<p> 100 W hydrogen-air open cathode proton exchange membrane fuel cell stack (FCS) consisted of 6 membrane-electrode assemblies (MEA) life cycle was studied in real operating conditions: dry hydrogen and ambient air. These conditions are usual for FCS operating as power source in unmanned aerial vehicles. Degradation of catalyst and ionomer in both catalytic layers, degradation of perfluorinated ion-exchange membrane in MEA after resource tests was studied by the use of ex situ and in situ methods: scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, voltammetry and electrochemical impedance techniques. The experimental data show that the main reason for the 27% decrease in the FCS power over 455 hours of operation is associated with a decrease in the ionomer content. Surprisingly, the degradation of anodic catalytic layer was much greater than cathodic. Nevertheless, the results of simulation of a single MEA work via 2D model built in the COMSOL Multiphysics^® approved that, as it was expected, the main reason for the voltage drop during the operating tests is the degradation of cathodic catalytic layer and the proton exchange membrane. The important non-evident explanation of experimental results was obtained via model simulation. Despite the greater degradation of anodic layer, the only thin anodic layer adjacent to the membrane is working in MEA with sufficient voltametric characteristics, enough for MEA functioning. This result of comparison of experiment and model is important for the optimization of MEA content in air-cooled open cathode proton exchange membrane fuel cell stack operating in stressed conditions.</p>