The U.S. Federal Aviation Administration (FAA) says it has found no legal or technologically feasible way to make cockpit electronics impervious to tampering.

The finding came in a written response to a National Transportation Safety Board (NTSB) recommendation to redesign black-box recorders and other critical electronics so they cannot be switched off during flight. The main impediment is that pilots sometimes need to cut the power in the event of overheating or fire, the FAA says in the letter obtained by Bloomberg News.

“There appears to be no safe way to ensure recorders cannot be intentionally disabled while keeping the airplane safe from electrical failure that could become hazardous,” FAA Administrator Michael Huerta writes in the April 22 letter.

The FAA also rejected the NTSB’s calls for adding video recorders in cockpits, saying there is “no compelling evidence” it would help investigations.

The FAA declined to comment to Bloomberg beyond the letter. The agency has been taking steps to address the risk of pilot tampering. In May it formed an advisory panel to examine how it can better screen pilots for mental illness. The FAA also is part of a United Nations group studying broader flight-tracking suggestions.

The NTSB has been seeking ways to make flight-data recorders tamper resistant since 2000, Bloomberg reports. Its original recommendation was prompted by the 1997 crash of a SilkAir plane in Indonesia that killed 104 people. The NTSB concluded that the captain, who had growing debts and was in trouble at work, cut power to the recorders and dived the plane into a river.

The issue was raised again in 2014 when a Malaysian Airline System flight with 239 people aboard vanished on a flight from Kuala Lumpur to Beijing. A search of the Indian Ocean was complicated because it appears the Boeing 777 was deliberately turned off course and its radios and tracking equipment were switched off, according to Malaysian Prime Minister Najib Razak.

Because of that case and pilot suicides that preceded this year’s Germanwings crash, the NTSB in January issued nine recommendations calling for better aircraft tracking, improved flight recorders and systems that couldn’t be disabled by pilots.

The FAA’s response to the NTSB laid out reasons why such changes were not feasible. Huerta cites aviation regulations that require aircraft designers to give pilots the ability to switch off electrical power from components in the event they overheat and threaten to cause a fire.

One example occurred in 1998 when pilots of a Swissair flight detected smoke in the cockpit but were unable to cut power to the in-flight entertainment system to diagnose and contain the problem, the FAA says. A fire spread and the plane crashed in the Atlantic Ocean near Nova Scotia, killing all 229 aboard.

“The FAA does not want to introduce design requirements that could expose the airplane to system risks that can lead to cascading failure and fires,” the FAA’s letter says.

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