There's been much discussion about US Airways Flight 1549 and the extraordinary circumstances that befell the Airbus A320-200 (N106US) and her compliment of 150 passengers and 5 crew. At least preliminarily, it appears a double bird strike disabled each of the CFM56-5B4/P engine forcing Captain C.B. Sullenberger III and his First Officer to ditch the jetliner in the Hudson River.
As the aircraft was making its 'final approach' to the Hudson, the crew was preparing the aircraft and its passengers for the water landing, including, some speculate, by activating the ditching system on the A320. The button, cleverly labeled 'ditching', is located on the 'Cabin Press' section of the overhead panel shown above.
So what does that infrequently used button actually do?
When pressed, it commands the aircraft operating system to close the outflow valve, emergency ram air inlet, avionics inlet, extract valve and flow control valve. In addition, it will immediately shutdown the cabin fans. The button itself has a guard over it to prevent accidental activation. The system is available on all A320 family, A340/A330 and A380 aircraft.
According to the A320 quick reference guide, the ditching procedure calls for Flaps 3 and a minimum approach speed of 150 kts. The system should be activated at 2000 feet AGL and Airbus recommends 11 degrees of pitch at the time of touchdown.
The ultimate purpose of the system is to seal the aircraft to prevent water from undermining the buoyancy of the aircraft to keep it afloat in the event that the airframe remains intact after impacting the water. Federal Aviation Regulation Part 25, Section 801 describes the safety requirements in the event of a ditching:
(d) It must be shown that, under reasonably probable water conditions, the flotation time and trim of the aeroplane will allow the occupants to leave the aeroplane and enter the liferafts. If compliance with this provision is shown by buoyancy and trim computations, appropriate allowances must be made for probable structural damage and leakage. If the aeroplane has fuel tanks (with fuel jettisoning provisions) that can reasonably be expected to withstand a ditching without leakage, the jettisonable volume of fuel may be considered as buoyancy volume.
It's not hard to imagine that this live test of the 'Ditching' system was a resounding success.
