Sequence of events
On 26 July 2004, as the Boeing 717 (717), registered VH-VQA, was being prepared a scheduled passenger service between Brisbane and Hamilton Island Qld, the cabin crew noticed water spilling from the hand basin in the aircraft's aft left lavatory, and notified the flight crew. The leakage was cleaned up, and the aircraft was dispatched from Brisbane with the aft lavatory locked and placarded to prevent its use during the flight, in accordance with the provisions of the CASA-approved aircraft minimum equipment list.
During the cruise at flight level 320, when the aircraft was about 60 NM south of Mackay, the cabin crew noticed a faint `electrical smell' in the aft area of the passenger cabin. The cabin manager notified the flight crew, and about 1 minute later advised the flight crew that the aft cabin interphone hand piece located on the outside wall of the aft left lavatory was very hot, and that the smell was becoming stronger. Shortly after, the cabin manager notified the flight crew that the aft cabin interphone handset flexible-wiring loom was `melting'.
The flight crew conducted the aircraft quick reference handbook (QRH) `Electrical Smoke/Fumes of Unknown Origin' non-normal checklist. They both donned their oxygen masks, and selected emergency electrical power, as per the checklist. The 717 QRH non-normal checklist procedure dictated that a flight crew should plan to land at the nearest suitable airport. At that stage, the aircraft was about 30 NM south of Mackay. The flight crew broadcast a PAN1 to air traffic control, and advised the controller that they required a clearance for an immediate descent and diversion to Mackay. The flight crew notified the cabin crew of their intentions, and the cabin crew prepared the passenger cabin for the landing at Mackay.
The controller in receipt of the PAN call notified the Mackay tower controller, who activated the aerodrome emergency plan and notified the emergency response services. The aircraft landed safely at Mackay about 9 minutes after the flight crew first notified air traffic control of the problem. After the aircraft landed, the flight crew confirmed with the aerodrome rescue and fire fighting service that there was no smoke at the rear of the aircraft. The flight crew also confirmed with the cabin manager that the electrical smell had abated and that the cabin interphone had cooled down. The aircraft was taxied to the terminal, and a normal disembarkation was conducted through the forward door of the aircraft. Eight of the occupants were treated for smoke/fumes inhalation after they had left the aircraft. There were no other reported injuries.
Following the occurrence, the flight crew reported to the operator that during the landing approach at Mackay, the landing gear `down' indication in the aircraft was different from that indicated in the 717 flight simulator during training sessions for flight on emergency electrical power. The operator conducted an investigation into the matter, and determined that the simulator landing gear `down' indication for flight under those conditions was incorrect. That was confirmed by the aircraft manufacturer.
During the subsequent engineering inspection of the aircraft, the B1-778 `Miscellaneous Cabin & Lavatory Occupied Aft' electrical circuit breaker was found to have popped, and could not be reset. The fault was traced to a short circuit in a connector plug located under the aft left lavatory hand basin. Water from the overflowing hand basin had seeped into the connector plug. The short circuit within the connector plug resulted in several pins within the plug becoming welded together. As a consequence, the aft cabin interphone handset flexible-wiring loom was overheated from a 115 VAC over-current within the loom and resulted in the in-flight `electrical smell' and overheating of the handset. The damaged components were replaced, and the aircraft was returned to service.
1 PAN is a radio broadcast indicating uncertainty or alert. It is a general broadcast to the widest area, but not yet the level of a MAYDAY, which is the international broadcast for urgent assistance.
At 1435 Eastern Standard Time on 10 August 2004, a Boeing Company 717-200 aircraft, registered VH-VQA, was climbing to cruise altitude on a scheduled passenger service from Melbourne, Vic. to Hobart, Tas. with six crew and 52 passengers on board. As the aircraft passed through flight level (FL) 110, the crew heard a loud bang, with a corresponding increase in indicated left engine vibrations. The left engine began to spool down and the turbine gas temperature (TGT) indications began to increase significantly.
The crew initially brought the left engine power lever back to idle. However, the TGT continued to increase, indicating a maximum of 1,149oC, before they shut the engine down and discharged a fire bottle into the cowling area in accordance with the operator's procedures. They then notified Melbourne air traffic control (ATC) of the engine failure and returned to Melbourne.
The operator examined the left engine and found metal fragments in the exhaust area and some metallisation1 of the exhaust duct.
At the time of the failure, the BR700-715 engine, serial number 13148, had completed 10, 321 hours and 8,888 cycles since new, and 6,474 hours and 5,417 cycles since repair.
The operator removed the engine and forwarded it to the engine manufacturer in Germany for detailed investigation, under the supervision of a representative of the German Federal Bureau of Aircraft Accident Investigation (BFU2), on behalf of the Australian Transport Safety Bureau (ATSB).
The manufacturer conducted a visual inspection of the engine's exterior, noting a bulge around most of the circumference of the high-pressure turbine (HPT) casing (Figure 1), in line with the Stage-1 HPT (HPT 1). A borescope examination of the engine interior showed that one HPT 1 blade was almost completely missing, with the remaining HPT 1 blades separated just above the blade platforms (Figure 2). There was also significant damage to the subsequent HPT and low-pressure turbine stages. Examination of the engine's compressor assembly revealed no significant damage. All of the high energy debris from the failure had been fully contained3.
A detailed examination of the engine revealed that the reason for the engine failure was the release of a single HPT 1 blade. The blade failed following the development of low-cycle fatigue4 (LCF) cracking in its internal cooling passages. All other engine damage was considered to be a consequence of the initial HPT 1 blade failure.
Figure 1: Bulged HPT casing
Figure 2: Damage to HPT 1 and HPT 2 rotors
Blade design considerations
The failed HPT 1 blade (Figure 3) was a life improvement package5 (LIP) blade. The blade was a shrouded-tip aerofoil design, with multi-passage internal cooling (Figure 4). There was a vapour aluminised coating on the blade's external aerodynamic surfaces and internal cooling passages.
The manufacturer indicated that there have been four similar failures of LIP HPT blades in the BR700-715 engine type, with another engine failure still under investigation. One failure occurred prior to this event in November 2003. The remainder occurred after this incident.
Figure 3: The failed HPT 1 blade (position 21)
Following those failures, the manufacturer conducted additional computer stress modelling on the LIP blades. That modelling found that there were stress levels in the larger trombone radius feature, within the blade's cooling passages (Figure 4) that were potentially in excess of the manufacturer's original design intent. The manufacturer also found that the thickness of the vapour aluminised coating inside the blade's internal cooling passages was variable and difficult to predict. In certain operational conditions, dependent upon high strains in areas of stress concentration and local temperature, the coating could crack with the possibility of subsequent growth into the coated (parent) material. The area from which the failure occurred was confirmed to be the most susceptible to this behaviour (Figure 5).
Figure 4: Intact HPT 1 blade (left); blade internal cooling passage showing trombone feature (right)
Figure 5: Computer generated stress diagram from the manufacturer indicating the point of potentially excessive stress and crack origin
Flight data recorder information
The ATSB's examination of the aircraft's flight data recorder (FDR) for the occurrence flight found that the left engine had surged as the aircraft passed through 10,240 ft. The engine pressure ratio (EPR) and engine rotational speed indications decreased abruptly, while the turbine gas temperature (TGT) for the engine began to increase. HPT vibration values for the engine increased from a level of 0.5 units before the failure to a maximum of 6.3 units over a three-second period. The manufacturer's high-limit for vibrations was 4.0 units.
The FDR readout indicated that the TGT for the engine continued to increase following the engine failure and remained at an indicated maximum of 1,149oC for 1 minute and 46 seconds before decreasing (Figure 6). It is likely that the maximum TGT reached during the failure was higher than 1,149oC, however the aircraft systems do not record above that temperature.
The FDR report indicated that there were no anomalies observed in the performance of the left engine prior to the failure.
Figure 6: FDR data plot of key engine parameters at the time of the failure
1 Metal pulverised by the
turbine becomes molten and flows rearward attaching to the
subsequent turbine and exhaust assemblies (US Department of the Air
Force (1987). Safety Investigative Techniques (AF Pamphlet
127-1, Volume II. Washington DC: Author).
2 Bundesstelle für Flugunfalluntersuchung (BFU).
3 FAA AC 33-5, paragraph 5.c. definitions state '…Contained means that no fragments are released through the engine structure, but fragments may be ejected out of the engine air inlet or exhaust'.
4 Fatigue that occurs at relatively small numbers of cycles. Brooks, C. (1993). Metalurgical Failure Analysis. USA: McGraw-Hill, Inc.
5 The Life improvement Package 3 (LIP3) was a suite of HP Turbine modifications that included the HPT blade P/N BRH20351. The manufacturer introduced the package by SB-BR700-72-100801.
On 2 September 2004, the aircraft manufacturer reported that it was reviewing the following:
- failure conditions that can affect lavatory hand basin water shutoff mechanisms
- design, panel assembly and installation of 717 aft cabin interphones
- electrical installations associated with 717 aircraft lavatory modules
- lavatory faucet reliability data.
On 5 December 2004, the aircraft operator reported that its 717 flight simulator landing gear `down' indication during flight on emergency electrical power was incorrect. As a consequence, the aircraft manufacturer made software changes to 717 flight simulators to correct that anomaly.
|Date:||26 July 2004||Investigation status:||Completed|
|Time:||1658 hours EST|
|Location:||37 km S Mackay, Aero.|
|Release date:||24 December 2004||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||The Boeing Company|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Minor|
|Departure point||Brisbane, QLD|
|Departure time||0603 hours EST|
|Destination||Hamilton Island, QLD|
|Role||Class of licence||Hours on type||Hours total|