WASHINGTON (AP) _ The Air Force has temporarily grounded more than one-third of its fleet of KC-135 air refueling aircraft after discovering a problem with a stabilizer part on the tail section, officials said today.

Some lower-priority missions that KC-135s normally would perform will be postponed or cancelled while the 198 aircraft are grounded, the Air Mobility Command public affairs office said. The Air Force has 546 of the planes.

The Air Force said the grounding of the KC-135s is ``in no way connected'' to the crash in January of an Alaska Airlines MD-83 jetliner whose crew reported problems with the aircraft's horizontal stabilizer and were trying to correct them when the plane crashed, killing all 88 aboard. No official cause of the crash has been determined.

The grounding of the KC-135s was described by the Air Force as a precautionary measure due to a manufacturing problem that was discovered in the process of replacing a stabilizer trim actuator, which is a gear. A failure of the gear could cause the stabilizer to jam, hindering a pilot's ability to make the airplane go up or down.

The Air Force did not say how long the planes would be grounded.

Officials at Air Force Materiel Command and Air Force Mobility Command, which announced the groundings last weekend, were not immediately available for a fuller explanation of the problem and the planned solution.