How to know fastest cruise altitude?

Hi fellow pilotes,

I’m quite a bit experienced with 350 flying hours right now but I still have one thing that interests me when flying planes like the TBM 930. How can I calculate or know the best (=fastest, with 100% power) cruise level for different planes? Right now I’m on 15.000 ft with 230 kts ias. Some website tells me it cruises around 290 kts (or are we talking gs here?)

Fori

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Cruise performance is usually measured in TAS.
To find out the fastest cruising altitude you need the performance section of the aircraft manual.

In case of the TBM at ISA conditions it’s between FL280-290. (330ktas at 5500lbs)

VMO on the TBM is 270kias, hence 290 can only be TAS.

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The biggest thing influencing you is the wind. Wind speeds of course vary signifcantly with altitude; so can wind direction.

Take a look on Windy https://www.windy.com/ … good visaulization of the wind with a slider for altitude. And of course since MSFS has live weather…

You have to consider that the wind forecast you are using during preflight planning can differ a lot from the actual wind.

As a very rough rule of thump. The higher the higher the TAS.

(Ground speed = TAS plus / minus Wind).

This moves you trough the air / over the ground the fastest. That does not mean however it is the shortest time to fly from A to B as you also have to consider the time to climb to altitude. But that is more involved.

Wouldn’t it depend on wind speed and direction at the varying altitudes?

Fastest wind from behind would get you fastest cruise correct?

Yes and no. That’s why TAS is so important.
If you are e.g. cruising with a 20kts tailwind component at an altitude which results in a 30kts lower TAS, it’s the wrong decision.

How can FL280 be ISA . My noob understanding is that ISA is 15Celsius at 0 Feet MSL

ISA means international standard atmosphere and it‘s defined as 15C at 1013,2hPa (at MSL). Now every 1000 feet of altitude you‘ll lose 2 Kelvin, every 18000 feet will halve the air pressure. So it doesn‘t matter at wich altitude you are, calculating with ISA means that at FL280 you fly in always the same temp and pressure.

So at FL280 ISA means 15C - (28*2K)= -41C and QNH is a calibration value so you won‘t do anything except press STD or dial in 1013,2 for ISA.

Keep in mind that ISA does not take wind into consideration and possible warm or cold air masses and variations from this rule. That‘s why it‘s called „standard“.

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Got it many thanks

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