’t agree. Rub an ice cube along the stick for a bit and see what happens.
Moral of the story
If an engine is running hot, why split the engine and jump into hot section sticker shock if it’s just a faulty temp stick? And the other moral of this story? Trend monitoring, trend monitoring, trend monitoring. Make that your new mantra. If you are regularly recording your engine parameters and the only change is a spike in temperature, chances are a new temp stick is all you need.
As always, fly safely.