Certain items are prohibited in any airline luggage, whether this is checked or carry-on luggage. These are mostly items which could cause a devastating explosion or fire in part of the aircraft, which could cause damage to the plane and possibly a crash. Such fully-prohibited item currently include flares of any kind and gun powder--including percussion caps and black powder.
Sharp items is a large category of items which are typically prohibited in carry-on luggage, although they may be checked in luggage traveling in the hold area of the aircraft. The reason behind restricting passengers from carrying sharp objects onto the plane is that hijackers may be able to use a sharp object as a weapon against the plane crew or passengers. Sharp items prohibited in carry-on luggage but allowed in checked luggage include box cutters, ice axes, knives, meat cleavers, razors (other than safety razors), sabers, scissors, and swords.
Various sporting goods which could be used as a weapon are prohibited in carry-on luggage, although typically permitted in checked luggage. These include baseball bats, bows and arrows, cricket bats, golf clubs, hockey sticks, lacrosse sticks, ski poles, and any other sporting accessory which has the potential to be used as a weapon.
Some airlines allow passengers to carry ammunition in checked baggage, but without exception guns and ammunition are prohibited in carry-on luggage. Prohibited items include the individual parts of guns, realistic replica guns, BB guns, and gun lighters.
Most tools, apart from those which are very small, are prohibited in carry-on luggage. They may, however, typically be carried in checked luggage. Tools which may not be packed in carry-on luggage include axes, hatchets, crow bars, hammers, drills, saws, and any tool longer than seven inches. Again, this is to prevent the possible use of such tools as hijacking weapons on board the plane.
Individual airlines place restrictions on the size and weight of carry-on luggage. This is to ensure that there is sufficient storage space in the aircraft cabin for all the passengers' carry-on luggage, and also to make sure that fuel calculations based on the weight of the aircraft, passengers and luggage are accurate.
Liquids are permitted in carry-on luggage, however their size and total volume is restricted by the TSA. This restriction was enacted in response to incidents where terrorists attempted to take liquid explosives on board passenger aircraft. The TSA limits the liquid allowed in each passenger's carry-on luggage as follows: each individual bottle must have a volume no greater than 3.4 ounces (100ml); all the passenger's liquids must fit into one quart-sized, clear, plastic, zip-top bag.
