Acclaim cruise CHT’s and TAS fluctuations

[Schllc]

Flew from Nashville TN to Driggs ID today and I saw something I’ve never experienced before. 
I was flying at 16,000, almost the entire time.  
I was flying LOP and all was well with one curious thing happening almost cyclically.
My MP was 29”, fuel flow was relatively stable at 16.9-17.1 gph and the TIT was stable, but my cht’s would cycle from 212 knts to low 190,s, and the hot cylinder #5, would go from 355-375. Not rapidly, but continuously drifting back and forth between the two extremes. 
Has anyone ever experienced this, or have an explanation?  
I have never flown over this terrain before and we had headwinds from 20-50+ knots right on the nose. 
I know there were some thermals at times, and I would sometimes see 10deg pitch up attitude with no real change in altitude. 
it could just be inexperience on my part but was hoping for an explanation. 

[carusoam]

Sch,

Looks like Siri may have edited your °F to something else...

Often, when temps like CHT’s oscillate there is something changing with some regularity...

Since CHTs are one part Heat energy coming in, and one part heat energy leaving...

We can look at a couple of things...

Heat in, is often seen as steady EGTs... if your EGTs were oscillating the same way... we would know where to look next...

Heat out, is clearly CHTs, OATs, Kias, and attitude...

If everything was stable in cruise except the CHTs... it is probably something like cowl seals opening a gap and some how closing back up...

Got any Engine monitor data that you can share?

PP thoughts only, not a mechanic...

Best regards,

-a-

[kmyfm20s]

At 16,000’ Heading west you were most likely dealing with mountain wave causing the cyclical air speed and CHT fluctuations. 

[carusoam]

Would a recent G1000 upgrade have airspeed data saved along with the engine data?

Good thinking k!

Thanks for sharing that thought...

The AP can really hide smooth mountain waves... instead of moving up and down... we would experience going faster and slower as if we were descending and climbing... CHTs will follow the kias’ inverse...

PP thoughts only, not a CFI...

Best regards,

-a-

[kmyfm20s]

I just departed from Driggs Monday. You getting to the area at a great time! The crowds dropped dramatically this past weekend.

[larryb]

I frequently experience this in my encore. One time the downdraft was so severe that IAS dropped to only 90 kts and I had to go to full power for 5 minutes to maintain altitude.

[aviatoreb]

On 9/2/2020 at 11:07 PM, Schllc said:

Flew from Nashville TN to Driggs ID today and I saw something I’ve never experienced before. 
I was flying at 16,000, almost the entire time.  
I was flying LOP and all was well with one curious thing happening almost cyclically.
My MP was 29”, fuel flow was relatively stable at 16.9-17.1 gph and the TIT was stable, but my cht’s would cycle from 212 knts to low 190,s, and the hot cylinder #5, would go from 355-375. Not rapidly, but continuously drifting back and forth between the two extremes. 
Has anyone ever experienced this, or have an explanation?  
I have never flown over this terrain before and we had headwinds from 20-50+ knots right on the nose. 
I know there were some thermals at times, and I would sometimes see 10deg pitch up attitude with no real change in altitude. 
it could just be inexperience on my part but was hoping for an explanation. 

I agree with the others - you may have just been flying through periodic waves of atmospheric effects of downdrafts.  I have gone through such things.

So when there is a down draft your autopilot works to keep altitude and pitches up and then now you are climbing through the airmass and your engine heats up a bit and you slow down a bit. Then you pass that down draft and your nose lowers and your cht lowers.  Repeatedly.  

If this happens on north south flights or on every flight then I would blame the airplane.  If it happens on this east west flight I will blame the mountain waves.