Incident Overview
On May 22, 2020, at approximately 19:58 Japan Standard Time, a Gulfstream Aerospace G-IV, registration N146BG, operated by Prime Jet, LLC, experienced a serious incident at an altitude of about 13,200 m (FL446) approximately 300 km southwest of Tokyo International Airport. The aircraft was on a positioning flight from Phnom Penh International Airport to Tokyo International Airport with three people on board: the captain and two crew members. No injuries were sustained, and the aircraft sustained no damage.
Flight History
The flight departed Phnom Penh at 14:46 JST. After climbing stepwise to FL450, the aircraft cruised with smooth air. Approximately one hour before landing at Tokyo International Airport, the Engine Indication and Crew Alerting System (EICAS) displayed a “DADC MISCOMPARE” message, indicating a mismatch between the two Digital Air Data Computers (DADCs). At that time, the left-seat Primary Flight Display (PFD) showed data from No. 1 DADC, and the right-seat PFD showed data from No. 2 DADC. No failure flags were displayed.
Crew Actions
The flight crew followed the Quick Reference Handbook (QRH) procedure for DADC MISCOMPARE. They determined that No. 2 DADC was correct because the standby airspeed indicator showed a Mach number of 0.81, while the left PFD showed 0.79 and the right PFD showed 0.82. At 19:29:14, the captain selected No. 2 DADC for the left-seat display and deactivated No. 1 DADC by pulling its circuit breaker. At 19:30:00, the crew suspected a failure of No. 2 DADC when the autopilot functioned partially, so they activated No. 1 DADC and deactivated No. 2 DADC. After that, the autopilot operated normally. Both crew members selected No. 1 DADC on their display controllers and continued the flight.
At 19:57, the aircraft began its descent from FL450. Shortly after, the standby airspeed indicator showed increasing airspeed while the left and right PFD airspeeds decreased. At 19:58, the crew disconnected the autothrottle because the standby airspeed increased further when the autothrottle increased engine thrust. The crew determined that the PFD airspeed indications were unreliable and continued the flight using the standby airspeed and altitude indicators, following the QRH procedure for Suspected Erroneous/Unreliable Airspeed Indications. They re-activated No. 2 DADC and selected it on the left display controller. At 20:08, the crew requested priority handling from air traffic control and landed at Tokyo International Airport at 20:30.
Investigation Findings
The Japan Transport Safety Board (JTSB) investigated the incident. The DADC miscompare and subsequent unreliable airspeed indications were identified. The investigation did not specify a probable cause. The aircraft’s weight and center of gravity were within limits. Meteorological conditions included high water vapor content and cloud tops up to 10 km near the flight route.