Unusually high cruise speeds

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salim
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Unusually high cruise speeds

Post by salim »

Hello,

I observed on two separate occasions cruise speeds of around 169-170 kt at FL140, 85% power, and ISA / ISA +11 on my DA42 tdi (standard configuration, with TKS)

Approx 1700kg at the time of the observations.

Seems significantly higher than POH. Any idea why ?

Could it be that 2.0 engines can sustain 85% at higher altitudes than the 1.7 engines on which the POH is based ?

Thanks !
Salim
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CFIDave
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Re: Unusually high cruise speeds

Post by CFIDave »

Take a look at your attitude indicator relative to the G1000 horizon line -- if the plane is pitched down more than "normal" in cruise, you're going to cruise faster. Changes in cruise pitch (and hence cruise airspeed) are often due to almost imperceptible updrafts and downdrafts -- caused by flying through high (sinking air) or low (rising air) pressure areas.
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dgger
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Re: Unusually high cruise speeds

Post by dgger »

Looks like both times you have been flying leeward of the Alps in somewhat windy weather. My guess would be you caught a mountain wave that carried you upwards while flying level.

I have seen performance vary significantly when I have been flying in comparable conditions.
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Re: Unusually high cruise speeds

Post by klang »

I see similar performance figures with my 42TDI with TKS (e.g. here FL170/ 75%/ ISA++/ 2-3° cruise pitch up/ 164KTAS
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ememic99
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Re: Unusually high cruise speeds

Post by ememic99 »

Mountain waves :)
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Re: Unusually high cruise speeds

Post by klang »

Hello Emir
if it was riding a mountain wave or thermal updraft, it would result in zero, or even negative pitch attitude. But the AI shows a normal 2-3° pitch up and zero climb/descend.
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ememic99
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Re: Unusually high cruise speeds

Post by ememic99 »

When I had 2.0 engines my average numbers were:
- FL140 85% - 131 KIAS, 165 KTAS, 6.1 GPH (it's bit less than what @salim picture shows but his PFD also shows less pitch up than in horizontal flight)
- FL170 75% - 122 KIAS, 162 KTAS, 5.4 GPH (it's similar to what @klang picture shows)
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Re: Unusually high cruise speeds

Post by klang »

What are the respective numbers now with your CD155 engines?
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ememic99
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Re: Unusually high cruise speeds

Post by ememic99 »

- FL140 85% - 142 KIAS, 177 KTAS, 7.4 GPH
- FL170 75% - 132 KIAS, 175 KTAS, 6.5 GPH
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