What’s an “acceptable cooling rate” in the shock cooling view of the world ?
I often hear 50°F/min
What is a good way to deal with this issue during flight training where you often want/need to quickly apply or reduce power?
Reduce power to get CHTs below say 320F. This means briefing an examiner on a checkride, and it means he can’t give you a totally surprise engine failure 
On the TB20 it means flying at say 20" instead of the 23" normal at low level.
I often hear 50°F/min
Probably reasonable. IIRC an EDM700 flashed the warning at 70F/min. But it does this irrespective of the CHT value, so you often see it during landing, when it doesn’t matter.
I am in the camp that doesn’t believe in shock cooling, and even if it existed it wouldn’t be an issue in the aircraft I normally fly (Aquila A210 and C172)
@Pilot_DAR another good topic. Typically the POH called for 15 to 17 inches MP and keeping the CHT in the green during descent, with cowl flaps closed. So the POH seems to believe there is a risk of excessive cylinder cooling.
Even the lowly fixed prop Worrier says 2500 RPM and 126 KIAS in descent.
Most schools have SOPs which have power settings in descent, and while not explicit on ‘shock cooling ‘, are in line with POH recommended minimum power in descent.
I expect old radial engine types were even more prescriptive on descent planning and power settings, cooling risk.
There are two types of people: Those who believe in x, and those who don’t – no problem.
For the people who choose to not believe in the possible harm resulting from shock cooling, it’s unlikely that they’ll be offended if you fly their plane with due care and regard for the possible risk, and just go easy on it. On the other hand, for those who do believe shock cooling is a risk, they will immediately notice someone slamming the throttle closed, and think less of them for doing so. Then probably not offer the use of the plane to them again! So which kind of pilot would you like to be thought as being? The kind who takes it easy, and takes care, or the pilot who does not demonstrate care?
I agree that O-200’s in 150’s, O-235’s in 152’s, and O-320’s in 172’s and Cherokees seem to be very tolerant of training abuse. I too agree that they don’t run hot and hard as the larger engines. My O-200 struggles to get to 300F CHT. A Navajo I used to fly took care to keep CHT’s below 400F. I have certainly seen O-200 and O-320 cylinders cracked, but nowhere near as often as IO-520’s and IO-540’s. That said, the 172 I bought with an O-300 in it….. two cylinders were scrap for being cracked (as was the crankshaft flange, but that was a different story of abuse).
Sometimes during flight test maneuvers, I have been required to close a throttle much more quickly than I would like to (or would if I owned it) – so, morally, I do three things: I reduce power as much as I can, for a time before I do it, to let the temps stabilize, and maybe cool a little (because I planned), I slow the plane down as much as I can before I do it, to reduce the cooling airflow over the engine, so it won’t cool as much after I close the throttle, and I think an apology to the engine. Doing so reminds me that I’m handling the engine out of the norm, and would rather not. I’ve never been directly accused of cracking a cylinder, but I can’t assert that I haven’t, but I know that I’ve never cracked one I own! (O-200/O-320/O360).
A nearby flying school operates several Beech Barons. I hear them coming… waaAAaaAAaa, then a throttle slammed closed, and bang pop pop pop from the exhaust. I commented on this to the Director of Maintenance, who I knew quite well from a previous affiliation. More recently, when I hear them coming, I hear nice, smooth power reductions…
For me, pilot owners have commented to me in the past that they noticed that I handled their plane with care, they appreciated that, and they were happy to let me fly it. Gentle power changes are certainly a part of that. I take that as a complement, and I fly anyone’s plane as though I owned it.
For the amounts you could encounter in “flyable weather conditions”, rain and snow contact will not harm the engine. The water mostly hits the fins on the cylinder barrels, not the cylinder heads. There’s just not enough cooling effect of drops of water to cool enough to do harm. Hot immersion will ruin cylinders, but it tends to ruin the whole plane!
It‘s not about belief but about facts!
Thermo-Mechanical processes in an engine are to a great extend symmetric! (Yes, there is some hysteresis, but for practical reasons they are neglectable).
Therefore if shock-cooling is a proble, shock-heating is the same problem in the same temperature range.
If closing the throttle faster than 2‘‘-MAP/min is a problem, opening the throttle faster than that is the same problem.
Unfortunately at my field the runnway is not long enough, that I can do the takeoff-run by opening the throttle only 2‘‘/min ;-) Therefore if shock-cooling or heating was a problem, my engine is doomed to fail anyways.
(And this shock heating is not avoided by warm up of the engine as we are talking about a completely different temperature range here – one does not warm up the engine to 350CHT…)
It‘s not about belief but about facts!
Indeed.
Thermo-Mechanical processes in an engine are to a great extend symmetric! (Yes, there is some hysteresis, but for practical reasons they are neglectable).
Therefore if shock-cooling is a proble, shock-heating is the same problem in the same temperature range.
It may be in the same temp range eventually but heating rate (dT/dt) during climb is a lot slower, no doubt partly because you have the airflow stealing a lot of the heat generated.
There was a report from a parachuting club whose Cessna 182 suffered from frequent cylinder cracks. They flew full power, low airspeed (Vy) all the way up to drop altitude, quickly dropped the jumpers, closed the throttle and glided at high speed and low power down to pick up the next jumpers. Then they changed the procedure, slightly. When reaching jumping altitude, they started reducing power gradually to some moderate value (cruise power) and flew level at one minut – much longer than the time for the three jumpers to jump. At the end of that one minut, the pilot did just as before, snapped the throttle closed and dived down at high speed. The new procedure all but eliminated the cracked cylinder problem. This is consistent with the theory that shock cooling can be a problem when cooling from very high CHT’s, but not at moderate temps.
It was in a magazine many years ago – I’ll come back if I find it.
