one thing - this isnt my suggestion. Its whats in the AFM.
Err.... no, it's not. The AFM for a 601 just says flaps up at
not less than 1.25 Vs. Therefore either method meets the letter of the AFM. Other types, as has been pointed out, do have at or not below 400' mentioned.
why don't we sort the wheat from the chaff and forget about noise abatement!
Because we're not allowed to. The original question was why do people climb to 1500' before flap retraction - answer, because that's the noise abatement departure as required by the operating rules. I've quoted this previously, but it's EU-OPS1.235