4.4 Purifying Innovation in Aircraft
Marcos Antonio de Lima Filho, PhD.
This section covers the design features that were replaced (made obsolete), discontinued (made redundant), or rejected as airliners evolved.
Obsolete Innovations
The turbojet was an important invention that inaugurated a new technologic foundation: It was the first gas turbine power plant used in aviation. Within a decade, the turbojet displaced the piston engine, which dominated until the early 1950s (see Figure 4.4.1). The section titled “The Jet Age Disruption?” explores how the disruption of the Jet Age led to the demise of piston engines in commercial aviation.
The heyday of the biplane lasted from 1903 to 1940. Although the monoplane surpassed the biplane in acceptance, it is still a viable option for many missions, such as in aerobatic or agricultural aircraft (Gudmundsson, 2013, p. 1049).
Flying Boats played a major role in the early history of commercial aviation (Figure 4.4.2). These seaplanes were substituted by land-based airliners after the Second World War, as the conflict encouraged investments in airports and runways on land. Other design features have also passed their prime. Tail or aft-mounting was once popular for all types of transport aircraft. This arrangement produces an aerodynamically cleaner wing, but the advantage is offset by the lack of wing bending-moment alleviation, and by problems arising from the engine intake being in the wake of the wing (Bernard & Philpott, 2010, p. 184). T-tail designs have become popular on many light and large aircraft, especially those with aft-mounted engines, because the T-tail configuration removes the tail from the exhaust blast of the engines (Federal Aviation Administration, 2016). However, this configuration requires additional design considerations. The vertical stabiliser has to be strengthened, resulting in a weight penalty over conventional tail designs.
Figure 4.4.3 presents more technologies or design features that were replaced or made obsolete in subsequent generations of passenger aircraft. The 1970s saw a revolutionary change on the flight deck with the reduction in crew members from three to two, eliminating the flight engineer and subsuming his role to avionic systems (Moir et al., 2013, p. 449). This was made possible by automating tasks to reduce crew workload. In the mid-1980s, colour cathode ray tube (CRT) technology, and in the mid-1990s active matrix liquid crystal display (AMLCD) technology, matured sufficiently to be suitable for use in airborne environments (Moir et al., 2013, p. 164). This led to the obsolescence of analogical (electromechanical) instruments. The aviation industry benefited from this purification. The introduction of microelectronics and the related software applications have dramatically improved the ability of avionics systems to introduce automation, improve flight crew situational awareness, and create operational efficiencies — all with ever-increasing margins of safety (Spitzer et al., 2015, p. 36-1).
The process of purifying selection operates at multiple levels of the design hierarchy. As demonstrated by the data, it can impact specific parts as well as major architectures or dominant designs.
The discontinuation of tri-jets is an example of purification (Figure 4.4.4). Three-turbofan aircraft prevailed from the 1960s until the late 1970s. However, as bigger and more powerful engines became available, this three-turbofan configuration was rendered obsolete by dual-turbofan (or twin-jet) aircraft. The new architecture enabled the A300 to offer airlines a 20% saving in direct operating costs per trip relative to the competing tri-jets (Airbus, n.d.). Besides fuel efficiency, having one fewer engine considerably reduces the capital cost involved in buying an aircraft.
The classic 747 and the modern A380 are among the most iconic aircraft, both featuring four-turbofan engines. Designed to accommodate heavier payloads, more passengers, and longer routes, these aircraft became particularly popular for transoceanic flights due to their increased safety. In a four-turbofan architecture, an engine failure poses a significantly lower risk compared to two- or three-engine configurations.
However, as turbofan technology attained new levels of reliability, the market shifted in favour of the dual-turbofan configuration. In addition, airline operators also shifted from the traditional hub-to-hub transoceanic model to the more convenient hub-to-secondary cities. For these missions, a smaller, long-haul twin-engined aircraft was more economical. As a result, the demand for 747s and A380s has declined in the last decade. Four-turbofan deliveries reached less than 1% in 2018, with the 2020 pandemic worsening this situation even further. Due to a lack of orders, Airbus had to discontinue the A380 production in 2021.
Turboprops are still in use and play an important part in regional routes. Modern turboprop airliners, such as the ATR, are equipped with the most advanced avionics, offering outstanding performance and reliability. However, turboprop aircraft have had a modest market share, accounting for only 3% of aircraft deliveries. In the past, its diffusion reached up to 42% of the market.
Discontinued Innovations
There is a subset of purified innovations in which features are not replaced by a direct alternative counterpart. Instead, they simply “disappear”. A designated smoking room, for instance, used to be a common feature of cabin interiors. This feature was discontinued when airlines and aviation authorities banned in-flight smoking in the 1980s.
To support operations in less well-equipped airports, commercial airliners used to feature a retractable stairway that lowers from the rear underbelly of the fuselage (as shown in Figure 4.4.5). The so-called airstairs were useful for boarding passengers and ground services. As airport infrastructure has developed, this feature has become less appealing. Its additional weight and complexity became a significant penalty.
Rejected Innovations
The Concorde was the first airliner to feature a fly-by-wire control system, besides flying at supersonic speeds. Despite the cutting-edge technology, supersonic aircraft accounted for only 1,4% of total airframe deliveries in 1975, their best year (Figure 4.4.6). As shown in this data, it is uncontroversial that supersonic airliners failed to gain mass adoption.
The Soviet Tupolev Tu-144 suffered from reliability and developmental issues, which restricted its service for regular use. Passenger service began in December 1975, but after a second Tu-144 crash in May 1978, it was withdrawn from passenger service. The Concorde, on the other hand, was regarded as the world's safest passenger airliner, with zero passenger deaths per kilometre flown. This changed on 25th July 2000, after the Concorde’s first fatal accident.
The Concorde demonstrated that supersonic flights could be done safely. It cut transatlantic journey times by half: instead of 7 or 8 hours, it took less than 3.5 hours to get from Paris to New York. However, doing so was not cheap: due to its higher fuel consumption, the Concorde was destined for the rich. While carrying a full load, Concorde averaged 15.8 passenger miles per gallon of fuel, compared to 33.3 for the older Boeing 707; 44.4 for the DC-8-61; 46.3 for the Boeing 747, and 53.6 for the DC-10 (Ross, 1978). Thus, despite being an engineering marvel, the Concorde could not achieve the economics required for broader adoption.
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