WASHINGTON — Although Boeing and SpaceX remain on schedule to have their commercial crew vehicles completed by 2018, an advisory group is worried about a potential gap in access to the International Space Station should they experience delays.

At a July 28 meeting of the NASA Advisory Council in Cleveland, members discussed the possibility NASA may have no means to send crews to the station should both companies fail to have their vehicles certified by the end of 2018, when NASA’s current agreement with Russia for seats on Soyuz spacecraft expires.

Wayne Hale, interim chairman of the council’s human exploration and operations committee, told the council that while both companies’ current schedules have their vehicles ready by 2018 to carry NASA astronauts, “there is very little margin” in those schedules.

“Human spaceflight development programs invariably suffer schedules slips due to their technical complexity, and integration of commercial providers into government service adds further obstacles,” he said. “It’s therefore prudent to assume delays in the post-certification missions from the schedule.”

Schedules presented at a July 26 committee meeting showed Boeing completing its certification review, the final milestone before operational flights, in May 2018. That comes after an uncrewed test flight of its CST-100 Starliner spacecraft in December 2017 and a crewed test flight in February 2018.

SpaceX currently has its certification review scheduled for October 2017. It has an uncrewed test flight of its Crew Dragon vehicle scheduled for May 2017, followed by a crewed test flight in August 2017.

With NASA’s July 29 order of a second post-certification mission from SpaceX, the agency now has ordered four such missions from Boeing and SpaceX for crew transportation to and from the ISS. NASA has not formally scheduled any of those missions, or specified if Boeing or SpaceX will get the first such mission.

Both companies are making good progress on development of their commercial crew vehicles, said Phil McAlister, director of commercial spaceflight development at NASA, at the July 26 meeting. “We are grinding through a lot of these very difficult activities,” he said. “Now we’re in the blocking and tackling phase of the program.”

He acknowledged, though, that there could be changes in those schedules as both companies run into issues. One example is Boeing, who earlier this year delayed its two test flights by several months because of technical problems, including acoustic loads on its spacecraft and Atlas 5 rocket during launch that McAlister said the company has largely resolved.

“They’re in the final stages of some wind tunnel testing. They think they have a good solution,” he said, which involves installing an extended skirt behind the capsule. “We think that’s a pretty good solution too, but we really want to see some of that final wind tunnel test data come through.”

More such problems could crop up as the companies move closer to their test flights. “Our partners are doing a great job actively building and testing their hardware. Even though they’re doing a great job, I would not be surprised to see some future adjustments to their schedules,” he said. “Their schedules are optimistic but achievable.”

McAlister said he felt there was sufficient margin in those schedules to ensure that at least one company was certified before the Soyuz agreement expires at the end of 2018. “I think we’ve got some margin today so that our partners are not feeling a lot of schedule pressure,” he said.

At the NASA Advisory Council meeting, though, Hale said his committee was concerned about a gap, particularly since NASA has typically had to arrange Soyuz flights two to three years in advance. “Due to the long lead time to procure Soyuz seats, a decision must be made really very shortly — before the end of 2016 — to guarantee access to the ISS in 2019,” he said, “or we may be forced to reduce or possibly eliminate its crew complement.”

Hale said that conclusion was an “area of concern” for his committee, but stopped short of offering a specific recommendation to NASA. “We make no recommendation here, frankly, because we don’t know what the solution would be other than to say that we need a backup plan,” he said.

“The future availability of Soyuz is not certain” beyond the seats NASA has purchased through 2018, McAlister said at the July 26 committee meeting. “We’re going to have to continue to monitor that and see whether we’re going to need, and if we can purchase, more Soyuzes.”