SACRAMENTO, Calif. — The Army is spending $2.6 billion on hundreds of European-designed helicopters for homeland security and disaster relief that turn out to have a crucial flaw: They aren’t safe to fly on hot days, according to an internal report.

While the Army scrambles to fix the problem — potentially adding millions to the taxpayer cost — at least one high-ranking lawmaker is calling for the whole deal to be scrapped.

During flight tests in Southern California in mild, 80-degree weather, cockpit temperatures in the UH-72A Lakota soared above 104 degrees, the point at which the Army says the communication, navigation and flight control systems can overheat and shut down.

No cockpit equipment failed during the nearly 23 hours of testing, according to the report, prepared for the Army in July. But it concluded that the aircraft “is not effective for use in hot environments.”

The Army said that to fix the cockpit overheating problem, it will take the highly unusual step of adding air conditioners to many of the 322 helicopters ordered.

The Army did not respond to questions about how much the retrofitting will cost and who will bear the expense.

California Rep. Duncan Hunter, the ranking Republican on the House Armed Services Committee, said the lightweight helicopter will still have too many weaknesses.

“In my view, we would be well advised to terminate the planned buy of 322 Lakota helicopters and purchase instead additional Blackhawk helicopters,” Hunter said in a letter this week to Army Secretary Pete Geren.

But Army spokesman Maj. Tom McCuin at the Pentagon said: “It’s certainly a concern to people out there in the field now because it’s hot in those cockpits, but it’s being fixed.”

The Army has received 12 of the Lakotas so far from the American Eurocopter Corp., a North American division of Germany’s European Aeronautic Defence and Space Co., or EADS. Testing on the first six by an independent arm of the Pentagon revealed the problems. The rest of the choppers are scheduled for delivery to the active-duty Army and the National Guard over the next eight years.

The Lakota represents the Army’s first major effort to adapt commercially available helicopters for military use. Air conditioning is standard in commercial versions of the aircraft, which have not had overheating problems. But the military usually avoids air conditioning in military aircraft to reduce weight and increase performance.

“We don’t need air conditioning in the Blackhawks, so we didn’t think it would be an issue” in the Lakota, McCuin said. “But when we got the helicopter into the desert, we realized it was a problem.”

The commercial purchase was designed partly to cut costs and quickly get aircraft into the field to replace two aging Vietnam-era helicopters, the Kiowa and Huey. The Army said the Lakota will also free up more Blackhawks to send to Iraq for medical evacuation flights.

The Army plans to use the Lakota for such things as search-and-rescue missions in disaster areas, evacuation of injured people, reconnaissance, disaster relief and VIP tours for members of Congress and Army brass. All of its missions will be in the U.S. or other noncombat zones.

The Lakota has another problem: Testers said it fails to meet the Army’s requirement that it be able to simultaneously evacuate two critically injured patients. The Lakota can hold two patients, but the cabin is too cramped for medics to actually work on more than one of them at a time, the testers said.

Also, the Lakota cannot lift a standard 2,200-pound firefighting water bucket, but can handle a 1,400-pound one. The Army said it had no intention of using the Lakota to fight wildfires.