The shortest scheduled jet flight I was on went -- years ago -- from San Francisco to Oakland, maybe 9 nautical miles. The flight continued to PDX.
The current UA schedule shows these short jet flights (I've left out duplicate jet flights on the same routes):
UA 786 BUF-ROC 55 miles Boeing 737
UA 5616 ORD-MKE 67 miles Canadair RJ
UA 1924 ORD-MKE 67 miles Boeing 727
UA 877 STT-SJU 68 miles Airbus 320
UA 2916 DEN-COS 72 miles Boeing 737
Some other airline must have scheduled jet service for a route shorter than 55 miles. Bay area inter-airport? MIA-FLL? Caribbean island-hops?
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"If you want to win the game, write the rules."