jumpseat
v.— «Someone at my company was jumpseating on FedEx and he said the crew told him he was on a flight that would do what was known as sweeps. Staying in the air for about an extra hour in case a plane broke down somewhere at an outstation.» —“FDX (and maybe UPS?) “Sweeps” – A Question from an Outsider” by contrails Airline Pilot Forums Dec. 29, 2007. (source: Double-Tongued Dictionary)
Further reading
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Active for many years in the aviation world I can tell you that “jumpseat” is the word commonly used for a reserve seat in the cockpit of an aircraft. Apart from the seats for the pilot and the co-pilot there is a third seat which is normally used when the check-pilot is on board. Sometimes it may be used, with the permission of the captain, by airline staff other than flying personnel or by air traffic controllers on duty trip. When, long before the safety regulations for cockpits, a passenger was allowed to visit the cockpit, he also used that “jumpseat”.