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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?

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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.

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    $\begingroup$ +1 This is the most relevant answer. Without APU many useful/important systems would be down. Also, I assume the APU allowed them to continue flying in normal law, rather than reverting to direct law. $\endgroup$
    – Thomas
    Commented Jun 30, 2017 at 19:43
  • $\begingroup$ @Thomas -- you are indeed correct that starting the APU kept them in normal law (instead of alternate law) which meant that Sully had the full benefit of the Airbus FBW protections $\endgroup$ Commented Jul 1, 2017 at 0:25
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    $\begingroup$ But he selected flaps 2, not flaps 3, because he was concerned about the drag at flaps 3... $\endgroup$
    – Koyovis
    Commented Jul 1, 2017 at 13:31
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    $\begingroup$ I think the answer could be edited. Only flaps 2 was selected, thus not even slats were extended, which would have allowed for even lower airspeed. The most important factor after making the correct decision to land in the Hudson in the first place, was in my opinion, still having normal law. As demonstrated already in the 80s, over 20 years earlier, you can successfully crash land an A320 in a forest without significant injuries from the impact when just pulling back on the stick and achieving the α-FLOOR or α MAX speed at touch down / impact. Without full hydraulics it's not so easy anymore. $\endgroup$
    – rkantos
    Commented Aug 21, 2019 at 4:04
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    $\begingroup$ aviationmatters.co/airbus-a320-hydraulic-system A320 hydraulic system has electric pumps for Yellow and Blue lines and PTU between Yellow and Green so it is enough that APU produces 320pilots.com/air-conditioning electricity. $\endgroup$
    – h22
    Commented Aug 23, 2023 at 13:32
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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.

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    $\begingroup$ You mean that switching on the APU allowed to retract the RAT ? It's kind of implied by your answer but it would be nice to write it down. $\endgroup$
    – Antzi
    Commented Jun 30, 2017 at 1:23
  • $\begingroup$ Have amended answer. $\endgroup$
    – Koyovis
    Commented Jun 30, 2017 at 2:04
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    $\begingroup$ Still unclear - how did switching on the APU "buy them time" exactly? Less drag? Better electrics/hydraulics than the RAT would have provided? Slight thrust? Other? While the APU would give them a shot at restarting a motor, that isn't how things played out, so I don't follow your statement. $\endgroup$
    – Ralph J
    Commented Jun 30, 2017 at 5:22
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    $\begingroup$ @RalphJ, the drag was all but irrelevant. The RAT has 5 kW electric power. I don't know the value for hydraulic power, but it will be comparable. Let's be generous and say they saved 20 kW. $P = mgv_v$ They had at least 60 t. That gives at most 7 ft/min. Less in practice, since I intentionally overestimated. That negligible compared to the over 1000 ft/min glide. Moreover, once they concluded they don't have enough energy to reach the closest runway, they were not limited by the energy anyway. What did help, though, was the ability to control flaps as mentioned in h22's answer. $\endgroup$
    – Jan Hudec
    Commented Jun 30, 2017 at 19:09
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    $\begingroup$ The main advantage of the APU over the RAT is more power, not less drag. $\endgroup$
    – Thomas
    Commented Jun 30, 2017 at 20:00
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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.

B777 Dual Eng Fail

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

enter image description here

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    $\begingroup$ No, the "Miracle on the Hudson" flight was NOT saved exclusively by the APU All three answers should have made that the "bottom line up front" as you did. $\endgroup$ Commented Dec 29, 2017 at 18:26
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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.

Airbus A320 HYD panel

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  • $\begingroup$ The chosen answer cites the official NTBS report but somehow your answer contradicts that answer: can you also support your answer with some official document? If so, then your answer becomes in my opinion the most complete (and correct) $\endgroup$
    – sophit
    Commented Aug 23, 2023 at 11:16
  • $\begingroup$ @sophit, sure. Need to split this into two comments. Part 1 - The answer you mention cites the NTSB report regarding using both slats and flaps (10.3) and that flaps will not operate solely on blue hydraulic power (9.3). The following sentence is where it goes wrong: the incorrect statement that the APU powers hydraulics. This is not in the NTSB report or in any Airbus FCOM (Flight Crew Operating Manual), as the APU and hydraulic systems are not connected. FCOMs are carrier-specific and not typically public material, but I've added a link to a generic Airbus FCOM to my answer. $\endgroup$ Commented Aug 24, 2023 at 17:58
  • $\begingroup$ Part 2 - The same answer also ignores a statement in the report a few lines later, also in section 9.3: “Preliminary information obtained from the DFDR indicated that none of the three (green, blue, or yellow) hydraulic systems indicated low pressure… [indicating] that the selection of flaps 2 in the accident scenario resulted in slats 22 and flaps 15.” Even if they were correct regarding the APU’s purpose, the hydraulics maintained pressure long enough to complete slat/flap configuration. Absent a leak, the systems don’t immediately lose all air pressure. $\endgroup$ Commented Aug 24, 2023 at 17:58
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    $\begingroup$ Perfect, thanks, +1 👍 $\endgroup$
    – sophit
    Commented Aug 24, 2023 at 18:03
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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.

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  • $\begingroup$ For the A320, emergency electrical configuration (RAT/flight on batteries only) with the gear up results in alternate law. When the gear is extended, the aircraft goes into direct law. $\endgroup$
    – user71659
    Commented Feb 27, 2019 at 21:44
  • $\begingroup$ sorry, did not research coerrectly. Alternate law 1/2 don't have AOA protection either, so the rest is correct. I will edit my answer and mention you, ok? $\endgroup$ Commented Feb 27, 2019 at 22:02
  • $\begingroup$ Sounds fine to me. $\endgroup$
    – user71659
    Commented Feb 27, 2019 at 22:15
  • $\begingroup$ APU was absolutely necessary to provide power to the electrical hydraulic pumps as the milling effect was much reduced by the damaged fan and other items in the engines, further the RAT alone when not stalling gives a slow rate to the surfaces $\endgroup$
    – user40476
    Commented Jun 3, 2019 at 18:51

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