Bad probe, or something more sinister ;-?

[FlyingScot]

 

 

I saw this recently on one of my flight data downloads. Note the ~10deg oscillation in CHT. Now, if this was EGT I would suspect a valve, but it's CHT, and I would expect the time constant of the system to be longer than 3 min - which makes me think probe (since a cyl head has a larger heat capacity) . Any thoughts?

 

[kortopates]

not a probe issue but baffling. 3-5f fluctuations are very common but 10F warrants investigating. It may clear up just removing and re-installing upper cowling if a piece of flexible baffling got pinched or pushed back. make sure baffling is tight and sealing well against upper cowling.


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[Fly Boomer]

25 minutes ago, kortopates said:

not a probe issue but baffling. 3-5f fluctuations are very common but 10F warrants investigating. It may clear up just removing and re-installing upper cowling if a piece of flexible baffling got pinched or pushed back. make sure baffling is tight and sealing well against upper cowling.

How would baffle seals cause a fluctuation?  I would expect temps to go up and stay up?

[Guest]

Loose inner cylinder baffles have caused CHT oscillations in SR22 airframes before.

[kortopates]

no, pressure builds to force baffling to give, causing leakage till pressure drops and baffle seals again, cycle repeats endlessly till change in airflow pattern.

Pretty common actually, many have them but quite small if you look closely.

regular small perfect fluctuations only in level flight can also be caused by AP as it ever so slightly pitches up and down to maintain altitude- not these those.

Clarence brings up another source of these with the inner baffling to verify their security.

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[Fly Boomer]

24 minutes ago, kortopates said:

Clarence brings up another source of these with the inner baffling to verify their security.

Good to know.  Thanks!

[MatthiasArnold]

Same here on cylinder #4 (flight at FL140):

• stable EGT, oscillating CHT with amplitude 5 - 10°F, period time 1-2mins.
• change of pattern in CHT#4 (higher amplitude and period time) correlates with changes in TAS at 01:48 onwards (not depicted)
• TAS changed due to very small mountain waves while crossing the main rims of the Alps (Gotthard route), Alt & power setting constant

Obviously caused by air flow and thus pressure changes in the upper/lower part of the cowling impacting the baffles as described @kortopates and @M20Doc above.
Reseating the upper cowling did not help, inner baffles looking good. Maybe have to replace the elastic baffling at the upper cowling some day in the future.

Best,

Matthias
 

[Pinecone]

2 hours ago, MatthiasArnold said:

Same here on cylinder #4 (flight at FL140):

• stable EGT, oscillating CHT with amplitude 5 - 10°F, period time 1-2mins.
• change of pattern in CHT#4 (higher amplitude and period time) correlates with changes in TAS at 01:48 onwards (not depicted)
• TAS changed due to very small mountain waves while crossing the main rims of the Alps (Gotthard route), Alt & power setting constant

Obviously caused by air flow and thus pressure changes in the upper/lower part of the cowling impacting the baffles as described @kortopates and @M20Doc above.
Reseating the upper cowling did not help, inner baffles looking good. Maybe have to replace the elastic baffling at the upper cowling some day in the future.

Best,

Matthias

Although, with the first thing listed, I would be putting a bore scope in that cylinder and taking a look at the exhaust valve.

According to Mike Busch of Savvy, that is the classic sign of an exhaust valve starting to have issues.

[MatthiasArnold]

2 hours ago, Pinecone said:

Although, with the first thing listed, I would be putting a bore scope in that cylinder and taking a look at the exhaust valve.

According to Mike Busch of Savvy, that is the classic sign of an exhaust valve starting to have issues.

Thank you for your response!

I bore-scope the valves at every oil change = every 30hrs.
For the time being nice concentrical pizzas.

Maybe the scaling/zoom-level of the upper chart is a little bit misleading - the EGT signal fluctuation/amplitude is in the range +/-2°F which is around +/-0.15% at 1450°F.
I'm not an expert but would consider this as signal noise rather than the periodic pattern mentioned by Mike Busch et al.

Maybe the MS-brain can share typical EGT pattern amplitudes caused by bad EGT valves or valve seats?

Best,
Matthias

[Pinecone]

It also might be the very early signs as the blow by starts.

Too easy to bore scope that cylinder.

[kortopates]

1 hour ago, MatthiasArnold said:

but would consider this as signal noise rather than the periodic pattern mentioned by Mike Busch et al.

exactly, it's can also be misleading to look at a single EGT since only when you look at them together can you get an understanding of the degree of noise. Their is a lot of variation there and depends how carefully your probe wires were routed without being bundled with or adjacent with ignition leads; a large source of noise. 

[carusoam]

Gents,

We are seeing the issue pretty clearly…

But, don’t have the supporting detail to help with the logic…

If you can…

Load data up to savvy, click the share button, post link here…

 

Usually, a failed sensor can’t make a fancy regular oscillating pattern… they typically fail going off in one direction…

Pretty stable EGTs and/or nice pizza pictures kind of indicate NOT a valve issue…

The rest of the shared data can help narrow things down…

Best regards,

-a-