IAS vs MACH Speed

[EHM-1945 Christophe]

Hello Dear all pilots,

I'm wondering this questions, about when to switch from IAS speed to MACH speed ? I was not able to find a clear answer on internet if there is a clear procedure explaining when to use IAS speed setting and when to switch to MACH speed setting and the other way around for descent.

Any input / information are welcome

Cheers
Kristoff

[EHM-2097 Andrei]

He he, this is an interesting question, I was wondering myself... And yes, indeed, not everybody gives the same advice.

Having bought recently my (first) "smart" aircraft, I noticed what the autopilot did in "managed" mode and now I am trying to do the same even when flying different aircraft.

So, what I noticed is that THIS autopilot tries to maintain a given - and high - airspeed while climbing, and switches to MACH mode when reaching cruise level. It looks like a good idea to me, because trying to climb at a too low speed results either in high angles of attack (and possible stall if no action is taken), or a painfully low climb speed.

This tactics works well with the EHM Airbusses.

However, there are other aircraft that just cannot maintain a high airspeed and a decent climb rate. In this case it could make sense to switch to MACH mode while climbing, because this reduces the "target" airspeed as you climb higher. Otherwise there is a huge gap between the target airspeed and the speed the aircraft is physically capable of.

For the descent, I think it's easier to tell because I don't see any good reason to descend in MACH mode: the lower you descend, the higher airspeed the aircraft will try to reach. And this sounds like something that generally doesn't end well

But of course, this is my opinion which I made myself, so here I am with you, eager to hear more opinions.

Andrei

[EHM-2029 Sotiris]

Well, here's the answer from a pro:

The classic (well, dunno about the 400, but 500 and 300 work that way) speed control switches form IAS to Mach at FL260 if in level change or V/S.

In VNAV climb it will keep the IAS in the mode selected (ECON most likely) until the corresponding mach number of that profile is reached and then keeps that. That way switching usually is done later, often around the FL280 to FL310 mark depending on cost index and entered top of climb wind.

During VNAV descent its just the other way round, the FMC keeps the mach number according to the selected cost index and switches to IAS when that is reached. Descent speed schedule is not variable but rather fixed speeds according to the selected CI. With a low CI it often happens that the switch to IAS is done immediately upon selecting VNAV descent, for example we use a fixed CI of 20 which corresponds to 266 kias descent speed and that is often enough reached at our normal cruising levels, since 266kias isn't really ATC friendy most of us change the descent speed on the descent page to 280kias which is reached a tad later.

Considering that you've done a great deal fo studying in the last months I believe you can understand all his points.
If not, let me know and I'll explain.

[EHM-1945 Christophe]

Sotiris, Andrei, thank you to both of you for your valuable comments and inputs. I understand the way it is working and will try to apply it starting from Now.

Cheers
Kristoff