OK,what is the subject of this thread, "when should we arm the APP mode in general" or "when to do it in the 139"?
If you are flying a WWII aircraft, like a '412 or a '76 and all you have is VOR/ILS/DME then, by all means push that APP or ILS button on your FD controller only when ATC has given you the final vector to "intercept the LOC" and cleared you for the approach, and you are in a position to do so.
For the rest of you in the 139, when using the PRV mode or on HDG mode because receiving vectors to the FAC, arming the APP mode works the same way allowing the system to capture the LOC first and the GS if correctly positioned and the
APPROPRIATE MODES are
ENGAGED and
CAPTURED in the FD, so caution ought to be excercised to have been cleared for the approach before pushing that button.
BB, even you folks south of LA82 ought to know the meaning of shooting an approach, so your remark is not clear.
Mark Six, that
note may be in the blue side of the QRH, it is however a
note to remind the crew that arming APP when flying in the wrong direction
(as in away from the localizer) may result in "startling" manouvering. You are right, you never implied it was a limitation because the only limitation for that purpose is the one I specified with regard to the intercept angle and distance from the localizer, so if you are whithin those angles you don't need to be "almost right on top of the GS".
One more word of caution applies to reporting information heard from someone who heard it from someone else, you know the drill.