I think that the output and impedance of an aircraft audio is very similar to that of a walkman/mp3 player etc. I have been successful by just plugging a patch cable in. Try this with you walkman or mp3 player at home. You have to stop the recording and play back to make sure that you don't have the volume set too loud. I did this by starting with the volume very low. It was actually very clean sounding, you will be surprised.
"Don't trust any statistic that you didn't falsify yourself"