In an effort to relieve some of the environmental and economic pressures placed upon operation of commercial air transport equipment, several future aircraft designs have been discussed which consider in-flight use of the auxiliary power unit (APU). Justification for such a system is logical considering the depth of analysis conducted to date. But it must be remembered that it is the ultimate consumer, the traveling public, who must be satisfied. There can be no compromise on safety and reliability. Future studies of in-flight use of the APU must take this into account.