An avionics system for an aircraft (100A, 100B) comprising: at least one inertial measurement unit (110, 120, 130) configured to produce a calculated pitch solution and a calculated roll solution; at least one monitor (105, 115, 125, 135, 200) coupled to the at least one inertial measurement unit (110, 120, 130) and configured to produce an estimated pitch solution and an estimated roll solution; a comparator, wherein the comparator determines the difference between the calculated pitch solution and the estimated pitch solution, and the difference between the calculated roll solution and the estimated roll solution; and at least one display device (140, 150) communicatively coupled to the comparator; wherein the at least one display device (140, 150) is configured to receive a warning message (214, 226) from the comparator when the difference between the calculated pitch solution and the estimated pitch solution is greater than a pitch threshold, or when the difference between the calculated roll solution and the estimated roll solution is greater than a roll threshold, or both; characterized in that: the at least one monitor (105, 115, 125, 135, 200) is configured to receive angle of attack data and GNSS flight path angle data and to produce the estimated pitch solution by adding the angle of attack data to the GNSS flight path angle data, and the at least one monitor (105, 115, 125, 135, 200) is configured to produce the estimated roll solution by calculating a double-differencing solution using data from dual global navigation satellite system antennas transversely mounted along the fuselage of the aircraft a known distance apart. at least one inertial measurement unit (110, 120, 130) configured to produce a calculated pitch solution and a calculated roll solution; at least one monitor (105, 115, 125, 135, 200) coupled to the at least one inertial measurement unit (110, 120, 130) and configured to produce an estimated pitch solution and an estimated roll solution; a comparator, wherein the comparator determines the difference between the calculated pitch solution and the estimated pitch solution, and the difference between the calculated roll solution and the estimated roll solution; and at least one display device (140, 150) communicatively coupled to the comparator; wherein the at least one display device (140, 150) is configured to receive a warning message (214, 226) from the comparator when the difference between the calculated pitch solution and the estimated pitch solution is greater than a pitch threshold, or when the difference between the calculated roll solution and the estimated roll solution is greater than a roll threshold, or both; the at least one monitor (105, 115, 125, 135, 200) is configured to receive angle of attack data and GNSS flight path angle data and to produce the estimated pitch solution by adding the angle of attack data to the GNSS flight path angle data, and the at least one monitor (105, 115, 125, 135, 200) is configured to produce the estimated roll solution by calculating a double-differencing solution using data from dual global navigation satellite system antennas transversely mounted along the fuselage of the aircraft a known distance apart.