What happened
On May 22, 2020, a Gulfstream Aerospace G-IV, registration N146BG, was conducting a positioning flight from Phnom Penh to Tokyo when the flight crew encountered significant instrumentation issues. During the cruise phase, the crew received a "DADC MISCOMPARE" alert, indicating a discrepancy between the two Digital Air Data Computers. Following the Quick Reference Handbook (QRH) procedures, the crew initially identified the No. 1 DADC as the faulty unit and deactivated it by pulling its circuit breaker.
Shortly thereafter, the crew observed that certain autopilot functions were not operating correctly. They reassessed the situation, concluded that the No. 2 DADC was actually the malfunctioning unit, and subsequently deactivated No. 2 while reactivating No. 1. However, as the aircraft began its descent from FL450 toward Tokyo International Airport, the airspeed indications on both primary flight displays (PFDs) became unreliable. While the standby airspeed indicator showed an increasing speed, the PFDs showed a decreasing trend. The crew was forced to disconnect the autothrottle and manage the aircraft manually using the standby instruments. Despite the technical difficulties, the crew successfully landed the aircraft at Tokyo International Airport with no injuries and no damage to the aircraft.
The investigation
The JTSB investigation focused on the sequence of DADC deactivations and the physical state of the pitot-static system. Investigators examined the flight data recorder (DFDR) and performed high-pressure air tests on the pitot-static system. During these tests, investigators discovered water droplets in the No. 1 pitot system drain and foreign objects, including leaf veins, in a static port.
The investigation also scrutinized the crew's decision-making process regarding the DADC units. It was noted that the crew's attempt to re-engage the autopilot did not allow for the manufacturer-recommended one-minute reset period after pulling the circuit breaker, which likely contributed to the partial autopilot failure. Furthermore, the investigation highlighted ambiguities in the QRH, noting that the manual's structure made it difficult for the crew to promptly identify the faulty component or follow a streamlined procedure for resolving the mismatch.
Findings
- The primary cause of the incident was multiple malfunctions in the aircraft's systems that rendered both airspeed indications unreliable during descent.
- It is highly probable that the No. 1 pitot line became blocked due to the presence of ice crystals in the airspace, leading to the erroneous data.
- The crew's decision to deactivate the No. 2 DADC left the aircraft relying solely on the No. 1 DADC, which subsequently failed to provide accurate airspeed data.
- The QRH lacked clear, organized instructions for identifying the specific failed DADC and did not adequately direct the crew to use the "Unreliable Airspeed Indications" procedure when the source of the error could not be determined.