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Piper Turbo Arrow III

Peter wrote:

And you can’t just close the throttle, because you will probably crack the cylinders…

Interesting thread which got me curious. How does one practice PFL on a turbo airplane then?

Fairoaks/EGTF

1. one usually doesn’t
2. when you do, make sure the engine is already well cooled off before you pull the throttle to idle

Mainz (EDFZ) & Egelsbach (EDFE), Germany

Peter wrote:

And you can’t just close the throttle, because you will probably crack the cylinders…

That is if you actually believe in shock cooling.

And since I read these two articles, the first one by my second favourite author John Deakin, and this one which takes a mathematical approach to demonstrating that shock cooling does not exist, I have chosen my camp.

LFPT, LFPN

I think shock cooling definitely does exist, and Deakin himself explains this in one of his many articles. The required conditions are rapid cooling combined with a high CHT at which aluminium is weakened. This occurs over 400F or so, with a further and substantial weaking at 450F.

This makes sense otherwise you would get the issue on takeoff also. But you don’t – because the metal is still cool.

If I am doing a PDF in my TB20 (on a BFR etc) then I get the CHTs well below 350F.

Administrator
Shoreham EGKA, United Kingdom

I agree Peter, but under normal engine operations like transitioning from cruise to descent, shock cooling conditions should not exist, hence you should be able to pull the throttle or the mixture to descend

LFPT, LFPN

Perhaps shock cooling is not an issue when you operate within reasonable limits, but I have an example from our flying club.
We operate a Pawnee for glider towing, like many other clubs around the country. Interestingly, our aircraft runs to and beyond TBO without much trouble while some neighboring clubs replace cylinders and other bits and pieces on a regular basis (by comparison).

The one big difference in how we fly the aircraft is engine management. “Their” profile involves climbing at full power to altitude and once disconnected they chop the power and glide back at near Vne to land. We typically climb at a slightly reduced power setting and after disconnect, reduce to a lower descent power setting while maintaining green arc or slightly above. This has been going on for years and years.

Maybe we’re just lucky but I think that climbing at low speed and high power output then suddenly cutting the power over and over and over again will fatigue the metal and lead to these kinds of problems. If you use gently transitions it just helps a lot to mitigate the risk.

ESSB, Stockholm Bromma

I don’t believe that you can shock cool an engine when you are within normal operating temps. Towing gliders and after a Vx climb, the engine may be much hotter, so it is wise to think about shock cooling in these circumstances, but the 1 inch a minute descents are beyond my understanding.

Krister_L wrote:

Maybe we’re just lucky but I think that climbing at low speed and high power output then suddenly cutting the power over and over and over again will fatigue the metal and lead to these kinds of problems. If you use gently transitions it just helps a lot to mitigate the risk.

I think you are right, I remembered this article which I found on Peter’s website (thanks for keeping it alive! ), which explains how they resolved this problem in a fairly similar manner.

Bushpilot C208/C182
FMMI/EHRD, Madagascar
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