The manufacturer of the jet engine involved in the Manchester, England, accident Thursday urged the world's airlines yesterday to be certain that they are properly inspecting the combustion chambers of the widely used engine.

Pratt & Whitney, a division of United Technologies Corp., told the airlines that they should be particularly careful to check JT8D engines that are slow to start or accelerate to make sure there are no cracks in the combustion chambers and to be certain the chambers are properly aligned.

About 12,000 of the engines are in service worldwide on Boeing 727s and 737s and McDonnell Douglas DC9s. It was a Boeing 737-200 that caught fire on its takeoff roll in Manchester Thursday, killing 54 people.

Aviation officials said yesterday that they do not expect the engine checks will disrupt airline service because Pratt & Whitney recommended only that previously suggested practices be followed, and most carriers presumably have done so.

The Federal Aviation Administration is not planning a mandatory inspection order for U.S. airlines at this time because "we don't see anything yet we should" warn about, a top FAA official said. "Like everything else it will take us a few days to really understand what happened."

The JT8D has nine combustion chambers, which look like portable fire extinguishers and are arranged around the sides of the jet engine between its compressor section and its turbine section. Compressed air is forced into the chambers, then mixed with jet fuel and ignited. Exhaust gases through the turbine provide the thrust for the airplane and the rotation for the engine. Pressures inside the combustors can reach 220 pounds per square inch and temperatures hit 1,700 degrees Fahrenheit.

For some reason, the wall of one of the combustors on top of the engine apparently failed, the sources said, then blew out the top of the chamber casing.

It is presumed but not established that the chamber casing shot into the underside of the 737's wing and punctured the fuel tank there. If fuel then spilled directly into the hot engine, ignition and a large fire would be assured. A piece of chamber casing was recovered along the runway in Manchester, the sources said.

The fan blades and discs in the engine's compressor and turbine sections were intact. They were early suspects because a classic jet engine failure is a broken fan blade that spins out of the engine casing to damage other parts of the aircraft.

FAA officials said there are four previous instances of JT8D combustion chamber cracking in the long history of the engine, but other aviation sources said the Manchester incident is the first that manifested itself in this way.

Pratt & Whitney released a brief statement late yesterday saying that its advisory to all JT8D operators reemphasized "earlier recommendations covering inspection and maintenance of the combustors. We felt that it was prudent to reemphasize these recommendations . . . . "

Pratt & Whitney experts are participating in the Manchester accident investigation. It is normal for engine and prime manufacturers to aid investigators.

British Airways yesterday ordered immediate computer checks of the engines on its 30 Boeing 737s, but did not ground any of the planes. British Airways owns British Airtours, the charter flight airline whose plane caught on fire.