A321 Switching Knots & Mach Speed indicator

[★Macman★]

Why on the Managed Speed is 260 & 285 when we switch between Mach & Knots?

As it purely is a means to see Mach Speed then what's the point and why are they set differently for Managed Speeds?

We can not go 285kts at 39,000ft and get speed warnings. It's weird. So why would you bother changing to Mach at all?

[yorran]

The higher you fly, the lower will the speed of sound be.
That's why you can fly at 300 knots at 10000ft while at cruise altitude your IAS will be lower.
Switching to Mach reference will prevent subsonic planes from overshooting Mach 1.

There might be more to it, in particular how knot speed is calculated at low pressure. But I don't know that part very much.

[★Macman★]

This is about why it selects 285knts as the Mach Speed whilst in Knots Mode it is at 260. Besides that I don't see why they bother to wish to see Mach Speed.

What I can see in this is that Selecting Mach then the Managed Speed is able to CLIMB to a higher speed than the Knots Manged one. However, it sends the Warning Alarm off. Then why? I set Airframe Damage = OFF to just enjoy learning but if set ON then it takes just 3 Beeps and I am destroyed.

[Rever]

Planes loose efficiency close to mach 1. It is just easier to know .95 verses air speed units that change with pressure and temperature. It actually makes a lot of sense, if you never want to exceed a certain mach number.

[★Macman★]

I know that Knots will decrease due to the readings of Air Pressure via the Pitot. However, it is more fuel efficient at 39,000ft. So although only ~Mach 0.86 is possible you are saying that Pilots like to see Mach instead to feel as though they are faster than they were at say 10,000ft and not get distracted by that OR prefer they see Mach so they feel they are going almost speed of sound but are safely under it which they would get destroyed?

Then why is the Managed Speed allowing the extra room to use to get into Airframe trouble?
OK, rather what is the actually altitude they should be switching to Mach? - then this extra added allowance may be of some use in reaching 39,000ft sooner due to the Managed Speed adds that extra 25knts to do the CLIMB rate faster without getting into the Warning Alarm. This surely is the reason for this difference on the Managed Speeds.

[yorran]

There is no static altitude for speed switching, if you want to do it manually.
An ATC friend of mine told me once to watch for .7M to make the switch, back in the days.
No idea if pilots do that, but I kept that as a reference.

[★Macman★]

OK I will play around with the Mach and test if it does allow faster CLIMB rates. It surely will make a difference above a certain altitude perhaps above 18,000ft from which we must calibrate Barometer to Standard 29.92 so that the MCDU will then have more of a calculation to allow for the extra available speed when it is climbing (different to forward Ground Speed therefore for the added allowance) but only up until the peak altitude for it's airframe which is where I began to wonder why Set Mach at all as once at Cruise it has no apparent benefit for the extra headroom the MCDU uses for it.

[★Macman★]

OK so I get what was going on now. This was actually in the A321 which is not the same behaviour as A320 for the Managed Selected Speed by the MCDU...

A320:
Climbing above 29,200ft it switches to MACH by auto.
Above 30,000ft the KNOTS begins to drop from the lower air pressure but MACH is constant.
At 32,000ft CLIMB MODE struggles and simply Levels out here.
You need to set Selected Alt using MACH to the highest it can reach under the red markers then use FLEX (or MAX) POWER to be able to climb to 39,000ft else it can take a long time on CLIMB and would need VS at 3-5% to get there.
Descending below 26,200 it switches to KNOTS by auto.

A321:
Same here except when at 39,000ft the Selected Altitude shows different for each. It looks like KNOTS is at 265 as it took the cap of what it reached set in MCDU Cruise and this lowered with the falling speed in knots also. It shouldn't have done this. The Cruise speed is set at the KIAS not what it would have been without altitude pressure dropping.
So you get 2 Selected Speeds for both MACH & KNOTS.
This is after all a mod so may have errors to reality.

[yorran]

As I said in the other thread about speed, you cannot rely on IAS at high altitudes. You must rely on Mach speed.
The only relevant information is the IAS stall speed as you come close to the coffin coner. But then, you get warned early enough when you see max IAS and stall speed coming too close to comfort.

Also, if you want to fly the Airbus way, when you engage CL don't touch it until flare. The rest is all about weight, weather conditions, thrust power and optimum altitude.
Not max altitude. And step climb.
This becomes even more relevant in heavy aircrafts and long haul destinations.
The goal is not to hit the roof as fast as possible. It is to use as less fuel and more range with the same specifications.

I know this doesn't answer all of your questions ; I'm thinking maybe those speeds in the FMC are related to speeds below FL300?
Maybe switching modes back and forth at high altitudes proves pitot it off for a nap ? I don't know.

Since I'm only a simmer, anyone with better knowlege is welcome to correct what I said. At this point, it's all what I can gladly contribute.

[★Macman★]

I'll look at A321 again but it rather looks like (reason of first query) that Plane has not got the correct behaviour due to using a Knots Setting for Cruise which it can not reach but MACH can thus we see the 2 Managed Selections spread whilst the A320 is the proper none-modded Model without any spread of the Managed Selections.

Having came across in favour of using the A321 as I like the Lighting & Sound changes more, it actually shows us how to get Cruise to go 25kts faster by using MACH. I wonder if this is to cater for the same Cruise Setting regardsless of what Altitude you happen to change to. BUT the A321 does list the faster Cruise Speed also so perhaps it just is not properly modded.

*YES it is due to KNOTS can no longer keep up with Managed Selected 285kts setting for Cruise.