I've heard of people unlocking it from various rank 50 sectors. Perhaps it is just discovered from a random sector for each fc? Like FC A wouldn't have a shot at discovering it unless they went to sector 19, and FC B wouldn't have a shot at discovering it unless they went to sector 21. This is just conjecture though, but I think it's worth trying out. If you have multiple airships, why not send at least 1 to one of the other rank 50 sectors while you send the other(s) to sector 22?