PEI-3721:
Taking your comments in order:
AFAIR the certification standard for the Dash 7 (and similar aircraft) was a special condition, ‘STOL category’, which may have originated in Canada.
Correct, but many aircraft have 'special conditions' applied to certificate unusual or unique features under FAR25. Nothing unusual there.
the airlines flying into Aspen, Vail, etc, operated to FAR 135
My recollection is that they operated under the short lived 'FAR135-II' regs, which were basically identical to, and eventual merged with FAR121.
Steep approaches to airports differ from those approaching a runway. The latter requires a full steep approach certification for the aircraft – straight in landing, whereas an approach to an airport can be steep (>4.5 deg), but is then followed by a ‘normal’ landing (<4.5 deg); this is often a circle to land procedure, e.g. old Aspen procedure.
All correct, I believe. The unique thing about the Dash 7's STOL operation was that it assumed a continued approach and landing with OEI. If I'm not mistaken, I believe aircraft operating to the LCY type of approval must have all engines operating at the start of the procedure?