SAN FRANCISCO — After two San Francisco airport close calls earlier this year and yet another plane nearly landing on an occupied taxiway last month in Atlanta, investigators now say the pilot conversations in all three incidents were lost because the cockpit voice recorders were not removed from the planes in a timely manner, this news agency has learned.

Congressman Mark DeSaulnier wants to know what it will take to preserve the piece of evidence that aviation experts call critical. He has proposed new legislation to find a way to safeguard cockpit voice recorders, but finds himself up against the very federal agency that investigates such evidence and aviation incidents — the National Transportation Safety Board. The NTSB calls it unnecessary.

“It was shocking to me that in this day and age with technology as it is how they don’t store almost everything. It’s just crazy to me,” DeSaulnier said by phone Friday. “It seems so corrective and it would be such an advantage for the NTSB to access.”

A challenging loophole also muddies when airlines must immediately save cockpit voice recorders. Because these airplanes didn’t actually land on the crowded taxiways — potentially killing thousands of people — the events are not classified as “serious incidents” which would require archiving of the pilot conversations.

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In October, DeSaulnier filed amendments to the proposed 21st Century AIRR Act, calling on the Federal Aviation Administration to issue guidance for air carriers on best practices for removing and saving cockpit voice recorders, as well as asking the federal agency to provide recommendations to prevent the loss of cockpit voice recorders.

However, it’s unclear when Republicans will bring the wide-ranging bill, which would privatize air traffic control, among other aviation measures, to the floor, the congressman said.

On Nov. 15, DeSaulnier wrote to FAA Administrator Michael Huerta asking the agency to take action to prevent the future loss of cockpit voice recorders in light of the July 7 incident at SFO. He has not heard back.

“From a public safety perspective, this is unacceptable, and I am concerned that we are missing opportunities to learn from all of the facts when safety issues arise,” DeSaulnier wrote.

He specifically wrote about the valuable information lost in the SFO incidents. In July, an Air Canada flight mistook a crowded SFO taxiway for a runway, barely missing four fully loaded planes awaiting takeoff on the ground. In October, another Air Canada flight crew landed on a SFO runway despite repeated warnings by an air traffic controller to abort because he believed another airplane had not left the area yet.

On Nov. 29, a Delta Air Lines plane lined up toward a taxiway at an Atlanta airport before performing a late go-around. A NTSB official told this news agency Thursday the cockpit voice recorder was not recovered following the latest incident last month.

All three incidents are under federal investigation without the dialogue between pilots available that might explain the confusion.

Cockpit voice recorders tape the dialogue among the flight crew and newer planes must have a two-hour tape limit, while older models are only required to have 30 minutes of tape. The recorder runs as long as the plane is powered, but once the recording reaches the end, it begins taping over at the beginning.

“We believe that we get all the information that we need when it’s reportable. We don’t think anything else needs to be done than what is currently being done,” NTSB spokesman Keith Holloway said, adding the NTSB is not recommending any changes to regulations.

He called the cockpit voice recorder an “added bonus, but not the end-all-be-all.” He also said the surviving pilots provided valuable information and can be compared to flight data, making the voice recorder less important.

Jim Hall, former NTSB chairman, disagreed in an interview earlier this year, calling the cockpit conversations “critical,” especially in such close-calls.

“It reflects the conversation in the cockpit of how this airplane might have ended up in this position,” he said, adding that requirements on saving cockpit conversations must tighten.

DeSaulnier also argued that relying on the crew is not ideal.

“The pilot is often the person who is the culprit in making the mistake and humans memories are not perfect,” he said.

Holloway stressed that the SFO and Atlanta incidents did not reach the threshold of a “reportable incident,” which requires airline notification when a plane “lands or departs on a taxiway, incorrect runway, or other area not designed as a runway.” Even for those, the NTSB must put in a request for the cockpit voice recorder to be saved, which can take time in during the short window before it gets overwritten.

In 2002, the NTSB sent a safety recommendation to the FAA saying it had a “longstanding concerns” about the availability of cockpit voice recorders, saying it’s one of the most valuable tools in investigations. During catastrophic accidents the plane’s electrical system cuts out which preserves the recorder, many investigations occur after incidents where the plane’s electrical system is operational, such as runway and taxiway incursions, rejected takeoffs or runway overruns.

In that 2002 memo, the NTSB said in those incidents “nearly every” CVR was overwritten. Often the only sound they get is background noise from an unoccupied cockpit from the plane sitting at a gate well after the incident.