Menu Sign In Contact FAQ
Banner
Welcome to our forums

Piper Turbo Arrow III

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

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 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

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

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

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…

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

Fairoaks/EGTF

…but IIRC there is a day/night switch that can dim the gear lights to a point where you cannot make out if they’re illuminated or not…

This is done by the nav-lights switch, common to all retractable Pipers. On some, nav-lights and panel lights are controlled by the same rotary dial which has no markings that show whether it is on or off, so whenever you don’t see the gear lights, you can start fiddling around on the panel…

Which brings up an important point, that applies to many other retractables as well: Whenever there is a (suspected) gear malfunction, perform no immediate action! If on approach, go around, climb to a safe altitude and follow the instructions in the manual. The Pa28 manual will tell you first thing to check the position of the nav-lights switch. It will then give figures for speeds at which the emergency gear extensions will work best, and what circuit breaker(s) to check or pull so that the half functioning mechanism won’t interfere with the manual extension. Abnormal systems operation like a manual gear extension are not supposed to be performed from memory. On no aeroplane. It is not safety or time critical in itself. But a hastily performed manual gear extension that fails (on some aircraft you have only one attempt, not so on the Pa28) will leave the gear in an undefined position and turn your abnormal condition into an emergency situation. (I write this because not long ago a student started to pull circuit breakers right away when I gave him a gear malfunction in the procedures trainer.)

Last Edited by what_next at 09 May 06:16
EDDS - Stuttgart

I did my complex endorsement (FAA) in one of these Pipers. Was a long time ago, but what I do remember was the issue of potentially overboosting during the t/o run and a rather weird thing to do with the landing lights. Memory is a bit hazy (these two hours were the only ones I ever flew that type), but IIRC there is a day/night switch that can dim the gear lights to a point where you cannot make out if they’re illuminated or not in broad daylight. Can really scare you when you drop the gear – and no greens in sight!

To quote from “Firefox” the epic Clint Eastwood movie:

“You must think in Russian! Can you do that Mr Gant?”

ESSB, Stockholm Bromma
27 Posts
Sign in to add your message

Back to Top