SEATAC, Wash.—At first, air-traffic controllers didn’t seem alarmed when Richard Russell climbed into the cockpit of a small airliner here Friday evening, spooled up its twin turboprop engines and trundled from its parking spot near a cargo area.

Ground-services workers like Mr. Russell sometimes shuttle planes between locations at the Seattle-Tacoma International Airport and other fields without promptly checking in with the tower as required.

This...