THE RAM AIR TUR­BINE

Flying - - HOW IT WORKS - By Rob Mark

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.

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