914 High Altitude Operation
in 912 / 914 Technical Questions
10 years ago
I fly a Europa with a 914 and Airmaster constant speed prop. I usually operate around 8000 ft at about 70% power, using 5000 rpm and 31 in mp and my throttle seems to be at about 67% per the book. My throttle has a physical stop at 100% and a detent to go to 115% per the install manual.
I recently completed a flight at 13000 ft. I was surprised that I was unable to maintain 70% power as usual (5000 rpm, 31 in mp) and also surprised that my throttle position was at 100% just to maintain about 28 in mp and 5000 rpm. I was also surprised that my fuel flow showed about 6.4 gph, where it usually shows about 5.5 gph. Part of the flight was also at 11000 ft, and there with the throttle position at 100% and 5000 rpm the manifold pressure was a little higher, say 29 in and the fuel flow was a little lower.
I reread the OM (Chapter 2.1, page 2-3) and noted that 16000 ft is the critical altitude, max continuous performance is available to that altitude at standard ISA conditions.
Then I looked at the non standard conditions max continuous performance chart (Chapter 5, page 5-5). If the temperature is ISA, that chart shows that max power will decrease with altitude from 74 KW at SL to 66 KW at 16000 ft. This seems to conflict with the critical altitude statement. I don't understand this.
In my case, at 13000 ft the OAT was +5 C so from the chart max continuous power would be 67 KW or about 90% of 74 KW. Is that why why I was only able to get 28 in mp?
Now if I look at Chapter 5, Fig 1, Engine performance, 90% of max power (90 hp) crosses the propeller line at 5350 rpm. If I then go to Fig 3, fuel consumption, it shows about 6.4 gph. Is this how these charts are to be used?
In my old turbo Mooney 231, I could advance the throttle to get my desired power (70%) at any altitude up to the critical altitude. I thought the 914 would do the same. Am I mistaken?
Thanks for any clarification.
Jim Butcher
Europa XS N241BW
I recently completed a flight at 13000 ft. I was surprised that I was unable to maintain 70% power as usual (5000 rpm, 31 in mp) and also surprised that my throttle position was at 100% just to maintain about 28 in mp and 5000 rpm. I was also surprised that my fuel flow showed about 6.4 gph, where it usually shows about 5.5 gph. Part of the flight was also at 11000 ft, and there with the throttle position at 100% and 5000 rpm the manifold pressure was a little higher, say 29 in and the fuel flow was a little lower.
I reread the OM (Chapter 2.1, page 2-3) and noted that 16000 ft is the critical altitude, max continuous performance is available to that altitude at standard ISA conditions.
Then I looked at the non standard conditions max continuous performance chart (Chapter 5, page 5-5). If the temperature is ISA, that chart shows that max power will decrease with altitude from 74 KW at SL to 66 KW at 16000 ft. This seems to conflict with the critical altitude statement. I don't understand this.
In my case, at 13000 ft the OAT was +5 C so from the chart max continuous power would be 67 KW or about 90% of 74 KW. Is that why why I was only able to get 28 in mp?
Now if I look at Chapter 5, Fig 1, Engine performance, 90% of max power (90 hp) crosses the propeller line at 5350 rpm. If I then go to Fig 3, fuel consumption, it shows about 6.4 gph. Is this how these charts are to be used?
In my old turbo Mooney 231, I could advance the throttle to get my desired power (70%) at any altitude up to the critical altitude. I thought the 914 would do the same. Am I mistaken?
Thanks for any clarification.
Jim Butcher
Europa XS N241BW
