Washington ? The Air Force’s refueling tankers are old, decaying and too expensive to maintain, Air Force officials told a congressional panel Tuesday.

But a congressional investigator said most of the KC-135 planes that make up the bulk of the fleet are functioning well. The Air Force said as recently as last year that it wouldn’t have to begin replacing the planes until 2009, said the investigator, Neil P. Curtin of the General Accounting Office.

The urgency of replacing the planes will be a key issue as Congress considers a $16 billion contract to lease 100 modified 767 jetliners from Boeing.

Supporters say the unusual leasing plan would allow tankers to be delivered three years earlier than a purchase would and would defer costs. Opponents question the need to speed up tanker delivery and say the plan will result in billions of dollars in additional expenses.

The lease agreement is crucial to Boeing and thousands of workers in Washington state, where the airframes are to be built, and in Kansas, where military modifications are to be done.

Curtin and the two Air Force officers appearing before the panel focused on the current state of the tanker fleet. The fleet’s average age is 42 and some of the oldest planes date to 1957.

Lt. Gen. Michael E. Zettler, deputy chief of staff for installations and logistics, and Maj. Gen. Paul Essex, director of programs at Air Mobility Command, said KC-135 maintenance costs were rising and it wasn’t worthwhile to invest more money in them. The planes have corroded and, with such old aircraft, there’s a risk of problems being identified in the future that could ground the fleet .

Zettler said maintaining the planes is like “doctoring people that are 100 years old.”

“What is it next that’s going to not perform like we want to?” he said.

But Curtain said the tankers hadfunctioned well in Afghanistan, Iraq and on missions inside the United States.

Despite their ages, most of the tankers wouldn’t receive their anticipated lifetime flying hours until after 2040, when some of the planes would be 80 years old.