I read "as a spare" and instantly had the mental image of the flight engineer, out on the wing adjustable spanner clutched in his mouth. A roaring 500mph wind blowing his hair all over the place.
Unbolts the burning engine, and watches it plummet into the sea thousands of feet below. Then swings the new engine across, nearly losing his grip and falling off himself, tightens a few bolts. Thumbs up to the cockpit window, it starts up. Film score goes from dramatic chords back into a major key for some 'success' music. He gets back in the plane, shuts the door, issues a wisecrack to the stewardess, and casually walks back to the cockpit.