IAS vs Mach philosophy

[A.FLOOR]

Can anyone explain why Boeing has chosen to use IAS over Mach in it's MCP design. Above the crossover altitude, in the high speed, high altitude regime, why, when opening the speed window is the default IAS?

[wiggy]

One explanation I've heard is that it's to keep things simple in the case of an emergency descent. By defaulting to an IAS you avoid the risk of accidentally descending down to lower levels at your cruise level Mach No and busting Vmo on the way down!

No doubt the truth is somewhat different, and is out there!

[BBK]

One reason could be an emergency descent where it's suspected, or even confirmed, that the aircraft has sustained structural damage. If you descend in Mach then as Wiggy says you'd end with a very high IAS. Perhaps not good in that situation!

Interested to hear any other ideas?

[RTO]

Can anyone explain why Boeing has chosen to use IAS over Mach in it's MCP design. Above the crossover altitude, in the high speed, high altitude regime, why, when opening the speed window is the default IAS?

Which Boeing is this?

[stilton]

Its certainly the case on the 757 / 767.

[Winnerhofer]

By T.S.
You want consistent behavior; when the pilot takes the same action the display system should respond in the same way, otherwise you get into situational awareness issues.
IAS is a better default than Mach because it's never wrong, just sometimes not optimal.
Having it open to Mach all the time would be confusing as hell below crossover altitude because nobody thinks in Mach down there.

[wiggy]

Its certainly the case on the 757 / 767.

RTO

I'll add the 744 (unless there's been a change) and 777 (unless there's been a very very recent change!) to that list.

[latetonite]

During take off and approach, when you are likely to fly with the MCP, I take it you do not want to fly with reference to Mach Nr.

[Derfred]

Doesn't seem to cause any issues on the 737, where it defaults to Mach above the crossover and IAS below.