The Federal Aviation Administration on Wednesday added Seattle-Tacoma International to its list of airports where most large jets are now permitted to land in low visibility without risk of interference from the new 5G wireless networks.

While that provides some relief for airlines and for travelers planning to fly in the coming days, the prospect of a large number of canceled flights if bad weather returns at Sea-Tac is not entirely lifted.

Smaller planes used on short-haul routes such as the Bombardier Q400 turboprops and the Embraer E175 regional jets flown by Alaska Airlines are not on the FAA’s list of cleared airplanes.

A statement from the FAA indicates that 38% of U.S. airline planes still face restricted flying.

“Even with these approvals, flights at some airports may still be affected,” the FAA said. “Passengers should check with their airlines for latest flight schedules.”

“Brinksmanship” between FCC and FAA

Wireless carriers already provide 5G cellphone service using different parts of the radio spectrum that don’t raise an interference issue.

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But to expand that service to many more users they bid for a big new slice of the spectrum called 5G C-Band that’s adjacent to the frequencies used by radio altimeters on aircraft — instruments that tell the pilots and other airplane systems a plane’s height above the ground.

The Radio Technical Commission for Aeronautics, or RTCA, did tests in 2020 that showed a risk of 5G C-Band interference with the altimeters on airliners, business jets, general aviation aircraft and helicopters. Over FAA objections, the Federal Communications Commission outright rejected the RTCA report.

Peter Lemme, a former Boeing flight-controls engineer who is now a consultant in avionics and satellite communications, said that instead of collaborating to find a solution, the FCC walked away and sold the wireless carriers licenses to use the spectrum.

“It’s gotten to brinksmanship here at the end, which is a shame,” Lemme said.

Concerned that interference could cause a crash, the FAA issued restrictions on flight operations that limit landings in low visibility where this latest 5G spectrum is in use — restrictions that took effect at 9 p.m. on Tuesday as wireless providers AT&T and Verizon turned on their nationwide 5G C-Band networks.

At the last minute, under pressure from the government, the two cell carriers agreed to temporarily defer turning on certain 5G C-Band cell towers within a 2-mile buffer zone around airport runways. (T-Mobile won’t activate its C-Band 5G towers until late 2023). That’s what allowed the FAA to partially lift some restrictions Wednesday.

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Extensive tests underway

To assess the precise risk, the FAA is scrambling to test all the different models of altimeters installed on commercial aircraft. At the same time, it’s examining the positioning of the 5G cell towers around the runways at each U.S. airport. And it’s evaluating each aircraft model to see which systems are potentially affected by a faulty altimeter.

The FAA said Wednesday it has now cleared five models of altimeters as safe from 5G interference. And it expanded the list of airports where low-visibility landings are now approved for aircraft using one of those five cleared altimeters.

In addition to Sea-Tac, the list now includes the major West Coast hub airports in Los Angeles and San Francisco as well as Dallas, Chicago, Portland and Salt Lake City.

This update will “allow an estimated 62% of the U.S. commercial fleet to perform low-visibility landings at airports where wireless companies deployed 5G,” the FAA said in a statement.

The list of aircraft with those altimeters installed includes “some Boeing 717, 737, 747, 757, 767, 777, MD-10/-11 and Airbus A300, A310, A319, A320, A330, A340, A350 and A380 models,” the FAA said.

Conspicuously, the word “some” suggests there may be a few models of the planes in that list that have different altimeters and so are not cleared to fly in low visibility.

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And the list doesn’t include Boeing’s 787 Dreamliner, even though the altimeter model supplied by Honeywell for the entire fleet of 787s is one of those that has been cleared. The FAA issued a separate Airworthiness Directive imposing restrictions on 787 operations at airports with 5G deployed.

On the 787, the altimeter reading is used to determine the timing and power of the automatic braking systems upon landing. The directive mandates revisions to the flight operating procedures, prohibiting certain landings, and a change to the procedures pilots use to calculate the distance needed to come to a stop on the runway at any airport where a 5G network is deployed.

The FAA said it “continues to work with manufacturers to understand how radar altimeter data is used in other flight control systems.”

In addition, the new FAA list of cleared aircraft doesn’t include any of the smaller planes that the big airlines use to fly passengers from less populous cities to their main hubs.

For example, Alaska Air uses Q400 turboprops and E175 regional jets flown by its sister carrier Horizon Air as well as regional carrier SkyWest for this purpose.

Many mainline Alaska Airlines flights depend on such regional airplane connections to feed passengers to its mainline Boeing 737 and Airbus A321 jets.

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As of now, those aircraft will not be able to fly in low-visibility weather.

The FAA website states that the agency is “reviewing testing data for altimeters used in regional jets.”

Lemme said the RTCA tests suggest that regional planes will be more susceptible to interference.

“There’s a good chance that we’re going to find most of the regional jets are going to be interfered with, that they’re going to have a problem,” he said.

For the near future, the weather forecast at Sea-Tac is good and so disruption may be minimized.

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The situation is changing fast as the FAA grinds through its tests and analysis of individual airports, of each different altimeter model and of the integrated systems on each different aircraft.

On Wednesday, both Japan Airlines and All Nippon Airways said they will resume flights into the United States on their Boeing 777s as the airports they fly into have been cleared, as have the altimeters installed on those jets.

Citing a notification from the FAA, All Nippon Airways said “there is no safety issue with the operation of Boeing 777 aircraft to the U.S. airports that we serve.”

Emirates, which on Tuesday canceled flights to Seattle and other U.S. cities through Thursday, had not updated its schedule by late Wednesday.