A main generator and or an APU failure could trigger an "auto deploy" on that aircraft. If the aircraft is in air mode but really on the ground an external power failure could do it too. Look out because it is not small! :0 A ram air turbine (RAT) is a small propeller and connected hydraulic pump, or electrical generator used as an emergency power source for aircraft. In case of the loss of both primary and auxiliary power sources the RAT will power vital systems (flight controls, linked hydraulics and also flight-critical instrumentation). Some RATs produce only hydraulic power, that is then used to power electrical generators. Modern aircraft generate power through the main engines or an additional fuel-burning turbine called an auxiliary power unit, which is often a small tail-mounted turbine engine. The RAT generates power from the airstream due to the speed of the aircraft, and if aircraft speeds are low the RAT will produce less power. In normal conditions the RAT is retracted into the fuselage (or wing), deploying automatically in emergency power loss. In the time between power loss and RAT deployment, batteries are used. In a controlled field, the local controller may instruct the pilot to go around if there is an aircraft, vehicle or object on the runway or some other unsafe condition. In both controlled and uncontrolled fields, the pilot in command may decide to go around at any time, for example if the aircraft is not lined up or configured properly for a safe landing; an aircraft, vehicle or other object has not cleared the runway; no landing clearance was received (in a controlled field); the landing gear is not properly extended; a dangerous meteorological condition is experienced on final approach (e.g. poor visibility, excessive cross-winds, etc.); or some other unsafe condition is detected. Flights conducted under instrument flight rules, including all airline traffic, refer to "executing the missed approach" rather than going around. The maneuver itself is the same, but the pilot instead follows a pre-defined navigational "missed approach" sequence, published on the approach chart, instead of entering a circuit or pattern. Absent further instructions from the controller, a missed approach sequence directs an aircraft around traffic patterns and terrain into a safe place to begin a holding pattern. P.S. Some manufacturers call them ADG"s (Air Driven Generators), and some run a reverse system called a HMG (hydraulic motor generator) which has a similar function but achieves it in a different way. Sorry wiki didn't have an article about that :( maybe I will write one!