777 Pilot Jaun Brown explains 5G C-Band Interference with Radar Altimeters in Aviation. Specifically, why Airlines are concerned with 5G installations near airports.

Bottom line? C-band gets too close to the radar altimeter's frequency – the crucial instrument required to auto-land planes in low or zero visibility landings.

For example, lets say you your altimeter tells your landing computer your at 1000 feet when you're actually at 10. I'm no pilot, but that sounds like a pretty hot landing.