U.S. government officials in 2014 revealed an alarming safety issue: Passenger mobile phones and other types of radio signals could pose a crash threat to some models of Boeing 737 and 777 airplanes.

More than 1,300 jets registered in the U.S. were equipped with cockpit screens vulnerable to interference from Wi-Fi, mobile phones and even outside frequencies such as weather radar, according to the Federal Aviation Administration, which gave airlines until November 2019 to replace the units made by Honeywell International Inc. Honeywell estimates that 70 or fewer planes with cockpit screens in need of repair are still flying.

Flight-critical data including airspeed, altitude and navigation could disappear and “result in loss of airplane control at an altitude insufficient for recovery,” the FAA said in its 2014 safety bulletin, known as an airworthiness directive.

A Honeywell spokeswoman said there have been no reports of display units blanking in-flight due to high-intensity radio frequency/Wi-Fi interference. Airlines and Honeywell have argued that radio signals were unlikely to cause safety problems during flight. The FAA, however, concluded there were safety risks based on assessments it had received from a vendor and an operator.

Boeing Co. found the interference in a laboratory test in 2012 and hasn’t seen similar issues on other aircraft, a company spokesman said. Honeywell is aware of only one case where all six display units in a 737 cockpit went blank, company spokeswoman Nina Krauss said. The cause was a software problem, unrelated to Wi-Fi or cellphones, that has been fixed and is currently being flight-tested, she said.

The affected 737s are the so-called Next Generation model, a predecessor of the Boeing Max, which was involved in two crashes in less than five months. Cockpit displays on the Max were made by Rockwell Collins, now a unit of United Technologies Corp., not Honeywell. Boeing’s 777s also were covered by the FAA order.

The FAA order didn’t quantify the amount of radio signals needed to cause interference problems. An agency spokesman said Thursday that the FAA bases the compliance time for its airworthiness directives on the risk that a condition poses. “A 60-month compliance time frame means the risk is low, and does not need to be addressed right away,” he said.

Still, the radio-signal threat extends beyond that specific display system and FAA warning.

Numerous mobile phones left on during any airplane flight “could be a real problem,” said professor Tim Wilson, department chair for electrical, computer, software and systems engineering at Embry-Riddle Aeronautical University. The greater the number of phones emitting radio signals, he said, the greater the potential for interference with a plane’s flight system.

Airplane Mode

Many airlines now permit passengers to turn their phones to “airplane mode,” which allows Wi-Fi transmissions. But mobile phones operate at higher power levels, Wilson said, since the signals must reach a cell tower and not just a local antenna or router. “So cellular service is potentially more impactful,” he added.

The FAA in 2013 began the process of allowing wider use of electronic devices on planes, provided airlines could demonstrate it was safe. That prompted an outcry from consumer groups concerned about passengers being subjected to the mobile phone conversations of seatmates.

No U.S. airlines allowed it and, in 2018, Congress barred the use of mobile phones for calls during flights.

Honeywell says that 70 or fewer planes with affected display screens require repair. That may leave a lot of screens unaccounted for.

A plane generally has six display screens. Back in 2014 Honeywell told the FAA that 10,100 display units — or the equivalent of nearly 1,700 planes — were affected worldwide. When asked this week about the progress of the fixes, Honeywell’s Krauss said that 8,000 of those screens were replaced and fewer than 400 components, or the equivalent of about 70 planes, still need to be fixed. That still leaves 1,700 units, or the equivalent of about 280 planes, unaccounted for out of the 2014 figure.

Honeywell says its calculation of 70 or fewer assumes that some airlines might have had the work performed at non-Honeywell facilities, and regulators in other regions of the world might not have ordered the units replaced. In addition, some planes might have been taken out of service due to age. The actual number of planes operating with faulty components couldn’t be determined by Bloomberg.

Krauss said that “even if a blanking incident were to occur,” the units are backed up by multiple redundancies.

Both Delta Air Lines Inc. and Southwest Airlines Co. have completed their overhauls, according to the companies. American Airlines Group Inc. has 14 more jets that need refurbished units, and United Airlines still needs to replace components across 17 aircraft, representatives from those companies said.

Ryanair Holdings Plc, the large Irish-based discount carrier, told the FAA in 2014 that its planes held 707 of the affected Honeywell units and argued at the time that changing out all of them “is imposing a high, and unnecessary, financial burden on operators.” A Ryanair spokeswoman said the airline hasn’t upgraded all 707 screens but that the carrier inspected all of its display units and “any affected DUs have been replaced.”

— With assistance by Thomas Black, Justin Bachman, Christopher Jasper, and Jonathan Morgan

(Corrects story first published on July 18 to clarify in the second and 13th-15th paragraphs the estimated number of planes still flying with affected display screens; rephrase Honeywell’s comment and clarify the FAA’s conclusions in the fourth paragraph; and provide more details on the cause of a blanking incident in the fifth paragraph.)

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