Sport Pilot Talk

How prone or resistant is the Rotax 912 ULS to shock cooling on descent?

designrs wrote:How prone or resistant is the Rotax 912 ULS to shock cooling on descent?

There are those who will tell you that "shock cooling" is an old wive's tale, even with more conventional engines.

With respect to the Rotax, the fact that it's water cooled should completely eliminate any fear of "shock cooling" .

Say you do a rapid descent of 4,000 feet at 1,200 ft/min with the power pulled back in colder air around 45 F OAT? Not a good idea of course, but can the Rotax with the oil / water cooling take that?

"There are those who will tell you that "shock cooling" is an old wive's tale, even with more conventional engines."
Perhaps that's the perception among some but, once you have full CHT/EGT instrumentation, the cooling rate seen by an engine e.g. during descent becomes observed fact. The issue is whether it's harmful - e.g. leads to cylinder head cracking - or not. With a partially liquid cooled engine, I would expect that consequence - to the extent it exists - is minimized.

Here's a classic article supporting the notion that shock cooling is a myth: http://www.avweb.com/news/maint/182883- ... directed=1

Nevertheless, I refuse to ignore Lycoming's caution - don't allow CHT cooling rate to rise above 50F - because they designed and built my engine, including the cylinders manufactured only a few years ago. What does Rotax say on the subject?

Rotax does not caution against shock cooling.
They do caution about over temps (obviously) and bank angles of 40 degrees (Surprising… I though it would be good to at least 45 degrees bank).

However, from the SportCruiser / PiperSport aircraft POH:

"CAUTION
Rapid engine cooling should be avoided during operation. This happens above all during aircraft descent, taxiing and low engine [rpm] or at engine shutdown immediately after landing.
Under normal conditions the engine temperatures stabilize during descent, taxiing and at values suitable to stop engine by switching the ignition off. If necessary, cool the engine at idle [rpm] to stabilize the temperatures prior to engine shut down."

Of course the bottom line is you have to watch your temps.
I was just looking from some additional input, perhaps also from our Rotax mechanics based on cases that they might have seen.

The 40 degree limit on bank angle is a new one on me. I suspect it's widely ignored in training service while doing steep turns. I've probably done over a hundred in mine, and I'm sure I've gone as far as 55-60 degrees briefly.

Wouldn't that 40 degree bank have to be uncoordinated before the engine 'knows' its banked?

CharlieTango wrote:Wouldn't that 40 degree bank have to be uncoordinated before the engine 'knows' its banked?

This has come up before. It's a weird one. Especially since steep turns are a standard training maneuver.

It MUST have been a limitation discovered on a test stand.

You're 100% right that as far as the engine goes, in a coordinated turn its still as if in level flight, albeit the engine would be "heavier".

FastEddieB wrote:. . . 'in a coordinated turn its still as if in level flight, albeit the engine would be "heavier'." . . .

As is the oil. . .

Wm.Ince wrote:

FastEddieB wrote:. . . 'in a coordinated turn its still as if in level flight, albeit the engine would be "heavier'." . . .

As is the oil. . .

Good point.

Is it even remotely possible that the concern is that oil weighing 1.4x it's normal "weight " in a 45° banked turn would be an issue?

I rule nothing out, but that sounds highly improbable. An aircraft engine that cannot be banked to 45° would be sorely limited.

You know, as much as POH and manufacturer literature is held as gospel, in some cases you really have to sort out reality from liability paranoia (not saying that is the case here, nor am I suggesting that we become engineers or test pilots)... but do your homework so to speak.

I have noticed that my Rotax, which runs on the cool side anyway (thankfully, as I'm in Georgia!) cools down *right quick* in a power off descent from any real altitude. I have seen the temps drop from my normal ~180-220F to 150ish -- quite a difference. I assume that as long as it does not drop below the 140F red line marking the engine is warm enough for takeoff, it's probably okay. I don't do those kinds of descents often, so it's not like I'm running it through brutal heating/cooling cycles all the time.

It's only mid-October, and already my engine rarely gets much above 190F...I am going to have to break out the radiator tape soon.

MrMorden wrote: I assume that as long as it does not drop below the 140F red line marking the engine is warm enough for takeoff, it's probably okay.

We're talking oil temperature, right? Where do you get a 140 redline? The Rotax manual says 50 Celsius minimum, which translates to 122F.

drseti wrote:

MrMorden wrote: I assume that as long as it does not drop below the 140F red line marking the engine is warm enough for takeoff, it's probably okay.

We're talking oil temperature, right? Where do you get a 140 redline? The Rotax manual says 50 Celsius minimum, which translates to 122F.

My factory oil temp gauge has a red line at around 140F. It might not be exactly on the 140 line, and could be a little lower. I was told that temps above that line are acceptable for takeoff. Maybe Flight Design likes to see higher temps than Rotax?

The CTSW POH says 124

http://documents.flightdesignusa.com/SW-POH.pdf

page 8.3