The latest generation of smartphones using 5G technology can interfere with aircraft altitude instruments and compromise flight safety, the French Civil Aviation Authority warned on Tuesday, saying the devices should be turned off completely during flight or put on "airplane mode."

Altimeters measure how close an aircraft is to the ground, a function that makes these instruments "extremely critical during landing," as the regulator, known by its French acronym DGAC, described them.

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5G technology uses a bandwidth which is apparently dangerously close to the frequencies used for air navigation, a proximity that could potentially cause altimeters to record the wrong altitude. Such errors at landing, when altimeters are key to the safety of the operation, could lead to fatal accidents.

The road to introduce the new wireless networks has been filled with mistrust and setbacks, amid conspiracy theories that falsely linked it to the spread of COVID-19 and real concerns that it could interfere with weather readings.

The issue of 5G potentially interfering with aircraft altimeters was initially raised in France in November last year, when the deployment of 5G around Nice airport and Paris-Charles de Gaulle was slowed down while regulators assessed its impact on flights.

The French regulator has now outlined the conditions for safely installing 5G networks while limiting the risk of interference with aircraft altitude instruments during landing, with the strength of signals from 5G base stations placed around airports being restricted. 

Regulators continue to study how the new technology interacts with aircraft instruments in the National Frequency Agency (ANFR).

_{Cover image: Zoltan Mathe / MTI via AP}