How Ram Air Turbines Harness Wind to Restore Flight Controls During Dual Engine Failures

On January 15, 2009, when US Airways Flight 1549 lost thrust in both engines at 2,818 feet over New York, the Airbus A320's electrical architecture fundamentally shifted. As the main generators spun down, the aircraft faced a total loss of the alternating current needed to drive its fly-by-wire flight computers.[2]

Before the flight crew could even reach for the emergency checklist, the aircraft's logic systems detected the voltage drop. A spring-loaded hatch beneath the fuselage snapped open automatically, dropping a small, two-bladed propeller into the 200-knot slipstream.[2]

That device, known as a Ram Air Turbine (RAT), immediately began spinning in the oncoming wind. Within three seconds, it restored 3,000 pounds per square inch of hydraulic pressure to the blue system, bringing the primary flight control surfaces back online.[2][3]

The deployment of a RAT transforms a 70-ton composite airliner from a falling object back into a controllable glider. It is the ultimate mechanical failsafe, relying on the aircraft's forward kinetic energy to replace the power normally generated by burning jet fuel.[1]

Harnessing the Slipstream

Modern commercial aircraft are entirely dependent on hydraulic and electrical power to move their elevators, ailerons, and rudders. When the main engines flame out, the engine-driven pumps and generators cease functioning, severing the physical link between the cockpit controls and the wings.

The Ram Air Turbine bridges this gap by harvesting energy directly from the atmosphere. As the aircraft glides forward, the relative wind strikes the turbine blades, which are typically constructed from lightweight carbon fiber and measure between 80 centimeters and 1.5 meters in diameter.[3]

To prevent the turbine from overspeeding at high altitudes or stalling during a slow approach, the RAT features a variable-pitch governing mechanism. Centrifugal counterweights continuously adjust the angle of the blades, ensuring the turbine maintains a constant rotational speed regardless of the aircraft's velocity.[4]

"The ram air turbine provides the ultimate electrical and hydraulic power backup, ensuring the flight crew retains control authority until the aircraft reaches a safe landing speed," states the Federal Aviation Administration's transport category certification framework.[1]

This constant rotation drives a central shaft connected to a hydraulic pump, an electrical generator, or both. The mechanical energy is instantly converted into fluid pressure, which flows through the aircraft's redundant hydraulic lines to the actuators that physically push and pull the control surfaces.[3][5]

Shedding the Non-Essential Load

A deployed RAT does not restore the aircraft to its normal operating state; it enforces a strict hierarchy of survival. A standard twin-engine airliner like the Boeing 777 generates roughly 240 kilovolt-amperes (kVA) of electrical power during normal cruise flight.

In contrast, the largest Ram Air Turbines produce a maximum of just 7.5 kVA of electrical output. This massive reduction forces the aircraft's power management computers to immediately shed all non-essential electrical loads to prevent the emergency generator from stalling.[3]

Cabin lighting, in-flight entertainment, galley ovens, and non-essential fuel pumps are instantly disconnected from the power grid. The remaining electricity is funneled exclusively into the essential alternating current and direct current buses.

This remaining 3.2 percent of the aircraft's original power capacity is just enough to keep the primary flight displays illuminated and the fly-by-wire computers calculating control inputs. The system proves that modern airliners require less than 4 percent of their designed power to maintain controlled aerodynamic flight.[5]

The transition is violent but effective. Pilots report that the sudden silence of the failing engines is immediately replaced by a loud, high-pitched mechanical whine vibrating through the airframe as the RAT spins at several thousand revolutions per minute just below the cabin floor.[2]

The Aerodynamic Cost of Survival

Extracting energy from the slipstream is not free; it comes at a direct aerodynamic cost. Hanging a spinning propeller into the airflow creates significant parasitic drag, which acts as an airbrake on the gliding aircraft.[4]

A clean, twin-engine airliner typically boasts a glide ratio of roughly 17:1, meaning it travels 17 miles forward for every one mile of altitude lost. When the RAT deploys, the added drag reduces this ratio to approximately 15:1, steepening the descent profile.[4][5]

Aerospace engineers accept this drag penalty because altitude is useless without control authority. The kinetic energy sacrificed to spin the turbine is the exact currency required to keep the aircraft's nose pitched at the optimal glide speed.[4]

Flight crews are trained to account for this steeper glide path when calculating their distance to a diversion airport. The drag penalty remains constant throughout the descent, forcing pilots to manage their airspeed meticulously to avoid falling short of the runway.[2]

Once deployed in flight, a Ram Air Turbine cannot be retracted by the crew. It remains locked in the slipstream until the aircraft is safely on the ground and maintenance personnel manually stow it using specialized ground equipment.[3]

Certification and the Final Flare

Regulatory authorities mandate strict performance envelopes for these emergency systems. Under 14 CFR Part 25, the FAA requires that a RAT must automatically deploy and provide full operating pressure within three to eight seconds of a total power loss.[1]

The system's greatest vulnerability occurs during the final seconds of the flight. Because the turbine relies on forward airspeed to spin, it loses effectiveness as the aircraft slows down for touchdown.[1][3]

Most Ram Air Turbines require a minimum airspeed of 130 to 140 knots to maintain their governed RPM. As the aircraft enters the landing flare and the speed bleeds off below this threshold, the turbine begins to stall, and hydraulic pressure drops.[3]

To cover this final gap, the aircraft's main lead-acid or lithium-ion batteries automatically take over the electrical load, while hydraulic accumulators provide the last few bursts of pressure needed to actuate the flight controls and deploy the brakes.

This seamless handoff from the engines, to the slipstream, and finally to the batteries ensures that the flight crew retains unbroken control of the aircraft from the moment of failure until the wheels come to a complete stop.[1][5]