Nit: If you're filing a flight plan, you do it with the country you're departing from. Even if you're piloting an aircraft departing into the US, it wouldn't have any effect on operations if you couldn't reach US websites. There's also several alternative ways for pilots to file flight plans outside of the web.
reply(The flight plans get passed between countries via AFTN/AMHS, which are dedicated telecommunications networks independent of the Internet.)
I thought airlines still had to file passenger manifests with CBP separately, no?
replyYes, though that's separate from the flight plan.
replyThere's also several different ways to transmit the passenger manifest to CBP - including over a CBP-provided VPN and IATA "Type B" messages sent through ARINC/SITA.
The network for Type B messages is also independent of the Internet (it was developed 60 years ago).