mutt:
Prior to the AC, there was no FAA guidance telling airlines to do anything apart from clear all obstacles in the takeoff flight path. For Missed Approach procedures there was even less information.
I find it disappointing that the FAA have only issued an Advisory Circular, as they are there for information, you don't have to adhere to them. I would much prefer to see the word "MUST" used rather than "SHOULD".
If AC 120-91 stated "must" or "shall" then instead of being an advisory circular, it would be a de facto amendment of F.A.R. 121.189 and related regulatory sections. The FAA cannot make rules via ACs. If they tried to do that, it would likely throw the entire issue into a "full press" rule-making process, including formal public hearings, etc. Simply put, there are marginal operators that do not wish to be bound by AC 120-91. Rather, they use a very sharp pencil to "comply" with the 600 foot wide 121.189 OEI path beyond the airport boundary.
Having said that, for benefit of the lurkers, a very good example of the missed approach case, and how it bears absolutely no relationship to FAR 121.189 is KBIH, Bishop, California. Bishop has a good RNP AR IAP to Runway 30, but Runway 12 would not qualify. So, two LNAV IAPs were developed to Runway 12, one with very low straight-in minimums for this airport and a climb-gradient missed approach, and the other with a missed approach that is 40:1 clear and, thus, with much higher minimums. These IAPs are strictly TERPs. The procedures folks who designed them know nothing about performance engineering, nor should they. They are making normal ops IAPs for every conceivable type of airplane, from a well-performing light airplane to an Airbus 320 or Boeing 767.
Low minimums with climb gradient:
http://aeronav.faa.gov/d-tpp/1108/05737RZ12.PDF
High minimums with 40:1 clear missed approach (assumes not less than 200 feet per mile CG):
http://aeronav.faa.gov/d-tpp/1108/05737RY12.PDF
As you can readily see the missed approach flight track for either of these IAPs, particulary the "Z" (low minimums) approach is very likely where the performance engineer would not want to send his airplane in the event of an engine failure at, or near, MDA. He might rather continue down the valley, where there are airspace and steeper terrain problems, nonetheless a fairly wide valley for 70 miles, or so.
What makes this airport and, in particular, Runway 12 very interesting is that IFR takeoff minimums are not authorized because of the close proximity of terrain to the D.E.R. (This issue is avoided with the IAPs by having the MAP prior to the runway threshold, plus even being at the lower MDA, you have a "running start" as opposed to an engine failure just above V1 on takeoff).
KBIH takeoff minimums (you have to scroll down a bit to located Bishop):
http://aeronav.faa.gov/d-tpp/1108/SW2TO.PDF
Now, let's say the missed approach engine failure occurs after turning east into the "merry go round" missed approach track. Do I want to continue around towards higher terrain or do I want to escape through the saddle into the valley to the east? Only performance engineering tailored to the specific airplane type can make that determination. Because engines can fail at varying points from MDA (or even prior to MDA) to infinately varying points along the missed approach flight track, is the very reason why AC 120-91 states that more than one OEI missed approach path may have to be provided for a given runway at a location as complex (terrain rich) as Bishop.
Another aspect, if the decision is to continue with the missed approach path back to BIH VOR because of an engine failure fairly late in the game, does holding have to be considered? Of course not, the holding pattern is what drives the 13,000 final altitude. Proceeding OEI up the valley to the north could result in a less demanding OEI path.
What is very apparent to the trained person is that none of this bears any relationship to TERPs, other than the performance engineer may elect to continue with the charted missed approach flight track, albeit without TERPs containment assessments.
In my considered opinion AC 120-91 should be regulatory and it should be updated to provide RNAV and even tight RNP OEI flight path options. But, this is a classic case of the "tombstone factor" at the FAA when something as critical as payload politics is involved. If the day comes when a marginal operator who does not use AC 120-91 pastes a bird on a mountain side while OEI then that would get things moving.
Finally, I can come up with many airports with OEI issues every bit as demanding as Bishop. And, as we know, these types of airports exist throughout the world. Latin American is rich with them.
Note: The URLs provided here will expire on August 25. But, the same charts would be available after that with a bit more searching at:
Digital Terminal Procedures/Airport Diagrams