What happened
On 29 March 2022, a Bombardier Challenger CL-600-2B16 (601-3A), registration 2-PAPA, was conducting a private flight from Guernsey to Oxford Airport. While performing a non-precision NDB/DME approach in instrument meteorological conditions (IMC), the aircraft experienced a significant increase in its rate of descent. As the aircraft passed the Final Approach Fix, the descent rate reached between 2,200 and 2,500 feet per minute, triggering a Ground Proximity Warning System (GPWS) 'terrain' alert.
During the descent, the commander adjusted the flight guidance panel and requested a 'vertical direct to the runway' via the Flight Management System (FMS). Following the GPWS warning, the aircraft emerged from the clouds. Although the co-pilot requested a go-around, the commander elected to continue the approach, noting that the runway was visible. The aircraft eventually landed on Runway 01 without damage to the airframe or injuries to the 5 persons on board.
The investigation
Investigators examined the flight path and the aircraft's FMS configuration. While no data was recovered from the Flight Data Recorder, broadcast data confirmed the aircraft deviated significantly from the published approach profile. The investigation looked into the FMS's behavior, specifically regarding a known manufacturer anomaly that can cause unexpected steep descents during profile recalculations.
The investigation also reviewed the crew's performance against the operator's manual, which mandates a maximum gradient climb following a GPWS warning in IMC and a mandatory go-around for unstable approaches where descent rates exceed 1,000 feet per minute. Discrepancies between the pilots' recollections regarding verbal interventions and the effectiveness of crew communication were also analyzed.