Boeing 747SR-46 which was operated by JAL, is an aircraft specifically configured for domestic flights with a high density seating arrangements. The Boeing authorities set safety as highest priority on this large passenger airplane, by designing it, using methodology called fault tree analysis. To address the safety and flyability needs 747 was designed by integrating the functions like structural redundancy, quadruple main landing gear, redundant hydraulic system and split control surfaces. The below figure shows the graphical design of Boeing 747 model
Figure 1: Graphical design of Boeing 747 model
Source:
With regards to JAL 123 crash, description of working system comes in two parts.
2.1) MAINTENANCE
The inspection and maintenance of aircraft in JAL fleet is done in line with hours flown, flight frequency and age of aircraft. JAL is organising periodical maintenance programs to maintain aircraft component in good working condition. The company carries out necessary repair or servicing operation in the event of malfunction or breakage.
JAL maintenance procedure is divided into two parts
Pre-flight maintenance: It is conducted between landing and takeoff and implies receiving report of problems, from the crew cabin of an arriving aircraft and then processing those reports appropriately. In addition, it also involves running predetermined check operation to make sure that there are no damages like scratches, leakage of hydraulic oil or aircraft fuel.
Regular maintenance: It differs from aircraft to aircraft and is generally carried out following the predefined flying hours of an aircraft. It is conducted under a regulation which closely governs which part is to be maintained and protocol used for required task.
According to the protocol set by Boeing, an air craft when damaged need to go through following stages before getting ready for flights. They are as follows
Repairing: It's a stage where skilled technicians are working to repair the damage parts as well as parts that are dependent on the damaged parts in order to make it work efficiently.
Inspection: It's an official examination that is conducted in order to check whether repairs made to an aircraft is done correctly or not.
Periodical Inspection: It's a recurring stage through which an aircraft had to go throughout its life cycle.
2.2) TECHNICAL
The system description of JAL 123 and its working environment can be determined by analysing the figure as shown below
Figure 2: Structure near the Empennage
Source: KOBAYASHI, HIDEO (Yokohama National University), TERADA, HIROYUKI (Japan Aerospace Technology)
The above figure describes the structure near the Empennage. It's a rear part of the plane which provides stability to an aircraft in a flight. All the parts that comprise the Empennage play an important role in keeping the flight in control. Their importance is described below
Central hydraulic unit: It maintains the flow of hydraulic fluid in hydraulic pipe. The main purpose of hydraulic fluid is to provide power to aircraft control. It's like an elevator that helps the plane to go up and down.
Vertical fin: They are use to reduce the aerodynamic side slip
Rudder: It helps the aircraft to take turn in any direction
Aft Pressure Bulkhead: It is a vital part of the plane whose purpose is to seal the rear part of the plane in order to maintain the cabin pressure
Auxiliary power unit: These power units provide electrical power in the operational absence of engine-driven generators or unavailability of external power
Hydraulic pipe: It's a medium, which allows the hydraulic fluid to flow around the aircraft.
DESCRIPTION OF CATASTROPHIC FAILURE
The direct cause of air crash of JAL 123 was the loss of central hydraulic controlling system, APU (Auxiliary Power Unit), and box beam structure of vertical fin as they were blown off by the huge mass of air moving a hypersonic velocity that was induced by the breakage of Aft Pressure Bulkhead.
In the past, aircraft had gone through large scale repair in June 1978, after it experienced tail strike landing at Osaka International Airport. Fatigue failure of Aft Pressure of bulkhead was due to an error made during the repairs at that time. During that repairing stage, Riveted joints were incorrectly repaired by Engineering Staff from Boeing as it was not conducted according to the protocol of Boeing Procedure. Figure of difference between directed repair that was done and the actual repair that needed to be done is as follows:
Figure 3: Aspect of Aft Pressure Bulkhead
Source: KOBAYASHI, HIDEO (Yokohama National University), TERADA, HIROYUKI (Japan Aerospace Technology)
As shown in the figure, the repair was made using two separate splice plate instead of single plate. As a result, the entire burden was carried through the centre rivet row only, and multiple fatigue cracks propagated by cyclic pressurization of the aircraft and finally caused the unstable fracture of the entire structure after a total of 12, 319 cycles. The mistake made during the repair was extended across 2 bays, which is about 1 meter length and as this was longer than the critical size essential for the unstable failure of the structure to survive, the crack propagated by the interlinking of adjacent small cracks without being arrested.
Figure 4: Part of aft pressure bulkhead seen from Tail side
Source: KOBAYASHI, HIDEO (Yokohama National University), TERADA, HIROYUKI (Japan Aerospace Technology)
Some of the most important points that were noticed are as follows:
Due to mistake in repair the entire membrane stress caused by internal pressure was carried through the centre row in a fast pace. By this it can be judged that Maintenance Engineer didn't understood the fundamental mechanism of load transmission
The inspection of repair made by the airline representative and air transportation authorities was in insufficient. That is even though there was improper repair conducted; the process of inspection should have been able to identify the problem
The issue of periodical maintenance and inspection conducted throughout the course of 12,319 flights was problematic, since the inspection was not conducted before accident
3.1 FAULT TREE
Figure 5: Fault Tree Analysis of JAL123 crash
Air crash
Loss of rudder
Loss of central hydraulic system
Loss of hydraulic pipe
Loss of Vertical fin
Aft Pressure Bulkhead burst
Loss of Auxiliary power unit
Unstable fracture of bulkhead
Inspection not done correctly
Miss repair
No periodical Inspection
Insufficient Knowledge
Lack of confirmation
Ignorance of procedure
Insufficient Communication
Carelessness of worker
Ignorance
Misperception
3.2 EVENT TREE
Figure 6: Event Tree Analysis of Accident caused by miss-repair of AFT Pressure
Source: KOBAYASHI, HIDEO (Yokohama National University), TERADA, HIROYUKI (Japan Aerospace Technology)
3.3 CAUSE AND CONSEQUENCE DIAGRAM
Figure 7: Cause and Consequence Tree of JAL 123 air crash
Irregular response of aircraft
Periodical Inspection
Error Inspection
Periodical Inspection
Yes
Other
Inspection not conducted
Inspection done correctly
False design Repair
Other
Source: KOBAYASHI, HIDEO (Yokohama National University), TERADA, HIROYUKI (Japan Aerospace Technology)
FAILURES AND THEIR CAUSES
Table 1: Failures and their causes
Number
Failure
Causes
1
Mistake in Repair
Unstable fracture of aft pressure bulkhead
2
Inspection not done properly
Major problem in the system was not identified
3
Periodical Maintenance and inspection of system was not conducted
Expiry date of the repair made in past was not known
4
Fatigue failure of aft pressure bulkhead
Burst in Aft pressure bulkhead due to which central hydraulic system and APU (Auxiliary Power Unit) which are located at the back of bulkhead and box stream structure of the Vertical fin were blown off by huge mass of air.
5
Loss of Hydraulic pressure and Vertical fin
Aircraft went out of control and finally crashed near the ridge of Mt Osutaka.
SIMILAR CATASTROPHIC FAILURE
In 1989 a similar type of accident occurred in USA. A DC 10 model with Flight number 232 operated by United Airlines lost the hydraulic pressure due to failure of engine's fan disk. However, in contrast to JAL 123 case where the cockpit crew didn't recognise the severity of situation and insisted on returning to original airport, the cockpit crew of DC 10 recognised that hydraulic unit was damaged. Therefore they decided to land to the nearest airport as soon as possible and finally end up with crash landing. Because of this 60% of people on board survived. If the cockpit crew of JAL 123 would have assessed the situation similar to what cockpit crew in DC10 did, then it would have been a different story overall.
In 2002, another similar mishap happened with air Flight CI611 due to tail strike and miss repair. In case of JAL 123, it had badly repaired aft pressure bulkhead which burst and resulted in blowing off the vertical fin. While CI611 had fuselage damage due to tail strike and was repaired with a faulty procedure which resulted in failure of lower rear side of the fuselage, and subsequently separated the whole tail of the plane.
It is also analysed that multiple site cracks were responsible for JAL crash due to the use of old rivet holes at the repair without considering resizing the holes. So it showed the potential danger of multiple site cracks. Three years later an accident caused by multiple site damage happened on 4th April 1988 in Hawaii, This time, major part of fuselage structure of B-737 operated by Aloha Airlines was blown off while cruising. These two accident caused by MSD (Multiple site damage) and MED ( Multiple element damage) showed that damage tolerant design concept based on conduct of single crack is deficient for such cases.
RECOMMENDATION
Some of the activities that need to be followed to avoid the accident are as follows
Periodical Inspection must be done, especially after a large-scale repair is conducted or design change has been carried out within the system.
Engineers working at repair site must be provided with fundamental training and education in areas like mechanics of materials.
The people responsible for approval of repair should check the site carefully to ensure that those repairs were conducted as desired.
The steps taken in order to avoid future accidents are as follows
The Aircraft accident investigation commissioner advised the manufacturer and operator to further strengthen the structure of torsion box by installing a plate to stop the high speed air flow from entering the box beam in the consequence of bursting of aft pressure bulkhead. The advice was put into practice
REFERENCES
KOBAYASHI, HIDEO (Yokohama National University), TERADA, HIROYUKI (Japan Aerospace Technology), Crash of Japan Airlines B-747 at Mt Osutaka [Online] Available http://shippai.jst.go.jp/en/Detail?fn=2HYPERLINK "http://shippai.jst.go.jp/en/Detail?fn=2&id=CB1071008"&HYPERLINK "http://shippai.jst.go.jp/en/Detail?fn=2&id=CB1071008"id=CB1071008 (22/03/10, 18:44)
