DA42 VI ownership decision questions

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BrandonH

DA42 VI ownership decision questions

Post by BrandonH »

I've been reading the forum for a year and benefited enormously from the many threads and 'thinking of buying' Q&A. I also bought the John Ewing book, downloaded the factory POH etc.
I've got ~140 student pilot hours in DA40 and eventually will schedule a checkride. My assumption has been i'd buy a 40 but I've been talking myself into a 42 ever since flying one. I routinely use air taxi Cirrus for business whenever feasible, I live in Norfolk, and a lot of flying north and south is over water, and I've taken note of the navigational choices my pilots have made, chute or not, which extend flight time.

Two questions I haven't found opinions on... (Austro related)

Emergency engine out restarts need to be completed under 2 min or 'the engines may not restart.' What is with that? I have also seen the new proscription against windmill restarts, which i assume is unrelated but does anyone know the mechanical issue preventing a longer restart period?

Secondly, also to the engine: I have huge admiration for this powerplant; my wife's e250 is one of the most satisfyingly powerful yet economic cars I've ever driven. However, auto engine designs come and go. Mercedes has actually exited the US market for all diesels at the moment, unwilling to risk the PR backlash against diesels for a car that sells so few units. I assume they are still popular in Europe, haven't checked, but the bigger question is future support for the engine. Eventually MB will sunset the design as they have with all prior diesels, where does that leave the NG/Austro owner?

Thanks for any opinions, and thanks again for the great forum!
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Colin
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Re: DA42 VI ownership decision questions

Post by Colin »

No idea on the engine supply issuer. The emergency restart prohibition is not an issue, in my mind, since if you lose an engine you will fly somewhere and land. (That's no generally what happens, it seems usually you lost some power (turbo waste gate issue, manifold pressure sensor issue, etc), and either keep the engine running or do a precautionary shutdown.

Restarts are done during training or sales demos. I did it twice on my plane and after further research will not be doing another one.

The gearbox, more on the Austro than the Centurion engine, does not like being driven by the propeller rather than the engine. It makes metal. So they have told pilots to stop doing that. Actually, they have only told them to stop air starting the engine. They should probably also tell them not to pull the engine to idle and dive to lose altitude. Using the propeller as a speed brake is probably rough on the gearbox.

I learned some about this while a friend was looking at Twin Bonanzas, some of which have geared engines and some interesting operational restrictions.
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Karl
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Re: DA42 VI ownership decision questions

Post by Karl »

If the engine has stopped for any reason other than fuel (mis) management or deliberately for training, it is probably not going to start anyway.

A DA42 flies well on one engine and is much less of an issue than it was with older designs.

I understand that the Centurion engine shouldn't be air started for similar reasons.
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Re: DA42 VI ownership decision questions

Post by CFIDave »

Colin wrote:The gearbox, more on the Austro than the Centurion engine, does not like being driven by the propeller rather than the engine. It makes metal. So they have told pilots to stop doing that. Actually, they have only told them to stop air starting the engine. They should probably also tell them not to pull the engine to idle and dive to lose altitude. Using the propeller as a speed brake is probably rough on the gearbox.
My understanding is that the issue with restarts (with the propeller driving the engine rather than visa versa) is related to wear on the timing chain for the DOHC (dual overhead cam) engine -- not the gearbox. This is why the AFM/POH now recommends in-air engine restarts (often performed during OEI training) using the key (DA40/DA42) or pushbutton (DA62) starter with the prop stationary, rather than putting the plane into a shallow dive and performing a windmilling start.

The prop RPM reduction gearbox (much larger on Austro than Thielert/Centurion engines) when subject to normal wear and tear can make small amounts of metal; this could potentially cause problems with the prop governor, since the governor shares the relatively clean gearbox oil (rather than dirty engine oil full of combustion byproducts) for its operation. To address this, Austro engines have a magnetic drain plug that catches ferrous metals; this plug is inspected and cleaned at each 100 hour servicing of the engine and gearbox -- when the gear oil is run through what looks like a coffee filter before being replaced. In addition, Austro engines built in the last 5+ years also include a separate gearbox oil filter that is inspected and cleaned every 100 hours, and replaced every 300 hours. The result is that "gearboxes making metal" is not an issue.
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Re: DA42 VI ownership decision questions

Post by Karl »

It appears for the TDI, windmilling restart was removed due to a gearbox drive shaft issue.
Windmilling was reinstated when it was found to be electric start technique that was causing the problem. The electric start procedure was amended to ensure that you keep the starter engaged until the engine reaches 500 rpm.

Diamond removed the windmilling restart for the NG but give no reason why.

If it was due to Cam chain you would expect it to affect both engines as they use the same basic core.

I have an employee at the factory this week so will ask for further information. No guarantee we will get it though.
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Re: DA42 VI ownership decision questions

Post by CFIDave »

Karl wrote:Diamond removed the windmilling restart for the NG but give no reason why.

If it was due to Cam chain you would expect it to affect both engines as they use the same basic core.

I have an employee at the factory this week so will ask for further information. No guarantee we will get it though.
Austro Engines published a Mandatory Service Bulletin MSB-E4-017/2 "Check/Replacement of timing chain" that explains:
Windmilling restarts cause high stress onto the timing chain. Therefore, windmilling restarts as part of normal operation has already been removed from the Operation Manual. In addition, a mandatory timing chain replacement for engines with known in-flight restarts by wind milling has been defined.
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Re: DA42 VI ownership decision questions

Post by Colin »

That seems rather rude to say, "Hey, you did it the way we were telling you do to it, now you have to replace this hundred dollar part, mandatory."

I'm guessing on the price. I might be a little low.
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Re: DA42 VI ownership decision questions

Post by Karl »

Interesting.

Do Continental Diesel and Austro fit their own cam chain?

If it is the origional Merc part I would have thought it gets a lot more stress in a car from engine braking and boy racer/granny driving styles.

I wonder if it is more to do with flywheel effect? TAE say you need to keep the starter engaged until 500 rpm, that seems to imply the engine can kick back if the starter is released at lower RPM. Which would obviously put some unusual loads through the chain.
Maybe during the initial stages of air start the prop does not have enough flywheel effect to prevent kick back when the engine starts.
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Re: DA42 VI ownership decision questions

Post by CFIDave »

Karl wrote:Do Continental Diesel and Austro fit their own cam chain?
Not sure abut Continental (Thielert), but Austro makes use of most of the original Mercedes engine -- just replacing the oil sump and external accessories. So Austro engines use the original Mercedes timing chain to drive the dual overhead cams located in the head.
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Re: DA42 VI ownership decision questions

Post by BrandonH »

Thanks this is great stuff. Still don't understand the 2 minute start, which really seems to limit single engine training, but there must be reasons.

The windmill restart prohibition makes sense to me: I assume that with the gear reduction, a windmill start would be like jump starting a car at high speed with a low gear. I might imagine there is a software fix perhaps where the prop could un-feather very gradually to reduce the shock, but that might be complicated. (and maybe it's a purely hydraulic system anyhow)

I believe the typical horizontally opposed aircooled engine has a gear driven valvetrain, (and also no gear reduction) so the shock is a lot less.

Anyhow, good stuff. Still musing on the fate of the 2.2L mercedes engine after a decade or so, but now that i've seen some production numbers it seems reasonable that whatever the future fortunes of Diamond, there will be support for the Austro program.
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