In our study, two factors are considered to be uncertain: overall pressure ratio of the engine, OPR, and the cruise Mach number, *M*. OPR represents engine performance, allowing for a trade-off between engine technology and mission parameters. For the Boeing 737-800, OPR and *M* are modeled as uniform random variables, with $OPR\u223cU[24.2,28.2]$ and $M\u223cU[0.77,0.79]$, see Refs. [43,44] for details on how those distributions were derived. For the D8.6, OPR and *M* are modeled as triangular random variables, with $OPR\u223cT[45,52,50]$ and $M\u223cT[0.73,0.75,0.74]$. These distributions are derived using a combination of historical data and expert opinion, as described in Refs. [44,45]. As the quantity of interest, we consider PFEI, the fuel energy consumption per payload-range.