An increase in the aircraft flights intensity leads to the need to improve the efficiency of the airfield radio equipment functioning. One of the ways to improve the accuracy of navigation equipment is to reduce the instrumental error. The article analyzes the methods of checking the instrumental error of the ADF. Is shown that the so-called “electric rotation” of the antenna system (AS) head used in the radio direction finders ADF-80K, ADF-80, ADF-85, “Platan”, DF-2000 for checking the instrumental error leads to incorrect results. The authors employed the linearity property of the Fourier transform in the simulation to prove the incorrectness of the method for determining the ADF instrumental error with the “electric rotation” of the AS head. The simulation results showed that in the ADF operating in the quasi-Doppler mode, the failure of the vibrators located along the bearing line to the radio source does not lead to the appearance of bearing error, while the failure of the vibrators located orthogonally to the bearing line can result in a bearing error reaching 3.750. The simulation results confirmed that unlike the ADF that use the AS head “electric rotation”, the ADF with mechanical rotation show reliable results of the instrumental error measurement. The paper proposes a new method for checking the instrumental bearing errors using the values of the AS phase non-identities which ensures the reliability of the measurement results.