Steve6443 wrote:
I thought idling turbos was supposed to allow the turbos to slowly run down, meaning that when the motor is switched off, the turbo isn’t turning that fast without oil to lubricate it. [snip]
Slow-down is a complete non-issue. It takes seconds, if that long – basically the “turbo lag” when adjusting power is the time for te charger to spin up or down. It will be spinning at whatever RPM is appropriate for the exhaust flow at idle well before you are through the shutdown checklist.
Steve6443 wrote:
A few seconds later, the turbo had cooked itself…. at least that was the explanation of the BMW mechanic…..
If the oil flow stops, and the turbo wheel and case are still very hot, it will burn/coke the oil.
So you want to shut down the engine when the turbocharger is cool enough. As mentioned above, depending on the engine, approach and taxi is sufficient, while an immediate shutdown after an extended run-up is definitely a bad idea. Different types will vary, depending on how well the TC case is colled, what material it is made of, etc.
Question for the engineers here: Wouldn’t a turbo temperature gauge be a simple tool to figure out when it is ok to shut down? That would prevent a lot of needles noise from idling engines… although in the DA42, where you sit on the ground for minutes on end to warm the things up before flight, this might not make much of a difference…
There is no such thing as a turbo temperature gauge. One might look at it in the lab, but not in operations.
If an rpm gauge was installed, one could see, as you perfectly explained above, that taxi time is more than enough to slow it down. Approach and taxi are low power situations, the turbo is not spinning vary fast to begin with. Pumping air against a closed throttle with very little exhaust gases coming the other side makes for a very efficient brake. If the turbo was hot, it will have given off all its heat to passing gases, and it can take whatever is left by design.
But those are a little pricey to get an accurate reading.
Interesting discussion. My POH says 5 minutes idle and my A&P says TIT below 800 at shutdown. The latter actually requires that I keep the fuel boost pump running otherwise TIT does not go that low.
I definitely believe in the concept that the innards of the turbo must be cooled down enough to avoid coking oil after shut down. Makes sense to me.
It seems to be a very different issue to the question of how fast the turbo is spinning and for how long.
It is really how fast this 14Kg heavy device will lose enough heat so that the oil that ends up in contact with the hot areas will not be coked.
Arne wrote:
There is no such thing as a turbo temperature gauge.
Why not? The turbo wheel temperature itself is not what we need to know, it’s the temperature of the bearing or wherever the thing is hottest. Wouldn’t a thermocouple in a little hole at least give an indication? Of course it will not be the exact temperature of anything important, but surely it would be possible to establish that if that TCT (“Turbo Casing Temperature”) is lower than X degrees, it is safe to shut down…
I let the turbo cool down per mr Continental’s instructions. The oil spillage seems to be due to the scavenge pump not scavenging enough at lower revs/idle. Thanks for your views!
Last week the MP reached just 33" where I should see 36" MP. I took the Mooner to my maintenance shop with this complaint and it took them 9.5 billable hours to come to the conclusion that the wastegate ‘may’ be at fault. So they shipped it over to Main Turbo (Westcoast US) to have it sorted. I suggested also to ship the controller to Main Turbo, which they did. The fault was with the controller, not with the wastegate. The articles by the hand of Mike Busch proved to be extremely helpful to get my knowledge up in order to understand what’s going on. I contacted Main Turbo myself, they told me what was wrong suggested a fix, quoted a price and I sent over my CC details and they shipped my overhauled wastegate and controller back (all within a week, nice). My maintenance shop told me off. They said I wasn’t suppose to do that. I said I paid the bills so I’ll decide that for myself. Is this normal practice for a reputable maintenance shop? Does a half competent mechanic need this much time to diagnose this problem?
I know a TB21 owner whose plane was grounded for 6 months while the “main dealer” was trying to fix his turbo wastegate. Lots of similar stories. Most GA maintenance companies don’t even know where to start looking. So, yeah, competence in these areas in Europe is to be found in, shall we say, narrow circles 
This is probably because of the reduced utility value of high performance IFR tourers (due to limited airport opening hours, etc, etc) so not many of them are flying “seriously” in Europe, and this is reflected in a lot more expertise in the US scene. In the SR22 sphere, most owners I know take it to a Cirrus dealer and they probably do know how to fix the SR22 engine.
Bobo wrote:
Does a half competent mechanic need this much time to diagnose this problem?
I’m assuming you’re referring to the 9.5H of troubleshooting or does that include removing the wastegate & controller as well ?
Troubleshooting turbo problems is often a trial-by-error exercise that may involve multiple test run-ups and flights.
So to answer your question: no 9.5 h does not sound outrageous, particularly if it includes R&Ring multiple components.
The OTHER solution is to overhaul everything from the get go and hope it sorts the problem out !
It’s a difficult call that Mx Techs need to make almost daily: Spend the time and do a very thorough & analytical testing session OR just replace anything that MIGHT be the culprit.
Needless to say, the latter is most often than not the most onerous.
Well, the Mooney has been grounded for five weeks now. The overhauled wastegate and controller didn’t do the trick. Last week the maintenance shop called me saying they had it sorted, only to discover that during my testflight the MAP dropped again after rotation and before passing 1.500 ft I was back at 26" with wide open throttle. After that flight I’ve made a few testruns with the engine warm and just warmed up enough, but it didn’t make any difference. I thought maybe the oil temp had something to do with it, I guess it doesn’t.
The engine delivers 36" of MAP when the wastegate is closed manually, so it appears that the automatic wastegate controller is not doing it’s job. It was suggested to me however that when my TSIO360 turns itself into a normally aspirated IO360 it should have around 30" of MAP at a pressure altitude around sealevel. So that could suggest some kind of blockage? My mechanic is running out of options, he e-mailed Mooney in Texas for advice, but I’m not holding my breath. Gary Main (Main Turbo) is also in the dark. They all use the same manuals anyway. Why is this so difficult to diagnose?
Is this system so complicated, I found this diagram very helpful!
Is the maintenace shop a Mooney Service Center in your country?
No it’s not. They do service a few other Mooneys and ‘high performance’ piston singles and twins. They have served me well over the past.
