Some of America's largest and best-known corporations have been significantly underreporting the cost of letting their executives use company aircraft on personal trips, a pattern that has come to light through reports the companies have filed with the Securities and Exchange Commission, a recent enforcement case and a warning by a top SEC official.

In reports filed with the SEC this year, some companies have put a higher price on past years' flights and disclosed that they were changing the way they determined the value of the benefit.

A regulation that SEC spokesman John Nester said has been on the books for more than a decade requires that companies disclose the "incremental" cost of executive perks above a certain amount.

Using a formula devised for a different purpose -- figuring out how much executives owe in income tax on personal use of the corporate plane -- some companies in past years reported much smaller amounts or provided no indication that the company plane was being used for personal travel.

It is not uncommon for top executives to use company aircraft for vacations and the like. Some executives negotiate that privilege as part of their employment contracts. Some corporate boards, citing security concerns, even require executives to use the company plane for personal trips.

Regardless of the justification, "I don't think there's any question that [some companies] were low-balling those numbers" when they disclosed executive compensation in reports to shareholders, said Patrick McGurn, executive vice president of Institutional Shareholder Services, which analyzes corporate governance for big investors. "It's almost as though there was this large-scale decision made to kind of ignore the guidance that was out there and go with what had been . . . the accepted corporate practice of valuation."

Spokesmen for some companies that switched methods, asked about it in April, said there was nothing wrong with their past use of the tax formula.

David M. Stuart, a branch chief in the SEC's enforcement division, said in an interview, "There's no question that the proxy [pay disclosure] rule requires incremental cost." Using the income tax formula instead will often understate the cost to the company, Stuart said. In a recent enforcement action, the SEC accused Tyson Foods Inc. of improperly using the tax formula.

The choice of which formula to use can make a dramatic difference in the numbers that shareholders and the public see -- roughly akin to the difference between the price of a first-class airline ticket and the cost of operating a private plane.

Maytag Corp.'s original report on executive compensation in 2003, for example, made no mention of chairman and chief executive Ralph F. Hake's non-business use of corporate aircraft. However, a report Maytag filed with the Securities and Exchange Commission in March said that in 2003, his personal use of the plane cost Maytag $55,472."We do not believe by making this change our earlier disclosure was in any way incorrect," Maytag spokesman John Daggett said in a written statement.

Qwest Communications International Inc. originally reported that chairman and chief executive Richard C. Notebaert's personal use of corporate aircraft represented $116,839 of income to Notebaert in 2003. A more recent Qwest filing said it cost the company $377,232, more than three times as much.

"We believe we are in compliance, and we have been in compliance," said Qwest spokesman Steve Hammack.

Eli Lilly & Co. originally reported $60,725 of personal aircraft use by chairman and chief executive Sidney Taurel in 2003. The company now puts the cost at $90,678, partly because it erroneously omitted from the earlier disclosure the amount attributable to Taurel's family members, Lilly spokesman Philip C. Belt said by e-mail. Calculating flight costs by the tax formula was "kind of what we thought was the standard practice," said James B. Lootens, assistant secretary and assistant general counsel at Eli Lilly.

Bank One last year valued chairman and chief executive James Dimon's 2003 personal use of corporate aircraft at $30,889. Bank One is now part of JPMorgan Chase & Co., which recently reported that under the incremental cost method Dimon's personal aircraft use in 2003 cost the company more than five times that amount, or $160,590.

JPMorgan also recently reported that personal use of company aircraft by its chairman and chief executive, William B. Harrison Jr., cost the company $194,258 in 2003. In its original report on 2003 executive compensation, the company didn't disclose the cost. JPMorgan Chase spokesman Joseph Evangelisti declined to comment.

Some spokesmen said their companies changed the way they quantify the perk in response to a speech delivered last fall by Alan L. Beller, director of the SEC's division of corporation finance. Companies are required to disclose details of executive compensation in annual reports to shareholders filed with the SEC.

"Too much executive compensation disclosure has become an example of the kind of disclosure companies should disavow -- disclosure that says as little as possible while seeking to avoid liability, rather than disclosure that seeks to inform," Beller said in the October speech.

Beller said in the speech that he feared companies were routinely omitting perks such as use of company planes, and he warned corporate lawyers that "the appropriate measure of value is the aggregate incremental cost to the company."

However, even companies trying to comply with the SEC rule have room for interpretation and variation. When companies attempt to estimate incremental cost, "there is no specified methodology anywhere," said Penn Holsenbeck, corporate secretary at Altria Group Inc., which has switched to the incremental cost method. "Each company is sort of left to its own devices."

In the enforcement action in April, the SEC alleged that family and friends of former Tyson Foods chairman and chief executive Donald Tyson regularly flew on company aircraft without him on board. From 1997 to 2001, the company failed to disclose more than $1 million of perquisites for Tyson, the SEC said. Without admitting or denying wrongdoing, the company agreed to pay a $1.5 million penalty, and the former chairman agreed to pay a $700,000 penalty.

Among other things, the SEC alleged in a complaint that the company "incorrectly valued Mr. Tyson's personal aircraft usage using the tax . . . method instead of the aggregate incremental cost method required by" SEC reporting standards.

Despite the regulatory scrutiny, Tyson Foods, which disclosed the SEC investigation last year, continued to use the tax formula to value aircraft usage in a report filed in February. Spokesmen for Tyson Foods declined to comment, saying the company's settlement with the agency prevented them from doing so.

Locally, federal investigators are probing the extensive personal use of a corporate jet by former Riggs Bank chief executive Joe L. Albritton.

For some executives, however, there is no option other than to use the company plane.

Drugmaker Eli Lilly, for example, reports that "for security reasons," board policy dictates that Taurel "must generally use the company-owned aircraft for both business and personal travel."

In an interview, Belt, the company spokesman, cited other factors, such as "business needs," "the ability to discuss confidential issues," and "the most productive way to move from Point A to Point B."

Invoking security "is a convenient justification" for executives to fly corporate planes on vacation, but one "that I don't think washes," said Vahid Motevalli, director of the Aviation Institute at George Washington University. "In terms of security of air travel itself or safety of it, I think the scheduled airline travel is going to be more secure and more safe."

"I think it just makes intuitive sense that executives are safe when they are flying with people they know," said Dan Hubbard, a spokesman for the National Business Aviation Association, which represents manufacturers of business aircraft and companies that use them. "These aircraft don't get hijacked. The executives don't get roughed up or kidnapped from the aircraft. Their luggage, which often contains sensitive company information, is not stolen or lost in transit."

Still, beginning this year, Taurel will pay Eli Lilly a fee for personal flights, the company reported.

"Mr. Taurel was not really comfortable with the idea that he would be getting this benefit for free," said Lootens, the company attorney.

The fee will be capped at either the cost of a first-class airline ticket or the value under the tax formula, the company reported.

Staff writer Terence O'Hara contributed to this report.

Qwest recalculated the cost of chief executive Richard Notebaert's 2003 jet use as $377,232, three times its original report.Maytag chief executive Ralph Hake's private use of the corporate plane in 2003 cost $55,472, the company now reports.