FAA now says 5G airports may interfere with Boeing 737s

'Radio altimeters cannot be relied upon to function' – gulp

The US Federal Aviation Administration warned on Wednesday that 5G C-band transmissions may interfere with landing operations at a limited set of airports for most Boeing 737 aircraft.

It issued an advisory calling for affected planes to observe modified operating procedures where 5G interference might occur. The FAA in January green-lit all 737 models to land at airports when using radio altimeters in low-visibility conditions even if 5G-C cellular towers are nearby.

The aviation watchdog's U-turn on the 737 "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 by a determination that attempts to deal with interference and the resulting pressure put on aircraft personnel "could result in reduced ability of the flightcrew to maintain safe flight and landing of the airplane."

The radio altimeter measures the aircraft's distance from the terrain below; it's used in conjunction with other equipment for approach and landing, particularly in low-visibility conditions.

Concerns about the effect 5G signals may have on aviation systems have been discussed for years but became more contentious with the US rollout of 5G service from AT&T and Verizon last month.

The FAA issued its Airworthiness Directive (AD) [PDF] because several of the systems on the Boeing 737 rely on the radio altimeter to function properly.

The Boeing 737 flew its first commercial flight in 1968. A version of the plane that entered into service in 2017, the 737 Max, was grounded in 2019 following the fatal crashes of Lion Air Flight 610 (2018) and Ethiopian Airlines Flight 302 (2019). The 737 Max returned to service in December, 2020.

The FAA directive affects about 2,442 aircraft in the US and 8,342 worldwide. That's around a quarter of the estimated commercial aircraft inventory in the US as of 2020.

The directive follows from a series of Boeing reports issued earlier this month on how its aircrafts' radio altimeter-dependent systems perform when faced with potential 5G C-Band interference. The FAA says Boeing's data helped the agency identify an additional hazard associated with 5G interference.

"The FAA determined anomalies due to 5G C-Band interference may affect multiple other airplane systems using radio altimeter data, regardless of the approach type or weather," the AD explains. "These anomalies may not be evident until very low altitudes."

"Impacted systems include, but are not limited to, autopilot flight director system; autothrottle system; flight controls; flight instruments; traffic alert and collision avoidance system (TCAS); ground proximity warning system (GPWS); and configuration warnings."

The AD covers Boeing Model 737-100, -200, -200C, -300, -400, -500, -600, -700, -700C, -800, -900, and -900ER series airplanes, except for Model 737-200 and -200C series airplanes equipped with an SP-77 flight control system (which lacks autoland and flare modes that might be affected by 5G signals).

The FAA directive, however, is not an issue at most US airports.

"The AD does not apply to landings at airports where the FAA determined the aircraft radio altimeters are safe and reliable in the 5G C-band environment," the agency said. "It also does not apply to airports where 5G isn’t deployed."

Rather than specifying which airports are affected, the FAA said it intends to issue Notices to Air Missions (NOTAMS) "where the radio altimeter is unreliable due to the presence of 5G C-Band wireless broadband interference)." These can be found by querying FAA's NOTAMS service for specific airport identifiers.

The agency previously identified 87 airports that have low-visibility approaches – making an altimeter check relevant – where 5G service has been deployed.

Britain's Civil Aviation Authority (CAA) has found no reason to believe that 5G signals interfere with aircraft equipment. The CAA contends the 3.4-3.8 GHz 5G spectrum allocation in Europe and the 4.2-4.4 GHz band used by radio altimeters has enough separation to avoid interference.

But in the US, where 5G service uses a higher part of the spectrum – the 3.7-3.98 GHz band – the FAA believes signals in proximate parts of the spectrum could be disruptive to altimeter readings, particularly because US 5G operates at higher power levels than in Europe.

"The receiver on the radio altimeter is typically highly accurate, however it may deliver erroneous results in the presence of out-of-band radio frequency emissions from other frequency bands," the AD says.

Boeing did not immediately respond to a request for comment. ®

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