The pilot of a Henson Airlines commuter flight that crashed into the rugged terrain near Shenandoah Valley Airport in September may have tuned the plane's navigational instruments to the wrong frequency, investigators reported today, repeating one of the first theories on the cause of the crash that killed 14.

Investigators testifying before the National Transportation Safety Board said an examination of the radios taken from the cockpit of the Henson Beech 99 found that all three may have been tuned to 110.5 megahertz instead of 109.5, a course that would have taken the plane into the mountain instead of to the airport.

Henson's chief pilot, Charles Wintermoyer, disagreed. "It is just not my opinion that fixing the radio to that frequency would have caused the plane to fly into the mountain. But I can't account for it," he told the board.

Henson officials also testified that Zilda Spodaro Wolan, the copilot who was flying the plane when it crashed, was using a prescription drug without their knowledge.

Although theories on the crash were in greater abundance than facts, much of the testimony confirmed that the equipment the airport uses to guide a plane to the ground was working smoothly on the date of the crash, Sept. 23.

When the plane first crashed, it was reported that the airport's localizer -- the beam that helps guide planes to the runway -- may not have been working. Evidence presented today by federal officials, airport employes and pilots strongly disputes that theory.

"I had no problem with it," said Lake L. Kirby, a pilot who was approaching the airport when Henson Flight 1517 went down. He was put in a holding pattern for 35 minutes by an air traffic controller while officials tried to determine the fate of the Henson flight.

"I got no signal interruption," said Kirby. Federal Aviation Administration investigators also testified that the airport's instrument landing system was working well.

Dr. Richard L. Masters, a medical adviser to the Air Line Pilots Association, testified that Wolan had only a small amount of a prescription drug -- the appetite suppressant diethylpropion -- in her blood system when the plane crashed. He said it was inconceivable that the drug could have been responsible for the crash.

Wintermoyer said that all prescription drugs used by pilots had to be cleared by Henson medical personnel, and, where appropriate, the FAA. He said Wolan would have been relieved of duty had he known.

The hearing, which also will examine the level of training given to Henson pilots, and the quality of the instruments in the Beech 99, will continue Thursday.