The Federal Aviation Administration (FAA) is issuing a new airworthiness directive (AD) on Thursday warning Boeing 737 operators that interference from 5G C-band wireless broadband operations could affect radio altimeters. The warning applies to all Boeing 737 models except 200 and 200-c series types.

5G saga continues

This is the latest installment in the 5G rollout saga that airlines argue potentially imperil the safe operations of planes near airports. The argument centers on telecommunications towers near airports using 5G C-band frequencies. Aircraft altimeters use nearby frequencies and the fear is any interference could make landings unsafe.

Despite having two years to sort the problem, the FAA left the matter largely unresolved. With telcos set to turn on their 5G networks in mid-January, the FAA suddenly acted and issued scores of NOTAMs that banned auto-landings and radio-altimeter-based automated flight manoeuvres at 100 airports across the US. Airlines warned of impending chaos.

After an intervention by US Secretary of Transport Pete Buttigieg, the telcos delayed turning on the 5G frequencies near airports. The FAA, the Federal Communications Commission, airlines, and the relevant telcos set to work to resolve the issues. Under fire for its underwhelming preparation before the rollout date, the FAA soon rescinded most of its bans and promised to pick up its game.

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The FAA's new airworthiness directive potentially impacts hundreds of 737s flying for US-based airlines. Photo: Boeing

All Boeing 737s except for -200 and -200C types affected

Now, in an AD available online and due to be formally published on Thursday, 5G and the popular Boeing 737 is back on the agency's radar. The AD applies to Boeing 737-100, -200, -200C, -300, -400, -500, -600, -700, -700C, -800, -900, and -900ER series airplanes equipped with a certain flight control system. Only some 737-200 and -200C series airplanes escape the net - being those equipped with an SP-77 flight control system.

"This AD was prompted by a determination that radio altimeters cannot be relied upon to perform their intended function if they experience interference from wireless broadband operations in the 3.7-3.98 GHz frequency band (5G C-Band), and a recent determination that, during approach, landings, and go-arounds, as a result of this interference, certain airplane systems may not properly function, resulting in increased flight crew workload while on approach with the flight director, auto-throttle, or autopilot engaged, which could result in reduced ability of the flight crew to maintain safe flight and landing of the airplane." the AD reads."This AD requires revising the limitations and operating procedures sections of the existing airplane flight manual (AFM) to incorporate specific operating procedures for instrument landing system (ILS) approaches, speed brake deployment, go-arounds, and missed approaches, when in the presence of 5G C-Band interference as identified by Notices to Air Missions (NOTAMs). The FAA is issuing this AD to address the unsafe condition on these products."

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American Airlines is one of the big operators of Boeing 737 aircraft in the United States. Photo: Boeing

The Boeing 737 is a workhorse across many US carriers' fleets. Southwest Airlines has around 700 aircraft captured by the AD. Delta Air Lines has over 230 of the planes, American Airlines over 300, United Airlines over 350, and Alaska Airlines over 160.

It should be noted this AD is a warning - not a flight ban. Boeing states it issued similar advice last month to its US-based customers operating 737 aircraft. Further, the FAA has not yet advised on steps to mitigate the potential problem.