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Comment by koliber | Hacker News Reader

So it turns out that passenger jets do not carry backup batteries. This poses problems when all engines fail, as many functions are fly-by-wire, and require power.

Th RAT comes to the rescue in such situations. Its a Ram Air Turbine which pops out of the body when called upon. The turbine spins and generates just enough electricity to power the crucial instruments and controls.

I was touring a Hamilton Sundstrand factory a long time ago that manufactured these RATs. They had a tally board up that listed situations where the RAT was deployed and the number of people that survived the incident.

Talk about being motivated to do a good job!

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Modern jetliners have batteries in case of engine or generator failure (you can buy a 767 battery here for about $11,000[0] http://www.skygeek.com/saft-018550-000-nicad-battery.html). And, of course, to start the plane should you not have any ground power.

Of course, they don't power much - typically a few backup instruments and the APU starter - so the RAT[1] is used to power certain hydraulics/instruments so that the pilots can maintain control.

[0]Why is the battery $11,000? Well, it has to undergo a lot of testing and contain thermal runaway. See the problems with the 787's new Lithium Ion battery https://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_...

[1]For fun, here's a test flight that shows the 787 RAT deployed: https://i.stack.imgur.com/xYQPP.jpg

The RAT is backup power for the hydrolics. Do you have a citation that there's no backup battery for the electronics, because I don't believe it. Even for the crappy 25 year old not-approved-for-IFR GPS I had in my Cessna needed the engine/alternator to fail plus two separate batteries to drain before it would turn off. I can't believe the electronics on a fly-by-wire airliner are less redundant.

RAT provides electrical generation too. Its all layers of protection, there are batteries for limited duration and limited equipment. RAT provides more (duration and functionality).

There are some components that are considered like that GPS that have small enough power requirements and are critical enough that they can have significant battery and others that are powered by the RAT, and yet others that just don't work with a main engine failure.

Highly suggest watching some of the Mayday episodes about the power failure incidents like Gimli, its very educational.

Yes - the electrical generators are generally powered off the hydraulic system.

I was trying to paint a picture of why the RAT is needed, in order to share my admiration about the people who build it.

I don't know exactly which systems it powers, but have been told that in the case of engine failure, the plane cannot fly without it. You may be correct about the specifics.

Apparently also for electronics: https://en.wikipedia.org/wiki/Ram_air_turbine

> So it turns out that passenger jets do not carry backup batteries.

Isn't "backup batteries" a loose description of an APU? https://en.wikipedia.org/wiki/Auxiliary_power_unit

The APU provides electrical power but only when there's fuel for it to burn. In the case of the Gimli Glider, it therefore would not have worked.

If we look at the case of Cactus 1549 [1], though, we see Capt. Sullenberger turned on the APU immediately after both engines failed. This has been hailed [1,2] as a good decision -- no doubt because the A320 relies so heavily on electronics for control and it could have helped them restart the failed engines.

Commercial jets certainly do also carry traditional batteries for various uses. Take the Boeing 787 as an example; its onboard batteries originally presented a fire hazard [3].

[1] https://en.wikipedia.org/wiki/US_Airways_Flight_15494

[2][PDF] http://www.ntsb.gov/investigations/AccidentReports/Reports/A...

[3] https://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_...

Thank you for the very detailed reply! I like reading aviation-related material even though I'm no expert by any means

FYI you have an extra 4 on your first reference.

No, an APU is (in most aircraft that have them) actually a small jet engine that GENERATES electrical power, as opposed to storage. In some aircraft, the distinction between the onboard battery and the APU is quite clear. I know one jet that this is true of the the A320 family.

The startup procedure for that aircraft relies on either a tether from the ground to provide external electricity, or on the battery to provide electrical power to the instruments/HUD before the APU is spun up.

You are right, an APU is a generator not a battery, thanks for the explanation.

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