In a large diameter (0.1 m), long transport length (153 m), pneumatic transport system the unstable region of flow on start-up of the system was explored with two different types of plastic pellets of different shapes. The solids flow rates are those found in industrial applications. One finds that there is a range of flow conditions, solid and gas flow rates that lead to unstable operations. A perturbation analysis was employed using the experimental data that were obtained, showing the region of stability as a function of the solid mass loading.