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#199 “Shock Cooling”



“We Have to Have Something to Argue About”


This is me, talking about something I know absolutely nothing about.  Now, everyone who operates airplanes has heard a little something on the subject of “Shock Cooling”.  What I find fascinating is that those whose opinions I respect seem to be on different sides of this issue.  Some believe that shock cooling is a real danger to the well-being of aircraft engine cylinders and some simply don’t believe there is such a thing.  Just so we’re on the same page, “Shock Cooling” is the cooling of a cylinder at such a rapid rate that it creates stresses on parts leading to cracking and other nastiness.  I wish I didn’t believe in it.  It would certainly make chopping power and descending a lot easier for those who sometimes tow gliders or haul jumpers.  But I happen to believe in it.  And, since several people have asked me how to deal with preventing such cooling.  I’ll answer their questions right here.

The Pawnee, with flaps extended, yet to roll and sideslip.

Most of my experience with rapid descents and the prevention of rapid cooling was when I towed a glider at Sunriver Soaring.  I didn’t have a lot of instrumentation in the Pawnee with the O-540 and fixed pitch prop.  I had oil temperature.  That was about it.  Right before the glider would release I’d start reducing the power by a hundred RPM or so.  Then I’d wait.  When the glider released, I’d put the flaps down, reduce the power a bit more, then roll into a really steep bank of about 80 degrees.  Then I’d push in top rudder to slip.  Basically I’d created drag by putting the flaps down and by slipping.  Then, to increase the drag even further, I’d apply back pressure while referring to my “G” meter.  I’d keep the load to about two or two and a half “G”s.  That “G” force would produce oodles of drag and I’d be coming down at over a couple of thousand feet a minute without increasing airspeed and subsequent cooling air.  While I was watching the earth spin, I’d lean my mixture to further slow cooling.  If I’d had cowl flaps, I’d have closed them shortly before release to slow the change of cylinder head temperature and decrease the cooling air flowing through the engine compartment.  But I didn’t.

With the Sunriver airport below and to the left, the Pawnee is about ready to roll to the left and enter the pattern.

By the time I was at pattern altitude, my temps were down and I was ready to land.  Now you know everything I know about shock cooling.  But the bottom line is that if Shock Cooling is a myth and not to be worried about, at the very least I decreased my flight time and thus fuel consumption and engine time.  The way I see it, no harm done and a bit more proficiency added.

4 comments to #199 “Shock Cooling”

  • John Carroll

    This is an interesting topic. My mechanical engineering degree tells me one thing and my flying experience tells me another. Once while ferrying an aircraft overseas, my first officer who I had never flown with before was unrated in the aircraft and there only for insurance purposes. He was so worried about shock cooling the engines that he would often not make the necessary power changes that I iwas instructing him to do.

    I fixed that by taking the throttles and slamming them full up and back along their travel range, after that he did as I asked rather than have me do that again. At the time, I was trying to get some of the failed navigation equipment working over the mid-Atlantic in moderate turbilence and couldn’t spare the time for a power mutiny.

    But rather than bore you with too many stories, someone made this argument to me and I could not refute it. If shock cooling is the problem that some people say it is, why then can a hot engine ingest the fire hose spray of heavy rain, without incident? Arguments like that, along with a lot of banner and glider tow time without issue, make me think that this is much to do about nothing, at least for most normal operations.

  • Ron Parish

    So, that’s what you were doing up there in the skies over Sunriver! We would spend our mornings drinking coffee and reading on our Meadow house deck, but I was regularly distracted watching and listening to your Pawnee descend after glider release. It was fascinating to watch through binoculars, and the sound was “music” like Chopin on the piano! Then, for a finale, you did that little jig on final after dropping the tow line – I learned from you later, with sweat on my brow, that the “jig” was a landing in a turn.
    By the way, I believe in and observed good shock cooling practices in my PA34-200T as well – just not so much of a “ballet” as you were doing!
    Best regards, Ron

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