High EGT on rapid descent

[0TreeLemur]

Our C has developed a case of cold-cabin since we redid the interior this fall.   Yesterday I pulled the plastic overhead panel and went up for a draft diagnosis.  

I trimmed for cruise (cowl flaps closed, 24" MP,  2350 rpm, pulled the mixture about 1" back from full rich) and it took very little time to diagnose the source of the cold air.   Sure enough, when we had the SB 208B inspection done last summer, they pulled some tape off the overhead air plenum where the tubular steel frame penetrates it and didn't replace it.  This results in a sizeable leak that pressurizes the plastic headliner panel and results in cold air flooding in. 

I was only a few miles away from the field and at 2kft-ish.    After turning around and heading back to the field I pulled the power to start my descent back to pattern altitude. Pulled MP back to 16" or so, and nosed over to get 800-1000 fpm descent and the EDM900 showed yellow EGTs.   Mixture still at 1".  So, pushed the mixture to full rich, and pulled the MP out some more.  What gives?  Can someone educate me about why the EGT would increase when the power is pulled to descend like that with what I consider to be the "standard" low altitude mixture setting?  I was just surprised.  I normally do cruise descents and don't see this behavior.   It seems that I did something wrong.  

Thoughtful replies appreciated.

[ArtVandelay]

My uneducated reasoning is as you descended, your mixture is getting leaner. I assume you were running ROP.


Tom

[gsxrpilot]

Are you sure it was EGT's in the yellow? I don't think EGT's have any limits or colors. But CHT's do. 

Regardless it's seems odd to me as well. It's been a while since I flew a C, but I don't remember seeing that condition.  And I would often descend without touching the mixture. If you were only 2000 ft, then you only had 1000 ft to descend. I don't think the mixture would make any difference in just 1000 ft.

Interesting...

[Ragsf15e]

20 minutes ago, gsxrpilot said:

Are you sure it was EGT's in the yellow? I don't think EGT's have any limits or colors. But CHT's do. 

Regardless it's seems odd to me as well. It's been a while since I flew a C, but I don't remember seeing that condition.  And I would often descend without touching the mixture. If you were only 2000 ft, then you only had 1000 ft to descend. I don't think the mixture would make any difference in just 1000 ft.

Interesting...

My Edm 930 is also programmed with yellow for egts.  It’s around 14-1500 degrees.  I see it after cruising lop at say 10,000’ and then descending without adjusting mixture at all. Egt in cruise is around 1350.  Egt slowly climbs in the descent until I hit the yellow band around 4,000’.  I’m not answering your question however...

[0TreeLemur]

18 minutes ago, Ragsf15e said:

My Edm 930 is also programmed with yellow for egts.  It’s around 14-1500 degrees.  I see it after cruising lop at say 10,000’ and then descending without adjusting mixture at all. Egt in cruise is around 1350.  Egt slowly climbs in the descent until I hit the yellow band around 4,000’.  I’m not answering your question however...

Yes- same here. I have an EDM900, and I think the EGT bars turn yellow very near 1450F.   I really like that feature.   Once I took off without pushing the mixture in, and it was obvious when those bars turn yellow.  Another time where having an engine monitor probably paid for itself many times over.

I don't often see the yellow bars unless I screw up, and I always run ROP.

-Fred

[DXB]

@0TreeLemurSomethin' ain't right. EGT should drop dramatically upon the power reduction to descend.  Unless you keep pulling back throttle to keep MP to keep it steady during the descent, it will EGT slowly rise again until you are getting too lean at some point and it might even run rough.  But if you keep MP steady during the descent EGT should stay stable and you will not get much leaner. Is EGT on all cylinders or just one cylinder affected?  If just one, I'd suspect there is a plug fouling on descent. If it is all of them, then an entire mag cutting out might be a concern.  Careful mag checks before next flight are in order.

BTW I set my yellow EGT warning to 1600 and find that it serves me very well.  What is ideal will depend on the probe install though.  My "target" EGT for max power takeoff is around 1430, so 1600 provides a good warning buffer.

[Shadrach]

Did pushing the mixture in decrease EGTs?  

Think of it this way. Piston speed is controlled by the prop control (RPM). The speed of the combustion event is controlled by the mixture control. It is not uncommon to see an EGT increase after a power change.  I can’t speculate as to your exact power setting but I suspect that your mixture setting was such that the speed of the combustion event (flame front) had slowed to the point that it was still burning during the exhaust stroke which would push burning gases into the exhaust manifold. Going very lean of peak will cause the same scenario as will running on a single mag (slow flame front due to single point of propagation). Was airflow driving the prop at your descent power setting?

Edited December 17, 2019 by Shadrach

[Pasturepilot]

An intake leak would introduce the most air into a cylinder’s intake at low manifold pressure. 
 

Someone back me up. I just woke up and am waiting on the coffee to make so obviously anything I say has to be double checked. 

[0TreeLemur]

6 hours ago, Shadrach said:

Did pushing the mixture in decrease EGTs?  

Think of it this way. Piston speed is controlled by the prop control (RPM). The speed of the combustion event is controlled by the mixture control. It is not in common to see an EGT increase after a power change.  I can’t speculate as to your exact power setting but I suspect that your mixture setting was such that the speed of the combustion event (flame front) had slowed to the point that it was still burning during the exhaust stroke which would push burning gases into the exhaust manifold. Going very lean of peak will cause the same scenario as will running on a single mag (slow flame front due to single point of propagation). Was airflow driving the prop at your descent power setting?

Thanks everyone for your replies. 

Ross, I think you might be on to something here.  Come to think of it, I think that I did push the prop control forward to high rpm to get the slight speed brake effect of the prop.  Pushing burning gasses out into the exhaust sounds like something to avoid for the sake of the exhaust valves.  

All cylinders showed increased EGT, not just one.  Yes, @Shadrach pushing the mixture in helped to bring all EGT's down.  The intake air leak theory is possible @Pasturepilot but wishful thinking, I think, because this was a one-off event indicating stoopid pilot trick not mechanical issue.

I think that if in the future  there is a need to lose altitude rapidly near the field I should use a slip to lose energy, not high prop rpm. 

Burnin' and learnin'

-Fred

 

[Shadrach]

4 hours ago, Pasturepilot said:

An intake leak would introduce the most air into a cylinder’s intake at low manifold pressure. 
 

Someone back me up. I just woke up and am waiting on the coffee to make so obviously anything I say has to be double checked. 

Affirm but my experience is that intake leaks occur at individual cylinders causing high EGT reading for just that cylinder. I suppose it’s possible that there is a leak up stream at the carb allowing “unmetered” air to lean the mixture going to all cylinders but it seems unlikely. You’re are absolutely correct that a throttling back will quickly reveal intake leaks.

You needn't worry about cooking a valve.

Edited December 17, 2019 by Shadrach

[Pasturepilot]

5 hours ago, Shadrach said:

Affirm but my experience is that intake leaks occur at individual cylinders causing high EGT reading for just that cylinder.

See? Reading comprehension before coffee is a concern in this house. Completely missed that this was all cylinders. 

 

[Shadrach]

24 minutes ago, Pasturepilot said:

See? Reading comprehension before coffee is a concern in this house. Completely missed that this was all cylinders. 

 

Happens to me and my friends all the time.  I had a buddy once promise to send me a couple of Brittain AP actuators before his morning coffee and I never heard another word from him...

[Shadrach]

On 12/17/2019 at 8:12 AM, 0TreeLemur said:

Thanks everyone for your replies. 

Ross, I think you might be on to something here.  Come to think of it, I think that I did push the prop control forward to high rpm to get the slight speed brake effect of the prop.  Pushing burning gasses out into the exhaust sounds like something to avoid for the sake of the exhaust valves.  

All cylinders showed increased EGT, not just one.  Yes, @Shadrach pushing the mixture in helped to bring all EGT's down.  The intake air leak theory is possible @Pasturepilot but wishful thinking, I think, because this was a one-off event indicating stoopid pilot trick not mechanical issue.

I think that if in the future  there is a need to lose altitude rapidly near the field I should use a slip to lose energy, not high prop rpm. 

Burnin' and learnin'

-Fred

 

Fred, 

if you really want to lose altitude quickly, slowing down is the best method. And I mean SLOW. On the back side of the drag curve any Mooney will descend rapidly with the power off. An interesting exercise is to hold pattern altitude on final and slow to just above stall in the landing configuration. When the the runway threshold disappears under the nose go to idle and pitch down just enough to keep the stall horn from sounding. If you do it right, the plane comes down so steeply as to be on the runway within about 1500 feet of the threshold.   I’m not saying you should try this but The method can be employed at cruise altitude as well.

[DXB]

On 12/17/2019 at 8:12 AM, 0TreeLemur said:

Thanks everyone for your replies. 

Ross, I think you might be on to something here.  Come to think of it, I think that I did push the prop control forward to high rpm to get the slight speed brake effect of the prop.  Pushing burning gasses out into the exhaust sounds like something to avoid for the sake of the exhaust valves.  

All cylinders showed increased EGT, not just one.  Yes, @Shadrach pushing the mixture in helped to bring all EGT's down.  The intake air leak theory is possible @Pasturepilot but wishful thinking, I think, because this was a one-off event indicating stoopid pilot trick not mechanical issue.

I think that if in the future  there is a need to lose altitude rapidly near the field I should use a slip to lose energy, not high prop rpm. 

Burnin' and learnin'

-Fred

 

Stimulating discussion here on mixture,  rpm, EGTs.  I bet it was the high rpm at a lean mixture that was the key difference in this event.  In principle, the advanced timing and longer spark duration provided by electronic ignition (like the newly available Surefly) would also limit the rise in EGT you saw.