Air Safety

Sin­gle vs Twin


TTHE VERY FIRST HEAV­IER-THAN-AIR fly­ing ma­chine in the world that took to the air was on De­cem­ber 17, 1903, at Kitty Hawk, North Carolina in the United States. At the con­trols were the two brothers Orville and Wil­bur Wright who had de­signed, de­vel­oped and man­u­fac­tured the plat­form all by them­selves. The air­craft was aptly named as the Wright Flyer. The maiden flight of the ex­per­i­men­tal ma­chine lasted for a mere 12 sec­onds and cov­ered a dis­tance of just 120 feet. But de­spite it be­ing of short du­ra­tion, the flight by this sin­gle en­gine aero­plane stands out as a ma­jor land­mark in the his­tory of avi­a­tion. But what must be noted is the fact that this first heav­ier-than-air ma­chine that got air­borne, was pow­ered by a sin­gle-pis­ton en­gine. Over the last 113 years since the his­tor­i­cal feat by the Wright Brothers, avi­a­tion tech­nol­ogy has come a long way es­pe­cially with re­gard to aero en­gines that power fly­ing ma­chines both in the regime of fixed- and ro­tary-wing air­craft em­ployed in the civil or mil­i­tary do­main. Apart from the in­no­va­tions in tech­nol­ogy per­tain­ing to power plants on ac­count of which mod­ern aero en­gines to­day are now ca­pa­ble of de­liv­er­ing higher lev­els of power or thrust for a given weight are far more fuel-ef­fi­cient and cer­tainly have much higher lev­els of re­li­a­bil­ity, the is­sue re­lated to the num­ber of en­gines that an air­craft has or must have, is of­ten a sub­ject of dis­cus­sion. This is­sue per­vades all the seg­ments of civil and mil­i­tary avi­a­tion and en­com­passes both fixed- and ro­tary-wing plat­forms.

Dur­ing the ini­tial years in the his­tory of de­vel­op­ment of air­craft, only sin­gle-en­gine plat­forms were pro­duced and that too largely for mil­i­tary use. Th­ese plat­forms were clas­si­fied as “fighter air­craft” and were em­ployed pri­mar­ily for short-range aerial com­bat. How­ever, in the pe­riod be­fore World War II, there evolved a con­cept of “heavy fighter de­sign”. One of the more no­table plat­forms based on this con­cept was the Messer­schmitt Bf 110, a twin-en­gine fighter air­craft de­signed, de­vel­oped and pro­duced by the Ger­man aero­space in­dus­try for the Luft­waffe which ac­corded greater im­por­tance to twin-en­gine con­fig­u­ra­tion as com­pared with their sin­gle-en­gine fighter air­craft .

The Luft­waffe em­ployed sin­gle-en­gine fight­ers pri­mar­ily for air de­fence over their own ter­ri­tory and em­ployed the twinengine heavy fight­ers for of­fen­sive mis­sions such as es­cort­ing bomber air­craft that were tasked for long-range bomb­ing mis­sions. The heavy fight­ers with their higher speed could out­run the en­emy lightweight sin­gle-en­gine fight­ers, but could eas­ily be

out­ma­noeu­vred by the lat­ter in close com­bat. The first twin-jet to take to the air was the Heinkel He-280, pro­to­type of a Ger­man fighter air­craft which flew in April 1941.

As de­vel­op­ment of aerial plat­forms across the world con­tin­ued both in the regimes of mil­i­tary and civil avi­a­tion, the num­ber of en­gines pow­er­ing air­craft in­creased to three, then four and even to six as on the Rus­sian An-225, the largest trans­port air­craft in the world ever built and even more as on the B-52 Strato­fortress, a heavy bomber of the US Air Force which is pow­ered by eight Pratt & Whit­ney tur­bo­fan en­gines.


As for the global air­line in­dus­try, the is­sue of choice be­tween a sin­gle-en­gine plat­form as against one with two en­gines is not rel­e­vant as there are no air­lin­ers op­er­at­ing any­where in the world that are pow­ered by a sin­gle-en­gine and nei­ther is it likely, at least not in the fore­see­able fu­ture. For the air­line in­dus­try, fu­el­ef­fi­ciency is of the high­est pri­or­ity and pos­si­bly an over­rid­ing con­sid­er­a­tion as this fac­tor con­sti­tutes a large pro­por­tion of the op­er­at­ing costs. As air­lin­ers pow­ered by two en­gines pro­vide bet­ter fuel econ­omy in op­er­a­tions, since the year 2000, there is trend amongst air­lines to re­place three-en­gine and four-en­gine air­craft with those with two en­gines. The three-en­gine jet­lin­ers were phased out first as the prob­lem of high op­er­at­ing cost was com­pounded by the in­her­ent de­sign com­plex­ity and main­te­nance is­sues as the third en­gine is mounted on the sta­biliser. The large four-en­gine jets con­tinue in ser­vice es­pe­cially for long-haul trans-oceanic routes, but in re­duc­ing numbers as newer mod­els of twin-jet air­lin­ers with longer range and higher pas­sen­ger or pay­load ca­pac­ity en­ter the mar­ket.


In the seg­ment of busi­ness and gen­eral avi­a­tion, choice of singleengine or twin-en­gine air­craft de­pends largely on which plat­form would best meet the needs of the com­pany or the in­di­vid­ual cus­tomer. Both cat­e­gories of busi­ness air­craft, i.e. sin­gle-en­gine or twin-en­gine, meet the re­quire­ments of mis­sions un­der 500 km equally well. As both types can op­er­ate from short run­ways, they can ac­cess most if not all the air­ports that the owner may need to visit. How­ever, the twin-en­gine plat­forms have bet­ter take-off and climb per­for­mance, higher cruis­ing speed, larger pay­load or pas­sen­ger ca­pac­ity and most im­por­tant of all, th­ese pro­vide for higher lev­els of air safety as in the event of loss of power on one en­gine, the air­craft can con­tinue to fly and land safely at the near­est air­field. This would not be pos­si­ble with a sin­gle-en­gine plat­form. De­spite the higher main­te­nance costs of the twin-en­gine air­craft, on ac­count of con­sid­er­a­tions of air safety, the pros­per­ous busi­ness houses would gen­er­ally pre­fer the twin-en­gine plat­form.


The de­bate on sin­gle-en­gine ver­sus twin-en­gine has been rel­a­tively more in­tense in the case of com­bat air­craft of mod­ern gen­er­a­tion. Com­pared to a twin-en­gine com­bat plat­forms, a singleengine fighter is of lower weight and hence has bet­ter thrust-to-weight ra­tio which pro­vides it su­pe­rior ma­noeu­vre ca­pa­bil­ity. A twin-en­gine fighter air­craft, on the other hand, though with lower ma­noeu­vra­bil­ity, has a sig­nif­i­cantly higher weapon load ca­pac­ity with the ca­pa­bil­ity of car­ry­ing a wider va­ri­ety of weapons and can un­der­take mis­sions over much longer ranges giv­ing it a much larger com­bat ra­dius. With the ad­vent of Be­yond Vis­ual Range weapon sys- tems, en­emy air­craft can be tar­geted and de­stroyed at much longer ranges. As such, with th­ese tech­no­log­i­cal ad­vances, the era of close com­bat ap­pears to be re­ced­ing. Also, the com­bat ra­dius of a sin­gle-en­gine fighter air­craft can be en­hanced through in­flight re­fu­elling if op­er­a­tionally fea­si­ble. From the point of view of main­te­nance and op­er­at­ing costs, the twin-en­gine plat­form is de­cid­edly more ex­pen­sive pri­mar­ily on ac­count of the se­cond en­gine, its larger size, du­pli­ca­tion of en­gine-driven sys­tems and greater num­ber of weapon sta­tions.

But where the twin en­gine plat­form re­ally scores over its sin­gle-en­gine ri­val is in the area of air safety and survivability. In a sin­gle-en­gine fighter, in the un­for­tu­nate event of en­gine fail­ure, there is prac­ti­cally no chance for the pi­lot to make it to an air­field for a land­ing and as such he is left with no op­tion but to eject from the crip­pled air­craft. Fail­ure of the power plant on a sin­gle-en­gine air­craft thus equates to to­tal loss of an ex­pen­sive ma­chine. The twin-en­gine com­bat air­craft, on the other hand, has a high de­gree of re­dun­dancy and hence in the event of fail­ure of one en­gine, the pi­lot has a good chance to fly and land at the near­est suit­able air­field ob­vi­at­ing loss of the air­craft. The fo­cus of the in­dus­try has there­fore pro­gres­sively shifted to twin-en­gine plat­forms.

One fac­tor to counter the pref­er­ence for twin-en­gine plat­forms that has been ad­dressed to some ex­tent by the in­dus­try is by en­hanc­ing the re­li­a­bil­ity of the power plant and sig­nif­i­cantly re­duce the pos­si­bil­ity of en­gine fail­ure in flight. Suc­cess in this ef­fort is re­flected in the Mi­rage 2000 fighter air­craft. In the over three decades of op­er­a­tion, the Mi­rage 2000 fleet of the Indian Air Force (IAF) has not been af­flicted by this malaise as has been the case with its other sin­gle-en­gine fighter fleets. The con­fi­dence of the in­dus­try in the re­li­a­bil­ity of the newly de­signed power plants is ev­i­dent in the fact that the US aero­space ma­jor Lock­heed Martin has opted to power its lat­est com­bat plat­form, the F-35 with just one en­gine. Nev­er­the­less, it will not be easy for sin­gle-en­gine plat­forms to in­spire the same level of con­fi­dence with re­spect to air safety that a twin-en­gine plat­form can pro­vide.


The ro­tary-wing do­main too is fre­quently em­broiled in a de­bate over pref­er­ence for sin­gle-en­gine or twin-en­gine ma­chines, an is­sue that once again is cen­tred on air safety. In the event of fail­ure of the en­gine on a sin­gle-en­gine fixed-wing air­craft, the pos­si­bil­ity of find­ing an air­field within glid­ing dis­tance is some­what re­mote and as such the chances of a dis­as­trous end to the mis­sion is ex­tremely high. How­ever, in the case of en­gine fail­ure on a sin­gle-en­gine he­li­copter, the pi­lot can switch to the au­toro­ta­tion mode and can still make a suc­cess­ful emer­gency land­ing as the he­li­copter re­quires only a small open space to do so. A few years ago, a pi­lot form the Ro­tary Wing Academy of Hin­dus­tan Aero­nau­tics Lim­ited, Ben­galuru, landed his sin­gle-en­gine he­li­copter on the roof of a high rise build­ing. This how­ever would not have been fea­si­ble in case such a sit­u­a­tion had arisen at night.

As in the case of fixed-wing plat­forms, the twin-en­gine he­li­copter pro­vides bet­ter per­for­mance in terms of speed,range and pay­load ca­pa­bil­ity. It goes without say­ing that com­pared to a sin­gle-en­gine he­li­copter; a twin-en­gine plat­form pro­vides higher level of air safety and survivability in the event of fail­ure of one en­gine es­pe­cially at night. How­ever, in the event of a tail ro­tor fail­ure, the twin-en­gine fea­ture of­fers no ad­van­tage when com­pared to a sim­i­lar fail­ure on a sin­gle en­gine he­li­copter as the pi­lot will be com­pelled to shut down both the en­gines to cope with the emer­gency.



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