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In the film Sully, (staring Tom Hanks) the APU is switched on straight after both engines rollback, why was this step necessary; Airbus said this was the "15th thing" to do on the checklist.
How or why did this step in particular prevent the loss of lives of over 100 people?
Yes, the solution to start APU was important.
The ditching procedure directs the use of maximum available slats and flaps for the final approach and touchdown (source, chapter 10.3). This is not possible without APU, as Airbus A320-214 cannot move flaps if only powered by RAT (only blue hydraulic line, same source, chapter 9.3). The running APU adds the green hydraulic line that can also move flaps.
Hence, without running APU, the optimal ditching procedure is not possible; that gives less chances for success.
The engines provide electrical and hydraulic power to the systems that control the flight path of the aircraft: elevator, aileron etc. If all power is lost, a Ram Air Turbine (RAT) is deployed which spins up and powers the systems from the airspeed. However, this creates extra drag.
The RAT deploys automatically upon loss of AC bus 1 and AC bus 2. This takes a while after engine fail since the engines spool down - if APU is switched on before the buses are detected down, the APU then powers the AC buses and the AC pumps that drive the Blue and Yellow hydraulic systems. Below 125 knots the RAT stalls.
The RAT was found deployed in the accident report. It has an automatic blade pitch adjust system that retains constant speed under load - if the RAT is deployed and needs to provide the backup power for Blue hydraulic and AC electrical, it extracts energy from the air stream and provides extra drag. If the APU provides power for electrical and hydraulic systems, the RAT windmills and there is less drag.
The A320 had just taken off and did not have much altitude yet. Aircraft without engine power can glide - the distance they can cover while gliding depends on the starting altitude of course, but also on Lift over Drag: the lower the drag, the further the aircraft can glide. Switching on the APU provided them with extra gliding distance due to less drag - although that may not have been the captain's main objective, it may have been an attempt at engine start.
They needed all the distance they could get, and all the clearheadedness that could be provided. And that is what the pilots delivered. Switching on the APU bought them time, and more choice in landing the plane at a suitable spot. The situation was not exclusively saved by switching on the APU, it was a factor in a chain of events where everything needed to be right - and it was.
No, the "Miracle on the Hudson" flight was NOT saved exclusively by the APU.
Airline flights are routinely dispatched with a non-operable APU. It is not needed or required to complete a safe flight.
On the Boeing B777 starting the APU(if avaialable) is number 5 in the checklist for dual engine failure.
In my opinion the starting of the APU had very little to do with the success of the water landing and survival of all the passengers and crew.
Sully is on record as saying he started the APU to provide an addition electrical source: Sully Speaks Out
I had taken by memory the first two remedial actions that would help us the most, that we would later get to on the checklist over a minute later, over a third of the way through the remaining flight time, and that was to turn on the engine ignition so the engines would recover if they could (it turned out they were irreparably damaged), and I started the airplane’s auxiliary power unit, the APU, to provide a backup source of electrical power. Since this is a fly-by-wire airplane and there’s no longer a direct mechanical connection with the flight controls, instead you have electrical impulses that are fed through computers that then move actuators.
In this particular case the APU may have allowed a slightly longer glide, but that would just have meant touching down on the water slightly upstream from where they did touch down.
Map Source: CNN Map
I realize this is an old question, but I came across it while looking for something else and it bugs me that the existing answers are conjecture and incorrect (though I'm sure well intentioned). So, in case anybody else comes looking some day...
The advantage of the APU in this situation was to provide greater power. When the electrical busses on an A320 lose power, a small windmilling propeller called the Ram Air Turbine (RAT) deploys automatically when the aircraft is above 100 knots. The RAT provides a minimum of power to support crucial aircraft systems, and it also powers one of the three hydraulic systems. The APU, on the other hand, can power the entire aircraft.
As far as the electrical side of things goes, this allowed the aircraft to remain in Normal Law. Airbus aircraft use a set of flight control laws that determine certain protections and operating margins. In this case, it allowed the aircraft to remain a safe margin above stall speed for as long as possible. Without Normal Law protection, which would've been lost if operating only on the RAT, some of those protections would be lost. In other words, it allowed greater control protections to help with a smoother landing.
On the hydraulic side, it gets a little complicated. The three hydraulic systems are variously powered by the engines or electrical supply. The RAT can only power one of these three systems (blue). With this single system powered, some controls would operate normally, some at half speed, and some not at all. The APU can directly power the blue system, same as the RAT, and could power the other two systems depending on some switch settings. The US Air crew didn't have time to change those settings, but as it turns out, they didn't need to. The hydraulic systems weren't leaking, and with the short time between the birdstrikes and forced landing, enough pressure existed to operate the control surfaces normally.
So, short answer: the APU was helpful in that it provided electric power, which enabled the crew to have better control of the aircraft. It didn't affect the hydraulics, which remained pressurized, and it did not add thrust. It was one of many things that the crew did right, but not the only one. How much of an impact it made it difficult to say, but in an emergency scenario, you'll take all the help you can get.
Edit to add: Another user asked for some documentation regarding the APU and hydraulic systems. All aircraft have flight manuals, including airliners, though the more complex they get, the longer the manuals. At the airliner level and on the pilot side, these are usually distributed as a Flight Crew Operating Manual, or FCOM. FCOMs are not typically publicly available, as some of the information inside is proprietary or otherwise sensitive.
With that said, you can find nearly anything via Google, and there are a number of Airbus A320 FCOMs floating around online. Here's a link to a website featuring some generic A320 FCOM sections which are not specific to any particular air carrier. These are typically issued by Airbus for companies to customize or for training purposes. Here's a specific link to the APU section and one to the Hydraulics section (these two are PDFs). Note that in the first, hydraulics are not mentioned among the APU's functions, and in the latter, the APU is not mentioned among the sources of power for the hydraulics system.
Finally, here's a picture of the overhead panel on the A320. One of the nice things about the A320 flight deck is that there is a basic systems diagram on some of the panels, acting as a quick reference for major components of the system. Note how Green and Yellow systems show engine pumps, Blue and Yellow show electric pumps, the PTU connects Green and Yellow, and the RAT can also power the Blue system. Were hydraulic power available from the APU, it would be shown here (and would require a pushbutton), consistent with Airbus design philosophy.
Jep, I think it was very beneficial to the outcome of this event. However I think that in addition to the reasons stated here, the APU also prevented Sully from stalling the airplane.
If I remember correctly, if all electrical power is lost/provided by the RAT, the airplane goes to "direct law" mode. This means, that all inputs by the pilots are translated directly into proportional movements of the flight control surfaces. The systems that usually keep the pilot from getting the plane into a unrecoverable situation are deactivated. As Sully pulled the plane up just before touchdown in order to execute a flare, he would have stalled the airplane, had the fly-by-wire system not prevented him from doing so. As this system would not have been active if he had not switched on the APU, he would have dropped into the water like a stone.
So yes, the APU saved the day, by reducing drag, burning a tiny bit of weight of, moving the flaps, and keeping Sully from stalling.
