In-flight recording on iphone

When training pilots in light aircraft, I have previously used an old MP3 recorder to record the conversations. I have a splitter that allows me to connect the device between the mic of my headset and the aircraft with a TRS jack into the MP3 device.

I would now like to do this job using an iphone 4S. I have tried using Voice memos on the iphone but the problem is that it reduces the levels of audio within the headset, and the voice recordings are barely audible above the background noise.

Is there an app that will allow me to play with settings to sort this problem, or is it a hardware problem requiring TRRS coonector. If so, isthere a suitable adaptor I can buy?

Many thanks