Hero of flight 1549 is a RAT

Posted on 23. Jan, 2009 by in Featured

Ram Air Turbine that is. OK I’m being cute; the Rat is a back up source for electrical and hydraulic power. I have had a lot of questions on how a fly by wire aircraft could continue to fly with little or no power or hydraulics.

First, what is fly by wire? Basically, very basically; the flight controller or yoke is like the controller of a video game. The pilot sends an input to a flight control computer which then transmits the request to the flight control surface through an electrical connection. At the control surface hydraulic pressure is activated and it moves. The design saves a lot of weight. The down side is you need both electricity and hydraulic pressure to move the surface and thus control the aircraft. These are normally provided by the aircraft engines accessory driven generators and pumps. With both engines out they drop off line. Below is an airbus video on the A-320 fly by wire system (same aircraft as 1549).

In an aircraft like the MD-80 the flight controls are moved by actual steel cables (poor man’s fly by wire). So if everything fails, even the emergency battery, you can still fly the aircraft. And you can even drop the gear by opening the doors manually. There are other backup systems for brakes, thrust reverse etc, that also do not require electricity or hydraulic pressure.

In a hydraulically powered aircraft, if the engines continue to windmill and you maintain a minimum speed depending on the aircraft type (for an EA-6B Prowler it is 140 knots, useless info I no longer need to know), sufficient hydraulic pressure will be maintained allowing you to control the aircraft. On airliners you also have electrical back up pumps that you can power via the APU (auxiliary power unit) a small jet engine that powers its own generator and hydraulics. However if you lose all hydraulic fluid or pressure you no longer can move any of the flight controls. The United DC-10 that crashed in Sioux City Iowa is an example of that. The crew did an incredible job of guiding the aircraft to the runway using only differential engine thrust saving many lives. But in the end could not maintain the control necessary for landing.

Back to the RAT. In the airbus a Rat deploys (automatically) that powers the flight controls. See below.

This RAT can provide power for as long as the aircraft maintains sufficient airspeed to turn it (in the EA-6B it was 110 knots, more useless info). Designed to allow the crew to maintain control while they attempt a re-start; or in this case a ditching. They have been around for a long time; the A-4 designed in the 50’s had one. Below is one from an F-102 Delta Dagger also designed in the 50’s.

It is for electrical only, but you can see the action and relative size. Remember they are only for emergency systems that have to be powered, the movie is going off. An ingenious and simple device, and the hero device of the day.

One Response to “Hero of flight 1549 is a RAT”

  1. Joy 21 March 2014 at 00:00 #

    That happens all the time aruond here, too. Except they’re usually army green or dark grey. There’s a Lockheed aruond here, too, about a 20-minute drive from my house. My brother interviewed with them but didn’t get in, obviously (but he teaches me how to identify Boeing planes and what model they are).

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