A UPS plane crash that killed 15 people last year might have been prevented if an original inspection schedule hadn’t been relaxed, but mechanics didn’t get a close look at the parts that should have kept an engine from flying off its wing because federal regulators allowed Boeing to recommend checking them less frequently, according to testimony Wednesday.

The National Transportation Safety Board’s questions also drew out that Boeing relied on older data when it asked to extend the inspection schedule in 2015, and didn’t seem to account for seven instances on other planes of the same model when the key engine mount parts were failing. The Federal Aviation Administration, for its part, approved the request after a month’s review without seeking more information.

“Safety is a shared responsibility between the airline, the manufacturer, and the regulator. And the NTSB is attempting to parse out the roles and responsibilities of each of those three entities,” aviation safety expert Jeff Guzzetti said.

The two-day hearing made clear that key safety information wasn’t being shared among everyone involved, and the former crash investigator said the FAA should have been more skeptical about Boeing’s request.

Boeing and FAA officials acknowledged that they misunderstood the risks related to the potential failure of a steel bearing and metal sheath in the engine mount before the crash, not realizing that it could lead to the lugs that secure engines to an MD-11’s wings breaking. The bearings are tucked deep inside near the pylons, so problems are hard to spot without removing each engine for detailed inspections.

Boeing succeeded in extending the required inspections from once every 19,900 cycles of takeoffs and landings, to once every 29,260, so that airlines could complete more of the major maintenance tasks simultaneously, with less down time. The planemaker sought the change even after receiving reports about seven of the flaws in the bearings well before the planes had reached their original inspection limits. In the years after the schedule was relaxed, three more instances were discovered before the crash.

The plane that crashed after losing its engine while accelerating down the runway at Louisville’s Muhammad Ali International Airport had flown 21,043 cycles, so it would have been thoroughly inspected under the original schedule. The crash killed all three pilots and 12 people on the ground. Twenty-three more were injured. There has been only one other crash, decades earlier, involving a similar plane model losing an engine, but that one was blamed on improper maintenance and not the same flaw.

Plane operators aren’t expected to deviate from federally approved maintenance schedules, said Greg Raiff, who owns several aviation maintenance companies and operates a fleet of planes at Elevate Aviation Group.