Airline maintenance crews worked throughout today to check engines on Boeing 737 aircraft following an order by Britain's Civil Aviation Authority based on preliminary results of the investigation into last Thursday's crash in Manchester of a 737 whose engine exploded.

The authority's order, issued late last night, effectively grounded about two dozen 737s until the checks were carried out.

By this evening, most of the six airlines using the U.S.-manufactured Pratt & Whitney JT8D15 engine had completed the process, and the planes were again in the air.

One air charter company, Orion, reported finding a crack in an engine combustion chamber, and that plane was withdrawn from service.

After the British directive was issued, the U.S. Federal Aviation Administration announced last night that it would order U.S. airlines to check engines in the JT8D series, used on about 2,000 planes.

Flight delays of up to six hours were reported at airports throughout Britain while the planes, primarily used for charter flights to the Mediterranean, were checked.

Passengers were bused to different airports as planes became available. Few canceled, but airlines reported calls from passengers asking if the planes were safe even after the checks.

Following last week's crash in Manchester, in which 54 persons died, British Airways independently ordered computerized examinations of engines on all 44 of the Boeing 737s it operates. The crash followed the explosion of an engine during the takeoff of a charter flight operated by British Airtours, a British Airways subsidiary.

Air accident investigators, working with officials from Pratt & Whitney and Boeing, have attributed the engine explosion to a faulty combustion chamber.

The independent British Airways check revealed what the Civil Aviation Authority said were "various combustion chambers with extensive cracking to the cooling rings and chamber heads" in six engines.

Based on these results and on the initial indications of the investigation into the Manchester crash, the authority ordered checks of all British airliners using similarly equipped 737s.