Aircraft emit CO2, Nitrous Oxides, water vapor & particulate matter from their engines which forms Contrails or Chemtrails. The main source of efficiency in modern aircraft engines (turbofan) is derived from having high bypass ratios. Bypass Ratio is the amount of air bypassed around the engine's core to the amount that enters the core. Even though there have been significant improvements in fuel efficiency, these are being eclipsed by a constant surge in passenger volume. Various companies & organisations are working towards developing technologies to reduce aviation's impact on the environment. Experts believe, achieving sustainability is only possible in the long term. Many industry leading companies are working for future of zero emission flights. Rolls-Royce completed ground testing the world's fastest all-electric plane. Airbus revealed 3 concepts for the world's first zero emission aircraft. Boeing and Etihad are testing innovations & biofuel on ecoDemonstrator 787-10 before delivery.