Definition
An FAA Advisory Circular that provides guidance for operators on developing obstacle clearance procedures for takeoff, including the analysis required to ensure an aircraft can safely clear obstacles after losing an engine on departure. It describes acceptable methods for identifying obstacles, calculating climb performance, and constructing departure paths, particularly for turbine-powered aircraft operating under Part 121, 125, and 135 rules.
Plain English
An FAA guidance document that explains how operators should plan a takeoff so the aircraft can still clear nearby obstacles even if one engine fails during departure.
Context Anchor
Seen in discussions of one-engine-inoperative takeoff obstacle clearance, engine-out departure planning, and commercial operator takeoff performance procedures.
Derivation
Advisory Circular' (AC) is the FAA's term for non-regulatory guidance documents. The number 120-91 identifies the series (120 covers air carrier operations) and the specific document. Knowing it's an AC tells you it's accepted guidance, not a regulation itself — but following it is a recognized way to comply with the rules.
Why Pilots Care
Allows operators to develop compliant procedures that maintain required obstacle clearance margins during engine-out takeoffs.
Intuition Check
“Advisory” does not mean “unimportant.” An Advisory Circular is not usually a regulation by itself, but it can show a method the FAA accepts for meeting regulatory requirements.
Example Sentence 1
The company's engine-out escape procedures for that mountainous airport were developed using the methods outlined in AC 120-91.
Example Sentence 2
Following the analysis methods in AC 120-91 confirmed adequate terrain margins during the OEI departure.