Definition
A multiengine airplane procedure performed after an engine failure has been confirmed and the engine has been identified and verified, in which the pilot completes the manufacturer's published steps to shut down the inoperative engine and configure it to minimize drag. Typical actions include moving the mixture to idle cutoff, the propeller to the feather position, and the fuel selector and magnetos off, along with isolating the failed engine's electrical, hydraulic, and fuel systems as specified in the Airplane Flight Manual.
Plain English
It's the written list of steps a pilot follows to fully shut down a dead engine in a twin and clean it up so it stops dragging on the airplane.
Context Anchor
Used in multiengine engine-failure training and procedures, especially after takeoff or at low altitude once the pilot has control of the airplane and enough performance to keep climbing.
Derivation
Securing' comes from the Latin securus, meaning 'free from care.' In aviation it means making something safe and stable so it no longer needs attention -- in this case, taking the failed engine fully out of operation so the pilot can focus on flying the airplane on the remaining engine.
Why Pilots Care
Correctly completing the checklist reduces asymmetric drag, lowers the risk of fire or mechanical damage, and allows the aircraft to maintain safe single-engine climb or level flight.
Intuition Check
Do not read securing as tying something down. Here, securing means making the failed engine safe, shut down, and as low-drag as practical.
Example Sentence 1
After confirming the right engine had failed, the pilot called for the securing failed engine checklist and feathered the propeller.
Example Sentence 2
After leveling off on one engine, the instructor directed the student to run the securing failed engine checklist to simulate a complete powerplant shutdown.