Editing Airliner (section)

===Passenger service units===
Above the passenger seats are [[Passenger Service Unit]]s (PSU). These typically contain reading lights, air vents, and a flight attendant call light. On most narrowbody aircraft (and some [[Airbus A300]]s and [[Airbus A310|A310s]]), the flight attendant call button and the buttons to control the reading lights are located directly on the PSU, while on most widebody aircraft, the flight attendant call button and the reading light control buttons are usually part of the [[in-flight entertainment]] system. The units frequently have small "Fasten Seat Belt" and "No Smoking" illuminated signage and may also contain a speaker for the cabin public address system. On some newer aircraft, a "Turn off electronic devices" sign is used instead of the "No Smoking" sign, as smoking isn't permitted on board the aircraft anyway.

The PSU will also normally contain the drop-down [[oxygen mask]]s 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.