Airbus MCDU vs. Boeing FMC V-speed calc...

[pez_man]

Hi!

I have a question I've been wondering about for a long time as a humble flightsimmer.

If I understand it correctly, the Boeing FMC calculates the V-speeds automatically, based on the input data regarding weights and so on. How come that the Airbus MCDU (better get the terminology correct in here! :D ) doesn't do this? Since it being the more modern ac and everything? If I've understood it correctly the pilot must look up the V-speeds in airport-dependent charts and input it into the MCDU manually, or input it via the datalink. How can this data be airport dependant? It all seems a bit strange...

If anyone of you could shed some light over this I'd be a much happier man! (Guess I don't have too many things to worry about then! :D)

ciao!

/fredrik ESGG

[SKYYACHT]

pez man,

The airbus FMGS (made by Sextant) will calculate many different factors, to ensure that flight predictions, such as fuel burn and so on are highly accurate. The airport dependant charts are used to ensure that the aircraft will actually get off the ground at the predicted take off weight.

Aircraft instruments, and aircraft performance are always calibrated against a datum called the International Standard Atmosphere, (ISA) or Jet Standard Atmosphere(JSA). The standard day is as follows

Temp 15°C at mean sea level
Pressure 1013.25Mb (hPa)
Density 1.225Kg/m3
Lapse rate (loss of temperature with altitude) 1.98°C/1000 feet
Tropopause at 36,090 feet (11,000m)

The aircraft manufacturer will calculate the weights that the aeroplane will lift and how much runway it requires to do so assuming the standard conditions. They will also publish a set of correction figures to the weights and published speeds to be applied when the true conditions deviate from the ISA Standard Day.

To illustrate this, if the take off is to be made, from, say, Mexico City, which is thousands of feet above sea lavel, and in summer more than 15°C then the air will be less dense, so the aircraft wing becomes less efficient and so will the engine. Therefore the aircraft will lift less weight. This means that a longer runway may be needed to get off the ground.

Most major airlines will publish their own performance manuals that have a simple table format to enable crews to calculate their take ff performance, including V speeds, and weights for any given runway.

It is interesting to note that the B777 (as operated by my company) has a data uplink available to transmit the performance information directly into the FMS.

Our Airbuses may well have the ability to do an uplink too, but I believe that Airbus philosophy is to keep the crew in the loop.

I hope that helps. If not, send me a Private Message.

Blue skies!
:ok:

[abuspilot]

It can't be explained better than that! :D

[mutt]

Both manufacturers use different philosophies when it comes to calculating takeoff weights and speeds.

Boeing generally uses a BALANCED FIELD method for calculating SPEEDS, in simplistic terms this means that following an engine failure at Vef, the aircraft will either continue the takeoff and reach 35ft by the end of the runway, or stop by the end of the runway. This sort of calculation is constant, for a given WEIGHT/ temperature/ pressure altitude, the V-speeds will always be the same. These basic V-speeds can quite easily be calculated by the FMC. There are some limitations with calculating adjustments for MELS etc.

The Airbus philosophy is called OPTIMIZED VSPEEDS and is based on using the actual runway and optimizing the weights and speeds for that airport/runway/obstacles. In other words while the Boeing may only use 10,000 feet of a 12,000 ft runway for its takeoff calculation, the Airbus will use all of the 12,000 ft. This calculation is a lot more detailed and cannot be done by the FMC.

Boeing also offers the OPTIMIZED VSPEEDS option, these can’t be calculated by the FMC and will therefore be charted for each airport, as Airbus, or uplinked to the aircraft.

Like everything else in aviation, there are pros and cons for both philosophies.

Mutt.

[pez_man]

Wow, thanks a lot for your very good answers! It all seems a little bit clearer now!

ciao!

/fredrik ESGG

[SKYYACHT]

Abus pilot,

Thanks for the compliment! Thats what happens when you spend in excess of four hours a day in the sim teaching the thing...!!

Good luck with it Frederik.



Blue skies....

:O