Mystery pilot and communications lawyer Thomas L. Root of Alexandria pleaded guilty yesterday to five felony counts of forging and counterfeiting federal documents and of defrauding five clients seeking radio station licenses.

Root's guilty plea in U.S. District Court here means he faces a maximum of 35 years in prison and $1.25 million in fines. Sentencing is set for Aug. 6. Prosecutors said Root probably will not receive the maximum sentence.

"As Kathy {Root's wife} just said, 'You do what you have to do,' " Root said yesterday from his home, which his family is planning to leave in the next few months. "It was the right course of action to take."

U.S. Attorney Jay B. Stephens said Root's plea was "a substantial admission" that would subject Root to "significant penalties."

A federal grand jury indicted Root in March on 33 counts of counterfeiting federal documents, forgery, making false statements to the Federal Communications Commission and defrauding five clients seeking radio licenses from the FCC.

After initially pleading innocent to all charges, Root reached a plea agreement with the government on May 10 and pleaded guilty yesterday before U.S. District Judge John Garrett Penn. The government agreed to drop the remaining charges after Root is sentenced. He is free until then.

Although Root's conviction was only one of several legal troubles he faces, the 37-year-old Ohio native is better known for a bizarre plane ride he took last July 13.

After taking off in his Cessna 210 from National Airport to meet with some clients in North Carolina, Root radioed he was having trouble breathing and said he blacked out.

The plane, monitored by air traffic controllers and military aircraft, continued on automatic pilot down the Eastern seaboard before ditching in the Atlantic Ocean near the Bahamas. Root bobbed to the surface, alive but with a gunshot wound in the stomach.

The cause of neither the accident nor the shooting has been determined. Root has denied he was trying to kill himself because of the legal and financial problems he faced.

Root's attorney, Eugene Propper of Washington, told reporters yesterday, "The plane trip is irrelevant to any of this . . . . Quite frankly, all this interest {in Root} may be because of the plane trip."

Apart from the federal charges, Root was indicted Monday by a grand jury in North Carolina on 445 counts of securities violations and other charges. He was disbarred from the D.C. Bar in April, but said he has been able to continue practicing law because he still is a member of the Ohio Bar.

Root is one of a special breed of Washington lawyers who specialize in representing clients who are battling with competitors for TV and radio station licenses.

From 1987 to 1989, the grand jury said, Root made a series of fraudulent transactions in representing clients before the FCC. Among the charges to which he pleaded guilty:

Submitting to the commission a counterfeit FCC document with a forged signature on behalf of a Kirksville, Mo., client who was seeking a radio station construction permit.

Defrauding a client in Louisburg, N.C. Root took a $10,000 settlement from one of his client's competitors in exchange for dismissing the client's broadcast license application. Root had told his client the competitor would pay $50,000.

Forging a counterfeited order of an FCC administrative law judge. After Root asked the judge to dismiss his client's license application, he filed a forged and counterfeited order reinstating the application.

Facing the judge in a gray suit, with Propper to his left, Root was precise, earnest and lawyerly, acknowledging guilt on each of the five counts but several times correcting factual details stated by Penn.

On the count accusing Root of defrauding a client by telefaxing him a fake lease option agreement, Root said: "What I did as my client's demands became more insistent was to write this document in order to satisfy my client's demands."

Penn then asked Root whether he knew the document was not what he claimed it was. Root said yes.

Staff writer Barton Gellman contributed to this report.