The pilot of a Cessna 310 was likely spatially disoriented when their aircraft diverged from its instrument approach path and flew below the minimum sector altitude near Alice Springs, an ATSB investigation has concluded.
On 1 July 2024, the pilot of the twin-engine Cessna was repositioning the aircraft back to Alice Springs, after conducting an earlier passenger transport flight to Willowra, NT.
During an instrument landing system (ILS) approach to Alice Springs runway 12, while in instrument meteorological conditions (IMC), the pilot reported observing false indications on the attitude indicator and directional gyroscope.
"Testing after the incident found the artificial horizon did exhibit some deviations outside the manufacturer's tolerances, but not to an extent consistent with the sudden and absolute failure described by the pilot," ATSB Director Transport Safety Dr Stuart Godley explained.
"Rather, with no visual cues due to the IMC, the pilot likely became spatially disoriented and interpreted real instrument indications as false, as they mismatched the pilot's sensed orientation."
Flight data showed the aircraft turned perpendicular to the ILS approach path, travelling outside its tolerances, and therefore below the defined minimum sector altitude (MSA).
During this extended flight below MSA, the aircraft exhibited high angles of bank, and at one point came within 810 ft of terrain.
After the pilot notified air traffic control of their perceived instrument issues, they received clearance to track back to the initial approach fix.
The pilot then conducted an uneventful landing after returning to the start of the ILS approach path.
"Underestimated in both prevalence and severity, spatial disorientation is a very common problem for pilots flying in IMC," Dr Godley said.
"In this case, the pilot believed the aircraft was under control, and attributed the unusual attitudes indicated on the artificial horizon to an instrument error, rather than the high bank angles evident in the flight data."
The ATSB investigation also noted the pilot, in their state of urgency, did not broadcast a 'PAN PAN' call during the occurrence, and air traffic control (ATC) did not issue a safety alert.
"Pilots should not hesitate to report an urgent condition to ATC, who can provide immediate assistance," Dr Godley said.
"Pilots should also conduct a missed approach - including an immediate climb to achieve safe altitude - once their aircraft is no longer on the established approach path and doubt exists as to lateral position and location.
"In addition, ATC has a duty of care to provide safety alerts to pilots on becoming aware that an unsafe situation such as proximity to terrain has occurred, or may occur."
Since the incident, the operator has introduced a policy for the use of autopilot in IMC, and in high workload single-pilot operations.
"In this case, the pilot's choice not to use autopilot for this approach may have increased their workload, and the subsequent risk of spatial disorientation during the instrument approach procedure," Dr Godley concluded.
You can find here the final report: Flight below minimum sector altitude involving Cessna 310R, VH-ZMB, 14 km west-north-west of Alice Springs Airport, Northern Territory, on 1 July 2024
