British Airways 747-400 Cargo Fire
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Air Safety Occurrence Report

 

Occurrence Details
Occurrence Number: 200203671
Release Date: 18-Dec-03
Occurrence Type: Serious-Incident
Location: 6 km N Sydney, Aero.
State: New South Wales (NSW)
Occurrence Date: 10-Aug-02
Time/Zone: 1607 hours EST
Highest Injury Level: None
Investigation Category: 3
           
Aircraft Details
Aircraft Manufacturer: Boeing Co
Aircraft Model: 747-436
Aircraft Registration: G-BNLK
Serial Number: 24053
Type of Operation: Air Transport, High Capacity, International, Passenger, Scheduled
Damage to Aircraft: Minor (Damage to the forward cargo bay and heat affected fuselage structure)
Departure Time: 1500 hours EST
Departure Point: Sydney, NSW
Destination: Singapore
Crew Details: Role Class of Licence Hours on Type Hours Total
  Pilot-In-Command ATPL 873.0 12930


FACTUAL INFORMATION

History of the flight

Shortly after take-off from runway 34L at Sydney, the flight crew of the Boeing 747-400 aircraft received a forward cargo compartment fire warning on the Engine Indicating and Crew Alerting System (EICAS). On receiving the warning message the crew actioned the appropriate checklist, activated the fire suppression system and transmitted a MAYDAY. At the same time, flight attendants noticed a fine mist and the smell of smoke in the passenger cabin. The crew then returned the aircraft to Sydney, where an uneventful overweight landing was conducted.

Prior to landing, the EICAS fire warning message ceased. This indicated that the aircraft fire suppression system may have successfully extinguished any fire, however the cabin fumes were still evident. After landing, the flight crew stopped the aircraft on the runway where emergency services came to their assistance. After confirming with the flight crew that the fire warning message was no longer present, the emergency services assessed the aircraft from the ground, then allowed the passengers and cabin crew to disembark to a safe distance via mobile stairs positioned at the aircraft’s front left door. Once the passengers and cabin crew were clear of the aircraft, the emergency services opened the forward cargo door.

A hot spot was detected on the left side of the forward cargo bay at body station STA900, where the side wall lining was found to be heat affected. Removal of the lining revealed burned insulation blanket material, discolouration of the aircraft skin and burned/broken electrical wires that powered the forward galley chiller boost fan situated in the area (see Fig 1). As the fire was no longer evident, ground engineers isolated the chiller boost fan electrical circuit and towed the aircraft clear of the runway.

 

FIGURE 1: FIGURE 1:
FIGURE 1: Forward cargo bay with expanded view of chiller boost fan location

Aircraft structural damage

Non-destructive testing to check for cracking and conductivity of the aircraft skin adjacent to the affected area was carried out. No cracks were detected, however the conductivity test revealed three locations where the skin had been substantially affected by heat (see Fig 2). The most severely affected area required a temporary skin repair before the aircraft could be flown back to the operator’s maintenance facility in the United Kingdom, where the heat-affected aircraft skin was replaced.

 

FIGURE 2: FIGURE 2:
FIGURE 2: Heat affected areas

Sidewall lining and insulation blankets

The fibreglass sidewall lining between STA880 to STA900 was visibly heat damaged with discolouration observed on the side facing into the cargo compartment. Inspection of the reverse side revealed burned layers of fibreglass confined to a localised area approximately 30cm x 45cm (see Fig. 3). The insulation blankets that lined the aircraft skin were made of a fibreglass core with a metallised TedlarTM film on one side and a MylarTM film on the other and had been subjected to localised heat and fire (see Fig. 4).

Samples of the sidewall lining and insulation blanket were sent to the United States of America, Federal Aviation Administration (FAA) technical centre and the aircraft manufacturer for analysis and testing.

 

FIGURE 3: FIGURE 4:
FIGURE 3: Sidewall lining FIGURE 4: Insulation blanket

The examinations determined that both the sidewall lining and insulation blanket samples complied with the appropriate material specifications for aircraft use.

The flammability testing, conducted by the FAA, on samples of the insulation blanket included a vertical Bunsen burner test, which was mandated in Federal Aviation Regulation FAR 25.853 – Appendix F. The samples tested met the requirements, but due to their limited size, the result was not conclusive as to the integrity of the entire blanket.

The aircraft manufacturer’s tests revealed contamination on the insulation blanket samples. This contamination consisted of environmental dust, fibres and corrosion inhibiting compound. These contaminants were consistent with general contamination found during evaluations of other in-service insulation blankets and were considered to be normal.

The aircraft manufacturer’s ‘flame propagation cotton swab tests’ found areas on the blanket samples that were self extinguishing while other areas showed “flame propagation uncharacteristic of that expected for new insulation blankets”. It was unknown whether contamination, in-service ageing, or heat exposure, or a combination of these, altered the blanket’s flame propagation characteristics.

Boost fan system

A galley chiller boost fan system was installed in the aircraft to provide forced air circulation over the forward galley chiller units increasing their cooling efficiency. The system incorporated a vaneaxial-type three-phase fan, powered by the aircraft’s number-3 alternating current electrical system. Control power was supplied by the aircraft’s direct current electrical system, with operation being automatic on selection of the galley chillers to ON. Circuit protection was provided by a 20 ampere circuit breaker and a cargo fire cutoff relay.

Chiller boost fan

An inspection of the boost fan revealed a burn hole and sooting on its casing adjacent to the electrical terminal (see Fig. 5). The electrical wiring to the fan was found to have four of its seven wires broken, with all of the wires displaying sooting discolouration (see Fig’s. 6 and 7). The soot marks corresponded to those on the fan casing and when positioned together, revealed that the wires had separated at a point adjacent to the corner of the electrical terminal. The failure of the wires produced electrical arcing, which melted the casing, resulting in the burn hole observed.

Further inspection found that all of the fan impeller blades had failed just above their roots (see Fig. 8). Neither the impeller nor the fan shroud showed signs of hard body impact damage.

 

FIGURE 5: FIGURE 6:
FIGURE 5: Electrical terminal FIGURE 6: Broken wires
FIGURE 7: FIGURE 8:
FIGURE 7: Sooting evident FIGURE 8: Fan impeller blades failed

Technical examination of the fan found that the impeller was made from a moulded resin material. There was no evidence of any pre-existing defects or cracking found on the blade fracture surfaces. However, a number of blades showed breakage of a curved lip of material from their forward corner. This condition was consistent with overload fatigue possibly due to the blade tips contacting the fan shroud. Such a condition may have occurred prior to the blades total failure. The inside surface of the impeller was coated with a brown powder, determined to be primarily iron oxide.

The aluminium alloy shroud contained several circumferential wear marks that were adjacent to the impeller blade path. Although there were random scratches, no evidence of gouging or penetration of the shroud skin was found.

Further disassembly of the fan revealed wear on the electrical motor stator, indicating that it had been subjected to armature rubbing. The armature did not display similar wear patterns. Rubbing of this nature usually occurs as a result of bearing failure or excessive wear, leading to armature oscillations. For the full technical report see Attachment ‘A’

Chiller boost fan service history

The chiller boost fan entered service in 1994, with the last overhaul being in June 2000, after removal from service because of electrical failure. The maintenance records for that overhaul stated: “Unit noisy due worn bearings, all other parameters ok. Reported defect not confirmed. Disassembled, cleaned and inspected, bearings renewed, unit reassembled and tested to spec”. The fan was then fitted to the incident aircraft on 2 August 2000. No subsequent maintenance was recorded.

Chiller boost fan circuit breaker and electrical relay

The installed circuit breaker was a 20 ampere three-phase, push-pull high performance, trip free type, designed for aircraft installations. It’s design allowed for increased amperage through the circuit for a specific time before tripping (breaking the circuit) and was used in large motor load applications where the inrush current would trip a standard circuit breaker. The length of time taken to cause the circuit breaker to trip varied according to the current it received. The aircraft manufacturer advised that “At 385 per cent or 400 per cent [load rating], this breaker will trip between 2.3 to 10 seconds”. This prevented aircraft electrical power surges from “nuisance” tripping of the circuit breaker and rendering the boost fan inoperative.

A number of tests were conducted on the circuit breaker, including a ‘load withstanding test’. This required the controlled increase in current through the circuit breaker, with time to trip recorded. This test was conducted at 105 per cent, 140 per cent and 200 per cent values, as per the manufacturers test procedures.

The installed relay was a 25 ampere, electromagnetic, three pole, single throw, normally open type. This was also subjected to a number of tests including ‘Coil resistance’, ‘Coil hold and drop voltage’ and ‘Voltage drop and switching test across all three phases’.

These tests were performed under the supervision of the United Kingdom Air Accident Investigation Branch. Both components were found to comply with their operational specifications, with no adverse mechanical or operational functions found during the testing. As a result, both components were considered to be serviceable.

Quick Access Recorder

The aircraft’s Quick Access Recorder (QAR) data was analysed by the Australian Transport Safety Bureau with the following information retrieved.

During climb the number-3 alternating current system showed a momentary increase in load from a nominal 31 per cent to 54 per cent, which equated to an increase in current draw of 57 amperes.

Four seconds later, the load was again recorded and had returned to the nominal 30 percentage range, where it remained for the rest of the flight.

Approximately 1 minute later the QAR recorded a forward cargo fire.

Approximately 3 minutes later, the first cargo fire bottle low quantity message appeared, indicating that extinguishant had been discharged successfully.

Other recorded data received from the aircraft’s central maintenance computer (CMC) confirmed the arming of the fire bottles approximately 2 minutes after the fire warning and the discharging of the last two fire bottles after the aircraft landed.

Cargo fire detection/extinguishing system

The aircraft incorporated two dual loop smoke detectors in each cargo compartment. Air from throughout the compartment was drawn through the detectors and sampled. In normal operation, both loops must sense smoke for a fire warning to be activated. If the system detects a loop fault during self-test at aircraft power on, it would reconfigure to a single loop operation.

The cargo fire module located on the overhead instrument panel in the flight deck incorporated forward and aft compartment ARM buttons and a DISCH discharge button. On sensing smoke, the relevant ARM button, along with an EICAS message would be illuminated, alerting the crew of the fire. The crew must then push the ARM button in. This action disables electrical power to a number of circuits, including the galley chiller fan circuit. Extinguishing is then achieved by pressing the DISCH button (see Fig. 9).

 

FIGURE 9: FIGURE 9:
FIGURE 9: Overhead instrument panel with expanded view of cargo fire panel

Four fire extinguisher bottles (A, B, C and D) service the cargo compartments, each having discharge lines to both the forward or aft compartments. On depression of the DISCH switch, bottles A and B discharge flooding the selected compartment with extinguishing agent. Bottles C and D are not discharged until 30 minutes later. If the aircraft reaches the ground before the 30 minutes are up, the bottles will discharge on touch down. The system was designed to give up to 180 minutes of discharge time.


ANALYSIS

Chiller boost fan

The impeller blade’s fracture surfaces did not show evidence of material flaws, foreign object impact, or any other pre-existing damage. However, as no blade remnants were found, these conditions could not be ruled out. The resin materials used to manufacture the impeller made it prone to cracking and failure under impact, or excessive stress conditions, due to its lack of ductility. Some blades displayed preferential tip breakage before complete blade failure.

Maintenance records for the fan indicated that, prior to its fitment to the aircraft, it had been removed from service and overhauled because of electrical failure. The records stated “ Unit noisy due worn bearings”.

Due to the close tolerance between the impeller blades and the fan shroud, excessive wear in the bearings most probably led to oscillations of the armature and impeller, resulting in armature and blade tip rubbing. There was evidence of rubbing wear on the fan shroud and the motor stator. The lack of wear on the motor armature indicated that the rubbing condition had occurred prior to the fan’s last overhaul. The brown iron oxide dust observed on the inside of the impeller was believed to be from the worn bearing. This confirmed that the impeller was the same one that had been fitted at that time.

The shroud rubbing may have predisposed the blade tips to cracking that could have resulted in their subsequent failure in service. Once the tips were released, their impacting on additional blades would have led to the total failure of the blades due to their brittleness.

Fire initiation

The position of the broken/ burned wires and the localised burning and soot marks on the fan case electrical terminal housing, indicated that a probable chafing event had occurred, leading to electrical arcing. The chafing was most likely the result of the excessive vibration induced by the fan’s imbalance after the impeller blades failed.

Molten material from the fan case dripped onto the adjacent insulation blanket where it smouldered and burned. It is possible that the fan continued to operate for a short period of time after the arcing had initiated. This condition would have provided a positive airflow into the confined area, feeding the fire.

Insulation blanket and sidewall lining

Although the insulation blanket had been subjected to in-use contamination, the material composition of the insulation blanket (and sidewall lining) was able to prevent a rapid spread of fire. However, due to the temperatures involved, localised burning had occurred.

Boost fan circuit isolation

The electrical short-circuiting of the boost fan was indicated by the momentary increase in current load on the aircraft’s number-3 generator. Due to the circuit breakers “trip free” design, the short time frame of the overload condition would not have resulted in the circuit breaker tripping and isolating power from the fan. Ground engineers reported that they isolated the fan circuit prior to towing the aircraft confirming that the circuit breaker had not tripped.

Electrical power would only have been removed from the fan circuit when the forward cargo ARM button was depressed. This was recorded on the CMC approximately 2 minutes after the fire was detected.


SIGNIFICANT FACTORS

 

  1. Worn bearings led to impeller and shroud rubbing, weakening the blade tips.
  2. The fan blade tips failed, creating an out of balance condition and vibration.
  3. Chiller boost fan vibration resulted in the wires chafing and electrical short circuit initiating the fire.
  4. The trip free capability of the circuit breaker in the chiller boost fan electrical circuit prevented rapid electrical isolation.


SAFETY ACTION

As a result of this incident the following local safety actions have been carried out.

Aircraft manufacturer

The manufacturer issued alert service bulletin SB747-21A2427, directing the inspection and corrective routing of the electrical wire loom to the boost fan.

Operator

The operator conducted a fleet inspection of the fan wiring for condition and routing and has undertaken to pay particular attention to the balancing of the boost fan assembly during overhaul. The incorporation of SB747-21A2427 on their fleet was scheduled for commencement from June 2003.

Related Documents: |Technical Analysis Report 200203671|  (see below)


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TECHNICAL ANALYSIS REPORTS

Examination of a Failed Air Cooling Fan

Boeing Commercial Aircraft Group, 747-436, G-BNLK

1. FACTUAL INFORMATION

1.1. Examination brief

The disassembled components of an electric air-cooling fan (figure 1) were received by the ATSB Technical Analysis unit for examination and analysis of the damaged fan impeller. The fan unit had been fitted to a Boeing 747 aircraft (registration G-BNLK) to provide forced air circulation for a forward galley chiller unit. During the early stages of a flight on 10 August 2002, a small fire developed in the forward cargo compartment adjacent to this unit (figure 2). Physical and recorded evidence suggested the fire had initiated from electrical arcing that was a result of a wiring short-circuit near the fan terminal housing.

 

FIGURE 1: FIGURE 2:
FIGURE 1: Cooling fan assembly. Inlet is at top. FIGURE 2: Cooling fan location (arrowed) behind cargo hold wall.

1.2. Samples received

Data plates affixed to the fan housing (figures 3 & 4) identified the unit as a three-phase unit (part number 73259E, serial number 3676), manufactured by Sunstrand (San Diego, California) in 1994. The fan was an axial flow design, with a single bell-shaped impeller manufactured from a moulded resin material. The motor and fan outlet guide vane assembly shared an integral housing which also carried the (damaged) electrical terminal housing. The motor was a brushless (induction) design, with the armature supported on sealed rolling element bearings. Surrounding the impeller was an aluminium shroud, which formed the fan intake and also provided for the mounting and support of the unit and its associated ducting.

 

FIGURE 3: FIGURE 4:
FIGURE 3 & 4: Data and wiring information plates affixed to the fan housing.

1.3. Visual examination

1.3.1. Impeller

The impeller unit had been effectively ‘cobbed’, with all eleven blades fractured at or immediately adjacent to the impeller hub (figure 5). The uneven, irregular nature of the fractures suggested the failure occurred as a cascading fragmentation event, with multiple sections of blade breaking away and striking others, causing further break-up. A study of all fracture surfaces failed to identify any evidence of pre-existing defects or cracking that may have precipitated the initial blade failure, nor was any indication found of unusual hard-object impact damage that may have suggested foreign object ingestion. There was some evidence however that suggested early damage to the impeller blade forward corners – many blades showed breakage of a curved lip of material from the corners in a manner that suggested possible contact with the fan shroud (figure 6).

The inside surfaces of the impeller body carried a heavy coating of a powdery brown dust (figure 7), which was also evident inside the armature core and around the end of the motor housing. This material was loose and easily wiped away by hand and a sample was taken for later qualitative chemical analysis.

 

FIGURE 5: FIGURE 6:
FIGURE 5: ‘Cobbing’ of all the blades from the impeller body. FIGURE 6: Scalloped fracture form at the forward edge of several impeller blades.
FIGURE 7:  
FIGURE 7: Dusty brown deposit on inside of impeller body.

1.3.2. Fan shroud

Produced as a machined die-casting from an aluminium alloy, the fan shroud encased the full impeller length and showed no evidence of having failed to fully contain the fractured impeller blades. Around the blade tip path, the shroud internal surfaces showed several circumferential wear bands that indicated significant tip interference (figure 8). These were most clearly defined at the forward and rear limits of the blade path. Random indentation and scratching damage was noted around the shroud ‘throat’ region – this was consistent with the effect of multiple fragment impacts produced by the failing impeller blades. No specific evidence of hard-object impact damage was found across the blade path. Dark, waxy stains produced by an unidentified liquid draining through the fan shroud were noted at the low-point of the assembly (figure 9) and a series of tide-marks were formed on the front face of the shroud as the liquid had accumulated and later drained away.

 

FIGURE 8: FIGURE 9:
FIGURE 8: Fan shroud showing prominent evidence of rotational contact. FIGURE 9: Fan shroud with stains from liquid draining through the assembly.

1.3.3. Fan motor and housing

Being a brushless three-phase induction motor, the unit was comparatively simple in construction, with a star-wound stator containing a compact cage-type armature. Internal inspection found evidence of rub and erosion of the iron stator former over the innermost twenty millimetres of the stator length (figure 10), however the armature did not reflect this and showed no evidence of operational damage (figure 11). Several areas where material had been removed by bevelling the core corners were evidence that the armature had been re-balanced at some time following original manufacture (figure 12). Both armature shaft bearings rotated smoothly by hand and showed no notable indications of distress or abnormal operation. The rear armature bearing housing (figure 13) contained particulate debris that appeared to be the fine brown dust mixed with lubricant lost from the bearing unit. The contact points of the leaf spring and the housing bottom showed light fretting damage (figure 14), with some evidence of rotation also noted.

 

FIGURE 10: FIGURE 11:
FIGURE 10: Motor stator inside surface showing clear evidence of rotational contact against the armature. FIGURE 11: Motor armature showing no evidence of contact with the stator.
FIGURE 12: FIGURE 13:
FIGURE 12: Motor armature with material removed for balancing purposes. FIGURE 13: Motor end bearing housing with contaminants found.
FIGURE 14: FIGURE 15:
FIGURE 14: Area of fretting damage found between the seating spring and housing base. FIGURE 15: Electrical arcing damage and associated metal loss.

The fan motor electrical supply was routed behind the outlet guide vanes and terminated in an external housing fitted with an eight-pole connector. A localised area of heavy electrical arcing was clearly evident adjacent to the connector and had resulted in the melting and loss of around ten to fifteen millimetres of material in a v-shaped notch from both the front and side faces of the connector housing (figure 15). The metal loss had exposed some of the internal wiring and had produced an appreciable level of heating around the contact area, as evident from the tinting of the surrounding surfaces.

1.4. Dust analysis

The sample of brown dust recovered from the underside of the impeller body was qualitatively analysed using energy-dispersive x-ray spectroscopy techniques under the scanning electron microscope. The results of this testwork (figure 16) showed the material to be comprised primarily of an iron-oxide compound, with traces of chromium, aluminium and silicon.

 

FIGURE 16:
FIGURE 16: EDS spectra for the brown dust found beneath the impeller. Strong Fe and O peaks.

2. ANALYSIS

2.1. Impeller failure

All of the failed impeller blades exhibited brittle overload fractures. No evidence of material flaws, cracks or other pre-existing damage was shown by the blade stubs, however this was not conclusive, given the opportunity was not available to examine all of the blade remnants. Resin materials such as that used to manufacture the impeller have an inherent lack of ductility and as such, are prone to cracking and fracture under impact or elevated stress conditions. While no direct evidence was found to suggest the ingestion of foreign object/s, the damage sustained was not unlike that expected from such an event, and thus this possibility cannot be discounted.

2.2. Motor damage

The abrasion exhibited by the inner sections of the stator assembly was believed to be evidence of armature contact, presumably caused by the failure and collapse of the end bearing unit. The dust under the fan impeller was believed to be an accumulation of wear products from this contact – both the stator former and the armature core were ferrous alloys, while the remainder of the motor and fan assembly was predominantly aluminium based materials. Given that the installed armature showed no indication of stator contact and the end bearing was sound, it was concluded that the stator damage was a product of a previous failure that had been repaired by replacement of the armature and the re-use of the remaining components, including the impeller.

The collapse and failure of an armature shaft bearing and the shaft misalignment that results would be expected to alter the impeller – shroud clearances, with a risk of contact between the shroud and the impeller blade tips if the misalignment became severe enough. Impeller blade tip contact, if it did not produce immediate blade breakage, may produce latent cracking damage that could lead to later blade failures if not detected. While there was no evidence found to suggest that pre-existing damage of this nature existed, the possibility remained that this damage had been sustained given the evidence of the rear bearing failure and the contact marks inside the fan shroud.

2.3. Vibration

Rotary equipment such as the cooling fan relies upon accurate dynamic balancing to minimise the vibration induced during operation. Events such as the impeller failure will disrupt the balance of the assembly and can lead to significantly increased vibration levels. Wiring or piping that is installed against or in contact with the vibrating equipment may sustain fretting or erosion damage if the external protection or insulation has not safeguarded against this event.

3. CONCLUSIONS

3.1. Findings

  1. The cooling fan impeller had sustained gross breakage of all blades at or adjacent to the body of the impeller.
  2. All fractures were brittle in nature and showed no evidence of pre-existing defects.
  3. No evidence of foreign object damage was found.
  4. The fracture profiles of some blades suggested preferential tip breakage before the complete blade failure.
  5. The fan motor showed evidence of a previous rear bearing failure that had produced contact and wear between the armature and the stator. The armature had subsequently been replaced, however the stator and other motor components remained in service.
  6. The previous bearing failure may have allowed operating contact between the impeller blade tips and the fan shroud. This contact may have produced blade cracking which predisposed the impeller to failure in the manner observed.
  7. After failure, the fan impeller would have presented a significant unbalanced load to the fan.

Related Documents: |Investigation Report 200203671|

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