Changeover from IAS to Mach speed climbing in 744....

[JSACKS 0]

...occurred tonight at exactly FL300 when IAS equalled M0.825 following a multi-step climb at IAS 314 from 10,000'.Does Mach changeover typically occur at this speed in the 744 or does altitude play a role too ?Jonathan

[Check_Airman 3]

The change occurs when the selected IAS= selected Mach during the climb/descent. This usually occurs between FL240 and FL280.

[JSACKS 0]

Erm,...sorry,... still not exactly clear on this. Do you mean when the selected IAS coincides with the equivalent Mach reading at a certain altitude, typically, FL240 to FL280?Jonathan

[markadeane 272]

That's mostly correct. A given IAS will equate to a different Mach velocity at different altitudes though.The speed of sound decreases as you go higher, so any given mach value represents a slower physical velocity the higher you go.The FMC has a target climb speed recorded in two different measures: Knots IAS and Mach. As you climb at a consistent target airspeed, say 320 knots, the mach value representing that speed will gradually increase.When you reach an altitude where the mach speed that 320 knots represents equals the target mach from the FMC, the changeover will occur and the aircraft will hold that Mach value and the IAS in knots will decrease slightly.Hopefully that explains it?What I've never been taught was WHY we use mach instead of Knots IAS.Anyone?

[JSACKS 0]

Clearer, Mark, thanks.Perhaps Mach meter is more accurate than IAS....?Jonathan

[Guest 777jockey]

>What I've never been taught was WHY we use mach instead of Knots IAS.

[Guest Darren Howie]

To put it simply guys at high alts mach is more critical than IAS at the upper end of the speed spectrum.IE the absolute fastest the aircraft can fly is determined by mach not by IAS.The max speed of the aircraft is generally based on a buffer between max mach number and the speed at which you will get local supersonic flow on the airframe.Hence we use mach.AT lower alts the max speed becomes an IAS issue hence mac speed is listed as an IAS/Mach number.On the 320 which i fly its 350/M.82 Hence max speed 350 below changover and 0.82 above.Comprondo??Darren