High Cruising Speed

I’m currently doing a flight using the 787-10 and my ground speed seems way too high. I’m at M 0.85 at FL330 with no tailwind and I’m reading 559kts over the ground. Shouldn’t this be considerably lower? Here’s an image of the situation:


Your outside temperature is causing this. 13C is pretty warm up there.

Probably due to this:


Ah you’re right, I just checked sqrt(gammaRT) and it all checks out down to the knot. It’s just that my wx hasn’t updated since takeoff, thanks :)

1 Like

sqrt(1.4286K287) = 339m/s = 659kts
0.85*659= 560kts so all is well 🙏

It’s so cool that the AP just goes off that 😮

1 Like

I have no idea what any of that means. Lol.


Honestly this is a super efficient way to fly haha
Feel free to close this, thanks for the help

1 Like

Wait wait wait before this gets closed I’d love to know what this means. I love learning new things like this!


PM him! I’d also like to know what it means :eyes:

1 Like

The sqrt(gammaRT) is the real world formula for calculating the speed of sound. Gamma is the specific heat ratio which is always 1.4 for air, R is the universal gas constant 287J/kg and T is the temperature in Kelvin. What’s cool is that the only thing that varies is the temperature. That’s why the speed of sound is different (typically slower) when you go higher and it gets colder. Doing that spits out the speed of sound in meters per second, since it’s all SI units. Then you convert it to knots and then multiply by your M number. That will give you your exact TAS.

So basically IF uses this formula once you’re above FL280 and that’s why it’s always exactly correct. It’s super fun to test it out while you’re flying, let me know how it goes! Feel free to hmu with any questions


Thanks for sharing! It’s sounds pretty much like something our developers would do, add those tiny details that makes a difference but that no one never really thinks about, until they do.

I just feel a bit more dumb than i did before, but that’s cool ;)


It just speaks to how good of a job you guys did with the physics! And man I know the feeling haha there’s always more to learn :)