What is allowed on airplanes?
Anything that isn't a weapon, can't cause damage, and is logical
to take with you. If you want to take a cell phone you can, but put
your cell phone into airplane mode before take off.
... only that which is absolute necessary on the flight. Rules vary between airlines, ask your airline or check their webpage.
The question nowadays is "What ISN'T allowed on airplanes?" Items such as aerosol cans; toothpaste; soda cans; and most (if not all) pressurized cans and aren't allowed on airplanes due to the fact people have miss used them as weapons. Then there's the obvious, guns; knives; sharp metal; any other type of weapon (blade.. etc.)
No Sikhs are not allowed to bring their kirpans on a plane because it is seen as a weapon .This can be justified as the aiport security has to check everyone's safety,but still i feel that a small kirpan should be allowed as its mandatory for a baptised sikh to keep kirpan with himself every moment.