ASAP’s enroute performance evaluates obstacles between takeoff and landing in the event of an engine failure. Loss of an engine at cruise causes the aircraft to drift down to the single-engine service ceiling, which our calculations model to ensure required obstacle clearance. Method One Drift Down plots the aircraft trajectory, calculates fuel burn for weight loss, identifies obstacles, and considers atmospheric conditions, temperature, pressure, and safety margins to determine maximum weight for clearance.

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Method Two Drift Down extends Method One for routes with limiting obstacles: if failure occurs before the obstacle, the aircraft must return to the departure or alternate airport; if after, it may continue to the destination, avoiding the takeoff weight penalty required under Method One.