It is our contention that shock cooling is a myth.  I routinely reduce power significantly and never worry about shock cooling the engine.  How can you shock cool something that's not hot already?  If my CHTs ar eunder 380 or so, I can't shock cool them.

This is an interesting Old Wive's Tale that will not die.  Prior to the introduction of the turbo Twin Cessnas, there is no reference to the phenomenon of Shock cooling anywhere in any of the aviation literature.  So how did it all get started?  In the 421, pilots were descending from the flight levels to land. The pressurized aircraft could descend quickly and not be an ear-popper.   The fuel in the tanks was very cold from being at altitude.  They'd reduce the power and when they got to pattern altitude, the common practice was to go full rich.  This blast of very cold fuel on the inside wall of the warm intake chamber caused cracking.  That's where all of this shock cooling stuff came from.  The cylinders were not having trouble at all.  The problem was the super-cooled fuel from flying high hitting the warm metal.

Shock cooling is a myth for operational purposes unless the CHT is near redline and you chop the power and go full rich.  That stupid pilot trick can cause problems. <g>

Walter Atkinson
Advanced Pilot Seminars

I agree with Walter, I think it's an old wives tale. In my game of AG flying there is a lot of throttling off to land, with aerial fertilizing there is a landing every 6 or 7 minutes. You go from full power to criuse to throttle off in that time with landing throttling right off until touchdown. I have flown behind Pratt&Whitney, Continental and Lycoming, none seem prone to "shock cooling". In all the old flight manuals and pilot notes on aircraft before WW2 the instructions are to just trottle off and land, even up to the early 60's that was the way to land. It's only been with the introduction of shallow approaches that power has been carried to landing.

I forgot to add there are always exceptions, glider towing and parachuting are very hard on engines. Low speed high power climbs then low power high speed decsents, you can run into trouble with those but "normal" operations OK.

**I forgot to add there are always exceptions, glider towing and parachuting are very hard on engines. Low speed high power climbs then low power high speed decsents, you can run into trouble with those but "normal" operations OK**

OK, let's take this at face value.

Why then do those engines generally make TBO? 


That reality is at odds with the assumption that those operations are hard on the engine.  They are not.  So, the question is, "Why aren't those operations as hard on the engine as it apprears they should be?"

Walter Atkinson
Advanced Pilot Seminars

The comments I made about Parachuting and Glider Tugging come from seeing the results. Seeing those results are not the full story though, are we to assume all those cracked cylinders were new before fitting? Both mobs are known for their penny pinching so fitting of dubious cylinders would be standard for some. Would those cylinders have cracked with use other than Parachuting or Tugging?

Another good story is about geared engines, your not supposed to throttle off and let the prop push the engine. There is no mention in operating handbooks with P&W engines, or mention about this with other older types. There is mention about it with the GO-300 in the 175 Cessna, the mention there is only because of the noise it makes not the damage it does. Has the same thing happened to  the geared engine? In some Aircraft configurations throttling right off can blanket the control surfaces as the prop disk becomes a big brake, good to use for a steep approach but a bit unnerving if your not used to it, is this part of the old wives tale with geared engines? Was it Continental that made the Tiara? good engine bad reputation, was it an old wives tale and gossip that cause people to avoid this engine?

Where in the 175 and the 421 POH's or owners manual's does it state the engine management for geared engines you suggest.

From a 1962 Owners manual.........Approach glides are normally made at 70 to 80 mph with flaps up, or 65 to 75 with flaps down........that's it, There is mention of cowl flap use to keep temps up, no mention of special throttling off proceedure or propblems with gear lash.

In the 421 owners manual there is no mention of special proceedures either, you would think if this is a problem the manufacturer's would have stated the problem and the cure.

With regard to Pratt's to which "oil port" are you refering to is uncovered by letting the prop drive the engine?

The 421 owners manual might have been written before the Aircraft was out and about but General Aviation had a bit to do with geared engines before then. The Queenair had geared Lycomings, Pigaggio had Geared pusher Lycomings and Cessna had the 175 with the Continental. There were also other not so well known Aircraft with like early PC-6 Pilatus, Dornier Do-27 and DO-28. All of these Aircraft had geared engines well before the 421 was released, if there is no mention of special operating proceedures in the manual it smacks of an old wives tale.


Are the geared Pratt and Whitney's the only ones with the "single port" crank? are you talking of oil pressure when you talk of "pressure on one side"?

I've got a couple of years behind 985's, I used to throttle right off to get braking from the prop to make steep or quick approachs. On the Op's we used them for there was a landing every 6 or 7 minutes. I've never heard of any problem with oil feed, if the prop is driving the engine then you would think the oil pump would be doing the same revs and the oil pressure would be OK.

The work was aerial fertilising, taking of with up to a ton of fertiliser then spreading it at a rate of 1 cwt/acre then back to land. The strips were no more than 2 mile from the job.

The engines would go full time and some of the operators used to only put half the time down, with some of the engines doing up to 2,500 oil switch time (at the time a complete overhauled engine was cheaper to buy than doing up your own). The main thing with the 985 seemed to be warm them up and they would keep on going. The blokes that didn't warm them up properly seem to do the supercharger bearing after a couple of hundred hours. They rekon the oil feed was a small diameter and the thick oil would'nt flow, from memory the drill was no more than 800 revs until 20 degrees then no more than 1200 revs until 40 degrees, then run up's. Gen online at 1200, prop at 1400 (full movement with counterweight prop) then mags 1750 and full power and pressure check at 2000 with MP at 30 inches. Ready set go.

To Walter A. You have enlightened me through the years on the shock cooling myth as well as running the engine lean of peak.
I routinely run my engine LOP in my Bonanza. The ABS service clinic commented on the cleanliness of my plugs, cylinder walls and exhaust. The oil stays cleaner longer too. Good, smooth pilot technique keeps me from "chopping" power. Not the fear of "shock cooling. It is funny to watch peoples' reactions when I teach them how to operate LOP and when I tell them shock cooling myths.

Dan R.

What is LOP?  Low Operating Power?

Dan

Thanks, I am a new pilot and one of the hardest things is the abreviation of almost everything, I went so far as to make a list of them and keep it with me just in case.

Dan