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-018550-0

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 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_problems 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 https://i.stack.imgur.com/xYQPP.jpg