"Shock cooling"

[Learjetter]

Fellas, I searched and didn't find an answer: what is "shock cooling", why is it a bad thing, and how does one avoid it? Finally, how is shock cooling different from engine start/runup--isn't that shock heating?

Haven't flown pistons in years...your insight appreciated!

[WebMaster]

Not a problem with Turbines.
If you are cooking along at FL040, it's not a problem. If you are cooking along at FL180, it can be.
Here's what happens.
The engine is at a temp, say 350. Then you descend quickly, and the temp RAPIDLY decreases, to say 180. That sudden decrease in temp to the cylinder head causes thermodynamic stresses, and frequently ends up in a crack.
That's why it's a bad thing.
If you come to Dayton, Dr. Brian Von Herzon will discuss shock cooling at length. It's really a problem for the 520's (which he happens to own) which normally operate at altitude, and then are subjected to slam dunk approaches by ATC.

[Learjetter]

Thanks Larry--I hope to make it to Dayton--even if only for a few hours one of the days.

Not to be argumentative, I get it that you need to reduce power to keep speed under control in the descent. But on engine start, let's say the motor's at 80 deg ambient temp, right after start it jumps to , say, 300+, right? That's tha same thermal stress you're describing, just in reverse, no?

[WebMaster]

Actually No.
If you watch the gauges, when you start an engine, you have max mixture, normally. That helps cool the engine. So, on start up, CHT and EGT are typically low. These temps gradually increase, which is why you want to warm up the engines, before take off. When you are at cruise, you have the mixture leaned out, and that makes the engine more efficient, and raises CHT and EGT.
Hal Stoen has some discussion on this. http://www.stoenworks.com/Aviation%20home%20page.html

http://www.stoenworks.com/Tutorials/...res%20and.html

[Learjetter]

Good stuff, Larry. Thanks!

[Red Air Rambo]

If you are getting a T or P model the speedbrakes really help getting down from alt without pulling the engines back too far...I think the rule of thumb is 2 inches of manifold pressure every 2 minutes.

Brent

[Learjetter]

Brent, thanks...the mission calls for Turbo so a T or P (or 310 HP Riley!) is what I'm looking for...some mountainous routes & airstrips...so speed brakes and gear door mods are also high on my list of wants...

[Red Air Rambo]

Originally Posted by Learjetter

Brent, thanks...the mission calls for Turbo so a T or P (or 310 HP Riley!) is what I'm looking for...some mountainous routes & airstrips...so speed brakes and gear door mods are also high on my list of wants...

Why the gear door mods?

[Learjetter]

Originally Posted by Red Air Rambo

Why the gear door mods?

I thought I read (somewhere on this site, or on the rtaerospace site) that the gear doors opening creates a significant drag penalty...so I think a mod that eliminates that adds an extra layer of safety (especially during an engine failure after takeoff). Also, if I recall correctly, getting rid of the doors simplifies the gear system a little (making it easier to maintain). Www.rtaerospace.com still advertises the STC for about $4G + labor...

[Red Air Rambo]

Originally Posted by Learjetter

I thought I read (somewhere on this site, or on the rtaerospace site) that the gear doors opening creates a significant drag penalty...so I think a mod that eliminates that adds an extra layer of safety (especially during an engine failure after takeoff). Also, if I recall correctly, getting rid of the doors simplifies the gear system a little (making it easier to maintain). Www.rtaerospace.com still advertises the STC for about $4G + labor...

Ok....I can understand that and I asked the same question to a 210 pilot I know years ago and his response was "just maintain it". Now does it make it safer....I'd say yes since you are removing components so less to go wrong. However, It does look cool not done since a million miracles are happening....but is it worth the price?...I don't know that one either. Others will have to chime in on that one. As for drag penalty if everything works right I was tought -240 fpm for up to 8 seconds...in the sim it was a non-event above blue line.

When I was shopping it would have narrowed the field down 2 small since I could not find any that had that done....My advice is concentrate on the big mods and if you need the smaller add them later...just my 2 cents.

Brent

[JeffAxel]

big difference between the drag from the gear doors on a P210 and the doors on a P337. I probably wouldn't take the doors off a 210, but would consider it on a P337 if you fly in marginal situations a lot. That said, if you don't cycle the gear until you have altitude to loose, it shouldn't be a problem either way. The plane will climb okay (200fpm or so) on one engine with the gear down and the doors closed.

[sns3guppy]

the mission calls for Turbo so a T or P (or 310 HP Riley!) is what I'm looking for...some mountainous routes & airstrips...so speed brakes and gear door mods are also high on my list of wants...

Why speed brakes? Shock cooling is a debatable subject (and the source of much "hangar flying,"), but is irrelevant if proper power management is used.

One shouldn't try to fly a piston airplane like a turbojet, making idle descents and disregarding proper use and treatment of the piston engine, but the piston engine isn't a sacred cow, either.

Generally, if you keep adequate power on the motor such that the engine is driving the propeller and not the other way around, you'll be carrying enough power in the descent. A common rule of thumb is that one shouldn't retard power below the bottom of the green arc on the tachomoeter (about 15" of manifold pressure, for most light airplanes), but one may also want to carry considerably more than that during an enroute descent for thermal reasons and for speed.

I've heard some argue in the past that whereas a 3:1 descent in a turbojet is the norm, a 6:1 descent in a turbocharged piston twin is a better idea. I disagree. Especially if one is cruising at any respectable altitude, trying to descend slowly over a 6:1 plan isn't practical in most places and will only end up pissing off controllers and creating a flying roadblock. Instead, a normal 3:1 descent with a little forethought and planning works wonders.

I've seen a lot of skymaster motors go right to TBO with people doing idle descents in them, however, and those were turbocharged models. In fact, I've seen idle, dirty, diving descents to landing used as a normal part of the daily routine by an operator that rarely sees anything but motors lasting to TBO. If shock cooling were indeed an issue with those airlplanes, one might have expected to see parts raining down off the runway end, but not so. Such operation was never my first choice, but the TSIO-360's in the Skymasters are surprisingly tough, versatile engines, and not the glass wrist-charms that some might think them to be.

I'll also add that they run extremely well lean of peak, though in my opinion that's best reserved for operations at or below barometric pressure, and not in the boosted range of operations (especially on hot days, and especially during a climb).

Mixture management is an important part of power management. I was once told by a former military aviator that he had just experienced an engine failure in his skymaster. When I asked how he knew he had an engine failure, he told me that he was in cruise in the "jet" (this is how he referred t the skymaster), and stated that he saw the manifold pressure "roll back to zero," and that this is how he knew he had experienced an turbocharger failure, and therefore an engine failure. He stated that the aircraft lost 300 feet, and there was the proof.

I was somewhat bewildered by the accounting, as manifold pressure can't roll to zero, nor would a turbocharger failure result in an engine failure (except in catastrophic circumstances which become immediately apparent), nor would a turbocharger failure result in what he experienced, or thought he had experienced. I had never heard of a 300' altitude loss representing an engine failure, but he was absolutely convinced that he'd experienced an engine failure.

I was asked to investigate the matter, consult with the mechanics assigned to the airplane, and then to ferry the airplane a short distance for an inspection. Setting aside the notion that ferrying an airplane with a failed engine might not be the brightest act of the day, I sought out the mechanics associated with the airplane, and these rocket scientists informed me that the airplane had a failed turbo, because the manifold pressure had been reported to roll back to zero.

I tried explaining to them the concept of barometric pressure and that manifold pressure couldn't have gone below that with a failed engine, but they didn't seem to grasp the point. I had a fairly good idea what really happened, so I loaded up both mechanics, and taxied to a run-up area. Exactly as predicted, the airplane started and taxied fine, but had roughness on one mag, and this was easily cleared up during a power run. The ex-fighter pilot had been flying with the mixture rich all the time, fowled the plugs, and had a rough engine and power loss. The manifold pressure never rolled to zero; it was the action he thought he saw because it was what he expected based on his turbojet experience; he saw what he wanted to see. Not at all an uncommon experience.

The point there is that mixture management is just as important as power settings when it comes to issues of climb and descent, and this also applies to the question of "shock cooling."

Continued (due to length)...

[sns3guppy]

...(continued)

When you fly into cold rain, you're doing a lot more to cause cooling issues, especially on the surfaces exposed to the rain, than you are by pulling the power back on a descent. When you land and shut down the engine, the engine temperature climbs, with a lack of cooling airflow, and a lack of oil circulation. Some parts begin to cool, while other parts continue to heat up. When you apply power for takeoff, a significant thermal change occurs. One doesn't normally expect to see cracking during these times, but it can rarely happen. Likewise, pulling the power to idle at altitude is poor practice, but one shouldn't normally expect one's engine to begin cracking and falling apart. Thermal abuse of the engine, however, is to be avoided, and it's always a good idea to baby the engine to some extent.

There are strong arguments to be made against partial power climbs and operation at too low power settings, but there's also no need to do idle descents if one plans ahead. There's no need to worry about speed brakes in the skymaster; it's draggy enough as-is. The airplane is one big speed brake. It's got two propellers to do the job, and struts, and if you need it, the gear does very well, too. If you're worried about shock cooling and plan to carry power while extending speed brakes, it's a noble thought, but baseless in reason. Save your money.

In a large turbofan airplane, retarding power at the wrong time or adding power at the wrong time can cause fan case rub. Pulling back power while nosing over into a descent, for example, can cause fan case rub and deterioration of the rub strip and the fan disc. Retarding power initially, losing a little speed, then making it up by dropping the nose makes more sense, though it's not commonly taught insofar as I've seen. Likewise, in the piston airplane, retarding power slightly before beginning the descent, easing over into a gentle descent, then eeking the power back a little at a time once established in the descent, makes sense, and is good airmanship and power management in a turbocharged piston airplane.

If you're used to descending in the flight levels, you know that in a turbojet airplane a descend at lower altitudes, say FL270 and below, can usually easily be done at idle and as a straight forward descent. At higher altitudes, especially approaching FL410 and above, one generally just makes small power reductions, starts with a small rate of descent, and comes down maintaining one's mach number. The initial part of the descent is a ginger one with respect to descent rate, speed, and power.

A similar approach can be taken to descents in piston airplanes; reduce the power a minute or two before beginning the descent, then start down and recover the speed. A minute or two later reduce power a little more, and increase descent accordingly to maintain speed. With a little finesse, there's only a small incremental change occurring thermally. In this way one can keep speed up throughout the descent, then leave the power alone when preparing to set up for the approach in the terminal area. Level off, the airplane slows to the desired speed without having to monkey with the power, and as the airplane naturally slows, the cooling airflow decreases, the engine begins to warm somewhat, and one can begin configuring for the terminal area arrival and approach/landing.

No need to throw out speed brakes.

Barometric is the manifold pressure setting that corresponds to the altitude at which one is operating. At sea level with the engine shut off, read barometric pressure on the manifold pressure gauge; it's close to 30" just as the altimeter is 29.92 at sea level of a standard day. At five thousand feet, no power on the engine, sitting on the ramp, the manifold pressure gauge reads about twenty five inches of mercury. This is barometric. At 10,000', barometric is 20".

Aside from the general rules of reducing power an inch to two inches per thousand feet or per minute, try to abide by the rule of thumb of keeping power at barometric or better, until needed lower for work in the terminal area (landing). This doesn't mean the initial power reduction should be right to barometric depending on one's cruise situation, one may be operating well below barometric from the outset, and may need nothing more than dropping the nose and starting the descent. Convsersely, operating at a high power setting, one needn't pull power back to barometric initially, but should make a reduction of an inch or a few inches before starting a gentle descent.

If you happen to be flying a geared engine, there are good reasons to keep the power up and keep the engine driving the propeller, because despite what some will tell you to the contrary, allowing the propeller to drive the engine through the reduction gears can lead to a very early demise in your engine. The issue is not as critical with non-geared engines, and is hotly debated by some, but it's still a good operating practice to keep your power up high enough that the slipstream doesn't end up driving the engine through the propeller; keep the engine driving the prop, instead. If your power is high enough to do this, and you aren't making large, rapid power changes, then you're in good shape.

Another good habit to get into when landing is not to through the propeller forward. Too often I see people run a checklist on the downwind or on final, and when they get to propeller, they push the prop all the way forward. The engine wraps up in speed and screams, a surge is heard, and the engine has pointlessly been abused. Wait until the power is pulled back far enough that an RPM increase won't occur, before advancing the propeller.

Once on the ground, don't be in a big hurry to shut down. This is as true of turbojet airplanes as turbocharged airplanes. Give at least five minutes between reaching idle power, and shutdown. If you have to sit in the tie downs or sit at the end of the runway (off the runway, obviously) after landing, then do it; keep the oil circulating through the engine and turbo bearings while they cool, before shutting down. Shutting down too soon is a great way to coke the turbo bearings and ruin the turbocharger, as well as the engine. Don't do that.

If you're trying to slide into Aspen, Colorado, you might benefit from the speed brakes. Otherwise, for the most part they'll simply be crutches for poor power management, and are unnecessary. You can save money and do without the brakes, just fine, by planning ahead.

[Roger]

Originally Posted by sns3guppy

I was once told by a former military aviator that he had just experienced an engine failure in his skymaster. When I asked how he knew he had an engine failure, he told me that he was in cruise in the "jet" (this is how he referred t the skymaster), and stated that he saw the manifold pressure "roll back to zero," and that this is how he knew he had experienced an turbocharger failure, and therefore an engine failure. He stated that the aircraft lost 300 feet, and there was the proof.

I was somewhat bewildered by the accounting, as manifold pressure can't roll to zero, nor would a turbocharger failure result in an engine failure (except in catastrophic circumstances which become immediately apparent), nor would a turbocharger failure result in what he experienced, or thought he had experienced. I had never heard of a 300' altitude loss representing an engine failure, but he was absolutely convinced that he'd experienced an engine failure.

I was asked to investigate the matter, consult with the mechanics assigned to the airplane, and then to ferry the airplane a short distance for an inspection. Setting aside the notion that ferrying an airplane with a failed engine might not be the brightest act of the day, I sought out the mechanics associated with the airplane, and these rocket scientists informed me that the airplane had a failed turbo, because the manifold pressure had been reported to roll back to zero.

I tried explaining to them the concept of barometric pressure and that manifold pressure couldn't have gone below that with a failed engine, but they didn't seem to grasp the point. I had a fairly good idea what really happened, so I loaded up both mechanics, and taxied to a run-up area. Exactly as predicted, the airplane started and taxied fine, but had roughness on one mag, and this was easily cleared up during a power run. The ex-fighter pilot had been flying with the mixture rich all the time, fowled the plugs, and had a rough engine and power loss. The manifold pressure never rolled to zero; it was the action he thought he saw because it was what he expected based on his turbojet experience; he saw what he wanted to see. Not at all an uncommon experience.

Perhaps you should have considered that the aircraft most likely experienced partial or complete induciton icing (or some other transient induction/butterfly obstruction) which created Compression Braking due to the spinning prop. This would have drawn down the MP to a negligible reading, i.e. zero +/- as well as caused a decrease in thrust and the subsequent loss of altitude.

The fact that someone misinterpreted the event as an turbo failure would be consisent with a non-mechanic, but to assume that the MP can't roll back to virtually nothing is I beleive an errant review of the possible events.

[sns3guppy]

Perhaps you should have considered that the aircraft most likely experienced partial or complete induciton icing (or some other transient induction/butterfly obstruction) which created Compression Braking due to the spinning prop. This would have drawn down the MP to a negligible reading, i.e. zero +/- as well as caused a decrease in thrust and the subsequent loss of altitude.

I did consider that possibility. Given the lack of visible moisture, time of year, desert location, and impossibility of induction ice in the injected airplane under those conditions, I discounted that possibility. However were the induction to be partially blocked, manifold pressure still would not roll to zero. Piston engines 101:

Manifold pressure is a function of a)ambient air pressure, or current real barometric pressure, or b) boosted pressure by turbocharging, supercharging, or both, or c) suction by the engine which has drawn manifold pressure down to a lesser value.

While it's not done with most horizontally opposed recip engines these days, part of the checking on a piston engine should be idle manifold pressure checks. Most are, or should be, familiar with idle mixture checks, and idle RPM checks, idle manifold pressure is part of the equation. Most engines cannot drop below about 12" of manifold pressure except for a complete occlusion. In such cases, the engine can't continue to run, and still won't show zero inches of manifold pressure. In fact, in cases I've seen in which the induction manifold failed, or in cases of icing occlusions, I've never seen anywhere remotely close to zero manifold pressure. I've seen manifolds collapse and induction doors and alternate air doors come loose and plug the manifold, and still it doesn't drop that low.

What the pilot saw wasn't what he actually viewed; he saw what he expected to see, which was a manifold pressure rolling back to zero. In truth, whereas a turbojet aircraft (or the advanced tactical fighter he had been flying previous to this airplane) don't have manifold pressure gauges, but instead deal with other parameters such as EPR, various spool RPM's, and turbine interstage temperatures or exhaust gas temperatures. He saw what he thought he expected to see, not what really happened.

The fact that someone misinterpreted the event as an turbo failure would be consisent with a non-mechanic, but to assume that the MP can't roll back to virtually nothing is I beleive an errant review of the possible events.

The problem is that the two individuals who interpreted the event as a turbocharger failure were both mechanics, assigned to the airplane. The individual who was flying interpreted the event as an engine failure. None of the three individuals were correct. The event was a partial power loss due to improper use of the mixture, and subsequent spark plug fouling.

The "possible events" as I described them are the actual events, as I not only interviewed each person involved, but examined the aircraft, ran it up, and flew it. I knew exactly what had happened.

The tendency for pilots with advanced aircraft experience to underestimate light aircraft is a dangerous and all too common one. Some months prior to the event above, an individual had a double engine failure. He lost power on one engine, switched tanks, and shortly thereafter lost power on the other. No checklists were used, before, during, or after the flight. When I queried the matter, I found that the pilot was insistent that the mechanics has "misrouted" his fuel. I took that to indicate that he thought the mechanics had somehow routed his fuel lines improperly. I checked with the mechanics; they'd done nothing to his fuel system, other than move the fuel tank selctor valves during maintenance.

Checking with the pilot, and his superior I learned that neither used the checklist for preflight, before takeoff, takeoff, after takeoff, cruise, descent and approach, landing, after landing, or parking. No checklist use. After all, they said, it's a light airplane. They were experienced in far more complex and demanding aircraft; these airplanes were like toys to them.

What actually happened was that the fuel selector positions weren't checked prior to takeoff. One engine was run dry, and shortly thereafter, for reasons unknown, the individual switched to the dry tank and killed both engines. He landed without power. He sharply blamed the mechanics for moving the fuel selectors, but placed no blame on himself for failing to use the checklist or verify his fuel routing before takeoff.

The light airplane can kill you, but just barely. So they say.