A King Air owner was waiting for his 200 to be released from the shop after having a phase inspection. It was his first phase since purchasing the aircraft in 2002. The shop refused to release the aircraft to him because the engines did not make book power – they “temped out” before reaching max torque. The owner directed the shop to “jet-cal” the instruments and after they did, the engine gauges were reading correctly. The jet-cal results further bolstered the shop’s position that the engines were not airworthy. Both engines were about 100 hours from overhaul, and the shop wanted to have them overhauled or discuss replacement, but the owner wasn’t having any of that. 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, both T1 temp sticks were bad. I ordered and installed new temp sticks on each side. Both engines made book power within allowable temperature parameters. The owner was pleased to say the least. A little later he put the engines on the M.O.R.E. program and ran them another 5,000-plus hours before selling the aircraft.
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 measurement of the resistance needed to bring the temperature reading into line. That measurement, stated in degrees of temperature drop (called 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, 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 readings inside their engines. Various methods were developed to make the engine gauges read a lower temperature while still preserving accuracy. That is my understanding of it.
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, taking OAT into account 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 is taking the real running temperature of the engine and recalculating it into what you see on your engine gauges.
Adjustable or Fixed
Back in the day, temp sticks were adjustable. There was only one type of stick to order, then you consulted the engine data plate, adjusted the new temp stick accordingly, and then sealed and installed it. 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 in order 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 pre-set. The technician consults your engine data plate and compares that information to a chart in the engine maintenance manual (see next page). The chart tells him/her which temp stick to order for your engine. Temp stick replacement is not something I do every day; I’ve changed four or five in the last three years. Whenever there is a temperature spike in an engine, one immediately starts to worry about hot section repairs and the inevitable expense that follows. So, if it turns out to be a faulty temp stick, you can breathe a big sigh of relief.
Diagnosis and Trend Monitoring
Successful troubleshooting is impossible without correct information. This is true for any squawk you have, but it is particularly crucial in diagnosing engine problems. Trend monitoring, accurately done, enables your technician to zero in on the problem, saving many hours in labor and thousands of dollars in parts. There’s no reason not to keep track of engine temperatures, N1, fuel flow and torque on a regular basis. Let’s say that you’ve been trend monitoring for some time. Both your N1s are close, both 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 records, I’d approach a temp spike on one side by calibrating the engine gauges first and verifying the internal probes in the engine. Next, I’d run the engine in question with the stick in and then I’d run it again, to the exact same torque value, without the stick. 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. I’d ohm it out to be sure, but that’s the sign of an open stick. To replace that stick, I’d 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 I did have a temp difference between the stick-in run and the stick-out run, I would compare the difference to the data plate information. If running that engine with the stick results in a drop in temperature consistent with the data plate, then the stick is doing its job. That means this hot-running engine has another problem; a borescope inspection would be next.
To Swap or Not to Swap
A customer called me in a frantic state of mind after seeing a sudden spike in engine temperature on one side while returning from a flight to Mexico. He was worried the hot section was going bad as the engines were past TBO. I quizzed him on the engine’s performance: Was the N1 slower? Did the fuel flow jump up? He didn’t think so. I told him to bring the aircraft over to the shop and right away I duplicated the discrepancy on the ground run. First, I calibrated the temp system and it was fine. Next, I checked 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. So, to illustrate the problem to the pilot, I swapped temp sticks and the problem went to the other side. Then I “un-swapped” the sticks and ordered a new one. We put it in and he was good to go. No issues reported since. Normally, swapping temp sticks from side to side just confuses the issue, because the temp sticks do not match from engine to engine. In fact, quite recently I unraveled an engine temp problem that had been complicated by an earlier troubleshooting attempt where they had 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. 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! So they were having problems even before that stick went bad. The wrong stick throws off the engine temp reading. Ultimately, we installed the proper stick on the right side and their engine temp problems went away.
On the Ground versus in the Air
Remember the 200, mentioned previously, with two bad sticks? Years later, I had a trim stick déjà vu on that King Air and it happened in a rather roundabout manner. A leak had developed with the cabin door that wasn’t resolving. It was bothersome to the passengers, so I flew in the aircraft on a short trip in order 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 doing regular maintenance on this King Air for many years and I had never seen this in any ground running. I asked the pilot about it. He had chalked it up to age and time past overhaul (by now, both engines had been on the M.O.R.E. program for some time). The pilot didn’t seem too concerned about it, but I couldn’t let it go. He wasn’t into trend monitoring, so I asked him to perform a number of tests during his next few flights. He didn’t really do what I asked, but the information I managed to eke out of him hinted that we were looking at another temp stick problem – this time it was one that didn’t show up on ground running. The next time that aircraft was in my hangar, I performed an ice test on the L/H temp stick to fool it into thinking it was at altitude. The stick opened right up. I checked the data plate, ordered the corresponding class of temp stick, and all was well. It’s something to think about when everything points to a bad temp stick but your 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 just needs a new temp stick? And the other moral of this story? Trend monitoring, trend monitoring, trend monitoring. Make that your new mantra. As always, fly safely.
After 40-plus years of “turning wrenches” and working on many King Airs, Dean Benedict is stepping down from the day-to-day business of maintenance and closing the doors to Honest Air March 31, 2016. He will continue to be involved in King Air maintenance on a consulting basis to former customers and the inevitable King Air magazine readers with questions. Besides consulting, Dean will also partake in pre-purchase inspections, expert witness testimonies and Director of Maintenance work under the name BeechMedic LLC.
If there is a particular maintenance issue you would like Dean to address in a future issue, please email Editor Kim Blonigen at kblonigen@cox.net.
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