What happened
On the morning of 10 December 2019, a Boeing 737-400, registration ZS-DMI, departed O.R. Tambo International Aerodrome for a scheduled domestic flight to Port Elizabeth. The aircraft was carrying 166 passengers and 6 crew members. During the climb phase, the flight crew issued a Mayday distress call. While the pilot-in-command initially reported a fire in the cockpit, they quickly corrected the transmission to state that smoke was emanating from the left side of the instrument panel.
In response to the emergency, the crew implemented the Quick Reference Handbook procedures for smoke, fire, and fumes. The pilots donned oxygen masks and smoke goggles to maintain visibility. The crew requested an immediate return to O.R. Tambo, which air traffic control approved. The aircraft descended and landed safely on Runway 03L. All passengers and crew disembarked without any injuries.
The investigation
The SACAA AIID investigation focused on the source of the smoke and the mechanical state of the cockpit environment. Investigators examined the aircraft's flight recorders and interviewed the crew regarding the sequence of events. The inquiry specifically looked into the area behind the left-side instrument panel where the smoke was observed.
Findings
The investigation established that the smoke was caused by water leaking onto the aircraft's instrumentation. Specifically, a water drain pipe connected to the overhead cockpit drip pan had become dislodged. This allowed water to drip onto the electrical components, leading to electrical arcing and subsequent smoke. The failure of the tubing was attributed to the material becoming hard and brittle due to prolonged exposure to environmental conditions and age.
Safety action
Following the incident, the operator conducted inspections of the drip pan and tubing on all Boeing 737 series aircraft within its fleet, in accordance with the manufacturer's maintenance manual. The inspection focused on ensuring the security of the tubing that drains the overhead drip pan to prevent a recurrence.