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An article in June 2018 Flying Magazine regarding the RAT by a Rob Mark explains that Capt. Sullunberger deployed the RAT for additional power to guide the plane down to the safe ditching on the Hudson River.

My understanding is, according to all the narratives, the book, and the movie, what saved the day (with the exception of the amazing skills of the flight crew) was the immediate starting of the APU by Capt. Sullenberger that was not even on the QRM check list,...and not the RAT.

I am wondering if both (APU and RAT) were used on that flight or the APU only? And if so, has Flying Magazine author made a mistake?

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The full NTSB report can be found here and is the official source of information on the incident. According to that report Sullunberger did immediately start the APU which did deliver power and they determined they did not need to manually deploy the RAT. However the onboard systems deployed it automatically likely as a result of the initial power loss and sufficient airspeed (over 100Kts).

Regarding step e, the pilots stated that they determined that electrical power was established and, therefore, that the RAT did not need to be manually deployed. Further, immediately after the loss of engine thrust, the captain started the APU.

This can be heard in the official transcript

At 1527:14, the first officer stated, “uh oh,” followed by the captain stating, “we got one rol- both of ‘em rolling back.” At 1527:18, the cockpit area microphone (CAM) recorded the beginning of a rumbling sound.

At 1527:19, the captain stated, “[engine] ignition, start,” and, about 2 seconds later, “I’m starting the APU [auxiliary power unit].”5

At 1527:23, the captain took over control of the airplane, stating, “my aircraft.”

However the RAT was found to be deployed (due to the automatic systems on the aircraft) when the airframe was recovered

When all of the airplane’s electrical power is lost and the airplane has an airspeed greater than 100 knots (kts), the RAT will automatically deploy and begin providing electrical power. The RAT was found in the extended position when the airplane was recovered from the water.

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    $\begingroup$ How can a pilot extend rat when there is no power? Which is the problem rat tries to solve. (afaik) Is there a handle directly attached to it? $\endgroup$ – Ali Erdem Sep 4 '18 at 19:09
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    $\begingroup$ @AliErdem: aviation.stackexchange.com/questions/1583/… $\endgroup$ – BowlOfRed Sep 4 '18 at 19:22
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    $\begingroup$ If I recall correctly, you do not need power to deploy a RAT, you need power to retract it. Like brakes on a truck or a train, the default state is actually deployed, not stowed. $\endgroup$ – MichaelK Sep 5 '18 at 7:37
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    $\begingroup$ RATs are normally spring-deployed (to get them out fast), and are held in by a latch. You need battery power to a solenoid to release the latch - twang! (They are retracted by hand to ensure the latch is hlding correctly, to avoid a twang at the wrong time.) $\endgroup$ – RAC Sep 5 '18 at 7:54
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    $\begingroup$ Thank you for linking the report. It was an interresting read. $\endgroup$ – Daan van Hoek Sep 5 '18 at 14:06
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The RAT only delivers 5 kVA on the A320. It supplies electrical power only to the essential buses. Therefore the APU is still required to power the normal AC BUS bars and the DC buses via TRU's.

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Before he decided to start the APU which needs power for the starter and some time to spool up and go online he was already on RAT provided power. As stated above if you loose both generators the RAT will deploy automatically in order to provide power to the busses. APU and its generator is just a good backup in case the RAT would fail. This one btw does not need AC but DC power to start the APU which is provided by the batteries or in this case inverters from AC bus bars when RAT is running.

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