Accoding to https://www.faa.gov/air_traffic/by_the_numbers/ https://www.faa.gov/air_traffic/by_the_numbers/ there are more than 2.6 million passengers a day. So

Accoding to https://www.faa.gov/air_traffic/by_the_numbers/ https://www.faa.gov/air_traffic/by_the_numbers/ there are more than 2.6 million passengers a day. So even with 1e-6 you get a handful of falls alarms each day. When dealing with large systems, even small probabilities regularly occur.