How did ‘wire replacements’ ground planes across the entire US?

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A single issue during maintenance grounded thousands of flights across the country.

On Friday, all flights across the United States were grounded for more than four hours due to what appeared to be a telecommunications issue.

The Federal Aviation Administration (FAA) issued an alert citing “extraordinary circumstances” and called for all departing flights to remain ground stopped until further notice. According to media reports, about 3,500 flights were in the air when the alert was issued – all landed safely.

With thousands of flights delayed and cancelled, millions of passengers were stranded in airports. The travel disruptions have been slowly resolving over the weekend.

The ground stop was attributed to an update to air traffic control equipment during planned maintenance. So what exactly happened?

US Transportation Secretary Sean Duffy attributed the disruption to maintenance undertaken by government contractor L3Harris at a remote FAA facility.

The work involved replacing copper telecommunications wires with fibre-optic links, as well as changing how the system sends signals from an older method known as time-division multiplexing (TDM) to the more recent internet protocol (IP).

During this maintenance, a router (a networking device) reportedly generated more than 1,000 connection requests every four to five seconds. This affected other routers on the network, disrupting aircraft communications and some radar data feeds.

Duffy described the upgrade as a transition from analogue to digital technology, but this conflates different aspects of telecommunications.

Copper and fibre describe the physical medium carrying signals, while TDM and IP describe how information is transmitted and organised. Copper cables can carry digital data, and TDM is widely used in digital telecommunications. Fibre-optic systems also carry digital data, using light rather than electrical signals.

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