Unité de service passager (PSU)

Présentation

The Passenger Service Unit (PSU) is the overhead panel directly above each passenger seat row that consolidates multiple passenger-facing services into a single integrated module. In a typical economy class installation the PSU contains: a reading light switch and spotlight, a flight attendant call button with indicator light, a "fasten seatbelt" sign and "no smoking" sign (or combined symbol), individual air gaspers (airflow outlets), and concealed behind a drop-down panel, the chemical oxygen generator and passenger oxygen masks for emergency use. Business and first class PSUs may also house individual air vents, mood lighting elements, or personal electronic device stowage.

Despite its modest appearance, the PSU is one of the most certification-intensive components in the cabin. The oxygen mask release mechanism must activate reliably within four seconds of a depressurization event; the reading light must meet minimum illuminance requirements; the call button must interface with the CIDS without creating electromagnetic interference; and the entire unit must be manufactured from materials meeting strict FAA/EASA flammability requirements to resist flame spread in a post-crash fire. Every PSU must be individually documented, traceable, and approved under its own Technical Standard Order (TSO-C149).

Fonctionnement

The PSU is typically a multi-piece composite assembly that spans two to four seat positions per module, attached to the cabin ceiling structure or integrated into the overhead luggage bin assembly. Its functional elements operate independently:

The reading light is controlled by a pushbutton or rotary switch on the PSU face that routes 28V DC through the cabin lighting network to a focused LED spotlight above the seat. The light is typically adjustable in intensity and can be dimmed by the crew from the attendant control panel during sleep periods.

The attendant call button closes a circuit to the CIDS when pressed, triggering a cabin chime and illuminating the call indicator light in the PSU and at the nearest attendant station. The CIDS tracks all active call lights and displays their positions on the attendant control panel to prioritize service.

The fasten seatbelt and no smoking signs are illuminated by LED elements controlled directly from the flight deck sign panel; when the cockpit switch is activated, all PSU sign lamps illuminate simultaneously via the CIDS or a direct switching bus.

The gasper air outlet connects to the cabin air distribution system; passengers adjust airflow direction and volume by rotating the nozzle. The gasper provides personal airflow that helps passengers feel cooler than ambient cabin temperature regardless of the overall ECS set point.

The oxygen mask compartment is sealed behind a hinged or spring-loaded door held closed by an electrical solenoid latch. When the CPC or the cockpit oxygen panel commands mask deployment, power is removed from the solenoid, the spring opens the door, and the masks drop on their lanyards. The first pull on any mask triggers the COG to begin generating oxygen.

Composants clés

• PSU Shell: Injection-moulded thermoplastic (FAR 25.853 compliant) housing all functional elements; available in airline-specific colors and surface finishes.
• Reading Light Assembly: LED spotlight with beam angle 15–30°; typically 1–2W power consumption; individually addressed by CIDS for per-seat dimming control.
• Call Button & Indicator: Illuminated pushbutton with blue or white indicator LED; wired to CIDS call input; reset by attendant pressing the button at the seat or from the galley panel.
• Sign Lamps (Seatbelt/No Smoking): LED-illuminated pictogram panels driven by the sign switching bus; amber or white illumination depending on aircraft type.
• Gasper Nozzle: Swivelling aluminium or thermoplastic air outlet connected to the distribution duct; adjustable from fully closed to 30+ liters/min open flow.
• Oxygen Mask Drop Mechanism: Solenoid latch and spring-loaded door retaining three or four passenger masks on individual lanyard-actuated COG inlets; emergency or manual cockpit activation releases all doors simultaneously in a depressurization event.

Applications aéronautiques

On the Boeing 737-800, PSUs span three seats per module in standard economy configurations and are integrated with the overhead bin lower face. The unit is relatively compact given the 737's narrowbody cabin width. The Airbus A320-200 features KSSU-compatible PSUs with the oxygen compartment on the inboard side above each seat pair.

The Boeing 787-9 introduced a redesigned PSU that incorporates the LED mood lighting elements of the cabin sidewall cove into a more aesthetically unified overhead panel, making the ceiling feel more open. The wider fuselage cross-section of the 787 allows taller PSU profiles that provide more oxygen mask stowage volume. The Boeing 777-300ER, in nine-abreast economy, uses a three-zone PSU arrangement — window, middle, and aisle sections — to cover all seat positions across the wide cabin.

Avantages et limites

Consolidating multiple passenger services into a single certified module reduces wiring complexity, simplifies cabin installation, and provides a single line replaceable unit that maintenance crews can swap in under an hour if a PSU develops a fault. The PSU's central position in the passenger experience — every passenger interacts with its reading light and call button — makes its ergonomic quality and reliability visible to passengers on every flight.

Limitations include the regulatory complexity of certifying a multi-function assembly where a safety-critical element (oxygen masks) cohabits with passenger convenience features. Any modification to PSU design requires fresh certification review. The oxygen mask compartment must be tested for reliable opening across a wide temperature range — from -20°C on a cold-soak aircraft to +50°C in a hot-day gate hold — without mechanical binding. Gasper fouling from dust and skin oils over thousands of flight cycles can reduce airflow and requires periodic cleaning or nozzle replacement at scheduled maintenance intervals.