RAT Switch on 777 Why "Press & Hold for 1 second?"
Hello Tech Guys,
During a Dual Engine Failure on the 777 part of the emergency checklist procedure is to push the Ram Air Turbine Switch - AND PUSH AND HOLD FOR 1 SECOND. Can anyone tell me why we have to Hold it for 1 second, but more specifically what is it actually doing mechanically/electrically?
The old Quick Reference Handbook (QRH) for the 777 says this action "backs up automatic deployment of the RAT." That's a general reason why we do it, but can anyone shed any light on what physically is happening and why we must push it for at least 1 second? I'd like to know and hopefully I'll never have to do it. Thanks!
You backup the automatic feature. The RAT should be deployed automatically when the Dual Engine Failure is recognized by the system. In case that fails as well you push and hold the button for one second firmly. The system opens the cover on the right sight of the fuselage and the RAT "freefalls" into the wind and the windmilling starts to supply AC Power and Hydraulic so you have enough time to glide the Bird thru the skies and restart one - or better - both engines again. The simple flameout should not happen but who knows ? I am sure I have my heart beating faster then ever on the treatmill in that moment. Did a lot of Pacific crossings East-West, North-South and vice versa, just imagine.
The RAT release mechanism has an electromechanical latch. I suppose if you don't keep your finger on the button for long enough, the latch won't completely disengage, and will probably spring back to the fully locked position.
This is not a case of microamps running around in a small solid state device
Same as the fire switch. Turn to stop and hold for 1 sec. To ensure that you hit the spot in your haste. An effort I think, to make it a bit more deliberate and reduce the chance of not making contact. There was an issue came up years ago with starting the apu. They wanted us to hold the switch in the start pos. for a beat as sometimes with a quick swing of the switch the apu failed to proceed.
So, I am sitting at work and this question has my attention. Pulled up a wiring diagram and I don't see anything (timing relay/logic) that would require the 1 second. Best educated guess (it being a critical task) would be to assure the switch is pressed and not punched in a rush.
Thank you to all for replying - esp to those who pulled out the books! What you guys came up with is pretty much also all that I could come up with (hence it most likely is the correct answer!) You'd better believe that if I ever had to push that RAT switch I'd be holding it down for probably 2 seconds and 'willing' it to drop out! (I've been watching too much Star Wars). Thanks and hope I never have to touch that switch ... ever.
Folks, The press and hold ---- holds good for virtually all modern aircraft ----- the reason is digital systems versus analog. A quick "flick" of a contact may not be enough for a digital system to recognize the signal being sent.
In the early days of the B757/767 is was a real problem to get crews to "press and hold" rather than a momentary press --- not easy to teach old dogs new tricks.