The PSU will also normally contain the drop-down oxygen masks which are activated if there is a sudden drop in cabin pressure . These are supplied with oxygen by means of a chemical oxygen generator . By using a chemical reaction rather than a connection to an oxygen tank , these devices supply breathing oxygen for long enough for the airliner to descend to thicker , more breathable air . Oxygen generators do generate considerable heat in the process . Because of this , the oxygen generators are thermally shielded and are only allowed in commercial airliners when properly installed – they are not permitted to be loaded as freight on passenger-carrying flights . ValuJet Flight 592 crashed on May 11 , 1996 , as a result of improperly loaded chemical oxygen generators
ValuJet Flight 592
ValuJet Flight 592
Airliner