Reports of a fire in the left engine of Northwest Airlines Flight 255 before it crashed in Detroit Sunday night lend urgency to investigation of an aircraft and a jet engine that have otherwise compiled an enviable safety record for seven years.

The McDonnell Douglas MD80, a quieter and more fuel-efficient version of the workhorse DC9, was certified for service Aug. 25, 1980, outfitted with an updated version of the standard Pratt & Whitney JT8D engine used on more than two-thirds of U.S. airliners. Since then, the fleet of more than 400 planes has proved popular with airlines and crews.

The MD80 had been involved in only one earlier crash, determined to have been caused by a navigational error rather than any fault with the plane. On Dec. 1, 1981, a chartered MD80 carrying Yugoslav tourists crashed in stiff winds and dense fog as it approached the airport in Corsica, France, killing 174.

Although considered reliable, the JT8D occasionally fails and, according to Federal Aviation Administration records, an unusually high proportion of failures on MD80s occurred on Republic Airlines, absorbed into Northwest last October.

Of 16 reported failures in the seven years of MD80 operation, five were on Republic. Two failures were on the plane that crashed in Detroit, a former Republic aircraft registered as N312RC.

In late 1985, a turbine blade fractured on the left engine of N312RC as it climbed away from Minneapolis, shutting down the engine and forcing it o fly 18 minutes on one engine to a safe landing. Early last year, a turbine blade on its right engine failed on takeoff from San Francisco, forcing it to fly 11 minutes on one engine.

In each case, the engine was replaced, so the one involved in Detroit was not the same one involved in the other two incidents.

Federal aviation officials are just beginning their investigation of the Detroit crash, and the cause probably will not be known for months.

Officials investigating the crash said last night that some of 25 to 30 witnesses they interviewed reported seeing fire in the left engine of the two-engine plane.

John K. Lauber, heading the National Transportation Safety Board (NTSB) investigation, said no initial evidence has been found to corroborate reports that fire penetrated the engine housing.

Engines may catch fire, or appear to, for various reasons, but fire in only one engine normally would not be enough to cause a crash, especially in the few seconds that elapsed between the takeoff and crash of Flight 255.

The MD80 is certified to take off and fly with only one engine in service.

That raises the question of "uncontained" or "catastrophic" engine failure, in which parts of an engine -- spinning at up to 8,000 rpm -- fly loose and penetrate the engine covering. In rare cases, the failure is so violent that the plane's fuselage is penetrated. Older versions of the JT8D were involved in several such incidents.

But only a few uncontained failures have been reported in the JT8D-217, designed specifically for the MD80, and those have caused no substantial fuselage damage.

The in-flight shutdown rate for the JT8D-217 is one in 20,000 hours, or about one every 2 1/4 years for an average commercial MD80, a good rate. The engine would qualify on a reliability basis for use in two-engine flight over water.

About three years ago, an Austrian Airlines MD80 had one catastrophic uncontained failure in which several turbine blades flew loose, according to federal sources. "It was an oddball event," caused by an improperly seated seal, one source said.

Three uncontained failures have been reported recently. In each, a shroud, which is not supposed to turn, began to rotate and penetrated its casing. Because such failure automatically shuts down the engine, no major damage was caused.

In each case, the cause of the trouble was found, and corrective measures have been or are being taken.

Just as a fire alone is not enough to cause a crash, a catastrophic failure alone will not cause a crash. A plane will not go down unless the fuselage is penetrated enough to damage such vital systems as hydraulic lines that control flight or fuel lines that could feed a fire.

In August 1985, an engine failed on a Boeing 737-200, which has under-wing engines, during its takeoff roll in Manchester, England. The pilot managed to stop, but 55 people were killed in a fire after engine parts punctured the fuel tanks.

A month later, in Milwaukee, a Midwest Express Airlines DC9 crashed on takeoff after a catastrophic engine failure, killing all 31 on board. However, the NTSB ruged that the plane was still flyable and faulted the crew for not taking proper action.

In both incidents, the engine parts that flew apart and caused the failure were ordered replaced.