The Federal Aviation Administration (FAA) issued an Airworthiness Directive (AD) prohibiting Boeing 747-8, 747-8F and 777 airplanes from landing at airports where 5G interference could occur. 

In the document, the FAA said that airlines operating Boeing 747-8s, 747-8Fs, and 777s are required to revise the existing limitations section of the airplane flight manual (AFM) “to incorporate limitations prohibiting dispatching or releasing to airports, and approaches or landings on runways” where the presence of 5G C-Band could cause interference.  

In the US, the C-Band telecommunication systems operates in the 3.7-3.98 GHz band, while commercial aviation radio altimeters operate at 4.2-4.4 GHz.

According to the FAA, radio altimeters installed on 747-8, 747-8F and 777 aircraft could display misleading data if affected by interference.  

The aircraft radio altimeter provides direct height-above-terrain and water information to various plane systems. This airborne electronic device is more precise than a barometric altimeter and is used to accurately measure aircraft height over the ground during manual landings, autoland, or other low-altitude operations. 

The FAA explained that while the receiver on the radio altimeter is typically highly accurate, it may deliver inaccurate data “in the presence of out-of-band radio frequency emissions from other frequency bands”. This means that the altimeter cannot sufficiently reject the out-of-band signals and could disrupt the device during critical phases of flight, such as during a take-off run, final approach, and landing.  

The authority highlighted that many systems on Boeing 747-8, 747-8F and 777 aircraft rely on the altimeter, including autothrottle, ground proximity warning, thrust reversers and Traffic Collision Avoidance System. 

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The FAA stated that it “is issuing this AD to address the unsafe condition on these products.” 

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

It is predicted that the AD will affect around 336 jets operating in the United States as well as 1,714 aircraft worldwide.