 Aft pressure bulkheads can either be curved, which reduces the amount of metal needed at the cost of reducing the usable space in the airliner, or flat, which gives more internal space but also more weight.Patents have been filed that propose deliberately creating cavities within the rear bulkhead with the purpose of providing more usable cabin space.On several airliners, production of the rear pressure bulkhead has been outsourced to third party manufacturers.