Despite two crashes involving the Boeing 737 MAX, the acting administrator of the Federal Aviation Administration said he has not come across any information that suggests his agency failed to properly review and certify the aircraft.
"I have not seen anything that suggests that," Daniel Elwell said in an interview with CNN's Drew Griffin Thursday, though he said ongoing audits and reviews may find ways to improve his agency's processes.
"We need to find out the chain of causes of these accidents. We know that there was a common thread. We are mitigating that common thread so that the 737 MAX is safe to fly," Elwell said.
"Like every accident I've ever examined in my career, there is never a single overriding cause. There's always a number of issues that all interplay to have something this tragic happen. So we are still in the process of examining that."
The 737 MAX aircraft was grounded worldwide after the crash of an Ethiopian Airlines plane in March -- an incident investigators have described as appearing to be similar to the Lion Air crash in Indonesia last year. A total of 346 people died in both crashes.
Scrutiny remains on the FAA for its prior certification of the aircraft and its Maneuvering Characteristics Augmentation System (MCAS), which is designed to push the nose of the airplane down if it senses an imminent stall and is believed to have played a role in both crashes.
Elwell defended the FAA's decision to wait about three days after the Ethiopia crash and months after the Indonesia crash to ground 737 MAXs in the US.
He said after the Lion Air crash in October, the FAA issued a directive with information about MCAS, and that after the Ethiopia crash, his agency appropriately waited to learn that the accident was MCAS-related before grounding the plane.
However, Elwell told CNN he was alarmed to learn that pilots were not originally informed of MCAS on the 737 MAX, and he said future manuals will include detailed descriptions of the software.
"The pilots didn't know about the MCAS because there was not an explanation of that software system in the flight-ops manual. And that -- as a pilot, I was concerned about that, quite frankly," he said.
While the FAA has authority over when the 737 MAX returns to the skies, the agency has been questioned in recent months over its relationship with Boeing and about a program known as Organization Designation Authorization (ODA), which allows manufacturers such as Boeing to help certify the safety of their own aircraft.
The FAA says that it "doesn't have the resources to do all the certification activities necessary to keep up with an expanding aviation industry," and therefore relies on designated companies like Boeing and their employees for such approvals.
In light of the 737 MAX crashes, the Department of Transportation established a special committee to review the FAA's procedures for certifying new aircraft, including the Boeing 737 MAX.
Elwell told CNN that includes a review of the ODA program, but he said the FAA will not discard the ODA program outright.
"We are going to look at the ODA process," Elwell said, adding, "We have to be careful that in our reaction to this incident we don't throw out a process that has made aviation the safest mode of travel in the world."
Elwell said the FAA has not committed to any timeline to recertify the MAX, even though some airlines have expressed hopes the flight restriction would be lifted by or before August.
He said Boeing's application to certify the planes' software fix was delayed by three or four weeks due an internal Boeing review that found outstanding issues. Although Boeing announced it had completed its software fix earlier this month, Elwell said the FAA has had additional questions, but he said he is "very comfortable" with how that process has unfolded.
"These airplanes aren't going to fly until we've done the necessary analysis to say it's safe to do so," Elwell said.
"They're working on a lot of things to make this exactly right."