After landing, the cabin pressure equalizes with the external air pressure when the outflow valves are fully open (fig.1a). There is one outflow valve on the A320 family, two on A300/A300-600/A310/A330/A340 and A350 aircraft, and four on A380 aircraft.

In some abnormal cases, outflow valves can remain closed, when the aircraft is on the ground, causing the air pressure in the cabin to be higher than the ambient air pressure outside the aircraft (fig.1b). In this case, there is a risk that an aircraft door could violently open and injure the operator or damage the aircraft.

The Residual Pressure Warning Light (fig.2), part of the residual pressure warning function, warns anyone who wants to open a door if the aircraft is pressurized. This device is installed on all Airbus aircraft except for some A300, A310 and A300-600 where it was offered as an option. On these aircraft not fitted with the warning light, a caution placard located on the door reminds the operator of the risk that residual pressure may cause violent door opening.

Depending on aircraft type, the Residual Pressure Warning Light is fitted on:

• Passenger doors,
• Emergency exits,
• FWD and AFT cargo-compartments doors,
• Main Deck Cargo Door on Freighter aircraft

Refer to the FCOM, CCOM and AMM for Residual Pressure Warning Light locations on your aircraft.

The Residual Pressure Warning Light flashes if the following conditions are met at the same time (fig.3):

• The slide of the door is disarmed (for passenger doors and emergency exits only) 
• Engines are off or the aircraft is on ground (depending on the aircraft type) 
• The residual cabin pressure exceeds a defined threshold

(fig.3) Residual pressure warning function activation logic

In several events reported to Airbus, an operator tried to open a door even though the warning light was flashing. Due to the overpressure, the door violently opened, causing damage to the aircraft and injury to the operator.

Recommendations

Anyone operating an aircraft door must be trained to the associated procedures. 

If the operator (flight crew, cabin crew or ground staff) observes that the Residual Pressure Warning Light is flashing, they must not attempt to open the door. If the cabin crew sees the light flashing from inside the cabin, they must immediately alert the flight crew (as per CCOM SOP). The flight crew should check the differential pressure and outflow valves position on the ECAM and take the necessary actions. Then, the cabin crew must wait for flight crew authorization to open the doors.

If the Residual Pressure Warning Light is seen flashing from the outside of an aircraft, the position of the outflow valves should be checked (fig.4). If outflow valves are observed closed, the ground staff should remove any connected external air source and check if an inward opening door, such as the forward avionics compartment door (fig.5) can be opened. If the avionics door cannot be opened, residual pressure still remains. The operator should then wait until the pressure dissipates by leakage. 

Emergency evacuation

In the case of an emergency evacuation, due to the logic of the system, the residual pressure warning light will not flash as long as the slides are ARMED (fig.3). 

The CCOM “EMERGENCY PASSENGER DOOR OPERATION" procedure provides guidance to identify residual cabin pressure such as a higher resistance of the door’s control handle when lifted, or a “hissing noise” around the immediate door area. In this situation, immediate communication must be established with the flight crew.

Limitation of the autonomous power supply of the residual pressure warning function

When there is no aircraft power supply and its main batteries are switched off, the residual pressure warning function is powered by the battery of the Autonomous Standby Power Supply Unit (ASPSU). 

The ASPSU battery autonomy is (fig.3):

• 4 days (5 days for A380 and A350) in “stand-by” mode 
• 50 minutes (45 minutes for A380 and A350) in “active” mode with the warning light flashing. 

Therefore, if the aircraft is unpowered for more than 4 days (5 days for A380 and A350), its residual pressure warning function is not available until the aircraft is re-energized.

In addition to the residual pressure warning function, the CAB PR EXCES RESIDUAL PR ECAM warning (fig.6) triggers if residual pressure is detected in the cabin after landing and the engines are switched off. The flight crew must follow the ECAM procedure, alert cabin crew and ground crew about the situation, and advise them not to operate the aircraft doors.

(fig.6) A320 CAB PR EXCES RESIDUAL PR ECAM Warning

Several reported incidents of violent door opening occurred after an unintentional pressurization of the aircraft on the ground.

Due to winter conditions, an A320 aircraft was secured for cold soak. As per procedure, the outflow valve was closed using the DITCHING pushbutton. The next day, a ground cart was connected to the aircraft for pre-conditioning while the aircraft doors and the outflow valve were still closed. This resulted in a cabin pressure build-up. An operator did not rely on the residual pressure warning light flashing and tried to open a passenger door. The door violently opened and the operator was injured.

When connecting a ground cart to the LP or HP ground air connectors, the maintenance personnel must ensure that the outflow valves are open and stay open, or one or more passenger doors are open and stay open, or the forward avionics compartment access door is open and stays open.

If all cabin pressure controllers fail, the ECAM procedure requires the flight crew to manually control the cabin pressure. 

Incorrect manual pressure control by the flight crew following landing can lead to the outflow valves to remain closed when the aircraft taxis to the gate. 

On A320/A330/A340 aircraft, Airbus developed a solution to this issue: the Residual Pressure Control Unit (RPCU). The RPCU automatically opens the outflow valves on the ground when the aircraft is operated in manual pressure control mode.