Expert: D. Norkus - 10/13/2004

Question
Hi
why above +/- 20.000 feet the speed in aircrafts is used in number MACH and not in IAS ?
thanks, from Bogota.

Answer
'Above +/- 20.000 feet, why is the speed of an aircraft measured in MACH number and not in IAS?'

Because aircraft climbing at a constant IAS will have an increasing TAS, Mach number is a more precise indicator of speed at the flight levels. The mach meter measures the airspeed of an aircraft relative to the speed of sound. It's basically made up of an Air Speed Indicator and an altimeter. It also has a sealed capsule containing ISA conditions and a pitot pressure sensor inside an (free air ) static pressure sensor. No temperature sensors are present in a machmeter because it uses the formula Mach Number = dynamic pressure. Since airspeed off an ASI must be corrrected for nonstandard temperatures and pressures, Mach is a truer indication of airspeed.

During climb jet aircraft generally conform to a schedule which specifies an initial climb speed as an indicated airspeed (IAS), and a Mach number for the upper part of the climb until cruise altitude is reached. Mach numbers are then used as a cruise schedule. The pressure altitude (FL) at which the IAS climb schedule ends and the climb Mach number schedule starts is called the "Changeover Level".

In the early stages of the climb at a fixed IAS, the Mach numbers rise until changeover level is reached. Changeover level is that flight level where the climb IAS value and climb Mach values occur simultaneously. For any given IAS/Mach climb schedule there can be only one changeover level. Different aircraft types may have different climb schedules specified in their Aircraft Flight Manual. (AFM).

When dealing with two aircraft at the same level, assigning the same IAS or Mach number will produce the same TAS. Mach numbers are normally used for this purpose at fairly high levels (FL250 and above). Mach 1 ranges from about 600kts TAS at FL250 to 572kts TAS at FL350. As a general rule of thumb at these levels 0.01 Mach equals 6 knots TAS. Mach 0.80 at FL290 will produce the same TAS as Mach 0.83 at FL370. This arises because the speed of sound is slightly less at FL370 than it is at FL290.

If a controller is trying to match speeds of aircraft at different levels, the same Mach number will mean that the higher aircraft will be a little slower.  An allowance of .01 Mach for every 2000-3000 feet level difference will achieve a closer match. Unlike Mach number control, the results of allocating IAS restrictions to aircraft vary substantially with altitude. An aircraft maintaining 280 knots IAS at FL370 will have a TAS of 459 knots while at the same IAS at FL270 will produce a TAS of 417 knots.

For aircraft operating at the same IAS, a rule of thumb is 7 knots for every 1000 feet level difference. The result of this is when two aircraft are assigned the same Mach number, the higher aircraft will be slower, however, when they are using the same IAS the higher aircraft will be faster. Above FL240 each 10 knots of IAS equals approximately 15 knots of TAS.

Using the more precise Mach number also allows for reduced longitudinal separation minima, according to the ATC handbook. This is obviously a benefit on high volume air traffic routes.  

Hope that helps

A note:  'Aircrafts' is a possesive form of the noun aircraft, which is both singular AND plural.