Fuel cell technology will play a decisive role in the process of achieving the ambitious ecological goals of the aviation industry. However, apart from its obvious environmental advantages, the integration of fuel cell technology into commercial aircraft represents a challenging task in terms of operational and economical aspects.Since fuel cell systems are currently exposed to an intense competition with well-established power sources onboard an aircraft, engineers are in pursuit of highly efficient and particularly lightweight fuel cell systems. Supported by model-based design in conjunction with elaborate optimization techniques this pursuit has led to highly specialized systems. These systems tend to use their components to full capacity, which typically implies marginal system robustness. In consequence, preliminary design studies propose fuel cell systems that are sensitive to partial faults, or even to the slightest deviation, or degradation of their components' behavior.Non-functional requirements such as reliability, availability, dispatchability and durability will become key factors for the acceptance of fuel cells as alternative power sources onboard aircraft. These requirements demand the taking of coordinated measures in both Systems Architecting and Sizing, since the potential of measures such as adding redundancy to the system's architecture or making use of modular approaches is directly bound to the reliability of subsystems and their components. A component's reliability, however, strongly depends on its operating point and thus on the operating point of connected components. Hence, an operation envelope-oriented sizing of subsystems and components becomes an important factor.This paper reveals and illustrates the crucial issue of highly sensitive, high-performance fuel cell systems and proposes and discusses measures regarding Systems Architecting and Sizing in order to enhance the system's operational robustness while still keeping track of key performance indicators such as weight and efficiency.