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Since the turbine needs to be deployed from within the aircraft to the outside, it will obviously induce aerodynamic drag (slowing the plane down?).

I mean I remember reading an article where a plane that for whatever reason did not stow it's landing gear and ending it burning significantly more fuel than planned.

So my question is what checklists and other factors to pilots need to account for before they decide to deploy the RAT

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    $\begingroup$ It is usually deployed automatically when power systems fail. I can't think of a reason a pilot would deploy the RAT unless they needed power (all engines-out). $\endgroup$
    – Ron Beyer
    Commented Dec 13, 2017 at 21:34
  • $\begingroup$ If you tell this to an engineer their answer is probably "why would you want to do it?" "Really? Computer X and subsystem Y is simply not designed to handle power loss." "Do you have any idea how much damage controller Z can do when it's losing power but not complete incapacitated yet?" "I don't know, for your safety we just make very sure RAT is deployed before that". $\endgroup$
    – user3528438
    Commented Dec 13, 2017 at 23:41
  • $\begingroup$ Inflight test for maintenance is a good reason (needed on some airplanes). The question can be better put as "when somebody accidentally hits the wrong button and deploys the RAT, do you need to divert?" $\endgroup$
    – user71659
    Commented Dec 14, 2017 at 2:15

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The reason why the RAT deployed is of primary concern. The RAT should automatically deploy when (depending on the design of the aircraft) you lose all primary electrical and/or hydraulic power. In these cases, an immediate diversion is necessary because you may be one failure away from losing control of the airplane.

However there are uncommanded RAT deployments due to system malfunction, or mistake. For example, the pilot hit the RAT button by accident, or somebody accidentally switched off both generators instead of switching on both bleeds. In that case, the problem is easily rectified and then the pilot needs to decide whether to divert/return or continue the flight.

For Airbus, they state

The uncommanded extension of the RAT is not considered to have any additional operational impact. The RAT is able to be extended in all flight envelope conditions. If there is an uncommanded RAT extension, then this is indicated to the flight crew, and hence the appropriate maintenance action, i.e. the resetting of the RAT, can be performed.

Fuel burn is minor, I think its +1% on the A320 family. The main concern operationally is that a RAT is loud. They sound like a prop plane or helicopter, so for passenger concerns, you might end up landing and having it restowed.

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    $\begingroup$ So it seems like the RATs used in commercial aviation have no way to be restowed without assistance from the ground? $\endgroup$
    – Kxy
    Commented Dec 14, 2017 at 12:38
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    $\begingroup$ @Ksery Correct. Basically the propeller only fits in the aircraft in a certain orientation, so somebody has to align it on the ground. $\endgroup$
    – user71659
    Commented Dec 14, 2017 at 16:45

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