CHT - do I need one?

[Big Ed]

Found it a bit surprising that I do not have a CHT. I do have an EGT.

When the aircraft was built, all the 2" gauge holes were filled with mandatory instruments, so there was only room for one optional instrument. I guess EGT won out. However, 2 years later the first owner moved the Tach to the center stack and replaced the 2" Manifold Pressure with a 3" MP, so now there is a free 2" hole.

I was considering installing a digital tach in the open 2" hole. Should I be looking at a CHT instead?

Some planes list the CHT as required equipment, but it is not so on the Decathlon. Any idea why?

 

[Bob Turner]

Bought my Dec in 2003. It has a CHT. I have this feeling that it has never worked. Engine is still running.

I used to worry about oil temp - on a hot day it would go to red line in a climb. Baffles need to be up and forward.

I would not say that the AEIO 360 is bulletproof, but with fine wires in the bottom I am beginning to think it is a pretty darn good engine. One quit the other day on takeoff, but so far it sounds like fuel starvation.

 

[Bob Turner]

By the way, have you considered a larger panel? You have seen mine, right?

 

[Big Ed]

By the way, have you considered a larger panel? You have seen mine, right?

I'm ok with the panel size I have; just making a few tweaks at this point.

 

[Big Ed]

Found the answer to my question. Cowl flaps. If you got 'em, the FAA sez a CHT is required equipment. Which makes sense. If no cowl flaps, like a Decathlon, CHT is optional.

 

[BB57]

Get both. In my 7KCAB high CHTs have never been an issue and I think the highest I've seen has been 367 on the #3 cylinder. 350-355 is the norm in the summer with the number 4 being 4-5 degrees behind and the #1 and #2 cylinders about 20 degrees cooler. However, without a CHT you don't really know that. You also won't obviously know if you develop an issue with the baffles and now have high CHTs on the #3 or #4 cylinders, and you won't know when you start running cylinders excessive cool (below 315 or so). These are air cooled engines and proper clearance between steel cylinders and aluminum pistons depends on running the engine at a high enough operating temp.

A four probe EGT is also very useful for both mixture adjustment as well as diagnostics and tracking valve and cylinder health.

I put a 2 1/4" EDM 700 in mine which includes both a four probe EGT and four probe CHT. IMHO, it's money well spent from an engine management and engine diagnostic perspective. I own a $60K airplane and an engine overhaul is about $25K. It's in my best interests to do what I can to make the engine last as long as it can to reduce the hourly set aside for a future overhaul.

The FBO that operated my 7KCAB before I bought it (a leaseback with the prior owner) ran it full rich all the time and at 150 hours since major, the number 2 and 4 exhaust valves already didn't have that nice bullseye heat pattern on them. Instead they were very asymmetric with dark sections on the edges (hot spots) to varying degrees that indicated they were headed for warped and eventually swallowed valve issues down the road. There were obvious excessive deposits on the valve seats from running 100LL, which is what causes the problem. Unfortunately, 100LL is the only option currently for the 7KCAB with it's IO-320 and Bendix fuel injection.

I flew it about 50 hours using traditional "lean it until it stumbles" and then enrich enough to smooth it out (or in it's case "loses lots of power", then enrich to get most of the power back). That more or less gets you somewhere between a slightly lean of best power to peak EGT "best economy" mixture, and you can get best power by leaning to peak RPM. But that's still potentially problematic. At power settings over 60-65% best power settings around 40 degrees ROP result in high internal cylinder pressures that tend to shorten engine life. At lower power settings under about 60-65% engine stress isn't an issue at any mixture setting, but CHTs and EGTs can be overly cool (especially at overly rich mixtures) which leads to excessive lead deposits on the valve stems and valve seats.

I installed an EDM 700 and started running it at a still conservative 100 degrees ROP in flight (and leaning aggressively on the ground to just short of a stumble). 50 hours later, the #2 exhaust valve now looks perfect, and the #4 exhaust valve is much improved, going from discoloration around just less than half its circumference to a small section over about a 15 degree arc. Lead deposits are also reduced on the valve seats and the compression numbers are 77/80 76/80 78/80 on 1, 2 and 3, with number 4 improving but still lagging a bit at 70/80.

One of the other benefits now that the weather is cooling is that it lets me set the power a little higher and adjust the mixture so the CHTs and EGTs stay high enough to prevent lead buildup on the valve seats and stems.

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In addition, the other day I had a spark plug fail on take off with the result that the engine lost significant power while operating on both mags. Since I was above the runway and had enough runway left to stop (about 800' down a 2800' grass runway), I pulled the power and landed.

Back on the ground, the EDM 700 helped to quickly isolate the problem. It ran normally on the left mag and the bottom fell out of the EGT for #3 when running on the right mag only. Which means I could identify the failed plug easily and only had to pull and replace the bottom plug in the #3 cylinder, rather than pull and test all 4 plugs connected to the right mag.

If it had occurred in flight with time to trouble shoot while pointed toward a suitable emergency landing site, I could have also tried each ignition system independently to see if it would run well enough to stay in the air, but again a 4 probe EGT and CHT system in concert with isolating each side of the ignition system would have immediately let me confirm it was a plug issue, and identify the failed plug.

In normal conditions when nothing is out of the ordinary it also helps me identify any changes that are occurring in terms of EGT and CHT patterns.

 

[Big Ed]

I chose to stick with a mechanical tach, so I still have an empty 2 1/4" gauge hole. Will eventually install an engine monitor there, most likely a bar-graph like the EDM 700. Just a question of when.

My engine sat for several years before I bought the plane, so most likely I am headed for an overhaul sooner rather than later. Not sure it makes sense to spend money to install a GEM to extend the life of an engine that probably has little life left, then spend more money to uninstall and reinstall it when the overhaul happens. The internet says that installing a GEM is a fairly labor-intensive project. Doing it once as part of the overhaul might be the most cost effective approach in my case.

That said, I do like the idea of having more data to diagnose cylinder problems. If my next few oil changes show no problems and I get a good distribution from my business at the end of this year, I might splurge on a GEM at my annual next May.

WRT my original question, a CHT gauge on a single piston doesn't appear to be of much use ... unless you have cowl flaps. Then it is a necessity.

 

[Bob Turner]

I am not sure yet, with only 18 years experience, but my friends say the 180 hp Lycs are bulletproof.

Well, in the late’70s we had a Mooney, and we ran the socks off it, but that was before instrumentation became commonplace. Not sure how many trouble-free hours we had, but somebody else crashed it before it had a chance to quit.

I can tell you that engines are routinely torn down for overhaul before they are worn out. Compression checks are done once a year, and if a bad reading is obtained either that cylinder or all of them come off. I beg my friends to wait a week and do another check - but nope. I do compression checks before each flight, and wait two weeks before deciding it wasn’t a piece of dirt under the valve.

 

[Big Ed]

Everybody says the one weak point of the IO-360 is the camshaft. If you wanted to ruin a Lycoming camshaft, you would treat it exactly like my plane was treated from 2014 to 2018. Fortunately, the price I paid reflects that expectation. Also fortunately, I am based 10 miles from Zephyr Engines, a top quality overhaul shop.

 

[BB57]

I can tell you that engines are routinely torn down for overhaul before they are worn out. Compression checks are done once a year, and if a bad reading is obtained either that cylinder or all of them come off. I beg my friends to wait a week and do another check - but nope. I do compression checks before each flight, and wait two weeks before deciding it wasn’t a piece of dirt under the valve.

I agree way too many top overhauls get done.

An unethical mechanic can make a compression check look worse than it is, or do it cold and get lower numbers that he or she would if it's done after the cylinders are brought up to operating temp.

Way too many pilots start panicking if a cylinder is below 70, when Lycoming advises that compression under 65 just warrants 100 hour compression checks. Even at 60 psi Lycoming only says overhauling the cylinder "should be considered".

IMHO a good mechanic won't pull a cylinder based only on low compression numbers, but will instead also look for the cause of the low numbers using a borescope, and for numbers in the lows 60s if there are no obvious indications of a problem, recheck after another 10 hours or so to see if it was an isolated event or a consistent indication of wear.

 

[Bob Turner]

Re-check! Always!
And remember that cylinders on average go bad six months before the annual compression check.

A local shop did a prebuy for a friend selling a six cyl aircraft. Four were below limits. He asked me to double check. I had the potential buyer hold the gauge and write down the numbers - all six above 70. I did verify, but the potential purchaser saw them in person. Do them warm.