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CAB825

Mach Crossover Altitude

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Hi,

 

It seems there is an error with the managed speed function. For no apparent reason, it will change from Speed to MACH at low FL's.

 

As can be seen here, FL180 and it has switched over from Speed to MACH, while in the cruise.

 

Regards,

 

1454992837_MachCrossover.thumb.jpg.15fc9e6f3f370e6417a7b4f6386f49a9.jpg

 

 

 

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What happens if You select SPEED......does it return to MACH by itself....??

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Its occurs while in managed mode, selecting speed has no effect.

 

It seems to occur when the cruise altitude is reached/active, even if the cruise altitude is below crossover altitude.

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I found that over CI 52 it shows MACH

Below CI 52 it shows SPEED

 

Why I can't find but searching

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Thanks for the report!

 

I haven't noticed this myself, but I'll give it another look in the next few days.

 

One thing to keep in mind is generally MACH crossover occurs around 24,000ft, however the altitude at which it occurs can also be affected by temperature and altitude corrected barometric pressure and I believe this is modeled in our aircraft.  I don't know where you were flying, but we're already seeing significant drops in tempretures in the Northern Lattiudes (winter is coming!).

 

If you wouldn't mind, please do another flight to see if it's repeatable, and if so please record the temperature at the altitude it occurs at (include SAT and TAT if possible).

 

In the meantime I will take another look at this sometime in the next few days (I'm away this weekend) and I'll let you know what i found.

 

Best wishes.

 

 

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Hi Dave,

 

Thanks for the repose, I have just run it one more time and noticed that the switch occurs when the IAS meets the corresponding Mach.

 

The managed speed values were 340/0.68 and as soon as the PFD Mach hit 0.68, the managed Mach kicked in.

 

As can be seen, ISA +3 so no real issue with weather/temp.

1.jpg

It does not seem to model the Airbus FMGC logics as below:

 

2.jpg

 

Thanks.

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