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27 Feb 2020 - On 10 Dec 2019, 737-400, ZS-DMI (24165/1720), FF 5/8/1989 (31 Years old), operated by Safair had smoke in the cockpit during a climb. They made use of their oxygen masks before declaring a mayday, requested an air turn back to OR Tambo International Airport, South Africa (FAOR). The aircraft landed safely at FAOR and there were no reported injuries.

The South African CAA published a preliminary report on 24 Feb 2020 and the final report on 10 Sep 2020..

 

History of the Flight

On Tuesday morning, 10 December 2019, a Boeing 737-400, registration ZS-DMI, performing flight FA369 departed OR Tambo International Aerodrome (FAOR) on a scheduled domestic flight to Port Elizabeth Aerodrome (FAPE). The aircraft took off from runway 03L at 0434Z. On board the aircraft was 166 passengers and 6 crew members. While climbing through 28 000 ft, still within the Johannesburg FIR water that have accumulated in the left drip tray (above the pilot-in-command head) started to leak from the drain pipe on the left side onto the avionics/instrumentation of the aircraft. The water caused some arcing, which resulted in smoke emanating from the behind the instrument panel on the left side. Following a brief discussion between the two members they declared a Mayday and requested to returned to FAOR. The crew members opted to make use of their respective oxygen masks and landed safely back at FAOR thirty two (32) minutes after take-off, at 0506Z.

After the smoke started to emanate from the instrument panel the crew members fitted their oxygen mask. The smoke dissipated by itself and no intervention was required by the crew (i.e., not having to make use a fire extinguisher).

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The Drip Tray and Drain Tube

Lining panels formed of thermoplastic sheets cover wall and ceiling areas. Insulation is provided by fiberglass blankets, some of which are part of a drip pan assembly. The drip pan is an assembly which is bolted to structure above the overhead instrument panel to insulate the area and assure drainage. The unit consists of a molded plastic pan with insulation blankets attached. Condensate collected on the outboard side of the drip pan drains thru tubing into the airplane drain system.

The drip pan and LH drain tube

Investigation reveal

This was the first flight of the day with this aircraft. While the aircraft was parked outside (at the aerodrome), and most probably during taxi and take-off roll, a small amount of water might have accumulated in the upper cockpit drip pan. Only once the aircraft levelled off at FL280 did the water start to drain via the tubing, which at that stage had most probably became dislodged from the fitting since it had been degrading over an undetermined period due to exposure to different environmental conditions. The PIC who was seated on the left seat most probably did not notice this as it was slightly out of his peripheral vision. Furthermore, he was pre-occupied in monitoring the PF as they entered instrument meteorological conditions (IMC) shortly after rotation as the cloud base at FAOR at the time was reported to be at 200ft AGL. Once the water made contact with some of the instrumentation, a flash/spark was seen, which the PIC first assessed as fire, however, shortly after, he changed his assessment of the situation to smoke in the cockpit. The crew then followed the emergency checklist for smoke, fire and fumes in the cockpit, and then broadcasted a Mayday, requesting the radar controller for an air turn back to FAOR.

A dislodge water drain tube as found during an inspection by the AMO. (Source: Safair Aviation Safety Division)

Probable cause

The tubing was found to have been exposed to prolonged environmental conditions as well as ageing, which caused it to become hard and brittle, and subsequently became dislodged from a connection fitting which allowed water from the overhead drip pan to leak onto the back of some of the instrumentation situated on the left side (pilot-in-command) of the cockpit. This caused arcing (flash was seen) followed by smoke in the cockpit.

Safety Recommendation

The aircraft operator advised that as a result of this serious incident, they had issued an immediate special inspection on all Boeing 737 series aircraft in their fleet. The inspection calls for the proper security of the overhead drip pan, which is bolted to the structure above the overhead instrument panel with special emphasis on the security of the tubing (PIC and FO sides) that drains water from the drip pan into the aircraft drain system. This inspection was signed off in the Aircraft Technical Log Report for each of the respective aircraft by the aircraft maintenance engineer (AME) responsible for the task, which was conducted in accordance with AMM 25-11-21, Task 25-11-21-002-031,

Safety Message:

This inspection should be conducted on a regular basis to prevent a recurrence of this serious incident. This inspection does not have to be limited to the operator in question but should be applicable to all aircraft owners/operators that are operating the Boeing 737 series of aircraft with this configuration installed as discussed in this report.

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