A320 speed/mach switching

[PhilRabbit]

Hi there,
can someone explain me how to read the table in FCOM 1.22.30 P12 (speed/mach cross over table)? I somehow don't understand when the speed/mach switching (and vice versa) actually occurs...:ugh:
Thanks!
Phil

[pensador]

Hi! lets assume you have speed 300kt during your climb phase.The mach number will gradually increase as altitude arises while the speed remanes constant. So, at altitude 28500 speed 300kt is equal to Mach number 0.78. The system switches to the Mach mode (usually in that moment I saw that the magenta target speed triangle temporary, within 1second, jumps down and then up). Then the system keeps the Much number 0.78 while the speed gradually decreases. Also you can use the QRH page 4.16.
Con respecto!

[aristoclis]

Hi to all,

let' s put it this way: I am descending with selected Mach .78. When will the switching occur according to the table? At 28500' with 300kt, at 26500' with 315kt or at 24500' with 320kt? Supposing I do not press the SPEED/MACH pushbutton.

Thanks.

[pensador]

aristoclis, as fare as I am concerned at At 28500' with 300kt.

[aristoclis]

Thank you Pensador,

Just wondering when the other cases apply.

[Flyman35]

Just wondered why did they put the end of using Vmo at FL24600 and the start of Mmo at that level isn’t it an indication that this is the trasition altitude between switching??!

[PhilRabbit]

Just wondering when the other cases apply.Well, this is exactly what irritates me... :ugh:

[TyroPicard]

What a marvellous question! In three years on type I never once looked at this table, just accepted that it happened and if I did not like the result I intervened...
Try this for my understanding of the AB logic in that table.....
In the climb, passing FL 245, if your SELECTED speed is 320,335, or 350, it will switch to the appropriate MACH.
If not it looks again at FL 255, and if the SEL SPD is 310,325, or 330 (or above I guess) it switches to MACH.
And so on.

In descent it will switch at the first spd/alt crossover, depending this time on MACH. So at M.78 it switches at FL 285 to 300 kt. If your descent started at FL 280 it would wait until FL 265/315 kt.

That is my interpretation - it could be wrong! On a cautionary note it is described as SPEED/MACH SWITCHING and NOT vice versa. The written words only describe SPD to MACH. The general precision of AB documents lead me to suspect it does not apply on descent - but again I could be wrong...

[396.8T]

Dont have my bibles to hand but from memory Airbus dont have a crossover level as Boeing do (FL245),the busses crossover Mach/Speed is dependant on a number of variables....GW,SAT etc

[aristoclis]

Tried it today. Descend with selected Mach .78 out of FL350. Automatic switchover to selected speed 292kt at FL303. Any ideas how to use the table? :}

[condorbaaz]

In descent,
The when the mach equals the speed for descent viz. .78/300, the changeover will take place.

Since the mach to ias conversion is function of absolute temperature, SAT also plays a role.

(If memory serves me right, local speed of sound = 38x Square root of absolute temp, with a correction for PA above 25000 ft.)

[john_tullamarine]

Since the mach to ias conversion is function of absolute temperature, SAT also plays a role.

Don't think so ?

.. Mach is a function of altitude and CAS only .. temperature doesn't come into play at all ... perhaps you are confusing Mach to IAS with Mach and speed of sound to TAS ? Speed of sound is a function of absolute temperature.

[condorbaaz]

you are right, john
if speed of sound is a fn of Absolute temp, then so will be the tas value for .78,
and maybe some logic based upon the econ cruise etc controls the change over..???

[john_tullamarine]

(a) The operator will do the sums to determine the desired numbers.

Once you have decided on the climb/descent Mach and CAS, the changeover level falls out of the Mach equation ... which is what you suggested earlier.

if speed of sound is a fn of Absolute temp, then so will be the tas value for .78

Absolutely,

a = a0 √ [T/T0],

so OAT determines the speed of sound and then

TAS = M a

but the OAT has naught to do with Mach.


(b) Looking at an earlier post ..

Just wondered why did they put the end of using Vmo at FL24600 and the start of Mmo at that level isn’t it an indication that this is the trasition altitude between switching??!

Exactly .. Vmo is a specific speed and Mmo a specific Mach Number. Define the two and the equation defines the changeover level for that combination of Mach and CAS.

[condorbaaz]

thanks john.