Applicability

This regulation introduces the performance operating limitations that apply to all Part 121 air carriers and defines two critical terms used throughout the subpart.

The "effective length of the runway" for landing isn't necessarily the full paved surface—it's measured from where an imaginary obstruction clearance plane intersects the runway centerline to the far end. This matters because obstacles near the approach end can effectively shorten the usable runway length.

The "obstruction clearance plane" is an imaginary surface rising at a 1:20 slope from the runway. It starts 200 feet wide (100 feet each side of centerline), widens to 500 feet at 1,500 feet from the runway, then follows the departure or approach path. Any obstacles penetrating this plane reduce the effective runway length available for performance calculations.

These definitions ensure airlines account for terrain and obstacles when calculating whether their aircraft can safely operate from a particular runway under Part 121's stringent performance requirements.

*This is a plain-English summary for study only. The official 14 CFR text on this page is controlling — always read the current regulation and consult a CFI.*