Definition
An FAA Advisory Circular titled 'Airport Obstacle Analysis,' which provides guidance and acceptable methods for operators to develop one-engine-inoperative (OEI) takeoff obstacle clearance procedures. It describes how to construct departure paths, identify and account for obstacles, and apply climb gradient requirements following an engine failure on takeoff.
Plain English
A guidance document from the FAA that explains how operators should plan a safe departure path if one engine fails during takeoff, including how to stay clear of terrain and obstacles.
Context Anchor
Seen in instrument procedure and performance discussions about one-engine-inoperative takeoff obstacle clearance requirements.
Derivation
An 'Advisory Circular' is an FAA publication that offers non-regulatory guidance — 'advisory' meaning it advises rather than mandates, and 'circular' from the older sense of a document circulated to inform a group. The number 120-91 is simply the FAA's filing identifier within the 120 series, which covers air carrier operations.
Why Pilots Care
Following its guidance keeps the flight within regulatory safety margins and prevents obstacle strikes after an engine failure on departure.
Intuition Check
Do not read advisory as meaning unimportant. Here it means FAA guidance: usually not a regulation by itself, but often an accepted way to show that the required safety standard is being met.
Example Sentence 1
The operator's takeoff obstacle analysis was built using the methods described in AC 120-91.
Example Sentence 2
During recurrent training the crew reviewed Advisory Circular (AC) 120-91 procedures for obstacle clearance after an engine failure just after liftoff.