With 5G broadband being rolled out in many countries as the next generation in telecommunications and broadband, concerns have been raised about the chances of possible interference with aircraft.

In this post, I share what I learnt from the FAA AD 2021-23-12.

What we know so far

C Band wireless broadband (5G) is being rolled out and authorised for use. C Band wireless broadband operates at 3.7-3.98 GHz. The United States Federal Communications Commission (FCC) has authorised flexible use of the 3.7-3.98 GHz band for next-generation services.

In April 2020 members including the FAA, aircraft and radio altimeter manufacturers, European Organisation for Civil Aviation Equipment (EUROCAE), industry organizations, and operators, performed “a quantitative evaluation of radar altimeter performance regarding RF interference from expected 5G emissions in the 3.7-3.98 GHz band”.

This work concluded that there was a major risk that 5G telecommunications systems in the 3.7-3.98 GHz band will cause harmful interference to radar altimeters on all types of civil aircraft.

The report also found that the chances of interferences increase with low altitude.

The interference could cause the radio altimeter in the flight deck to become inoperable or present misleading information.

The FAA determined that, at this time, no information has been presented that shows radio altimeters are not susceptible to interference caused by C-Band emissions permitted in the United States.

What is a radio altimeter?

The radio altimeter is a key instrument on the flight deck that provides accurate heigh information above the ground – typically below 2500ft as the aircraft gets close to landing.

Most radio altimeters operate in the 4.2-4.4 GHz band, which is separated by 220 megahertz from the C-Band telecommunication systems in the 3.7-3.98 GHz band.

The radio altimeter is significantly more accurate than the barometric altimeter of most aircraft. For this reason, radio altimeters are used for precision approaches or autoland.

737 classic

Taken from the FAA [4910-13-P] Air Worthiness Directive ‘The receiver on the radio altimeter is typically highly accurate, however it may deliver erroneous results in the presence of out-of-band radiofrequency emissions from other frequency bands. The radio altimeter must detect faint signals reflected off the ground to measure altitude, in a manner similar to radar.’

The radio altimeter must detect faint signals reflected off the ground to measure altitude, in a manner similar to radar. Out-of-band signals could significantly degrade radio altimeter functions during critical phases of flight if the altimeter is unable to sufficiently reject those signals.

How do airliners use radio altimeters?

Radio altimeters are mostly used in low visibility situations e.g. thick fog or low cloud.

Operators have to be certified and crews undergo specific training to be able to land in conditions where there are few if at all any visual references during the landing.

These operations need to be conducted close to the ground with the radio altimeter playing a key part in providing accurate height information.

Erroneous readings from the radio altimeter close to the ground may not be able to be detected by the pilots or may cause the aircraft to react in an unpredictable manner which could impact safe flight and landing.

Although the main worry is the interference of the radio altimeter close to the ground with regards to take-off and landing, the radio altimeter in most modern aircraft is linked to other automated systems. These systems can include terrain warning and avoidance systems.

Data is still being collected on how 5G C-Band may interfere with radio altimeters. That may drive the FAA to put additional measures in place.

Boeing and Airbus have deep concerns

“5G interference could adversely affect the ability of aircraft to safely operate,” said senior executives of Boeing and Airbus Americas, Dave Calhoun and Jeffrey Knittel, in a joint letter to US Transportation Secretary Pete Buttigieg.

Boeing vs Airbus

According to Bloomberg – The industry is concerned the 5G clash will cost as much as $2.1 billion in-flight disruptions, according to the trade group Airlines for America. “If you were to ask us what our number one concern is in the near term, it is the deployment of 5G,” Southwest Airlines Co. Chief Executive Officer Gary Kelly testified at a Senate hearing on Wednesday.

In contrast “5G operates safely and without causing harmful interference to aviation operations in nearly 40 countries around the world,” Ludlum said in an email. “U.S. airlines fly in and out of these countries every day.”

FAA Airworthiness directive

Radio altimeters cannot be relied on if they get interference from 5G wireless broadband 3.7-3.98 GHz frequency band (5G C-Band).

What is being done right now

FAA directive recommends limitations prohibiting certain operations requiring radio altimeters in the vicinity of areas of possible interference with 5G (C band). This would normally be identified by NOTAM (Notice To Air Men and Women)

The FAA has invited comments on the topic.


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