In BA our QRH (which I guess is derived from the Airbus QRH) says:
For a single failure
- determine the required ldg conf
- determine the delta Vref
- Vapp = Vref + delta Vref + wind correction
- determine the ldg dist factor
For a multiple failure
- use the lowest ldg conf
- use the highest delta Vref
- mulitply the ldg factors together, except where all failures indicatd by an *. In this case take the highest factor.
Example:
(1)
Flaps Fault (F<3, S>=1) Ldg Conf 3; delta Vref = 10kt; ldg dist x 1.15*
Brk Anti Skid Norm conf; delta Vref = 0; ldg dist x 1.5
Result Ldg Conf 3; delta Vref =10; ldg dist x 1.725
(2)
Altn Law Ldg Conf 3; delta Vref = 10; ldg dist x 1.2*
Flaps Fault (F<1, S>=1) Ldg Conf 3; delta Vref = 25; ldg dist x 1.3*
Result Ldg Conf 3; delta Vref = 25; ldg dist x 1.3
So, in summary, sometimes you multiply the factors sometimes just take the larger, it depends on the nature of the faults.....
HTH