Definition
The vertical and horizontal path that a multi-engine aircraft is certified to fly after losing one engine during takeoff, with the demonstrated climb performance reduced by a required safety margin. The reduction (typically 0.8% to 1.0% of climb gradient depending on the number of engines) is subtracted from the gross flight path to account for real-world variability. This net path is what must clear all obstacles in the departure area by the margins specified in the regulations.
Plain English
It's the climb path the aircraft is expected to fly after one engine fails on takeoff, but with a built-in penalty applied so the path looks worse on paper than the airplane can actually do. Obstacles must be cleared using this penalised path, not the best-case one.
Context Anchor
Seen in instrument departure planning, aircraft performance planning, and discussions of one-engine-inoperative takeoff obstacle clearance.
Derivation
"Net" comes from the Old French "net" meaning "clean" or "free of deductions" — the same sense used in "net income" (gross minus a deduction). The net flight path is the gross (demonstrated) flight path minus a required performance deduction.
Why Pilots Care
It guarantees obstacle clearance even when actual performance falls short of expectations after engine failure.
Analogy
It is like using take-home pay instead of gross pay when making a budget. The net number is lower, but it is the number you can safely plan around.
Grounding Statement
Picture the airplane just after takeoff with one engine failed: the OEI net takeoff flight path is the lower planning line that must stay above the obstacles ahead.
Intuition Check
Do not read net as the exact path the airplane will fly. In this context, net means expected one-engine-inoperative performance reduced by a required safety margin.
Example Sentence 1
The dispatcher checked that the OEI net takeoff flight path cleared the ridge east of the airport by the required margin before releasing the flight.
Example Sentence 2
Obstacle clearance is based on the OEI net takeoff flight path gradient published for the runway.