i was cruising at fl240 270 kias i changed to mach speed and it read 0.64 i wanted to cruise at .74 as it was a short flight, the speed went away up to 320kias which resulted in overspeed any suggestions to what was wrong. thanks, jim

you need to increase your altitude to be able to fly at .74, where the air is less dense and colder.I've done very short flights in the md-11 from EGBB to EIDW (45mins), i only go up as hgih as FL260 around the same mach speed as yourself.Paul

Higher where air is less dense, the mach number does not depend on temperature. In fact, at a given pressure the speed of sound would increase with a decrease in temperature (denser air).

In fact, at a given pressure the speed of sound would increase with a decrease in temperature (denser air).

doh! I should of known that :(Paul

thanks again, for the help. jim

Higher where air is less dense, the mach number does not depend on temperature.

Mach has everything to do with temperature .. Mach number is the ratio of TAS over Local Speed of Sound (LSS). LSS is calculated with following formula: 39 x square root of the temperature in degrees Kelvin. Sound passes through the air by compressing and expanding the distance between air molecules, transmitting energy to neighboring molecules. Molecules in warm air move faster, transmitting the sound energy faster than in cold air. Therefore the speed of sound decreases along with ambient temperature.JanF/O Avro RJ85/100MCC Instructor

You are referring to the Rayleigh compressible flow, which does involve heating due to impact... but I wouldn't say it has everything with temperature since in this case it is the temperature at the boundary layer or shock front but has nothing to do with ambient temperature. I didn't want to throw in this extra info just keeping it simple... and simply the mach is a ratio of speeds of sound between two densities.... it becomes interesting with compressible flows and that is what you refer to; however, sound also travels very well in non-compressible densities such as water.EDIT: I shouldn't have said it doesn't have anything to do with ambient temperature, which is an important factor in the density of air below the stratosphere in the troposphere... usually, we don't know the density, just the pressure and temperature so we use these two in a formula to get to a mach number where density is in there but buried in the derivation of the formulae we use. I don't think as fast as I type.