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While answering this question (How can reducing power too quickly on a piston engine damage it?), barit1 said that:

Skydiving planes see a LOT of shock cooling, and they pay for it at overhaul time

What exactly happens during a skydiving flight which causes a lot of shock cooling?

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Skydiving flights are abusive on engines for a number of reasons, but it's all tied to the fact that these flights are all about getting up to altitude, kicking the jumpers out, and putting the plane back on the ground as fast as possible so you can pick up the next group and do it all again, as many times as possible in a day.

While all skydiving descents aren't as dramatic as the video I linked to its not uncommon for skydiving pilots to make a very rapid descent at or near idle power. This causes a more drastic temperature change in the engine (particularly the cylinders, which are not producing as much heat but still have lots of air blowing through the fins).

In addition to the potential damage from rapid temperature changes the engine is also spending relatively little time at cruise power (it's either at full throttle climbing or near idle descending), and depending on the operation may be shut down between jumps. This is far from the ideal situation where the engine would run at 60-75% power as long as possible.

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    $\begingroup$ Just curious, are the acrobatics the pilot used in that video safe for the airframe? $\endgroup$
    – flyingfisch
    Commented Dec 21, 2014 at 22:07
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    $\begingroup$ @flyingfisch There is quite some discussion of that in the Youtube comments - short answer: "Well, the wings didn't fall off!" (Longer answer: I think everything demonstrated can be done as a 1-G maneuver, but I've no idea what the plane they used is certificated for) $\endgroup$
    – voretaq7
    Commented Dec 22, 2014 at 17:38
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    $\begingroup$ that video has been taken down. $\endgroup$
    – dalearn
    Commented May 1, 2017 at 15:33
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Because they tend to climb quickly to get to jump altitude (high power, hot engine) then descend very quickly to pick up the next batch of sky divers. Time = money.

Close the throttle, point the nose at the landing site.

However paul indicates in the comments that he worked at a centre where they took this into account and exercised caution (good airmanship) in how the engine was treated.

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    $\begingroup$ That's only half of it. Before the descent, the jump plane will make a full throttle climb to altitude. In other words, it heats the jugs up as much as it can during the climb, gets up to where the air is cold, and then dives throttle closed back to lower altitude. The dive starts up where the temperature is very low. $\endgroup$
    – Skip Miller
    Commented Dec 18, 2014 at 21:56
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    $\begingroup$ the PC-6's claim to fame was "10 jumpers to 10K in 10 minutes" $\endgroup$
    – rbp
    Commented Dec 18, 2014 at 23:16
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The aircraft in the referenced video showing a very rapid descent is a King Air (or similar). Turbine aircraft really don't have shock cooling issues - standard procedure is engines to flight idle, props to flat pitch, nose down to Vne. Planes like a Pilatus Porter can descend faster than freefalling jumpers and are often used as extras in films (you need to plan these shots with the pilot).

"Shock cooling" by definition is a temperature change faster than the metal can react, leaving you with a cylinder that is hot on one side and cool on the other. As I mentioned elsewhere, this is caused mainly by bad airmanship. My place covered this quite well in training, and we didn't have too many problems.

(pop quiz moved to separate question )

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  • $\begingroup$ Putting to the props in before the descent is a great way to upset the neighbors. Most places I've jumped at, the pilots seem to leave the props at the climb setting until in the pattern. The noise difference in the descent is dramatic, and our chief pilot says it doesn't make a huge difference in his turn times. Since every airport seems to be surrounded by NIMBYs these days, it makes sense to try to play nice. $\endgroup$
    – Brian
    Commented Dec 19, 2014 at 18:19
  • $\begingroup$ Everywhere I've jumped didn't have neighbors. $\endgroup$
    – paul
    Commented Dec 20, 2014 at 13:31
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Shock cooling is not a problem in reciprocating aircraft engines. In fact, today you almost only hear it mentioned in the context of turbocharged aircraft, and even then to avoid shock cooling of the turbocharger. Another common misconception.

You'll find no mention of shock cooling in the Pilot's Operating Handbook.

http://www.avweb.com/news/pelican/182107-1.html?redirected=1

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  • $\begingroup$ While I generally agree with John Deakin (and other smart folks who say shock cooling isn't a big concern), you will find it mentioned in Lycoming SI 1094D - Presumably they would not have taken the trouble to specifically call it out in this service instruction if it were completely mythical, but their limit of 50°F. per minute cooling rate is pretty generous for most operations. $\endgroup$
    – voretaq7
    Commented Dec 19, 2014 at 2:46
  • $\begingroup$ Shock cooling of Turbochargers is an old wife's tale? Excuse me while I giggle. $\endgroup$
    – Jon Story
    Commented Dec 19, 2014 at 10:52
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    $\begingroup$ Also, this wasn't a question about whether shock cooling exists or not, but about what parts of this particular flight activity would exacerbate the (potential) issue. $\endgroup$
    – FreeMan
    Commented Feb 24, 2016 at 16:52
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    $\begingroup$ If you run the cylinders to redline and then, without having a chance to cool to normal operating temp (~380°F max), pull ghem to idle and dive back down to the airport it is conceivable that shock cooling might be a problem. THAT is poor technique. Most skydiving engines that I've seen run past TBO and many with the original cylinders! $\endgroup$
    – acpilot
    Commented Feb 25, 2016 at 16:54
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    $\begingroup$ Shock cooling is a myth. Or at least in these kinds of applications. Multi engine trainers like the Piper Seminole have their critical engines routinely shut down during training flights with no damage as a result of shock cooling or reductions in TBO between the critical and non-critical engines. Maybe if you ran the engine at 85% power on a load cell for an hour to heat soak it, then quickly dunked the crankcase in a vat of ice water could you damage an engine this way, but not with air cooling. $\endgroup$
    – Romeo_4808N
    Commented May 1, 2017 at 1:26
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Having flown skydiving ops in both a C182 and a C208, I can make a few comments. First in the C182, the practice was to descend with cowl flaps closed, the throttle retarded but not at idle. "Clearing" the engine was done, and typically I would use a 50 degree (plus or minus) turn to maintain situational awareness and to increase the induced drag, and descend quickly.

In the C-208 the operation was similar, except that power was brought back, and there was no clearing of the engine, as it is meaningless with a turboprop.

On both aircraft I would frequently descend with full flaps. I did not use Vne descents, as often the air was not smooth, and the full flap descent at a lower speed offers adequate visibility and a steeper descent than just Vne.

The C182 was not turbocharged, and I know of no one in the area who uses a turbocharged recip for skydiving, although I am sure there are people who do elsewhered

Also, in the C182 normally there was a few minutes of flight at reduced power (75 or 70%) while ATC was contacted and the jumpmaster got happy with the traffic in the area, and where he was WRT the dropzone. This allowed CHT to drop more gradually.

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