An Air Force mechanic who was trained to handle high explosives was convicted by a federal judge yesterday of planting a bomb in a suitcase his wife took on a March 2 Braniff flight from Washington National Airport.

The mechanic, Airman Martin Thomas Bradley, 27, had previously mentioned to friends he planned to kill his wife, who carried life insurance policies totalling $145,000, according to a prosecution statement produced at his trial yesterday in U. S. District Court in Alexandria.

Bradley, shaking and weeping during his 45-minute court appearance, was ordered held under $400,000 bond for sentencing July 16 by Judge Albert B. Bryan Jr.

Bradley, arrested before dawn March 3 at Andrews Air Force Base, faces an 18 1/2-year prison term on a charge of placing an explosive device on the plane. The term is part of his agreement with federal prosecutors. A federal judge in Maryland recently dismissed three other charges against Bradley and ordered the case transferred to the Alexandria court, which has jurisdiction over National Airport.

Court documents filed there yesterday quoted a government expert as saying that "at the very least" a large fire and possibly destruction of the Texas-bound Boeing 727 jet would have occurred had the powerful bomb detonated in flight.

The FBI was unable to determine whether the device--a black cylinder containing gunpowder, explosives and two-inch nails and wired to a clock--failed to explode because of a malfunction or because the time-delay had not been reached, according to the documents.

The bomb was discovered ticking by Bradley's wife, Air Force Staff Sgt. Mary Jo Bradley, as she unpacked at her destination, Sheppard Air Force Base in Wichita Falls, Tex. The device was damaged during disarming by a bomb expert, making analysis difficult, the documents said.

The incident has touched off investigations by a congressional committee and the Federal Aviation Administration as to how the bomb passed undetected through luggage checks at National and Dallas-Fort Worth Airport.

Bradley spoke little yesterday, except to acknowledge in a soft, deep voice that he understood the proceedings. Although he pleaded innocent, Bradley waived his right to a jury trial and his right to appeal the verdict. His lawyer, U.S. Public Defender Fred Warren Bennett of Baltimore, said afterward that Bradley was deeply depressed and unable emotionally to plead guilty to planting the bomb.

Judge Bryan recessed the trial briefly to read a 20-page statement of facts presented by prosecutors and undisputed by the defense. There was no testimony.

The statement said the Bradleys' six-year marriage was unhappy and that Bradley had been unsuccessful in persuading his wife to agree to a divorce. On two occasions the airman mentioned killing his wife in talks with friends, according to the document.

The statement also said Mary Jo Bradley carried three life insurance policies totaling $145,000, naming her husband as the sole beneficiary.

Before enlisting in the Air Force, Bradley served with the Army from 1975 to 1979 as a scout and ammunition handler. During duty in West Germany, prosecutors said, he had access to C-4, a high explosive used in Claymore antipersonnel mines. Bradley recently confided to friends in this area he had managed secretly to ship home a pound of C-4, the same explosive found in the bomb in his wife's luggage, the government said.

The bomb was made from a chewing tobacco can, spray-painted black, and hooked up with green and black wires to a $7.50 alarm clock apparently bought by Bradley at the Andrews Exchange, documents said. The can contained gunpowder and C-4.

Mary Jo Bradley was on the way Texas for a three-month training course at the time of the incident.