What frequency should you attempt to contact when unable to reach Boston Center while cruising at FL290?

When flying at FL290 and you are unable to contact Boston Center, you should make every effort to reach them using the designated communication frequencies appropriate for your flight. In this scenario, both 348.7 UHF and 128.325 VHF are valid frequencies allocated for air traffic control communication, thus indicating that utilizing either frequency can be effective in attempting to re-establish contact.

348.7 UHF is typically used for military operations and is in the UHF radio band, which is standard for military aircraft communication. On the other hand, 128.325 VHF is a frequency commonly used for contact with ATC in civilian aviation. Since you are flying at a flight level where both military and civilian frequencies may be relevant, checking both can enhance your chances of establishing communication with air traffic control.

Thus, the choice that combines both frequencies aligns with the best practices of aviation communication, which emphasize redundancy in attempts to ensure safety and maintain situational awareness.