Airbus produced performance data is compliant with JAR-OPS 25 (ok now EU-OPS) at all times. In fact, there is nothing particularly odd about the assumptions made within the calculations. What is slightly unusual is the notion that for any given weight there are a range of take-off speeds that comply with ‘the law'. Picking the best speeds within this range is the job of OCTOPUS (the programme that uses algorithmic calculations to produce our performance data).
x OCTOPUS would like us to lift the most amount of weight (mass, actually) off the runway and it calculates Take-Off Performance Limit or TOPL not structural weights;
x It uses the excess margin to produce the highest FLEX figure possible. FLEX, or rather lack of FLEX, is so important to us (read costly!) that the Airbus Fleet uses 1 degree FLEX increments whereas other fleets only use 2 degree steps.
Pages in the performance manual are produced by OCTOPUS – they are not necessarily performance numbers based directly upon the stall speed but on a range of ratios of V1 to Vr or Vs to V2. In this way they are identical to the numbers produced by a CARD request.
If OCTOPUS cannot lift the maximum TOPL from a runway at ‘standard' speeds (i.e. a V2 defined by a fixed ratio on stall speed) then it will use excess runway to to push up V2 in order to improve second segment climb performance (essentially higher speed = less induced drag).
Obviously, this is only useful on runways that are more than long enough for the Airbus. Fortunately for us this applies to nearly every runway on the Airbus route network – therefore improved climbs are ‘the norm'.
There are a range of possible V1s before we consider the take off calculations. On the Airbus CARD and OCTOPUS always calculate the MAX and MIN V1s but we publish (and therefore use) a numerical mean value called MEAN V1.
At the upper end of the range we might be limited by brake energy or runway length before Vr becomes a limiting parameter. At the lower end we might be limited by the energy requirements of the ‘Go-case' considerations – remember from V1 we must be able to stop but we must also be able to go having suffered an engine failure.
When we are very light we can stop and go with ease, we have lots of excess power (even on one engine) and oodles of brake energy. OCTOPUS calculates the range of V1s and finds that we are limited by Vmcg (we can go) and Vr (we can stop). This range is nearly as wide as it can be and therefore MEAN V1 – the value that we use and CARD publishes is between Vmcg and Vr. A Split.
At high weight the picture is different – we can still stop from Vr (even using high, optimised speeds) because we have very powerful brakes but it is not possible to reach those high speeds with only 50% of our thrust remaining. Therefore we reach the following scenario. There is now only a small spread of speeds from which to take our average V1. In this case we are left with little or no V1/Vr split.
In summary, our spread of possible V1s is constrained by the need to go as well as stop. Our fleet policy is to calculate and publish the mean V1. This balances the risks associated with low speed GO (from min V1) against those of a high speed STOP. It is not possible to second guess CARD published V1s and therefore the V1s must be respected however small or large the V1/Vr split.
Hence the term mean V1. As to CG affect on V speeds, they don't. The CG does however affect our ASDR and thus the flex range. Hence why we input it on my fleet. Balanced V speeds are based on Balanced fields with no consideration for obstacle or climb limits.
Last edited by VNAVPTH; 19th Jul 2012 at 21:50.