Mach speed vs IAS - Flight Sim X question
 

In this example - I usually fly an A321... If I'm cruising at constant flight level 360, and use mach .81 as the speed reference the bug on the speed tape would be for example 275kts... Sometimes I have to change from .81 to .80 to .81 again to "reset" the speed so it doesn't go overspeed on me... sometimes comes back with a number different than 275. Almost as if the plane doesn't constantly calculate mach speed until you change it again. Is it this way in real life or just flight sim x? And just so I understand right, mach speed depends on temp?

Thanks all... let me know if the question needs clarifying.

At a given altitude and a given mach number, the indicated airspeed will be the same, no matter what the outside air temperature. I remember on the 727, at FL 350, mach .80 equals 272 knots. Always. At different altitudes, the indicated airspeed will be different. At different mach numbers, indicated airspeed will be different. When the temperature changes, and you stay at the same mach number, your true airspeed changes, but not your indicated. Good luck understanding all of that.

I understand believe it or not... Mainly concerned with IAS in this case, because of the overspeed situation mentioned above... So I guess I have to constantly 'reset' (set to .80, and back to .81 again) when I'm in a climb or descent since it doesn't seem to 'update' the bug on the speed tape.

Heres another question - you said it depends on altitude. Well FL350 is not always equal to true altitude... Or does it cancel this out because of the pressure involved is the same?

When do you switch to IAS to Mach? I heard that the magic number is FL260, but what determines this? (I know that for compressibility reasons you make the switch when at high altitude, but how do you know when?)

Are you sure this is correct? Mach is the ratio of the speed of an object through a medium / speed of sound. The speed of sound is variable with temperature. Thus, at different temperatures, the IAS will be different.

Since I've never paid attention to the IAS speeds for each individual MACH number, I really don't know. Perhaps the temperature is / was almost always the same when you noticed this?

Yeah, Mach vs. AS is a function of air DENSITY. Density varies with ALT, Temp, barometric pressure, and humidity.

In the real world, from day-to-day you would expect slight differences in Mach vs. AS.

A real ADC would constantly calculate mach and airspeed pretty much instantaneously. Sounds like a game-ism to me.

Talking only about the switchover from mach to indicated airspeed; it depends on the climb and descent speed schedule you are flying. On the 747-400 at heavy weights you may plan on 350 kias until Mach .85, and your switchover would be around FL 270. If you plan a speed schedule of 270 kias until Mach .83, your switchover will be FL 370.

Joe

Sorry, Not Correct
 

Carl:

For the same indicated and same altitude, variations in temperature will change the Mach number.

If it is warmer than ISA standard, then the Mach will be lower. Colder, then higher.

Looked at the other way, if always flying Mach 0.80, if warmer than normal, the indicated will be higher, and if colder, the IAS is lower.

Get out your old Jeppesen CR round flight computer. No matter what temperature you put in, if you are at FL 350 and doing mach .80, your indicated airspeed will be 272 knots. The charts in your manual will show the same thing. The relationship between mach and IAS is not changed by temperature. I cannot really explain why, but perhaps someone smarter than me can. The airspeed indicator in your airplane that has mach in the side also is just a mechanical relationship between mach number, altitude, and indicated airspeed. Temperature does not factor into it. The temperature will make your mach number different, but I am saying if you fly a constant mach, your indicated airspeed will not change.

Different Example
 

Carl:

I don't have a whiz-wheel with me, and I don't doubt that it says what you say...but I think there is a way to adjust for non-standard temperature on the EA-6B. I'll have to wait until I can get my hands on one and figure it out.

In the jet I currently fly (T-38), our standard cruise-mach is 0.78 at FL 280. On a standard day, this will result in an IAS of 320 knots.

In the winter, it happens at about 313 kts. In the summer, about 327 kts. The T-38C is a glass cockpit; the Mach-number is determined by current true airspeed, which uses an input from a Total Air-Temperature probe.

When I did functional check flights in the F-4, we did them at FL400. We accelerated to 500 kts indicated, where the intake-ramps started to program. There was a table in the flight manual that predicted the resulting Mach number where this occured. Since it was always at FL400, there was only one variable in the charts: OAT.

The lowest I ever saw was Mach 1.7; the highest was 1.8; same 500 IAS.

The F-4 had a mechanical-looking airspeed and Mach gauge, but it, too, had a temperature input from a TAT, and the Mach-scale moved independently of the IAS ring.

Rickair:

I was pretty sure of this earlier, but looked it up to make sure: temperature is really the only variable; pressure and density are negligible (I had thought the same as you until mid-career, then learned this; emphasis added by me).

Wikipedia Excerpt:

....This equates to 1,236 kilometers per hour (768 mph) or about one mile in five seconds. This figure for air (or any given gas) increases with gas temperature (equations are given below), but is nearly independent of pressure or density for a given gas. For different gases, the speed of sound is dependent on the mean molecular weight of the gas, and to a lesser extent upon the ways in which the molecules of the gas can store heat energy from compression.....

"The speed of sound is the rate at which small pressure disturbances will be propagated through the air and this propagation speed is solely a function of air temperature."
Mach# = KTAS/Speed of sound in knots.

Carl remember that when you fly at a "flight level" you are not actually flying at a fixed altitude. Since you are setting 29.92 you are actually changing your altitude above sea level to match the air density of that altitude on a standard day. Temperature and altitude are not directly related however most aircraft charts assume the standard lapse rate (2C/1000').
As for your wiz wheel are you looking at the density altitude or the pressure altitude window for these computations? I found an old wiz wheel to look at and mine actually has a note on it that says, "To obtain TAS from Mach number set true air temp agianst mach number index on "miles" scale read TAS corresponding to mach No. on "minutes" scale." Granted it's TAS but you can work backwards to get your IAS from there. It just shows the correlation between MACH and TEMP.