Mach VS IAS

[timzsta]

Speed of sound is only dependant on temperature. The reason airliners fly at IAS up to about FL260 and mach numbers above is so as not to get supersonic flow of air over aircraft or the aerofoils which would cause loss of lift and buffet over the horizontal stabiliser, all of which is not good!

The change over from constant IAS to mach in the climb takes a little explaining, here goes.

First you must understand when we talk about flow velocities over the aircraft becoming supersonic we are expressing their speed in TAS not IAS. As we climb at constant IAS our TAS is increasing. Also as you climb because the temperature is decreasing the local speed of sound is decreasing (mach number = TAS/local speed of sound)

There are two speeds that we have to consider so as no to run into problems. Vmo and Mmo. As we climb at constant IAS (say about 300 kIAS for a 737-300) our TAS is building and so we are approaching the local speed of sound problems. Due to the fact that air accelerates as it passes over the wing it is possible to achieve supersonic local flow velocities at mach numbers of about M0.8 over a typical airliner wing.

So at some point in the climb the flow velocities over the wing are going to start getting to close to the speed of sound so we could exceed Mmo. Usually the changeover point for IAS to Mach is at FL260. At this point we climb at constant mach, usually the aircrafts cruising mach number, so about M0.74 for the 737.

The autopilot mode for the vertical path/speed control engaged at this point would be the VNAV mode. Here the FMC will command the auto-thrust to climb at contanst mach number at the climb power setting. The ROC will be dependant on the excess of thrust available over that required to climb at the commanded mach number. Hence we have no longer a worry about getting a supersonic airflow over the wing as cruise speed will be below the speed this can happen. When changing over from constant IAS to constant Mach the rate of climb will initially increase and then decrease as the excess thrust decreases because the air is getting less dense.

Conversly in the descent as we descend the local speed of sound is increasing and the danger is you could exceed Vmo in the descent. So a descent is at constant mach again until FL260 then at constant IAS. Typical descent profile for a 737 would be descend at M.074, then around 300 kIAS, then at about FL120 decel to 250 kIAS to meet the speed restriction. Downwind would be about 220 knots, 180 on base, then approach speed (Vref = and must not be less than 1.3 x Vs) would be around 130 knots.

I am only ATPL student who just did PoF exam. Big jet drivers please correct me if I am wrong!