The Air Force’s decision to cut the total number of MH-139A Grey Wolf helicopters it plans to buy has incurred a cost overrun known as a critical Nunn-McCurdy breach.

The service originally inteded to buy 80 of the Boeing-made helicopters, which are slated to replace some of its Vietnam War-era UH-1N Huey helicopters and will be used for security patrols at nuclear missile fields.

But tight budgets prompted the service to dramatically cut its planned purchases of MH-139s in 2026 and beyond, and the service now intends to close out the program with 42 helicopters.

The Air Force expects to keep some older UH-1Ns and fly them at the Air Force District of Washington, Fairchild Air Force Base in Washington, Kirtland Air Force Base in New Mexico, and Duke Field in Florida.

A critical Nunn-McCurdy breach occurs when a major defense acquisition program’s costs increase at least 25% over the current cost targets, or at least 50% over the original expected cost.

An Air Force spokesperson told Defense News the service notified lawmakers about the breach on April 25 and said it was tied to a reduction in the helicopter’s quantities.

House Appropriations subcommittee on defense chairman Rep. Ken Calvert, R-Calif., said during a budget hearing Tuesday the critical Nunn-McCurdy breach of the MH-139A — following a similar breach earlier this year involving the nation’s next intercontinental ballistic missile, the LGM-35A Sentinel — is concerning.

“We also need assurance that your bet on modernization over sustainment will yield success,” Calvert said. “And unfortunately, the track record is not encouraging so far. This follows the Sentinel’s Nunn-McCurdy breach. We need to understand the implications of both of these breach reviews for fiscal year 2025 and beyond.”

A Nunn-McCurdy breach could lead to a program getting cancelled unless the Pentagon certifies to Congress that it is essential to national security, there are no cheaper alternatives that can do the job, the new cost estimates have been deemed reasonable, it is a higher priority than other programs that could get cut to pay for it and the management structure in place will be able to control additional cost growth.

The Air Force has said Sentinel program’s cost overruns were largely caused by its extremely complex construction and real estate development process.

The main difference between the two programs’ breaches, the spokesperson said, is that the Grey Wolf’s cost overruns stem from the plans to cut procurement quantities nearly in half, resulting in a higher per-helicopter cost. The spokesperson said the Pentagon will follow the Nunn-McCurdy process, but it could handle the quantity-related breach in a different way than those caused purely by spiraling costs.

If Congress restores more MH-139s back into the Air Force’s budget, for example, that could solve the Nunn-McCurdy breach on its own without restructuring the program.

The Air Force’s proposed 2025 budget has not yet been approved by Congress, but the spokesperson said it had to manage the program — and declare the Nunn-McCurdy breach — as if its request had already been approved.

The Air Force now has six MH-139s in its fleet, which it is using to wrap up the helicopters’ testing. Boeing is expected to deliver the first 13 low-rate initial production helicopters in 2025, which would bring the total fleet to 19 by the end of the year.

Seven more helicopters are on track for delivery in 2026, the Air Force said. Boeing on Monday announced it has received a $178 million contract to produce those helicopters as well as provide sustainment and training support.

The proposed budget for 2025 requests $333.5 million for MH-139 procurement.