Did the Mach speeds change?

Me and my henchman @Kamryn are doing a flight. We were at 293 KIAS in a 737-700. When it converted to Mach speed, I was at .80 and he was at .81. It used to be that 293 was Mach .78. Did that change?

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Rocco, that depend on the winds.

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Does it? It seemed that in previous updates it was always at .78 regardless of winds. Is this something new with this update?

Nope. We haven’t changed anything on this.

You’re right though, but not always. I usually target the same with narrows but set the AP to 285. Sometimes it was .78, next time .76. Always a few decimals hit or miss.

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That’s interesting, maybe I haven’t been paying attention close enough, or just seemed to get lucky with .78. Ty!

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I’m not sure if Infinite flight models it, but Mach speed should also be dependent on altitude and pressure at that location. Though IAS also decreases with altitude and I’m not sure if it is at the same rate, so that may cancel out.

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We do simulate that.

We get baseline pressure from standard and compute pressure in altitude based on that. Pressure at sea level is always 29.92 though for us. So we’re “cheating” a bit there.

Air density is also calculated based on altitude.

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Rocco is just a bad pilot, forgive him. What he forgot to mention is that he was going 293kts at 2,000ft 😬

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Cool topic rocco 😄😄😄

I’m sorry, but is this not incorrect? - isn’t mach speed independent of wind speed?

Rocco is referring to the switch on FL280 on the autopilot. So no, not in this specific case since the GS/TAS will not be the same if you have 100kts head wind as it would if you had 100kts tail wind.

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Well technically if you set the A/P to 293 and have a headwind or tailwind, it won’t affect what the A/P has set. Your ground speed will definitely be different but it shouldn’t affect your Mach speed at the end of the day (unless the winds changed right around FL280 and caused a IAS speed to change which the A/P then captures for Mach)

If I felt like climbing to FL280 in a 737, I’d do it and test it out, but this is a homework break.

But at FL280 the relationship between mach and ias should be independent of gs. If you have a 1000knot tail wind, you are no closer to the speed of sound from a mach point of view if your ias is only 300kts.

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As far as I remember, I never had a mach intercept from 293 go to anything but 0.78 (since it takes airspeed into account). I suppose if the wind suddenly dropped right before you crossed FL280 by like 10 knots (which is not unheard of) then you’d end up at around 0.8. It would sort of make sense that you guys ended up at different mach numbers, because if there was a differential in wind speed around FL280 due to gusts it’s highly unlikely you guys would catch the exact same magnitude of gust.

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You know what? I maybe completely in the wrong here.


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Mach is not determined solely by your IAS. It’s a measurement comparing your speed through the air to the speed of sound. The speed of sound changes depending on a few factors, some of which have been mentioned in this topic: Air Density and Wind Speed (to a much lesser degree, I’m pretty sure).

What determines air density? In aviation, we’re most concerned with temperature, pressure and humidity.

For any given time you’re passing through FL280 and changing to mach speed for A/P, mach .78 can therefore be measured at FL280 at a somewhat different KIAS.

Like @schyllberg said however, pressure is set to standard at sea level in Infinite Flight. So, all we care about in IF is air temperature, which I’m pretty sure is pulled from the winds aloft data that IF uses for winds as well. Obviously, IRL, the air at FL280 in California might not be the same temperature at FL280 in Florida at any given time, meaning the KIAS needed for .78 mach will not be the same, as the speed of sound in these two locations at this altitude will be different.

The airplane’s speed over the ground is also completely independent of it’s speed relative to the speed of sound at a given air density.

TLDR: For IF, I’m pretty sure since we have RL air temps pulled from winds aloft, we can see somewhat realistic differences in mach at identical indicated airspeeds and altitudes.

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It short term since air speed does decrease with altitude depending on what flight level you are at. So if you would set a/p at 293 let’s say around FL100 when you get to cruise it would be M.78 but if you set 293 at FL270 a/p would probably show M.80 during cruise.

Rocco needs to learn how to fly, 295 IAS is M.80 9 times out of 10 my friend. At least in IF where the switch from IAS to Mach occurs at FL280

No worries, there was also something wrong in what I said. I mentioned mach in relation to IAS. but mach is all about TAS. It’s the TAS ratio to LSS (Local Speed of Sound), not IAS.

If TAS is equal to LSS, you’re going mach 1.0

Safe flight has to consider both numbers separately. At high altitude, you might be approaching a stall according to IAS, while starting to exceed the mach turbulence over the wings due to how fast the air is traveling, TAS!

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