January 3, 2014 at 11:21 am #84720
I have a question for the engine experts here regarding shock cooling and harsh power reductions.
I know the subject of “Shock Cooling” has been beaten to death for some years now and know there is an overwhelming amount of data showing that cylinder head cracking as a result of what is known as “shock cooling”, is nothing more than an OWT (Old-Wives-Tale). There are several good articles out there showing the effect of rapid power reductions on CHT’s and EGT’s and the lack of correlation to cylinder head cracking or engine damage – some of it scientific, a lot of it anecdotal. I have also read John Deakin’s 6 part article on turbos, George Braly’s discussions on GAMI’s webpage and Kas Thomas articles on the subject. Nonetheless, I try to be disciplined in my energy management on descent and avoid rapid changes in power (up or down) or letting the props drive the engines which includes the use of spoilers and adequate planning. RAM also recommends being kind to our engines which includes gradual power reductions and letting the turbo cool-off before shutting down.
Having said this, I recently bumped into a fellow from San Luis Potosi, Mexico at a maintenance shop in San Antonio. The fellow is not a pilot but owns a 2008 Piper Malibu Mirage and has had the same pilot from day one who is an ex-corporate jet jockey with most of his thousands of hours in turbine powered aircraft. The owner of the Malibu was upset as his plane was in to have the third turbo replaced in less than 500 hours since he purchased the airplane. Odd if not alarming.
Not being a pilot, I did not expect him to know too much about aircraft operation, but I asked him if he had noticed how the engine was being managed. I was interested in how power was being applied and reduced. Take-off power, from his perspective, was applied smoothly – of course smoothly to him could be too fast for any of us – who knows. For power reduction, this gentlemen told me the pilot would just chop the power and dive for the airport – normal operation in a jet. Knowing the terrain, MEA’s, MSA’s and how ATC works in Mexico, this chopping of the power has to be fairly radical leading to rapid changes in TIT as they often fly you in high into your destination, and clear you down for landing from 8-10K above and 20 miles out.
Question is, could this be causing the turbo to fail prematurely? While shock cooling is a myth, could repeated cooling of a turbo 500F in 1 or 2 minutes (my assumption) be the culprit or is this just bad luck and/or poor QC from the turbo manufacturer?January 3, 2014 at 2:06 pm #102450
My feeling is that shock cooling might be a half myth item, but likely more one of those things like what Rick said in Casablanca. “You’ll regret it. Maybe not today, maybe not tomorrow, but soon and for the rest of your life.” In other words, I won’t expect an engine failure, but that in combination with other aspects of how you treat your engine may impact your maintenance and reliability issues. This fellow with a Malibu might be a good example, but I’ve also heard that some of these Lycoming turbos don’t last as well as Continentals. I’m not sure why, but anecdotally it seems to be true for T182s and Malibus. Malibus also use small turbos that are worked hard, and if he’s leaning to the max of 1750F TIT allowed (which is a terrible idea), that won’t help him any, either.January 3, 2014 at 2:58 pm #102451
Now that’s red hot!!! …and if in addition to that you chop the power…
That’s odd that Lycoming would recommend a FF that results in such a high TIT. You’ve said enough Tim. I think it all makes sense, and I can see this being a common theme with Mirages.January 3, 2014 at 4:13 pm #102454
The TIO-540-AE2A engine has a TIT probe in an unconventional location that is supposed to produce a roughly equal approximation for the two turbos. But basically, the 1750F number was a terrible idea and Piper wanted it to make their marketing numbers.
The last number I heard was that 10% of the Malibu fleet has had an engine failure.January 3, 2014 at 5:05 pm #102457
The small twin turbo’s used on some engines are very high speed. Cessna went with a single heavy turbo mounted to the airframe. The engine to airframe flex mounts are a source of trouble – but the turbo itself is excellent. Would not expect any significant issues with power cycling with the turbo on a Cessna.
I think the greatest risk of shock to the high power continentials is to the crank counterweights, connecting rods, etc. fast rpm changes put a whole range of stresses on the engine including the case.
On descent, I just keep the CHT’s in the green and I don’t change mixture.
500 hrs on three Malibu turbos sounds horrible. I would expect there is also something wrong with the setup (fuel flows, boost, etc). Even with poor technique a turbo should last longer than 170 hrs. What altitude are they cruising at too?
Turbo’s work the hardest in the FL’s. I would expect this pilot also has issues in the oil scavenge system (which would easily kill a turbo in short order). If they are just replacing turbo’s and not checking the rest of the system I hope he is budgeting for a 4th turbo….January 3, 2014 at 7:15 pm #102460
If you do what Piper wants with a Malibu, it’s fly high, run lean (hot), and run hard (nuclear). You’re right the small turbos used in the Malibu are more prone to heat and failure, too.
3 in 500 hours is really bad, but it seems like 500 hours isn’t uncommon on those planes. Again, comes down to how it’s run. Piper wanted a very efficient airframe, and they got it. They also got the repercussions of some of the design choices.January 3, 2014 at 8:18 pm #102465quote EPANNING:
I have no idea – I met this guy while getting some work at KSAT and he seemed upset (I would be too). It was a casual conversation with someone I really don’t know, but I was stunned when he told me he was in for his 3rd turbo in 500 hours! It looks more like an average of 250 hours between turbos (he was having the 3rd one installed), which is still horrible. Being from MMSP, they are flying the airplane in Mexico which means flying above FL200 if on instruments (probably). With everything you mention about the Malibu you add to the plate the fact that the pilot chops the power and take’s a dive for the airport, it doesn’t sound like they are being very kind to the engine. As you say, he may have to budget for a 4th turbo and maybe a crank-case….quote TDUPUIS:
Not surprising…January 3, 2014 at 8:59 pm #102467
I haven’t heard of wide-deck Lycoming cases cracking. I’m sure it happens, but it’s rare. The narrow-deck cases were the ones that cracked regularly. Turbos are a problem, and even on a Lycoming, cylinders are if you treat your engine that way. If you’re kind, though, the reliability isn’t bad.
I’ve spent about 3,000 hours running TIO-540-AE2As, and the things are very durable when treated properly.January 4, 2014 at 7:00 am #102478rwelshParticipant
Eric, Cessna changed their MO when they built the 303. They mounted the heavy turbo on two small rubber Lord mounts which tend to sag after a 100 hours or so which allows the one exhaust pipe to bang on the engine mount. This is a weak spot on the 303s along with an engine mount that has wall tubing that is too thin and tends to crack. The 303 has an engine mount like a Piper; made up of welded tubes and four dynafocal mounts on the back of the engine.January 4, 2014 at 7:30 am #102479
Dick, interesting details on the 303. Thanks for sharing. The amount of weight suspended off the engine beams on the other models is impressive.
EricJanuary 4, 2014 at 5:00 pm #102485
See inside cover of latest TC Flyer for Ram’s comments on “Shock Heating”. Basically on takeoff give the engines time to heat up before full power by using an intermediate power setting and letting it stabilize.
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