Question for Airbus Pilots

[Check Airman]

Reading about the Emirates A340 incident in Melbourne, as well as the Virgin A340 at Heathrow, I started wondering about the Airbus FMC functionality. In both incidents, the crews got their V speeds from an external source. Doesn't the Airbus FMC calculate V speeds for the given takeoff weight, similar to the Boeing FMC?

I realize that the numbers may differ from the performance data by a few knots, but surely it should be good enough for a gross error check.

Full disclosure- I've never flown airplanes from either manufacturer.

[RemoveB4Flght]

Take off speeds can be derived from Airbus performance charts, but the process can be rather tedious when numerous factors (meteorological, airfield conditions, aircraft technical issues, CG on the 320) are involved.

Most operators use a third party performance software like FOVE or LPCNG which is an approved alternative and much more user friendly. The Flight Management system calculates the V approach and V lowest selected airspeeds for landing, but does not calculate the takeoff speeds. The take off flap setting also needs to be calculated, along with a method of reducing engine thrust, known as Flexible temperature. This limits the take off thrust by allowing the engine computer to act as if the ambient temperature is much higher than it actually is, which reduces engine wear while still allowing normal safety and performance margins.

The incident you speak of is a prime example of a series of small errors and oversights that nearly lead to catastrophe.

While entering the flight's take off data into the performance program, the first officer made a simple error on one of the digits of the gross takeoff weight. This resulted in performance for an aircraft weighing 200 tons instead of 300 tons, a considerable difference. Most airlines SOP's call for both pilots to make separate performance calculations and then cross check to avoid these errors. In this case it was not done, despite there being two crews on the flight deck.

During the preflight preparation there were several other opportunities to cross check this data and see the inaccuracy of the result. One value the FMS calculates is the best lift over drag speed, known to Airbus pilots as "green dot speed" for how it is displayed on the speed tape. The heavier an aircraft is, the higher this speed will be aerodynamically. When running performance on the computer, the Green Dot speed computed by the software was around 225 knots, which any 340 pilot all tell you is extremely slow for a heavy fuel laden 300 ton aircraft. The FMS calculated a Green Dot above 252 knots. This should have been a warning bell to an experienced crew, especially since this number was recorded. It is thought because the numbers are easily transposed (2-2-5 and 2-5-2) that this could have also been overlooked.

The lower than normal V1, VR, and V2 speeds should also be a warning, however the crew had just flown a shorter leg from New Zealand with a much lower weight, so the speeds are not out of the realm of possibility.

Another gate should have been the abnormally long acceleration time, as a 340 training captain pointed out to me, the average time from stop to V1/Vr is around 45-50 seconds at that weight, and well over a minute had gone by on this roll... most pilots start a timer on the takeoff roll, but this can also be subjective.

As in most incidents, it can be seen as a series of errors in a chain of events rather that a major flaw in one part of the equation.

[Check Airman]

The Flight Management system calculates the V approach and V lowest selected airspeeds for landing, but does not calculate the takeoff speeds.

Thanks. In light of recent events, has Airbus indicated any intention of implementing this functionality?

In the Virgin incident, the crew calculated the takeoff speeds using the expected landing weight. Had the FMC calculated the takeoff speeds, the crew would probably have caught the error when their speeds were significantly lower than the FMC speeds.

[SMOC]

Didn't the crew enter the wrong weight?

Therefore any FMC generated speeds would have been incorrect also.

Rubbish in rubbish out as they say.

It needs a weight and balance system like the 747Fs* that way the FMC can tell you what it thinks as opposed to you giving it information.

* With the W&B option installed.

[OPEN DES]

Hi Check Airman!

Correct me if I am wrong but the speeds calculated by a Boeing FMC are just weight/config dependant?
It will be rather difficult to implement this feature in an Airbus FMGS because the Vspeeds in a FBW Airbus are always optimized in order to achieve maximum flex (=assumed temp). In other words: Airbus Vspeeds are nearly always based on improved climb technique and among many factors are largely dependant on the runway.
The consequence is less transparency for the pilots: very low TOW can produce some very high Vspeeds and vice versa.

Hope this helps!


PS: gross error check is done by many operators by comparing ¨Green Dot¨ speed (=minimum clean speed) between the FMGS and the Performance program. This happens to be the only speed calculated by the program which is purely TOW dependant!

[5LY]

Boeing's FMC does a simple balanced field calc. based on the ZFW that you enter. It doesn't weigh itself for this purpose. The provberbial Garbage in, garbage out is still posible.

[Check Airman]

SMOC & 5LY

The crew had the correct FMC weight, but the perf. computer had a lower weight. Agree that if the FMC weight had been wrong, they would have been no better off in a Boeing.

john_smith,

Very nice to know. Do you have any further information on this? Tolerances etc?

OPEN DES,

I believe the Boeing speeds are simply dependent on weight and configuration. I had no idea Airbus speeds were always optimized. Are the optimized speeds typically that far from the "standard" speeds? My airline doesn't use improved climb, so I don't have much experience with it.

[OPEN DES]

V2 scheduled 20kt above v2min is not uncommon.
True about the h2a update. AFAIK it only checks for vmcg/vmca/v2min. Still better than nothing. GW gross error check is still best achieved by crosschecking weight or green dot between FMGS and perf program.

Rgds

[CONF iture]

as well as the Virgin A340 at Heathrow
Would you have a link for the report please ?

[Check Airman]

Air Accidents Investigation: Download PDF document (http://www.aaib.gov.uk/cms_resources/Airbus%20A340-642,%20G-VYOU%2007-10.pdf)

[Fabo.sk]

Correct me if I am wrong but the speeds calculated by a Boeing FMC are just weight/config dependant?
It will be rather difficult to implement this feature in an Airbus FMGS because the Vspeeds in a FBW Airbus are always optimized in order to achieve maximum flex (=assumed temp). In other words: Airbus Vspeeds are nearly always based on improved climb technique and among many factors are largely dependant on the runway.

Airbus could (if they wished to) allow the FMGS to display balanced field speeds (to be overwritten if so desired).

It would then be same as in Boeing, where you can, of course, also use improved climb technique to allow for higher MTOM or higher AST (or higher fixed derate, or both). Values will differ from balanced field in such a case.

[Cough]

With the new standard of FMS (H2A) the computer will advise you to check performance figures again if the V speeds are inconsistent with the weight entered in the INIT B page.

This applies to A320 series aircraft. I'm not sure if the same feature is available on the A330/340.

We have the latest standard. Tried this today - Entered a V2 10kt below the min V2 speed for our takeoff config and weight. It was accepted without error (and a second one far lower too!) what it didn't like though was a V2 below the entered V1/R.

[9.G]

C.A. if the airbus is equipped with AOC function performance data can be requested to be uploaded. Same as with Boeing. All depends on the perks.:ok: