Depressurisation, 475 km north-west of Manila, Philippines, 25 July 2008, Boeing Company 747-438, VH-OJK
Today the Australian Transport Safety Bureau is releasing its Preliminary Factual report on the depressurisation of Boeing 747-438, VH-OJK, 475 km northwest of the Philippines, while being operated on a scheduled passenger flight from Hong Kong to Melbourne on 25 July 2008.
It is important to note that the information contained in the preliminary factual report, as the name suggests, is limited to preliminary factual information that has been establish in the initial investigation of the accident. Caution should be exercised that there is the possibility that new evidence may become available that alters the circumstances as depicted in the report.
To date, the Australian Transport Safety Bureau investigation, assisted by a number other organisations and agencies, including the Civil Aviation Authority of the Philippines, the National Transportation Safety Board and the Federal Aviation Administration of the USA, the Civil Aviation Safety Authority of Australia, Qantas and Boeing, has determined that about 55 minutes after departure from Hong Kong, and after the aircraft was established at its cruising altitude of 29,000 ft, the captain and first officer reported hearing a 'loud bang or cracking sound' with an associated airframe jolt. At that time, the autopilot disconnected, and the first officer, who was the pilot flying at the time, assumed manual control of the aircraft. Multiple warning messages were displayed in the cockpit, including warnings regarding the R2 door status and cabin altitude - that is the altitude corresponding to the air pressure inside the aircraft cabin. After donning oxygen masks the crew completed the 'cabin altitude non-normal' checklist. Approximately 20 seconds after the event, the captain reduced the thrust on all four engines and extended the speed brakes. The first officer commenced the descent, while the captain declared a MAYDAY on the Manila flight information region (FIR) radio frequency.
About 5 and a half minutes later, the aircraft reached and was levelled at an altitude of 10,000 ft, where the use of supplementary oxygen by passengers and crew was no longer required. After reviewing the aircraft's position, the flight crew elected to divert and land at the Ninoy Aquino International Airport, Manila. Landing preparations were subsequently commenced, including the jettisoning of excess fuel to ensure the aircraft's landing weight was within safe limits. The flight crew reported that many system failure messages were displayed, including all three instrument landing systems (ILS), the left VHF omni-directional radio-range (VOR) navigation instrument, the left flight management computer (FMC) and the aircrafts anti-skid braking system.
The cabin crew reported that shortly after the bang was heard, oxygen masks fell from most of the personal service units in the ceiling above passenger seats and in the toilets. Most passengers started using the oxygen masks soon after they dropped. All the cabin crew, who were engaged in passenger service activities at the time, immediately located oxygen masks to use. Some crew located a spare passenger mask and sat in between passengers, while others went to a crew jump-seat at an exit, and one used a mask in a toilet.
The flight crew reported that at all times during the ensuing descent into Manila, they were able to maintain the aircraft in visual flight conditions. With radar vectoring assistance from Manila air traffic control, the captain, who had assumed the pilot flying role, conducted an uneventful approach and landing on runway 06, with a smooth touchdown, full reverse thrust and minimal braking. Emergency services were in attendance after the aircraft was stopped on the runway, after which intercom contact was made with a ground engineer and the aircraft verified as being safe to tow to the airport terminal and disembark the passengers via a terminal airbridge.
None of the passengers or crew aboard the aircraft reported any physical injuries to the cabin crew immediately following the depressurisation event, or to the operators staff upon arrival in Manila.
An initial inspection of the external aircraft surfaces on the ground in Manila revealed the complete loss of the right wing forward leading edge-to-fuselage fairing, with separation occurring along the lines of interconnection between the fairing and fuselage skins. In the area exposed by the fairing loss, was an inverted T-shaped rupture in the fuselage skin, with several items from within the forward cargo hold partially protruding from the rupture.
Following removal of all cargo materials and lowering of the hold right-side curtain panels, it was found that the fuselage rupture was aligned with the nominal position of the number-4 passenger emergency oxygen cylinder; (slide2) one of seven such cylinders in a bank along the right side of the hold. The number-4 cylinder was missing from the bank.
On the basis of the physical damage to the aircraft's forward cargo hold and cabin, it was evident that the number-4 passenger oxygen cylinder sustained a failure that allowed a sudden and complete release of the pressurised contents. The rupture and damage to the aircraft's fuselage was consistent with being produced by the energy associated with that release of pressure. Furthermore, it was evident that as a result of the cylinder failure, the vessel was propelled upward, through the cabin floor and into the cabin space. Damage and impact witness marks found on the structure and fittings around the R2 cabin door showed the trajectory of the cylinder after the failure.
The following graphics illustrate the likely trajectory of the cylinder, based on the observed damage:
(slide 3) This graphic represents a cross-sectional view through the aircraft at the position of the R2 main cabin door - you will note here the oxygen bottle in situ in the cargo hold, the main deck floor, the R2 door and handle, the bustle that contains the escape slide and the overhead storage bins.
(slide 4) It is apparent that the cylinder failed towards its lower end, allowing a sudden and complete release of the pressurised contents that had sufficient energy to rupture the fuselage. It is likely that the lower part of the cylinder exited the aircraft through the rupture at this time, while the bulk of the bottle, complete with valve was propelled upward through the cabin floor. You can see the hole punched through the main cabin floor by the cylinder here, as viewed from within the cargo hold.
(slide 5) The cylinder then impacted the R2 door frame and the internal door handle - you can see in these photographs green paint transfer from the cylinder on the partition, vertically oriented scoring of the door escape slide shroud (bustle), impact damage on the door frame and the internal door handle rotated approximately 80% towards the open position. However, the door handle shaft had failed, as it is designed to do if an attempt is made to open the door in flight, so the position of the door handle is not representative of the position of the door lock mechanism or the security of the door. The investigation has confirmed that the door latches were still engaged. The portable walk-around oxygen cylinder that was normally located in an alcove just inside the R2 door was not present, and was not accounted for in a subsequent search of the aircraft.
(slide 6) It is apparent that the impact with the door frame broke off the cylinder valve and caused the cylinder to invert, while continuing to travel upward. Various items of debris were found around the aircraft cabin in the vicinity of the R2 door. Of note, that included fragments of the number-4 oxygen cylinder valve handle, the valve pressure relief assembly and the valve body itself. A fragment of the valve body was also recovered from within the damaged area on the door frame. However, a thorough search of the cabin and overhead ceiling void space failed to locate any part of the number-4 oxygen cylinder itself.
(slide 7) The cylinder then impacted the overhead panelling end on, producing the circular cut-out type damage observed in these photographs. The diameter of the cut-out region closely matched that of the passenger oxygen cylinder, as can bee seen in the lower photograph.
(slide 8) The still rotating cylinder then produced crushing damage adjacent to the cut-out opening. The semi-circular area of crushing damage to the partitioning panel was of a similar diameter to the cut-out section. A light fitting, normally present in the overhead panels had sustained upward crushing damage and presented clear green paint smears of a similar colouration to the marks on the partition panel and door bustle.
(slide 9) The cylinder then fell to the cabin floor, back through the hole into the cargo hold and exited the aircraft through the ruptured fuselage.
The investigation to date has also identified other damage to the aircraft, including severing and damage to numerous electrical cables and cable bundles, routed through the lower aircraft fuselage near the point of rupture. In addition, both right side (first officer's) aileron control cables, routed along the right side of the fuselage above the passenger oxygen cylinders, were fractured during the rupture event. However, the aircraft control systems have a redundancy arrangement whereby the first officer's aileron control cables are duplicated by the captain's system, the cables from which were routed along the opposite (left) side of the forward cargo hold. Interlinks between the aileron systems provided the necessary redundancy in this instance, ensuring the continued safety of flight after the event.
(slide 10) In terms of cabin safety issues, Investigators conducted a comprehensive walk-through examination of the aircraft's cabin and a survey of the safety systems; in particular, the status of the passenger oxygen masks and equipment. Preliminary observations included that:
- there were 353 passenger seats in the aircraft
- 476 passenger oxygen masks had deployed from their overhead compartments
- 426 passenger oxygen masks were pulled down (i.e. activated for use) - you will note in this photograph arrowed a number of those that dropped, but were not activated, and
- forward crew rest and customer support manager station masks had not deployed.
Cabin safety investigation aspects are ongoing and will examine the serviceability and functionality of the cabin oxygen apparatus and other cabin safety equipment, cabin crew actions, and passenger actions and problems. The investigation has interviewed all 16 of the cabin crew about their experiences, and a review of cabin crew procedures will be conducted.
The investigation is also conducting a survey of all passengers on the flight. The results of this survey will help the investigation determine what occurred and enable the investigation to document passenger and crew actions, equipment issues, and whether there were any resulting injuries. The effects of the damage sustained by the oxygen system on its capacity to function adequately and for a sufficient period will also be investigated. The survey will also help determine if any improvements in equipment design or crew procedures are needed to enhance safety. The survey has been emailed or posted to passengers where the ATSB could locate contact details. Passengers who have not received a survey but who would like to receive one are requested to provide an email or postal address to the ATSB, and this can be done through 1800 020 616 or to email .
The ongoing engineering investigation into the apparent oxygen cylinder failure will focus on (but not be limited to):
- cylinder design, manufacturing methods and type testing procedures
- manufacturing quality control processes and results
- modes and mechanisms of cylinder failure
- historical oxygen and pressurised cylinder failure experiences, civil and military, aviation and industrial.
- cylinder degradation mechanisms
- the adequacy and efficacy of inspection, maintenance and repair processes, procedures and equipment prescribed by the manufacturer and implemented by maintenance organisations, and
- cylinder filling processes and procedures.
As the failed cylinder was not recovered, the ATSB is currently working with the aircraft manufacturer, other aircraft operators and the oxygen cylinder manufacturer, to obtain samples of cylinders from the same manufacturing batch as the failed item, to facilitate the ongoing investigation of all relevant issues.
Examination of cockpit voice recorder, flight data recorder and quick access recorder information is ongoing and will include the
- Analysis of CVR audio regarding crew actions, aircraft handling and crew/cabin communications during the approach and landing at Manila
- Analysis of FDR data to produce a detailed sequence of events and assist in identifying secondary damage from the oxygen cylinder failure and the effects of that damage to aircraft systems and aircraft handling.
A number of safety actions have already been taken by the operator. On 27 July (2 days following the VH-OJK event), the aircraft operator, in agreement with the Civil Aviation Safety Authority (CASA), commenced a fleet-wide program of detailed visual inspections of its Boeing 747 oxygen system installations. The ATSB was advised that those inspections were completed by 1 August.
The operator has also completed a preliminary internal review of the event, addressing the crew and passenger response, the emergency passenger oxygen system operation, supplementary passenger oxygen requirements, and the functionality of the depressurisation emergency announcement system operation.
The ATSB has also considered, in consultation with the NTSB, US FAA, Boeing and CASA, the need for any immediate safety action. It is important that any corrective or precautionary action undertaken in response to a safety occurrence should be justifiable in terms of established or probable facts. However, in view of the nature of the depressurisation event and the implication of a possible mechanism or condition that could affect the structural integrity and safety of other oxygen cylinders used in the aviation environment, the ATSB has released a number of safety advisory notices on the basis of prudence, until such time that the mechanism/s contributing to the cylinder failure are established and understood.
Those safety advisory notices encourage all organisations performing inspection, testing, maintenance and repair activities on aviation oxygen cylinders, to note the circumstances detailed in the ATSB's preliminary report, with a view to ensuring that all relevant procedures, equipment, techniques and personnel qualifications satisfy the applicable regulatory requirements and established engineering best-practices. The ATSB also encourages other operators of transport category aircraft fitted with pressurised gaseous oxygen systems, to note the circumstances detailed in the preliminary report, with a view to ensuring that all oxygen cylinders, and cylinder installations, are maintained in full accordance with the relevant manufacturers requirements, statutory regulations, and established engineering best practices.
The investigation is ongoing and the ATSB continues to work closely with representatives from the US NTSB and FAA, Boeing, CASA and Qantas. It is always difficult to predict how long an investigation such as this will take. While it is likely to take some months, should any critical safety issues emerge that require urgent attention, the ATSB will immediately bring such issues to the attention of the relevant authorities who are best placed to take prompt action to address those issues.Media contact: 1800 020 616