WASHINGTON -- The Senate voted overwhelmingly Thursday to allow commercial pilots to carry weapons in the cockpit after the Bush administration dropped its opposition to the idea.

The administration, though, said a number of safety and logistical issues needed to be resolved.

In a letter to two senators, the White House recommended giving pilots lockboxes for the weapons so they won't be left in the cockpit. It also said only pilots who volunteer to carry weapons and receive extensive training should be armed.

Al Aitken, a pilot speaking for the 14,500-member union representing American Airlines pilots, which supports arming pilots, said the 87-6 vote meant the Senate recognized that all the security layers the administration is putting into place are inadequate.

"The people who need the weapons as a last line of defense are the pilots," he said. "They're the only ones they're trying to keep the gun from," he said, adding that thousands of state and federal law enforcement officers travel on planes while armed.

The heads of 21 airlines, which oppose the measure, sent a letter to each senator Thursday saying they wanted to discuss the idea of arming pilots with Congress and the administration.

"It must be noted, however, that while we are spending literally billions of dollars to keep dangerous weapons off of aircraft, the idea of intentionally introducing thousands of deadly weapons in to the system appears to be dangerously counterproductive," the letter said.

To address some of the airlines' concerns, the administration suggested a "detailed, effective" training program be designed from scratch and tested before an estimated 85,000 pilots are allowed to carry weapons.

The administration also warned the cost would be significant -- $900 million to start and $250 million annually thereafter -- and said there is no money now in the Transportation Security Administration budget to cover the expenses.

The letter from Transportation Security Administration chief James Loy was delivered to Sens. Ernest Hollings, D-S.C., and John McCain, R-Ariz., as the Senate debated the measure that would allow all pilots to carry guns into the cockpit. Hollings is chairman of the Senate Commerce and Transportation Committee; McCain is the committee's ranking Republican.

"If there is to be responsible legislation establishing a program to allow guns in the cockpit, it must address the numerous safety, security, cost and operational issues," Loy wrote.

Sen. Bob Smith, R-N.H., offered the amendment to the homeland security bill that would prohibit airlines and the federal government from barring armed pilots.

"We prefer a more comprehensive approach in our amendment, but are grateful for any efforts by the administration to roll the ball down the field," said Smith's spokeswoman, Lisa Harrison.

The chairman of the House Transportation Committee's aviation subcommittee, Rep. John Mica, R-Fla., said pilots should be armed at least until bulletproof cockpit doors are installed in all planes.

The Federal Aviation Administration said Thursday that manufacturers and airlines agree an April 9 deadline to install the new doors can be met.

Mica said the administration realized that the momentum in Congress favoring arming pilots is strong. A bill to create a program that would train and arm some pilots passed the House 310-113 in July.

Transportation Undersecretary John Magaw, who headed the TSA until July, said in May he would not allow pilots to carry guns. Reinforced cockpits and armed air marshals provide enough protection against terrorists who try to take over an airplane, Magaw said.

After Magaw's departure, Transportation Secretary Norman Mineta said he would re-examine the issue.