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Airbus MCDU

Posted: Mon May 16, 2022 12:48 pm
by stephen88
I’ve noticed that on the Airbus MCDU when calculating the V1, VR and V2 speeds for take off there is nowhere to input the airport QNH, temperature and winds which all effect your take off speeds, how comes the MCDU doesn’t give you this option to input this but it does for when calculating approach speeds.

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Re: Airbus MCDU

Posted: Mon May 16, 2022 12:50 pm
by rockafella306
Because irl Airbus recommends using the fly smart app to compute the above mentioned speeds. This is in the modern era, older methods required you to use charts and tables.


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Re: Airbus MCDU

Posted: Mon May 16, 2022 3:54 pm
by zeke
stephen88 wrote:
I’ve noticed that on the Airbus MCDU when calculating the V1, VR and V2 speeds for take off there is nowhere to input the airport QNH, temperature and winds which all effect your take off speeds, how comes the MCDU doesn’t give you this option to input this but it does for when calculating approach speeds.


The MCDU does not calculate V1/VR/V2, the teal/blue colored items can be modified by the pilot, the green number are calculated by the aircraft.

Re: Airbus MCDU

Posted: Mon May 16, 2022 7:20 pm
by Woodreau
The environmental inputs are done in an approved performance app external to the MCDU or manually from the performance manual…. (Gag me with a finger)

After takeoff performance is calculated the v speeds flap settings are finger inputted manually or uplinked automatically or manually to the MCDU.

Re: Airbus MCDU

Posted: Mon May 16, 2022 11:25 pm
by Starlionblue
As mentioned the QNH, temperature, runway condition and such are inputs to the takeoff performance calculation. The V speeds, flap setting and thrust setting are the output.

This calculation is nowadays typically done in FlySmart, but can also be done with a remote system via ACARS/web/SMS.

Re: Airbus MCDU

Posted: Tue May 17, 2022 2:06 pm
by stephen88
Thanks for all your replies everyone, that clears it up :)

Does anyone know why then the MCDU doesn't ask for winds, QNH, temperature on the takeoff performance but then allows you to enter this information for the approach/landing phase?

Re: Airbus MCDU

Posted: Tue May 17, 2022 8:02 pm
by Alias1024
stephen88 wrote:
Thanks for all your replies everyone, that clears it up :)

Does anyone know why then the MCDU doesn't ask for winds, QNH, temperature on the takeoff performance but then allows you to enter this information for the approach/landing phase?


Isn’t that the same question?

To summarize, the MCDU does not have entry lines for those parameters on the takeoff page because it does not calculate takeoff performance. That is done through another program and the resulting speeds and thrust setting are either uplinked or manually input.

It has those parameters on the Approach page because it IS calculating the speeds based on weight and environmental conditions. Wind is input to calculate ground speed and allow the ground speed minimum function to operate on approach.

Re: Airbus MCDU

Posted: Tue May 17, 2022 11:15 pm
by Starlionblue
As Alias1024 says, it comes down to how the calculation is made.

Backing up a bit, I think the basic reason we do the calculations differently is because of the rather different situations, before takeoff compared to before landing.

Before takeoff, you're at zero airspeed and zero AGL. Full attention can be given to a performance calculation. Plenty of time and attention span to plonk the numbers into the iPad/MCDU/chart and figure it out. Takeoffs are also almost always more performance-critical than landings. The company doesn't like it when you leave payload on the ground, meaning the calculation needs to be rather precise and not infrequently right up against the performance margins.

Before landing, you're in the air, so you need to also pay attention to aviating, navigating and communicating. This is especially true if you're already on approach and for some reason need to change the inputs, for example due to runway change or weather change (new ATIS). Besides, you need less runway than on takeoff, and more importantly the situation is dynamic. A gust of wind can push your touchdown point 50 metres down the runway. So the calculation by necessity has more padding and does not need to be quite so precise. Many operators don't even require a landing distance assessment if it is obvious you have more than adequate margins, e.g. a long runway with good weather at normal weights.

Re: Airbus MCDU

Posted: Wed May 18, 2022 12:21 am
by zeke
stephen88 wrote:
Thanks for all your replies everyone, that clears it up :)

Does anyone know why then the MCDU doesn't ask for winds, QNH, temperature on the takeoff performance but then allows you to enter this information for the approach/landing phase?


The environmental conditions for approach and landing are used for approaches which have barometric aided vertical profiles, the tower wind is used by the autothrust for a feature called “ground speed mini”, essentially ensures the aircraft IAS is automatically adjusted on approach based off the actual wind and the tower wind to have near constant ground speed on final.

The wind, temp, and QNH on that page have no bearing on landing performance calculations, they are performed in flysmart much like the takeoff data. The MCDU does not know things like runway surface conditions, temporary adjustments to the landing distance available, or obstacles in the missed approach.

Re: Airbus MCDU

Posted: Wed May 18, 2022 11:07 am
by stephen88
Very informative responses everyone, thank you for taking the time to explain it all :)