This story was updated April 14, 2021, at 12:43 p.m. EST with a statement from Air Mobility Command.

WASHINGTON — The Boeing KC-46 tanker currently can’t carry the palletized toilet normally used by mobility aircraft without the risk of waste water leaking into the cabin, leaving the tanker unable to conduct long-distance flights with a large number of passengers until the lavatory is modified.

The issue, according to U.S. Air Mobility Command, involves the Air Transportable Galley-Lavatory, a combined kitchen and bathroom that can be rolled on and off mobility aircraft like the KC-46, C-17 and C-130.

A source with knowledge of the program told Defense News on Wednesday that the problem arises because the ATGL cannot be loaded and stored in its normal orientation. The source said the issue stems from the KC-46′s cargo rails and locks, which are not as wide as other Air Force cargo aircraft. Instead, the ATGL is turned 90 degrees to fit inside a KC-46, but the lavatory system’s anti-spill valve does not work correctly while in that orientation.

That could allow toilet waste to drip into the cabin whenever the tanker climbs or descends in altitude, resulting in a messy problem. The source indicated that the issue was discovered during ground testing.

The problem is not considered a technical deficiency against the KC-46 aircraft because the Air Force’s 2011 contract with Boeing did not forbid the company from delivering a tanker with a narrower cargo rail system, the source said. Ultimately, the Air Force intends to fix the issue by developing a new valve for the ATGL, but so far the service has not designed, tested or fielded a replacement.

The Air Force intends to start testing a solution to the issue imminently, an Air Mobility Command spokeswoman said in a statement.

“The KC-46 program office plans to modify six ATGLs over the next twelve weeks to address the growing demand by the KC-46A enterprise as it begins limited operational use in support of U.S. Transportation Command taskings,” Lt. Emma Quirk said. “These six modified units will be evenly divided between McConnell [Air Force Base], Pease [Air National Guard Base] and Seymour Johnson AFB.”

The ATGL has been in service since the 1980s, Quirk added. It provides the space and power to furnish two meals and one snack for 160 people, as well as two lavatories that can handle about 15 hours of use, according to an Air Force instructional video about the system.

The KC-46 can still carry 112 passengers, even without the ATGL onboard, the source said. However, because the lavatory built into the KC-46 has a limited capacity, not having the option to add a palletized toilet system could make it more difficult for the Air Force to accomplish long-haul flights with many passengers onboard.

The problem was first highlighted during a Tuesday morning hearing with the Senate Armed Services Committee, when Sen. Jeanne Shaheen, D-N.H., mentioned that ongoing issues with the Air Transportable Galley-Lavatory “makes missions problematic” for the KC-46.

Army Gen. Steve Lyons, who leads U.S. Transportation Command, said he did not know enough about the problem to comment on the issue, adding that he was “not aware of any impact” that would make the KC-46 incapable of taking on limited operational missions sometime this summer.

Generally speaking, however, Lyons pointed to improvements to the military’s aerial refueling capacity over the past year, which he attributed to Boeing’s agreement to fix ongoing KC-46 technical deficiencies and adjustments in the retirement profiles of legacy tankers such as the KC-10.

“The Air Force has stepped forward with interim capability with the KC-46. We still have a long way to go, but I am very, very comfortable with where we are,” he said.