Flybe DH8D near Edinburgh on Jul 21st 2011, cargo smoke indication

Last Update: April 12, 2012 / 14:40:52 GMT/Zulu time

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Incident Facts

Date of incident
Jul 21, 2011

Cause
Cargo smoke indication

Airline
Flybe

Aircraft Registration
HL7734

Aircraft Type
De Havilland Dash 8 (400)

ICAO Type Designator
DH8D

The AAIB released their bulletin releasing following summary: The aircraft generated a spurious smoke warning from the forward baggage compartment, which was probably caused by a short circuit in the smoke detector connector. The pilots decided to treat the warning as valid even though there was no evidence of smoke or fumes. The pilots decided that, with the possibility of a fire on board, an evacuation was required and, after landing, the aircraft vacated the runway and the passengers were evacuated onto a taxiway. Safety action was taken by the aircraft operator to prevent a similar short circuit in other smoke detector connectors, and to ensure pilots received training with respect to aircraft evacuation that reflected company policy. Safety action was taken by the airport authority to address issues that arose during and after the evacuation. The aircraft was enroute at FL240 about 80nm south of Edinburgh when the crew received a master warning and a smoke warning from the forward luggage compartment. The crew donned their oxygen masks and smoke goggles and worked the relevant checklist including discharging the fire suppression system into the forward luggage compartment, the smoke warning light extinguished shortly after. The crew declared PAN and advised cabin crew to look for smoke from the forward luggage compartment. As there were no signs of smoke or fire on the flight deck the crew removed their oxygen masks and goggles, the captain (55, ATPL, 8,010 hours total, 1,960 hours on type) advised cabin crew they had had to fight a fire and would do an emergency descent into Edinburgh instructing cabin crew to secure the cabin and advising he would decide later whether an evacuation was necessary. The smoke warning light illuminated again and remained permanently lit until after landing. Captain and first officer agreed that the occurrence was real even though no smoke had been seen. The crew carried out the fuselage fire or smoke checklist and carried out actions associated with a fuselage fire or smoke from unknown source including the shut down of DC and AC generators and main, auxiliary and stand by batteries effectively shutting down the first officer's instrument displays. Power was also removed from the autopilot, ILS receivers, transponder, black boxes, the aircraft had begun to depressurize. The crew estimated the landing to take place in seven minutes when the aircraft was about 55nm from touch down, the air traffic controller attempting to compute a more accurate estimate was unable due to the loss of the transponder signal. The first officer inquired with cabin crew whether they would be able to prepare cabin and passengers for the landing in seven minutes which the cabin crew replied to in the negative. The first officer therefore made an emergency announcement via PA indicating they would evacuate after landing. The crew advised ATC they would vacate the runway and evacuate onto the taxiway. The crew performed a surveillance radar approach (SRA), touched down and rolled out safely and vacated onto taxiway L about 15 minutes after advising cabin crew of 7 minutes to landing, where the aircraft was evacuated, no injuries occurred. Emergency services found no trace of fire or smoke. The crew later reported that following the smoke indication extinguishing after the fire suppression system had been activated they were convinced they had a real smoke event on board. When the smoke warning light subsequently illuminated again they believed the fire might have re-ignited through an electrical loom or the smoke warning system might have been incorrectly wired and the smoke was actually in the rear luggage compartment. The commander did not believe the warning was spurious. A post flight examination of the aircraft found no sign of fire or smoke in any part of the aircraft. The forward luggage compartment contained no items that could have generated smoke or other airborne particles. The smoke detectors passed their tests, insulation and wiring checks of the smoke detectors also revealed no fault. The fire control amplifier was removed and sent to the manufacturer for further testing under NTSB supervision. The unit passed 15 of 17 tests, the remaining two tests including the temperature stress test were failed. The board had been exposed to cooling with a cooling spray followed by heating with the heat gun and produced an occasional smoke warning. These warnings were abnormal, the manufacturer stated, and were produced only after the board had been exposed to more moisture than would be encountered in service. Deposit was found around pin B of the smoke detector connector. The manufacturer stated that if those deposits were products of corrosion then moisture must have been present, and if such moisture caused a short circuit between pin B and the shell a self test would be triggered resulting in smoke warnings and the fire suppression system being armed. Several of the blanking pins of the connector were found missing. The detector passed all manufacturer's tests however. The detector was subsequently taken to a company specialised in electric failure investigation. The company identified the deposits were products of corrosion. Using a high resistance meter the resistance between pin B and shell was measured at 400.000 MegaOhms, in comparism pin A and shell was measured at 200.000 MegaOhms. When humid air was blown onto the connector, pin A to shell remained constant at 200.000 MegaOhms, pin B to shell however reduced to 4 MegoOhm. The connector was manufactured to military specification and would have been tight had all blanking pins been installed. With the blanking pins missing however moisture would be able to enter the connector. According to maintenance documentation the connector had never been replaced, the pins therefore must have been missing since production. Inspections of other aircraft found the blanking pins in place on those aircraft. The airline commented that they do not encourage crews, nor should ATC encourage crews, to vacate the runway before evacuating. The aircraft should rather be stopped on the runway and evacuation been done there. The airport authority stated that the loud speaker of the passenger evacuation management system mounted on a vehicle had been inoperative, a loudhailer was used instead. The bus to drive the passengers to the terminal was summoned by a radio call using a keyword that wasn't recognized by the driver forcing a phone call instead, however, that did not delay the bus significantly. The aerodrome manager declined the request to use the passenger receiption center believing the situation wasn't serious enough.