Latest victim of the jet squeeze: The Netherlands’ KLM, one of Europe’s few privately managed airlines, showed a loss of $21 million, the biggest in its 42-year history.

In Europe as in the U.S., the jet age is a convenience to passengers and a financial headache to the airlines. Between the high costs of the switchover to jets and the bitter competition for passengers to fill the bigger jets. West Germany’s Lufthansa last year lost about $25 million,…