It's easy enough to test, record some video with your cellphone during a flight, see how much voice you can pick up after some simple filtering. Now imagine what an intelligence service could do if they had access to test runs on actual planes, could engineer their own microphones, and could tune filters to the engine noise.
You might as well say that it's unlikely that tapping 60s era cars on the highway will work. I'm fairly certain it did.
You might as well say that it's unlikely that tapping 60s era cars on the highway will work. I'm fairly certain it did.