After Asiana Flight 214 crashed at San Francisco International Airport on Saturday — killing two and injuring over 100 passengers — early evidence suggested that the plane was simply flying too low and too slow due to operator error. But on Tuesday, National Transportation Safety Board (NTSB) chairwoman Deborah Hersman told the press that pilots may have relied on a device called an "auto-throttle" to maintain speed, and may have failed to notice that the automated system wasn't actually doing its job.

"He recognized that the auto-throttles were not maintaining speed."

According to an interview with the flight crew — whose contents, Hersman stressed, have not been independently confirmed with cockpit recorder data — the instructor pilot said that the crew had initially set the automated system to a safe approach speed of 137 knots. But at 200 feet from the ground, "he recognized that the auto-throttles were not maintaining speed."

While that sounds like it could be a mechanical issue, it might be more complex than that. More than one pilot has come forward in the wake of the crash to suggest that the Boeing 777's auto-throttle isn't necessarily active in all autopilot modes, and thus the crew could still be responsible.

When NTSB investigators finished their initial sweep of the aircraft, they discovered that the auto-throttles were in the "armed" position, but that doesn't necessarily mean they were in use. "Armed means that they are available to be engaged, but depending on what mode is used, we really need to understand that a little bit better... what the expectation was for the auto-throttles," said Hersman.

Whether or not the crew expected the plane to automatically maintain speed, though, might be beside the point. "The crew is required to maintain a safe aircraft," Hersman stressed. "One of the very critical things that needs to be monitored on an approach to landing is speed."