WASHINGTON — What began as an inquiry into a mysterious sound in the background of an airplane cockpit voice recording escalated into an unexpected challenge for the nation's top safety investigators.
The National Transportation Safety Board temporarily pulled down public documents for thousands of investigations last week after the agency inadvertently allowed the reconstruction of audio recordings from the cockpit of UPS flight 2976, which crashed shortly after takeoff in Louisville, Ky. last year, killing 15 people, including all three pilots.
Recent advances in artificial intelligence have made it easier to reconstruct audio from digital images that were published as part of the NTSB's investigation. And that's making it harder for the NTSB, which is forbidden by law from releasing those recordings, to stop them from being made public.
As investigators at the NTSB listened back to the cockpit voice recording from the crash flight, they heard a "high pitch ringing sound" that began just after the plane rotated for takeoff. But they couldn't figure out what it was.
"They're not just looking at the things that people are saying. They're looking at the other ambient sounds, right? Because that might give us some clues," explained Scott Manley, an engineer and YouTuber, in a video he posted last week.
Manley noticed NTSB investigators had tried to identify the mysterious sound. In the process, they created what are called spectrograms — basically, visual representations of the frequencies in an audio recording — and made those images available to the public on the NTSB's website.
Manley suspected it would be possible to reconstruct the audio of the original recordings from those spectrogram images.
"I basically made a post on Twitter saying, I think the NTSB has accidentally released cockpit audio recordings for this particular thing," Manley said in the video.
Some of Manley's social media followers took that as a challenge.
"I know nothing about audio, but I was able to figure it out pretty quickly," said John McElhone. He runs a small company that makes electrical turbines, where he often uses new AI tools to write computer code. McElhone said it did not take him very long to turn those spectrograms back into audio — about ten minutes, to be precise.
"The AI is just using tools that already exist," he explained in an interview. "So I'm sure somebody could do this just by writing code by hand. But it would take much, much longer."
McElhone did not post his audio reconstructions online. But other people did release theirs.
The NTSB's response was dramatic and unprecedented. The agency temporarily pulled down the public docket from the UPS crash — and for all its other crash investigations — while it conducted a review.
"The NTSB has longstanding procedures in place to protect cockpit voice recordings and other sensitive onboard audio and video materials obtained during investigations," NTSB spokesman Peter Knudson said in a statement to NPR.
"After becoming aware that artificial intelligence may allow approximations of cockpit audio to be digitally reconstructed from certain sound-spectrum imagery, the NTSB temporarily removed public access to its docket system while it reviewed investigative materials and evaluated additional safeguards," Knudson said.
The NTSB has since restored access to most of its public dockets, including UPS flight 2976, Knudson said, but 41 dockets remain under review.
In January, the NTSB offered reporters an inside look at its headquarters in Washington, D.C., including one of the soundproofed audition rooms where the NTSB analyzes the audio on cockpit voice recordings, and prepares the written transcripts it releases to the public.
"The reason why we do it here specifically is we provide a really high level of protection to the actual audio," said Chris Babcock, an engineer at the NTSB.
The origins of the law that prevents the NTSB from releasing CVR audio date back to the crash of Delta Air Lines flight 1141 in 1988.
Shortly before takeoff, the flight crew was joking about the dating habits of the flight attendants, "so that we can get it on the recorder, you know, in case we crash." Moments later, the plane did in fact crash after the pilots failed to properly configure the wing flaps and slats for takeoff, according to the NTSB.
The release of that cockpit audio was a major embarrassment for the pilots and the airline, leading Congress to pass a law prohibiting the release of cockpit recordings.
For decades, the NTSB has prevented those audio recordings from leaking to the public — until now.
"I didn't intend for this to be this kind of wild thing," said Manley. In an interview for this story, Manley said the NTSB should not have published those spectrogram images in the first place. But Manley conceded that he made a mistake, too.
"The mistake was for me to idly speculate in public on something that could have serious legal consequences," Manley said. "In retrospect, yeah, if I'd really thought about it, I would have tried to do a bit more on it myself. Or I would have just notified the guys at the NTSB."
Manley, who is a pilot himself, says he understands why the NTSB does not release recordings of the final moments before a crash. But in the case of UPS flight 2976, he says the reconstructed audio reveals that those pilots kept their cool as they responded to a catastrophic engine failure.
"They sounded a lot calmer when I heard the audio when I read the transcripts. So it made them seem more professional," Manley said.
Still, NTSB chair Jennifer Homendy says the agency has good reasons for not releasing cockpit audio to the public.
"Laws against releasing CVR audio exist to protect privacy, preserve the integrity of NTSB investigations, and out of respect for accident victims and their families during a time of tremendous loss," Homendy wrote on X. She called posts using the reconstructed audio "disgusting" and "manipulated," and she urged social media platforms to take them down.
The NTSB held a two-day hearing on the UPS crash last week. Investigators still have not identified the source of the mysterious sound.
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