The 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.

The Airline Industry

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.

Business and General Aviation

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.

Combat Aircraft

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 systems, 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.

WHERE THE TWIN-ENGINE PLATFORM REALLY SCORES OVER ITS SINGLEENGINE RIVAL IS IN THE AREA OF AIR SAFETY AND SURVIVABILITY

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 systems, 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.

Rotary-wing Platforms

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.