What happened
On March 20, 2001, a Saab 2000 operating a scheduled flight from Paris (CDG) to Cologne-Bonn was performing a descent from FL 210 to FL 180 under instrument meteorological conditions (IMC). While flying the route segment between ARCKY and the VOR NORVENICH, the flight crew and cabin crew both heard a sudden, loud noise, described as sounding like an explosion, originating from the rear of the aircraft.
Following the event, the crew declared an urgency signal (PAN PAN) and requested an immediate landing at Cologne-Bonn airport, requesting emergency services to be on standby. Upon landing, the crew performed a full systems check and found no deviations in aircraft parameters. There were no injuries among the 27 passengers and 4 crew members on board, though the aircraft sustained minor damage.
The investigation
The BFU recovered the flight data recorder (DFDR) and cockpit voice recorder (CVR) for analysis. A physical inspection of the aircraft revealed a hand-sized area of chipped paint (approximately 10 x 15 cm) on the fuselage near the vertical stabilizer attachment. Additionally, a pressure hatch in the rear cargo compartment, secured with Velcro, was found open.
Analysis of the radar recordings (RDQC) confirmed there was no conflicting traffic in the area at the time of the event. The descent had been ordered by ATC to avoid crossing traffic that was approximately 10 NM away. The DFDR confirmed the acoustic event, recording acceleration forces across all three axes at 14:36:48, shortly after the aircraft left FL 210. While the cabin pressure regulation showed a higher amplitude of oscillation during this descent compared to previous flights, the manufacturer determined this was not the cause of the disturbance.
Investigators also ruled out several external factors. There was no evidence of turbulence, icing, or lightning strikes in the vicinity. Potential collisions with weather balloons (radiosondes) were also ruled out. While the aircraft flew over the Elsenborn Shooting Range in Belgium, Belgian military authorities confirmed that all firing activities had ceased before the aircraft passed over the area.
Findings
- The investigation could not establish a link between the chipped paint on the fuselage and the loud noise.
- Technical inspections by the operator's maintenance personnel found no malfunctions in the aircraft's systems or equipment.
- The cause of the disturbance could not be determined.