The optimal cruise altitude of LNG-fuelled turbofan aircraft

The effect of fuel type (liquefied natural gas vs. kerosene) on the optimum cruise altitude for a single-aisle, medium-range transport aircraft is investigated. An automated aircraft synthesis program including a detailed mission analysis module was used to assess the impact of liquefied natural gas (LNG) on fuel burn and equivalent CO₂ emissions. Verification studies for four different reference aircraft showed that the mission analysis module was able to predict the fuel weight for various missions with an error between –1.5% and +5.9%. A baseline, kerosene-fuelled, mid-range, single-aisle aircraft was designed which was estimated to have an optimum cruise altitude for minimum fuel burn of 11,400m (37,500ft). It also showed the altitude for minimum equivalent CO₂ emissions to be at 12,500m (41,000ft). Changing to LNG, the aircraft showed a maximum reduction in CO₂-equivalent emissions of 21% at a cruise altitude of 10,400m (34,100ft). At altitudes above 9,000 meters, the higher H₂O emissions rapidly increased the equivalent CO₂ emissions. It was therefore concluded that, to a much larger degree than for kerosene-fuelled aircraft, cruise altitude is an important design parameter for LNG-fuelled aircraft if equivalent CO₂ emissions are to be minimized.