The FAA is calling for a retrofit to certain Honeywell flight deck display units that will eliminate the possibility that interference from Wi-Fi signals could cause the units to go blank.

The proposed directive, set for publication today, would require upgrading so-called Phase 3 display units (DU) to Phase 3A DUs, according to service instructions developed by Boeing late last year.

Operators of some 157 U.S-registered aircraft—139 Boeing 737NGs and 18 Boeing 777s—are using the affected displays.

Those operators will have five years to make the changes, which include both a new display and new software. The cost of the upgrade is about $20,000 per aircraft, including $10,200 for each new DU.

Most fixes are expected to be completed well before the FAA’s deadline, as the problem has prevented some operators—including American Airlines and Southwest Airlines—from deploying Wi-Fi services on their flights. In Southwest’s case, the problem also prevented pilots from using Wi-Fi-enabled tablets as part of their electronic flight bag application.

The FAA discovered the issue two years ago, during testing to certify a Wi-Fi system on a Boeing 737. “This testing determined that certain Honeywell phase 3 DUs exhibited flickering and blanking when subjected to radio frequency emissions in Wi-Fi frequency bands at radiated power levels below those that the displays are required to tolerate for certification of a Wi-Fi installation,” the agency explains in its proposed directive.

“Display blanking durations of as long as six minutes were observed during testing,” the regulator adds.

While both Honeywell and Boeing made changes based on the tests, it is not clear whether the displays pose an in-service threat.

“No display units have ever blanked in flight due to Wi-Fi interference,” a Honeywell spokesman says. “The only known occurrence was during a developmental test conducted on the ground. We worked with Boeing and addressed any concerns in 2012 with new display hardware.”

The affected DUs show pilots critical aircraft status information, including airspeed, altitude, attitude and heading.