What happened
On February 3, 2003, a scheduled flight operated by Ryanair, involving a Boeing 737-2T5 with registration EI-CKS, departed Dublin Airport for Bristol. Shortly after takeoff, a loud noise was heard in the cabin, followed by a sudden influx of hot air and what appeared to be smoke near row 10. While the flight crew initially observed no engine abnormalities or fire warnings, the cabin supervisor reported intense heat and visible smoke.
In response to the cabin conditions, the captain declared an emergency and initiated an immediate return to Dublin. Upon landing, the crew performed the necessary checklists and, following the supervisor' and the continued presence of heat and smoke, ordered an emergency evacuation. During the evacuation, wind conditions caused the forward escape slides to become unstable, forcing many passengers to utilize the over-wing exits and rear doors. While most passengers exited safely, six individuals were transported to the hospital for observation following the incident.
The investigation
The AAIU examined the aircraft's pressurization system to identify the source of the heat and smoke. Investigators discovered that a metal duct, which transports engine bleed air to the air conditioning mix chamber, had separated from the chamber itself. This failure allowed high-pressure, high-temperature engine air to vent into the under-floor area known as the "snake pit," subsequently entering the cabin through sidewall vents.
Physical inspection of the hardware revealed that all eight rivets securing the flange to the mix chamber had failed in shear. The investigation focused on the design of the rivet installation, specifically noting that the countersunk holes in the flange created sharp edges. These edges had caused significant wear, or fretting, on the rivet shanks, reducing their cross-sectional area by up to 90% in some instances. Additionally, the investigation looked into the evacuation process, noting that confusion at the over-wing exits was exacerbated by a lack of clear instructions on the aircraft's safety cards regarding how to descend from the wing.
Findings
- The primary cause of the duct separation was the failure of all eight rivets due to excessive wear caused by the sharp edges of the countersunk flange holes.
- The presence of accumulated dust and lint in the ventilation ducts, which was dislodged by the blast of air, created the visual illusion of smoke.
- The aircraft's age, high flight hours, and the specific design of the rivet installation contributed to the structural failure.
- The evacuation was complicated by wind affecting the forward slides and a lack of clear pictorial guidance on the safety cards for using over-wing exits.
Safety action
Following the investigation, the manufacturer, Boeing, issued a service letter recommending the use of stronger fasteners for the flange-to-mix-chamber connection. Additionally, a recommendation was made for operators to review safety cards to ensure passengers are clearly instructed on how to safely exit the aircraft via the wing surface.