Method and device to determine fault of a pressure measuring system aboard a flying device, the system determining static pressure pAC(t) and/or barometric altitude zAC(t). The method includes: determining position POSGNSS(t) and altitude zGNSS(t) of the flying device at time t using satellite navigation system GNSS; determining a geopotential altitude zAN/PROG(t) related to pAC(t) for position POSGNSS(t) in data ANDAT or in data PROGDAT of prediction model (NWP); and/or determining static pressure pAN/PROG(t) related to zGNSS(t) for POSGNSS(t) in ANDAT or PROGDAT; determining altitude deviation Δz(t)=zGNSS(t)−zAN/PROG(t) and/or pressure deviation Δp(t)=pAC(t)−pAN/PROG(t); determining altitude deviation Δz* averaged over time Δt from Δz(t)=zGNSS(t)−zAN/PROG(t) and/or pressure deviation Δp* averaged over time Δt from Δp(t)=pAC(t)−pAN/PROG(t); and generating a warning signal if a fault of the system is detected, the fault detected if |Δz*|>G1, or |Δp*|>G2.