Avionics control systems are typical real-time systems -- automatic control algorithms are realized as periodic computations where strict requirements on the timing can be derived by a control analysis. The requirements are effectively end-to-end timing requirements on the computation -- from sensory input to actuator output -- and have to be transformed into a detailed control software design. This analysis, together with a resource and performance analysis, is required early in the development process -- neither hardware nor prototypes of the software can be assumed to exist -- in order to estimate the cost of development and the cost of hardware to meet customer requirements. The paper investigates the applicability of a method (the period calibration method) to transform the end-to-end requirements of an avionics control software, into a design consisting of a set of periodic tasks with buffered data flows. The experience is discussed and shortcomings of the method are analysed. 1....