Good work, we've destroyed an old submarine using tactics that would get us sunk under normal conditions. Let's sharpen things up a bit.

Your frigate is equipped with anti-submarine rocket-launched torpedoes (ASROC) which have been temporarily disabled (they will be enabled when you are cleared to fire on your next target). The ASROC type are distinct from anti-submarine rockets like the RBU series employed on Soviet/Russian built ships, and are essentially a light-weight torpedo (LWT) attached to a booster rocket with a guidance system. The booster flies to the target and once in position the LWT is deployed and begins a search pattern. This is no different from dropping an LWT from a helicopter, and is an effective way to engage a submarine at arms-length if a helicopter is not available.

There is a (noisy, old) nuclear submarine operating at reference point (RP) Echo. It is constrained to be within 12nm of RP Echo, but this still gives us 452 square nautical miles to search!

Set a course to RP Echo and set your speed to flank. Ensure your sonar is set to passive to avoid giving away your presence as you transit.