The Va increment du jour on the DH8 depends on more factors than just wind. IŽll try to keep the answer as short as possible. And of course, I refer to the book as used in my company, the OM-B of FlyBE may vary as Bombardier allows for a bit more freedom in operating its aircraft than many other manufacturers.
For a beautiful, calm/cavok day on a non-limiting runway, we are generally requested to fly Vref+10 for flaps 15 and Va+5 for flaps 35. This applies to about 90% of approaches, but certainly not to this one.
But then along come the exceptions. I will restrict myself to those that may be applicable to the situation the Flybe colleagues faced, there are some others as well.
- For wind, the recommended correction is 1/3 the steady wind OR gusts (whichever is higher), but not more than +20kts. Below 9 kts wind speed this is covered by the standard +10kts (flaps 15), but above that, the actually flown speed will be increased.
- In icing conditions (on the DH8 inflight, they are defined as OAT below +6°C AND visible moisture), the icing speeds Va-Ice apply. Those are generally the basic Va speeds plus a fixed increment of 15 (Flap 35) to 20kts (Flap 15). This speed is flown as it is and not increased for other factors any more: the risk of a nose wheel landing would become too high otherwise especially with flaps 35°. The reason for this increment is the de-icing equipment of the aircraft: the boots will not clear the leading edge completely the way a "hot wing" would, and the extra speed caters for some residual ice.
The weather (...FEW022... 8/4... -RA) at the incident time as reported on ASN would probably show icing conditions as per above definition at the stabilisation gate (1000ft AAL). So if they faced icing conditions, they likely chose Va 15 ICE with the corresponding increment of 20kts. If not, the increment derived by above formula will probably not have been far below it.