Federal officials investigating the crash of a Henson Airlines commuter flight last month that killed all 14 people on board are examining the possibility that the pilot may have tuned the plane's navigational instruments to the wrong frequency.

Flight 1517, which slammed into the rugged mountains six miles from Virginia's Shenandoah Valley Airport on Sept. 23, was off course as it approached the airport, according to a transcript of the final radio transmissions between the pilot and air traffic controllers. The transcript was released yesterday by the Federal Aviation Administration.

As the plane neared the airport, a controller who was no longer able to monitor the flight by radar asked the pilot to "say your position."

"Ah, we were gonna ask you," Martin Burns, the captain of the plane, responded at 10:14 a.m., only six minutes before the flight was scheduled to land. "We're showing a little west of course, the inbound course here."

Copilot Zilda Wolan was flying the Beech 99 while Burns was handling the radio communications.

Less than two minutes later, Burns, who apparently thought the plane was off course, radioed the Washington Air Route Traffic Control Center and said, "You're showing us east of the inbound course over the valley." Within minutes, the flight crashed into the west face of Hall Mountain, six miles east of the runway.

"We took the pilot as far as we could," said Fred Farrar, a spokesman for the FAA. "The plane was in perfect position [for the pilot] to complete the rest of the flight and land by himself."

Officials from the National Transportation Safety Board said yesterday that they are still in the initial phase of the investigation, and that they also are looking into the possibility that the airports' localizer -- a radio beam that guides a plane to the center of the runway -- was not working properly.

But the FAA said that, on the day of the crash, several other planes used the airport without incident and a special test flight found that the localizer was working perfectly.

Under a normal instrument landing, the localizer emits a radio beam that guides the plane into the center of the runway. If the plane's navigational instruments were picking up the wrong frequency, Wolan could have flown into the mountain while believing that the plane was back on course.

Officials involved in the investigation said yesterday that there was reason to believe that the pilot was tuned to the wrong frequency. However, it is impossible to be sure without physical evidence, which probably does not exist because damage to the plane was so extensive.

By 10:18 a.m., two minutes before the flight was to land, the air traffic controller at Leesburg, obviously alarmed -- possibly by the lack of response from Flight 1517 -- told Wolan that if she had not yet properly aligned the plane with the runway she should attempt to "execute a missed approach," or an aborted landing attempt.

There was no reply to that message.

Thirty minutes after the control center in Leesburg lost contact with the plane, it initiated a search and rescue operation.

Like many of the smaller airports in the country, Shenandoah Valley has no control tower to guide a pilot to the runway. In such situations, the pilot assumes the responsibility of aligning the plane for the approach. The Leesburg control center was communicating with Flight 1517 after it had put the plane in position to land because two other flights were waiting to approach to the airport, according to a spokesman for the FAA.

If the pilot had tuned the landing instruments to an improper frequency, as some officials have suggested, then the flight could have been locked into a radio beam taking it away from the proper course for an approach.

"We still can't say for certain what happened there," said Brad Dunbar, a spokesman for the NTSB. "The flight was on course when it was on the radar, but it ended up six miles from where it was supposed to be.