THE RAM AIR TURBINE
The ram air turbine is often referred to as the RAT, a moniker offered up with much affection by the pilots who understand its purpose. Deployment of the RAT in actual flight operations signifies a cockpit crew has nearly run out of power-source options, usually because all engine-driven generators as well as the APU have become inoperative. With no electric power except for ship’s batteries, there is little energy left to operate the hydraulic or electrical systems necessary to move the flight controls and power critical systems.
The RAT is a small turbine connected to a support bracket that allows it to hang beneath the aircraft once it is deployed. RATs come in a variety of power options, including hydraulic, electric and hybrid, and include a variable-pitch propeller that normally faces into the slipstream to spin an internal electric generator or hydraulic pump. The propeller’s pitch varies in order to maintain a constant output from whichever device is connected.
A RAT remains invisible inside the belly of the aircraft until it’s needed. Deployment is normally automatic, triggered when other power sources drop below a specified threshold that varies by aircraft. Aircraft that use a RAT normally include a cockpit switch that allows the
flight crew to manually open the tiny doors beneath the aircraft and deploy the turbine when they believe it is needed. Usually there is no upper speed limit at which the RAT may be deployed, but there is a low-speed operational limit, below which the RAT ceases producing a constant electric current or hydraulic pressure.
Because the RAT is an emergency generator, its power output is normally quite minimal, often just enough on a hydraulically operated aircraft to move only the ailerons, elevator and rudder. Spoilers and speedbrakes might remain inoperative. Inside the cockpit, unnecessary avionics and lighting also cease. On a fly-by-wire airplane, the RAT provides just enough energy to operate a single computer needed to operate the flight controls.
Not all transport airliners or business jets employ an emergency RAT. Many Airbus and early Boeing aircraft do, including the 757, 767 and 777, while some recent Boeings do not. The latest 737-800, for instance, does not use a RAT. In the rare situation where all electrical power ceases, the -800 uses manual reversion to allow the flight crew to move the flight controls. By contrast, Airbus aircraft use a RAT as an emergency electrical source as well as a backup hydraulic pump to operate the flight controls.
Gulfstream uses RATs on its fly-by-wire business jets, the G500, G600 and G650. The RAT provides an immediate and persistent source of power to the FBW system in the unlikely event of a dual engine generator and APU generator failure. The FBW systems are, of course, backed up by time-limited batteries, but the RAT provides unlimited backup power while its turbine is spinning.
In one memorable example of a RAT saving the day, an Air Canada Boeing 767 at 41,000 feet ran out of fuel on a trip between Montreal and Edmonton due to a fuel loading miscalculation. When both engines flamed out, the generators dropped offline. With no fuel aboard, operation of the APU was also impossible. As the aircraft descended through 35,000 feet, the cockpit’s electronic flight instrument system also went dead.
The RAT deployed automatically, offering the pilots some control of the aircraft, at least until, while on approach, the Boeing slowed below the RAT’s minimum required speed for operation, making the aircraft extremely difficult to control in the final moments before touchdown. The Boeing landed safely at an abandoned air force base near Winnipeg after gliding for 17 minutes. US Airways 1549 also deployed the Airbus RAT when both engines essentially shut down after the jetliner collided with a flock of Canada geese shortly after takeoff from La Guardia Airport. Somewhat under control, the flight crew managed to ditch the aircraft in New York’s Hudson River with no loss of life.