Created Monday, Apr 24th 2023 08:43Z, last updated Thursday, Nov 21st 2024 18:22Z
A Pel-Air Saab 340A freighter on behalf REX Regional Express, registration VH-KDK performing freight flight ZL-9982 from Wagga-Wagga,NS to Charleville,QL (Australia), was enroute at FL220 about 70nm northwest of Cobar,NS (Australia) when the crew received a smoke indication on board of the aircraft followed shortly by smoke appearing in the cabin and cockpit. The crew performed an emergency descent to 6000 feet, turned around and diverted to Cobar and landed on Cobar's runway 23 about 14 minutes (!) after leaving FL220. Emergency services needed to dismantle parts of the aircraft to extinguish the fire. There were no injuries, the aircraft sustained substantial damage however.
Cobar Fire Fighters reported:
Today our crew responded to an incident at the Cobar Airport where an aircraft carrying freight had to make an emergency landing.
A smoke indicator alerted the pilots of smoke onboard and shortly after, the cabin started filling with smoke. Crews arrived to find smoke issuing from the right side of the aircraft under the wing area.
Fire and Rescue crews quickly got to work dismantling the plane to locate the source of the fire and extinguish. The batteries were quickly isolated as initially thought it was caused by an electrical harness.
After a second inspection inside the aircraft it was found to have significant damage to the flooring in which the fire had started. The floor was removed and checked for further signs of heat and determined safe, the aircraft was secured, ventilated and the site handed over to airport staff.
Excellent job by all involved, the quick action by firefighters and aircraft crew prevented total loss of the aircraft.
On Nov 21st 2024 the ATSB released their final report (no PDF) concluding the probable causes of the accident were:
Contributing factors
It was likely that an electrical component of a control circuit board on the recirculating fan failed, resulting in an in‑flight fire under the cabin floor.
The smoke curtain was not fitted as required for the cargo configuration, and the flight deck door was open, which allowed smoke from the in‑flight fire to enter the flight deck.
The underfloor fire caused weakening of the fuselage structure, which led to a subsequent depressurisation of the aircraft during the descent. However, the depressurisation aided in the removal of enough smoke from the flight deck on approach to allow an unhindered visual approach at Cobar.
Crew were not familiar with the cargo configuration and were unaware of the smoke curtain requirements and location of the cross-valve handle.
The Pel-Air and Rex Saab 340 flight crew operating manuals did not include reference to the location and operation of the cross-valve handle or the operation and use of the smoke curtain. (Safety issue)
Rex did not ensure its flight crews received training in the differences between passenger and freight‑configured Saab 340 aircraft, prior to being scheduled to fly freight operations. (Safety issue)
Saab did not include the smoke curtain fitment in pre-flight documentation for the cargo‑configured Saab 340 aircraft to inform flight crew of this difference from the passenger-configured version. (Safety issue)
Other factors that increased risk
When the flight crew donned their oxygen masks, the first officer's oxygen mask microphone did not function correctly. This led to difficulty in communication between the flight crew and a delay in responding to the emergency.
Due to the combination of the smoke density and lack of prior knowledge, the flight crew were unable to locate the cross-valve handle during the
emergency, therefore delaying the removal of smoke from the flight deck.
The ATSB analysed:
Origin of the in-flight fire
The source of the in-flight fire was traced to the right recirculating fan assembly. Although the fan was not damaged internally, the fire damage was most significant at the box sub-assembly, which was mounted external to the fan and housed the electrical control circuit boards. It is likely that an electrical component or components within the box sub-assembly failed, resulting in the underfloor fire. The fire damaged underfloor insulation and plastic air conditioning ducting components, which led to thick smoke filling the cabin and cockpit and aircraft structural damage.
The avionics warning received by the crew during the diversion was most likely associated with the avionics cooling air that was being drawn from the now smoke-filled cabin. This was also stated in the ‘cargo compartment smoke’ checklist.
The ATSB examination of the recirculating fan could not determine a cause for the failure in the electronic control cards which led to the fire.
When the crew received the air conditioning system right duct over temperature caution light, it was most likely due to the distribution duct over temperature being affected by the fire and the melting which occurred as a result. When the over temperature was sensed, the right bleed valve closed automatically as a function of system logic for over temperature protection.
Smoke barrier curtain
Cargo operations can have a greater fire risk than passenger operations due to the carriage of cargo that could be the source of a fire and the lack of cabin crew available to fight a fire. As such, additional protection was available to minimise flight crew exposure to cabin smoke in the form of additional smoke detectors and a smoke curtain.
However, the smoke curtain was not installed into position by anyone involved in the flight preparation. The flight crew, who normally operated the same aircraft type but in a passenger configuration, did not notice there was a placard at the aircraft entrance stating the smoke curtain was to be fitted for all cargo flights. The flight crew remained unaware of the smoke barrier curtain and its use for cargo operations. Further, the engine being transported was positioned in the cargo area of VH-KDK, on both occasions, by Rex engineers. As the curtain was usually installed by freight handlers during normal cargo operations, it is possible the Rex engineers were also unaware of its requirement to be fitted.
In this accident, the source of the fire was an aircraft component rather than the cargo being carried. If a similar fire occurs in a passenger‑configured Saab 340 aircraft, then the smoke curtain would not be in place. However, the smoke curtain was available and was required for use for this flight, so its non-use increased risk for this event.
The flight deck door was not closed during flight as prescribed in the operator’s FCOM operating limitations and checklists. Having the door closed would have likely prevented the smoke being able to flow into the flight deck.
The result of not having the smoke barrier fitted and the flight deck door closed as part of the aircraft pre-flight preparation was that smoke from the fire was not contained to the cabin area and was able to move forwards toward the flight deck.
Oxygen mask fault
The flight crew fitted their oxygen masks and smoke goggles when alerted of the presence of smoke by the central warning panel. This decision may have prevented the crew from being overcome by smoke and fumes in the cockpit in the next several minutes. However, once fitted, the crew had difficulty communicating with each other, as a result of the mask microphone being very faint and difficult for the captain to hear the first officer (FO). This appeared to be an internal fault only, as the cockpit voice recording (CVR) showed that the FO was able to be adequately heard by ATC.
This breakdown of communication delayed the crew by 57 seconds, in which emergency checks were not initiated due to the breakdown of communication. It created confusion and distraction between the crew while trying to execute the emergency checklist.
A review of the CVR captured prior to flight could not positively determine if the pre-flight check action in relation to the oxygen mask was performed. This is an important check of the emergency communication system whilst on oxygen and was designated as a mandatory check item for a daily inspection as required by the Rex and Pel-Air FCOM.
Cross-valve handle
While the flight crew were conducting the emergency checklist items for cargo compartment smoke, they were unable to locate the cross-valve handle. This was due to the combination of the thick smoke obscuring their vision and their lack of knowledge of the differences in the cargo‑configured aircraft.
Had the location and function of the cross-valve handle been known by the flight crew, the time taken to identify it during completion of the emergency checklist would have been minimised, which would have limited the delay of smoke removal from the flight deck.
In this case, the subsequent depressurisation resulted in the smoke dissipating even in the absence of the cross-valve.
Cabin depressurisation
The weakening of the fuselage structure due to the underfloor fire resulted in a breach of the fuselage skin, which led to a subsequent depressurisation of the aircraft during the descent. Although adding to another caution alert indication for the flight crew and subsequent checklist to be conducted, it also benefited in the removal of smoke from the cabin and flight deck.
At the time of the depressurisation, VH-KDK was at FL 160 and descending. The crew, when alerted to the depressurisation, increased the rate of descent to below 10,000 ft. The cabin depressurisation occurred 4 minutes after the initial smoke warning occurred.
Due to the size of the hole created, the smoke removal most likely occurred at a greater rate than using the aircraft pressurisation outflow valves alone. The resulting fortuitous reduction in the amount of smoke in the flight deck improved visibility and allowed the crew to carry out a safe landing into Cobar.
Crew familiarity of cargo‑configured aircraft
The flight crew had not flown or had any prior training on the cargo‑configured Saab 340 and were not familiar with the differences of the passenger configuration. The captain chose to liaise with a colleague to gain information on the cargo‑configured aircraft instead of accepting the company offer for a briefing.
This non-formal approach to understanding the differences between the 2 aircraft types ultimately did not pass on the required operational differences and potential safety aspects of the change of aircraft configuration.
Rex and Pel-Air manuals
Both operators’ (Rex and Pel-Air) manuals, which were designed to provide essential information to flight crews, did not include the required information to enable the pre-flight checks to be conducted adequately. While the weight and balance chapter of the FCOM showed the smoke barrier curtain location, there was no information on its importance to cargo operations, and as the crew had not been informed of any differences, they would not have been expecting that this section contained this information. The smoke barrier curtain installation information that was contained in the Saab service bulletin and flight manual supplement was not included in the pre‑flight checklists. As a result, the flight crew did not have awareness of its use.
Saab had no specific checklists for cargo‑configured aircraft, so the operators also had no specific cargo checklists to bring awareness to flight crews about the cross-valve handle which was unique for the cargo‑configured aircraft. As discussed above, when the checklist called for the crew to use this handle when the aircraft was already filling with smoke, the crew could not locate it.
Flight crew training
The Rex ground school provided type rating training on the Saab 340 series aircraft to both Rex and Pel-Air pilots. This training was based on the passenger‑configured aircraft. Pel-Air pilots undertook further training which gave them the knowledge and skills for the cargo‑configured aircraft.
In scheduling their flight crews to operate the cargo‑configured Saab 340, Rex did not have a process to ensure that the additional training or knowledge sharing for their crews in the differences applicable to aircraft operated by Pel-Air was delivered.
Saab pre-flight checklists
As discussed above, both operators’ manuals had no inclusion of a pre-flight interior check for the smoke barrier curtain or the cross-valve handle. Likewise, there was no pre-flight interior check for these items in the manufacturer’s documentation. Saab confirmed that there were no checks in the pre-flight checklist for the crew to specifically verify that the smoke barrier curtain was correctly fitted.
The result of the manufacturer’s pre-flight and interior checklists not detailing information for the smoke curtain was that the operator did not detail these in their own FCOM. This information was not available for the flight crew who, even without prior knowledge of the cargo‑configured variant, would have been alerted to these changes while conducting these pre-flight checks in accordance with the FCOM.
