Pugilistic Animus
From the Instrument Procedures Handbook:
Typically, the AFM for a large turbine powered airplane should contain information that allows flight crews to determine that the airplane will be capable of performing the following actions, considering the airplane’s landing weight and other pertinent environmental factors:
• Land within the distance required by the regulations.
• Climb from the missed approach point (MAP) and maintain a specified climb gradient with one engine inoperative.
• Perform a go-around from the final stage of landing and maintain a specified climb gradient with all engines operating and the airplane in the landing configuration.
One must be mindful that the procedure-specified "gradient" in a missed approach is linear -- compare to a sheet of plywood.
But we know that Transport Airplanes don't climb along a linear path in the real world.
The AFM data for approach climb specify the operative engine(s) at takeoff thrust. There are time limits for takeoff thrust. And we also know that available thrust will be decreasing with altitude as the jet climbs in the clean configuration (enroute and final) with thrust set at max continuous. So the actual climb path will be anything but linear.
An IFR procedure's missed approach may include instructions to "fly runway track to _______MSL, then turn to (
heading), intercept _____ radial until leaving______ MSL, then proceed direct_______"
In WAT-limiting conditions, the aircraft may be many, many miles downstream prior to reaching the first turn's altitude, may find itself up against time limitations for the approach climb thrust and may require a transition to enroute and final climb configuration (yet another set charts).
So it's a little simplistic for the Instrument Procedures Handbook to state that "crews can determine that the airplane will be able to maintain a specified climb gradient with one engine inoperative." (paraphrasing)
The key sentence in AC 120-91 paragraph seven may be:
For the purposes of analyzing performance on procedures developed under TERPS or PANS-OPS, it is understood that any gradient requirement, specified or unspecified, will be treated as a plane which must not be penetrated from above until reaching the stated height, rather than as a gradient which must be exceeded at all points in the path.
The OEI performance analysis for locations with lengthy and complex missed approach procedures can tax or exceed the skill sets and competence of average cockpit crew. (Such an analysis is certainly beyond my own abilities.)