I have mixed feelings on this:
When flying VFR in PA28s etc I have sometimes inadvertently left one on and have heard a bit of a buzz in the headsets, and had some VOR interference (will depend on the a/c cabling condition and layout more than anything I would guess). I would expect that a portable GPS would be likely to be affected too -- very sensitive receiver meets loud pulse-modulated RF noise generator in close proximity! Possibly less risk if good sheilded coax with loss-less / leak-less joints connect the GPS to an external antenna. Anyway, I am much more careful about the issue when flying in IMC is a possibility and have "PHONES OFF" on my pre-flight checklist...
When flying BA across the pond to the US (about once every month or two as part of the day job) I
usually remember to turn off my GSM/GPRS Blackberry and mobile phones... but twice in the last year or so, I am sorry to say, I have landed at SFO or ORD and then realised that I have left the phone or Crackberry on in my jacket which the nice BA ladies had hung in some store cupboard during the flight (747 or 777), with no apparent ill effects..!
I see that US adverts for cell-phone / headset adapters (e.g. here:
http://www.mypilotstore.com/MyPilotStore/sep/1767) provide the following caveat:
Note: The FCC does not allow the use of a cell phone in an airplane while in flight. Current regulations allow cell phone use in the cockpit while the plane is on the ground.
Andy
