Census has published a blog post about the AES Downtime policy.
The AES Downtime Policy provides uniform procedures when the AES or the ACE AESDirect program is unavailable for transmission. During this time, the Census Bureau and CBP will work together to determine the cause and resolution for the problem. If the issue is not fixed within two hours, the Census Bureau and CBP will evaluate and determine if the use of the AES Downtime Policy is appropriate.
Read more about it here. Lots of detail!