Twin-jet and trijet aircraft: a study for an optimal design of regional aircraft

Aircraft with three engines, as known as trijet became a standard design among manufacturers after 1964 when the FAA's 60-minute rule was established for these aircraft. This regulation restricted the flight path to 60 minutes' flying time to a suitable airport, therefore affecting the operation costs and limiting the range of twin-jet aircraft. However, improvements to the engine's reliability in the following decades allowed ETOPS certification for twin jet aircraft. The traditional trijet designs were slowly retired, the last commercial trijet flight was in 2014.The industry abandoned the trijet design as solution for commercial aviation; however, executive jets such as the Falcon 7x and Falcon 8x, certified in 2016, show that this configuration might still be advantageous for specific markets. The certification at one engine inoperative condition presents advantage for trijet aircraft reducing take-off thrust, since when one engine is inoperative, 75% of the installed thrust is available for the trijet aircraft, while this value is only 50% for a twin-jet aircraft. An initial study conducted showed a trend of lower thrust to weight ratio for trijet aircraft when compared to twin-jet aircraft, being particularly evident for MTOW (maximum take-off weight) lower than 75,000 lb. The aim of this work is to study the viability of a trijet aircraft configuration and potential advantages for a regional aviation scenario. The trijet configuration performance was evaluated and compared to twin-jet configuration in a multidisciplinary design environment considering disciplines such as aerodynamics, noise, performance, flight mechanic, weight, and structure. An aircraft with MTOW of 48,500 lb was studied and the results show that a reduction up to 9.4% of installed thrust might be achieved with the trijet design compared to twin jet aircraft when the balanced field length is the critical constraint. However, flyover noise and structural weight might increase slightly in trijet design. (C) 2018 The Authors. Published by Elsevier B.V.