Hallo,
I m preparing a DOC and i wanted to ask you what's the best way to record the audio comunication inside the helicopter with an external recorder.
Thank you so much

You will be able to record using a lead made up with the correct connectors so you can plug into their coms system.

This will either be a NATO plug: UK NATO Plug - UK Military Helicopter | eBay (http://www.ebay.co.uk/itm/UK-NATO-Plug-UK-Military-Helicopter-/171303284080?hash=item27e278ad70)

or it may be a twin GPO type jack for the headset and mic.

I would ask them which connectors they have and it may be that they have a cable that you could use with adaptors to suit your recorder.

If this is for some long-term shooting schedule, then plugging into the helicopter com system is a good idea. There are companies that specialize in selling recording adapter cables specifically for this scenario.

But if it is just a temporary or transient production, many people just take their tiny lav mic (like a Tram or Countryman, etc.) and and put it inside the ear-cup of the one of the aircraft's headsets.

I have used the lav mic in the earpiece many times, and never really wished for better. It lets in just enough helicopter nats to make it believable, without having to mix it in post.

To follow up on Richard's comments.. you slip the lav mic in while you're wearing the headset. So you put on the headset, then slip a lav inside the earpiece. This insulates the lav from the rotor noise.