Aart, can you give me fuel consumption data for 80% BHP and 65% BHP for one engine please?
80% = 6.2 USG/hr
75% = 5.8 (that was my reference for 135 KIAS)
65% = 4.8
Rounded to decimals
80% BHP is 1.23x the power compared to 65% and needs 1.29x the fuel. In my Lycoming 540, the ratio is almost identical but of course it cannot be operated at 80% continuously (and strictly speaking not at that fuel flue because you’re not supposed to run at peak with 80% BHP).
Now we need to know what the combined consumption with both engines running is for 107 KTAS.
I do love you Achim… Yet another excuse to go fly tomorrow. Keep the questions coming. 
Extrapolating from the AFM, the total fuel flow should be 5.5-6 USG/hr at 107 KIAS but of course this calls for verification.
C172 speeds at 30% less fuel burn, and way cheaper fuel. But way more expensive maintenance of course..
Not entirely stable atmosphere yesterday, but looks like 6 USG/hr total burn at 107 KIAS (=KTAS at 1.000 ft)
That is basically what we expected: producing 107KTAS with two engines gives better fuel economy than producing 107KTAS with OEI. The difference is smaller than I thought.
I believe that you would get different results if you really turned off one engine and feathered the prop as opposed to running it at 10%. One keeps reading these claims that a 10% engine has the same effect as a shutdown engine with feathered prop but one also reads reports from pilots that tried and saw different behavior. I tend to believe the latter.
Sorry, won’t be able to check tomorrow. Wife says no.
(Run to airport very early in the morning, when wife still sleeps! :-))
Aart, damn it, wake up – you’re being setup in an elaborate feathering-scheme here. Achimha and Alexis are 2 notorious aviation board Internet predators (both flying single engine!)baiting unsuspecting twin drivers into feathering their props. First they make you go 10%. Now they ask you to go behind your wife’s back. And before you know it, you’re sitting at 3000 ft AGL with the critical feathered!
Thanks Hodja.. Wary for their questions to ‘now feather both engines and do a steep turn while maintaining altitude’. You think one of these guys could be after my wife?
As to Achim’s comment: I do remember seeing that 10% power is about the same as a feathered engine. So the DA42’s relatively good OEI performance could really be true. The GFC does a really nice job, maintaining heading and altitude while banking into the good engine, kicking in opposite rudder, minimizing drag.
