History of the Flight
The Bombardier Challenger CL-600-2B16, registration 2-PAPA, departed Guernsey Airport at 1515 hrs on 29 March 2022 for a private flight to Oxford Airport. The crew anticipated flying the NDB/DME approach to Runway 01. En route, they received a shortened routing direct to the Compton VOR then the Oxford NDB. Upon contacting Oxford Approach, they were given a radar vector toward the Final Approach Fix (FAF).
Pilot Actions
As the aircraft approached the FAF in IMC, the commander changed the selected altitude to zero, engaged VNAV, and directed the co-pilot to lower the gear and select flap 30. Passing the FAF, the commander requested a 'vertical direct to the runway' in the FMS. The aircraft began a rapid descent; the co-pilot observed a rate of 2,200–2,500 fpm and called "vertical speed," but the commander did not recall hearing this. Both pilots then heard a GPWS warning—the commander recalled 'terrain terrain,' while the co-pilot recalled 'terrain terrain, pull up pull up.'
Responding to the warning, the commander disconnected the autopilot and reduced the descent rate. The aircraft broke out of cloud at approximately 900–1,000 ft amsl according to the commander, or 700 ft amsl according to the co-pilot. The commander saw the runway with four red PAPI lights, judged the aircraft to be 300 ft below the profile, and elected to continue. The co-pilot called for a go-around multiple times but the commander did not acknowledge. The aircraft landed normally at 1638 hrs with no damage.
Operator Procedures
According to the operator's Operations Manual, a GPWS warning in IMC required an immediate maximum gradient climb to the minimum safe altitude. For an unstable approach (e.g., descent rate exceeding 1,000 fpm or the approach not stabilised by 1,000 ft above decision height), a go-around was mandatory. Neither action was taken.
Technical Aspects
The FMS database contained the NDB/DME approach, with the first waypoint at the FAF requiring 1,800 ft or above and a 3.0° descent profile. The crew selected a 'vertical direct to the runway,' which deleted the existing profile and commanded a new descent path. An FMS manufacturer Service Information Letter (2020, revised 2021) described an anomaly that could cause a steeper-than-expected descent when VNAV mode recalculated after passing a waypoint and the next constraint was an 'at' or 'at or above' value. No flight data recorder data were available; broadcast data showed the aircraft's profile deviated below the published approach.