Approach controllers at class bravo airports usually assign 170kts to the final approach fix. My company has a stable approach criteria that I must be Flaps 45 (landing flaps) at the FAF. Flaps 45 have a speed limit of 170kts. This is therefore an impossible setup.
This is the correct solution and it should be done early. On initial contact with approach, they say "expect X approach", reply with "Roger and unable 170 to the FAF, will need to slow 1NM prior".
Waiting until the speed assignment is given is too late because the approach sequence and traffic separation will already be setup out.
This method is to slow just above 160kts. The justification is ATC speed assignment tolerances:
"Pilots complying with speed adjustment instructions (published or assigned) should maintain a speed within plus or minus 10 knots or 0.02 Mach number of the specified speed."(https://www.faa.gov/air_traffic/publications/atpubs/atc_html/chap5_section_7.html)
The problem is... ATC knows. They know because everyone else's FMS and autothrottles are flying the assigned speeds exactly correct. They know because the spacing infront of our aircraft seems to get wider, and the spacing behind us seems to get tighter. They know because sometimes the separation becomes too small and they have to call go arounds.
Another method is to cheat the stabilized criteria; fly the 170kts as assigned, and configure past the FAF.
There are two reasons why I think crews choose this method:
Regarding belief #1, the other operators which configure and slow past the FAF probably have certified vertical guidance. The CRJ's vertical guidance is only advisory. The CRJ pilot flies a CDA non precision approach by observing ground speed and setting the vertical descent rate. Changing speeds from 170kts to Vref requires a varying descent rate.
Regarding belief #2, lets look at my SOPM and see what it has to say about applicability and compliance:
"This manual provides mandatory guidance."
"[deviation allowed when] Immediate alternative action is required to maintain the safety of flight."
"When a conflict occurs between its contents and the compliance with any FAR, Ops Spec, or foreign regulation, the latter takes precedence."
Does a speed assignment from ATC count as compliance with a FAR that conflicts with SOPM? Possibly, except that...
"Prior to accepting ATC requests for fast or short-approaches consider the ability to meet [stabilized approach] criteria,"
and...
"crews must notify ATC if they are not able to comply with an assigned speed restriction."
I think it's quite clear, if you got caught, there would be no defense here. ATC speed assignments on approach are specifically mentioned as a situation where the stabilized approach criteria must be followed.
Considering recent industry incidents, stabilized approach criteria are going to be a hot item for the FAA when inspecting regional airlines.