As of this writing, these items are legal and useful in an emergency situation on a plane (or a downed plane)

Swiss Army Knife blades must be less than 1/2″ by 2.36″
30′ Paracord
Magnesium firestarter block
LED Penlight with a spare battery
Opteka 4000mAh Solar Power Battery Mobile iPhone/iPad/iPod Android Phone Charger
Zip cuffs (how do you restrain a terrorist?)
EMT Shears (multi-use including the magnesium firestarter block)
Voyager SW Survival Radio (has solar panel charger and chargers tablets, phones, etc)
MRE drink powder, save the stuff you don’t normally eat and take it
2 little bottles of Potable Aqua pills (water purification)
2 of the Israeli compression bandages (available on Ebay) for an emergency wound, the self clotting bandages are also useful
Tourniquet

Think “Lost” the ABC series that lasted 6 seasons…what did they scavenge the wreck for that was invaluable?  Antibiotics, bandages, food, water purification, tools…