According to Air Force Times, Defense Secretary Robert Gates will soon announce that cost overruns on the C-5 Galaxy transport modernization program have now reached 15% of the contract’s value, triggering a reporting and review process that raise long-term doubts about the aircraft’s operational future.

Two retired senior Air Force leaders, speaking with the Times on the condition of anonymity, confirm that the modernization has “tripped” cost overrun limits set in the Nunn-McCurdy Provision. Under that regulation, the Pentagon must notify Congress when cost increases on major acquisition programs reach 15%, or schedule delays hit six months. If cost overruns hit 25%, the military is required to justify continuing the program, based on its importance to national security and other criteria.

Why does this matter? The C-5, along with the newer C-17, forms the backbone of our strategic airlift fleet, ferrying equipment and supplies to U.S. forces around the globe. There are a total of 111 Galaxies in the Air Force inventory (61 of the original A models and 50 C-5Bs, built during the Reagan Administration); the service hopes to upgrade all aircraft to the C-5M standard under the modernization program, which includes new landing gear, safety equipment, communications systems, and–most importantly–new engines.

With its existing engines, the C-5 is anything but fuel efficient, adding to operating costs that are (reportedly) the highest of any military aircraft. By replacing existing power plants with General Electric CF6-80C2 engines, the C-5M will have significantly greater thrust (22,000 lbs per engine), resulting in shorter take-off rolls, improved climb performance, and longer intervals between in-flight refueling.

But the cost of attaining those capabilities is in dispute. Lockheed claims it can modernize the C-5 fleet at an average cost of $83 million per aircraft, well below Nunn-McCurdy thresholds. However, the Air Force estimates that each modernized C-5 will now run $120 million, meaning that cost overruns may reach 40%, casting a shadow over the future of the upgrade program–and the C-5.

At the Lockheed price, the service can get a modernized C-5 for less than half the price of a new C-17. But the Air Force estimate ($140 million) is roughly two-thirds the cost of a C-17. For a service scrambling to recapitalize its aircraft fleet–in a tough budget environment–it becomes difficult to justify a modernization program that offers only a 33% savings over a newer, more capable transport.

Without the upgrade, the Air Force will be stuck with C-5s that are expensive to operate and maintain, as jet fuel prices continue to climb. Retiring the Galaxy and buying more C-17s would be an optimal solution, but at more than $200 million a copy, it will be difficult to purchase more of the new transports and still fund Air Force acquisitions of other weapons systems, including the F-35 Joint Strike Fighter (JSF).

Meanwhile, the demand for strategic airlift has never been greater–and there’s already a shortfall between what we “have” and what the armed forces need to meet global commitments. And that leaves the Air Force with a real conundrum; the service needs modernized C-5s to sustain present capabilities, but the program may be too expensive to sustain. And without the upgrades, current C-5s may be too costly to operate and maintain, hastening their departure from active service. That, in turn, would put more pressure on a smaller number of C-17s and C-5s left to shoulder to load. The purchase of more C-17s would relieve some of that strain, but that would take years–and money the Air Force currently doesn’t have.