Always thought the D suffix was used for significantly delayed flights, to potentially avoid having two flights airborne with the same flight number at the same time. Pretty sure that is ICAOs guidance.
As for the D being used in the flight number to indicate that it is a DH8D and therefore a different wake turbulence player than the A, B and C models, surely simply knowing that a D is greater than 25000kg MTOW, as most professionals in this occupation should, will suffice.
Anyway, long time lurker, first time poster. Nice to be on board.
