CHT's & Leaning

[judgee]

In one of the 206's I fly the CHT is always very high especially after take-off and climb, sometimes going into the red. I have written it up on our MR and the engineers replaced the CHT probe and sent it on its way. Now Im not sure how it works when engineers clear an endorsement but common sense would suggest that if you assume the probe is the fault then you replace it and then check that this has fixed the problem by means of some sort of test. But engineers are a rant for another time.

Anyhow, the CHT has not changed one bit. Now before you say I am not flying it correctly, I have tried to overcome this by climbing at around 100 knots in a shallow climb, Cowl flaps are always fully open, mixture fully rich and it still gets up to redline. I have noticed that during the takeoff run the fuel flow only registers 22 GPH at 28-29' 2850RPM and the POH states in one of its placards that it should be 24GPH at sea level on take-off. I then told the engineers and they told me they had set the fuel flow to factory specs.

Now the aircraft returned with the same fuel flow so I told them again and they said 'oh sorry I set it to 22GPH as I thought that was the figure off the top of my head'. So it went back and they said they had bumped it up. Once again it was still at 22GPH and they said there isnt much more they can do as it is at its maximum.

So I have been flicking the low pump on during the takeoff roll which gets me 24GPH and I can now keep the CHT's out of the red, however they are still very high.

So today I leaned in the cruise to find peak EGT to set the needle. As I reached peach EGT I glanced at the CHT and it had reduced considerably. I then set 100 rich of peak and watched the CHT rise back up to around 3/4 scale which is where it usually sits. This has me stumped. The only logical reason that this would happen is if the cylinder that has the CHT probe has an injector that is either slightly blocked or runs leaner than the others and is therefore reaching peak EGT earlier and then going lean of peak and cooling down.

Any Ideas?

[aerobat77]

stating your MAP at takeoff i think you are talking the non turbocharged lycoming, right?

my best quess is your CHT instrument is erratic - it seems very unusual that the non turbocharged version runs that hot. normally you should not come even close to redline.

in every case you should clarify if you deal with an erratic indication or the engine runs truly that hot. when for whatever reason the engine is really that hot you may burn through a cylinderhead in a situation you don,t need it - especially at a single engine .

the lowering CHT at leaning to peak indicates for me also an erratic gauge or probe. otherwise it would mean that at least the cylinder where the probe indicates is in real far over peak and in this case you should see much engine roughness.

cheers !

[judgee]

Engine is a Continental IO-520. I too believe it could be an erroneous gauge. When fully cold the needle sits just before the start of the green at 200F. Maybe the needle is out of line with the gauge background. I would assume that it would have done some serious damage if it was running that hot as it has approx done 250 hours since our company got the aircraft.

Im still puzzled as to why it would drop temp so much when i lean towards peak EGT.

Thanks for the reply

[stevef]

You can probably discount a partially blocked injector as this would give a high fuel flow indication. Spark plug tip colour might give you some idea as to what the individual cylinder mixtures are like.

[judgee]

Good point about blocked injector. Might have a look at the sparky to see whats really going on. Quite annoying that with proper instrumentation this could be easily diagnosed.