Closing the throttle in flight - nicely (shock cooling)

Snoopy wrote:

Shock cooling is a myth

Well, fly the plane you own, the way you like. Fly everyone else’s planes as though care for their engine has some importance.

The referenced article makes a number of unsupported statements, who knows if they are true? I have personally changed cylinders, many of them. Someone suggests that it’s less costly to change cylinders than to operate the engine with care (and take longer in the air) – well, wait ’till you pull a stud installing the next cylinder, and now have to remove and split the engine for crank case repairs – which I have also done!

I worked for a year disassembling and inspecting incoming engines for the engine overhaul shop. Both of the engine shop co owners were pilots, and I flew their planes. They were very clear as to how to operate their engines, and why, including preventing shock cooling by good engine handling techniques! I’ve seen the cracks myself, and with very well instrumented engines, I seen the alarmingly fast temperature changes resulting from rapidly closing the throttle.

I have never had a cracked cylinder in a engine I have owned (3800 hours of operation). I bought a 172 with an O-300, and tore down the engine. Gee, the two rear cylinders were scrapped for cracks.

So suggesting here that pilots need not exercise care for someone else’s engine is unhelpful. Shock cooling is a real risk, and the article which Peter links relates well to experience I was aware of when I flew jumpers. I trained the other pilots as to how to not shock cool, and they went a season with two 185’s, and no cracked cylinders for the first time! Sure, one can go overboard in being cautious, everything in moderation. If you must close the throttle quickly, do the engine a favour, and also slow the plane as much as practical to reduce cooling.

Shock cooling is a mythQuote

Maybe tire and brake wear are myths too? And ride and lock up brakes without worry?

Snoopy wrote:

Why make this about me @pilot_dar
Where was I suggesting anything regarding how to treat other’s planes „here“? I posted an article. That’s all.

The answer to that question is that sure, there are all sorts of articles “out there”, which may be posted/linked etc. The fact that it can be found, does not in and of itself make it valued reading, and perhaps promoting it in conflict with conservative opinions does not serve the greater interest. In “the old days”, articles were typically printed in magazines, and thus subject to some qualified review. I think that today’s freedom to publish – here, for example, allows less authoritative information to appear to be credible. Yes, AvWeb, has some credibility, and, having met Paul Bertorelli, I am generally supportive of what they “publish”. That article (not written by Paul), no.

I have a lot of experience with “other ways” to do things in airplanes, which I do not post at all. I’m concerned that others might read it, interpret it, perhaps differently, or in a different context, and an unfortunate outcome could result. A few too many times, a wise old pilot has said: “Pilot DAR, gee, I know you can do that, but someone else will see you doing it, and try it themselves. They don’t know what you’re doing to mitigate risk, and they may have a problem, how would you feel if that happened?”

My first exposure to concern with shock cooling (as it was never mentioned during my PPL training) was when I was invited to fly Piper Turbo Aztecs. There were three extremely well cared for Aztecs in the fleet, and the rule was when planning descent (particularly in the winter), a power reduction of 1" of MP per minute, use the cowl flaps effectively, and watch the CHT’s. This was as much planning for passenger comfort, as shock cooling prevention. A well planned decent prevents one having to dive for the ground, and upsetting passenger’s ears. I find 1" per minute MP to be conservative, though I have no recollection of them ever changing a cylinder either. When they sold an Aztec, and bought the Cheyenne, one of the conversion training points was “hey, it’s turbine, so shock cooling is not a risk any more! It’s pressurized, so with a well planned descent, you can just point it down, neither the engines, nor the passengers will mind!”.

I started flying jumpers in 185’s decades later, and with lots of dirty hands engine shop experience behind me, when the operator complained about frequent cracked cylinders, I did some mentoring of their pilots, and it appeared that the problem was reduced, if not eliminated.

The problem with cylinder cracking, is that unless there is only one pilot who flies the plane, it’s hard to know who to hold to account for damage found later. In fairness, even the pilot might not realized that in one rapid motion of the throttle, they’d done $6,000 damage to the engine. So training should be broad through all pilots, with a consistent message of conservatism. When I flew the company Cessna 310 for a year, I was its only pilot, so I knew that if there was a problem, I would be accountable for it. It was years later that I learned enough about the vulnerable alternator drive couplings to realize that it had been my poor engine/electrical system handling technique which had required them to be changed. I didn’t know then, as I’d not been trained that, and the company didn’t know either, so they didn’t blame me – but I was responsible, and could have handled the plane better to prevent that premature failure.

Sure, cylinders are designed to be changed, and they are not horribly expensive, compared to some engine accessories. But, the labour to change a cylinder, and the risk or ruining an exhaust or cylinder base stud makes cylinder removal very undesirable. I’d owned my 150 only a couple of months when a stuck exhaust valve meant I had to remove a cylinder. Sure enough, while reinstalling, it, and (having been warned) using a torque wrench, I pulled a stud – worse, a through stud. Now I had the cost to remove the engine, and split the case to repair that. There was no doubt that the cylinder had to be changed, so it was inevitable in that case. but I sure became motivated to do whatever I could to prevent needing to change cylinders (and I helicoiled all the studs in that case, just to be sure for next time).

When I flight test a plane, there are some things which require sudden power changes. I just cringe, and do it as gently as possible to still meet the test point. So, I extra like flight testing turbine planes, they are less sensitive.

On this topic, as any other, I will post based upon my experience, and the properly conservative way I know things should be done for safety, and mechanical sympathy with the machine. When someone reads my post, they can agree or not, but should they choose to follow what I have posted, they will be following the path of less risk, a little less adventure, and considerably less to worry about for maintenance!