Recommended 75% cruise vs max 92% cruise

[LimaZulu]

Hey folks,

The AFM states: "The engine manufacturer recommends a cruise power setting of 75 %". What are the practical implications of using 92% power all the time, instead of the "recommended" 75%?

Yes the the fuel burn will be disproportionally higher, but what effect does it have come maintenance time?

[CFIDave]

I presume you're asking about an Austro DA40NG model, rather than Lycoming.

I know of owners who always cruise at 92% because the fuel burn difference is so low (8 gph vs. 6.5gph). If you're not as worried about fuel efficiency, there's not much harm in running at 92% -- it doesn't seem to hurt the engine. While I haven't done the math, the per-hour maintenance cost on the engine, combined with putting fewer hours on the airframe when operating at 92%, might neutralize most of the extra fuel cost -- not to mention that you'll arrive at your destination a few minutes earlier.

Of course you also need to consider range. If 75% cruise lets you avoid a fuel stop, or carry more reserve fuel if flying in IMC, it may be better to use 75% for that flight.

Finally, now that Diamond/Austro have eliminated the ability to overhaul Austro engines, there may be less reason to "baby" your engine by flying at a lower power setting, and you'll have an even greater incentive to minimize engine time in service.

[Colin]

I fly at 76% because I like it a little quieter and a few more minutes in the sky is just a better day for me.

[LimaZulu]

Yes, should have mentioned - NG model.

I'm a little puzzled by the "now that Diamond/Austro have eliminated the ability to overhaul Austro engines" comment. Is this related to the MSB from Oct 2022? If so, my engine is not subject to this as it was delivered last month.

I personally don't plan to be in the air for longer than 3hrs at a time unless I bring a piss jug for myself and wife (but she's not gonna be happy to use it), so extra endurance isn't a factor. I find 88% is my sweet spot right now as that gives me just under 8gph and a decent reserve to spare.

[CFIDave]

I'm a little puzzled by the "now that Diamond/Austro have eliminated the ability to overhaul Austro engines" comment.

When Austro engines were first introduced in 2009, they were sold as having a TBO (time before overhaul), extended to 1800 hours. This was in contrast to Thielert/Continental diesel engines that had a TBR (time before replacement). Austro has more recently stopped offering engine overhauls, only replacements. To some owners this means that you might as well run your engine hard at 92%, since you'll be replacing it with a new one.

[LimaZulu]

Hmm interesting. I was not aware of that. Doesn't that effectively 2x the "overhaul" cost?

[Boatguy]

Hmm interesting. I was not aware of that. Doesn't that effectively 2x the "overhaul" cost?

The most recent quote from Austro is €50K.

[pisymbol]

WOW! That is insane. Why would anyone want an NG model then?

[alanhawse]

FWIW yesterday at 9000 (density altitude 7812)
% TAS Fuel
70 132 6.2
80 140 7.1
85 144 7.6
90 146 8.1
92 148 8.3

[pisymbol]

140@7.1! Wow, can't really beat that!