On 17th July in 1996, a Boeing flight 747 operated as Flight 800 by TWA (Trans World Airlines). Near East Moriches, New York, US, it struck the ocean of Atlantic. All the passengers were killed in that accident. The plane was flying to Paris from New York at the height of around thirteen thousand feet when the explosion taken place after the breakage in structure of airplane. It was claimed that the terrorist attack was behind the attack on the following morning of the accident. However, the claim was put down in the final report by the US NTSB (National Transporting Safety Board) in the final report of incident that the possible cause of the Flight-800 was the setting of centre-wing fuel tank on fire resulting from the ignition in the fuel tank.
It was found in the initial investigations that contributing factors to the incident were the design and the idea that says explosion can only be avoided only by preventing the all sources of ignition and the design of the Boeing 747 with locating heat sources under CWT (Centre Wing Tank) without reducing the amount of heat transferring to the CWT or reducing the fuel vapour in the tank fireproof. The airplane took-off from Athens Greece to JFK International Airport. A crew change occurred at that airport according to the schedule. The airplane was refuelled with auxiliary power unit and two out of three AC packs were running for 150 minutes before its departure (Sanders, 2013). The flight squad altitude assignment from the ATC and a cockpit voice recorder started to record. The flight squad was told to fly towards Boston Air Route Traffic Control Centre Controller directly at 20:23 local time. The crew of flight was asked to maintain the height of 13 thousand feet at 20.26 due to traffic. However, the controller was told by the PNF that they could not detect the traffic in sight. The voice of captain was heard at 20:29 asking to have a look at indicator of fuel flow of fourth engine followed by the words “I better trim this thing, somewhere in here”. It was the moment where everything happened in the wink of an eye. The crew was asked to maintain 15,000 feet to set climb thrust and within thirty seconds a sound of mechanical movement was heard, a nontangible word at 20:31:03 and a noise of damage at 20:31:05 (Zinner, 2013). The CVR stopped recording at 20:31:12 along with the end to FDR recording and ATC. It was suggested by the initial report that witnesses and widespread distribution of the pieces show that structural breakage had taken place. It was considered by the investigators on the basis of initial information that there was no sign of any pre-existing condition that might have triggered the structural decomposition and failure (Vidoli and Mundorff, 2012). It was shown by the investigation that structure of the airplane did not have any prior damage that could have caused the breakage of the airplane. It was also revealed by the reports that no sign of missile or bomb was found because only 5% of the fuselage of airplane was not recovered and none of the missing pieces were big enough to show the signs of missile or bomb attack. Based on the similar explosion of Philippine Airline Flight 143 and Avianca Flight 203 concluded that the explosion of CWT would have led to the destruction of TWA 800 (Vidoli and Mundorff, 2012). The CWT was full at the time of departure from Athens and the arrival in New York, it was found that CWT contained 300 pounds of fuel. The tank was not refilled before the departure from New York. As shown by the reports, the fuel vapour at the time of accident in the unfilled space of tank was flammable. The temperature of the fuel vapour ranged between 38 C to 53 C. “Fuel vapours of a jet under the circumstances of such pressure, altitude and fuel mass loading at temperature of 35 are flammable of TWA Flight 800” (Kuhns, 2011). It was then found according to the damages incurred to the plane that the flammable vapours possibly would have generated such level of pressure that the explosion occurred. Investigations by FBI and NTSB revealed that a streak of light was perceived following the appearance of fireball which was witnessed by observers (Smail, et al. 2014). The streak of light could be burning fuel that was releasing from the fuel tank of airplane and the fireball was perceived was the burning part plunging to the ocean. The nose of airplane was detached from the plane following the explosion of CWT tank within three seconds (Vidoli and Mundorff, 2012).
The reports said that the FQIS (fuel quantity indicating system) could have played major role in the igniting the flammable vapours in CWT. It could be caused by two factors, supply of higher voltages to FQIS wiring outside the fuel tank and the elimination of energy from the wiring of FQIS into the inner part of tank that it ignites the fuel. It was showed by the studies that the energy released from the wiring of FQIS inside the fuel tank of Boeing was about 0.12 mJ which was less way less than the sustainability of Boeing (Sanders, 2013). A study revealed that even if all the RF transmitters were emitting the highest level of electromagnetic signal levels, the maximum energy would have been less than 0.1 mJ and the energy coupled to wiring of Jet would also be way less than that level (Kuhns, 2011). Therefore, it was said in the reports that neither the energy was released nor the voltage was supplied over the sustainability of Boeing-747. However, the CVR recording reveals the background noise of electric sparks right before the explosion. That indicated that a short circuit was impacting the energy in the electrical system. Additionally, the comments of captain to have a look at have a look at the unusual behaviour of number four engine led the investigations more focused on the routes of wire system for the fuel flow indicators (Zinner, 2013). It was theorised by the NTSB that the concern of captain regarding the indicator of fuel flow might in fact has been the electrical sparks from the wire of fuel flow meter to the FQIS. The same routes of wire were found to other possible sources of energy such as cabin lights under the cockpit as shown in figure 1. The cabin lights for instance of the Boeing-747, had required maintenance multiple times within a month before the incident took place. Any of these possible sources could play major role in the excessive energy to the FQIS system of CWT (Zinner, 2013).
It was determined by the NTSB that the cause of TWA flight 800 misfortune was proved to be an ignition in the fuel tank resulting from the flammable mixture of fuel and air. The sources of explosion could not be examined although the sources determined by the investigation direct to the short circuit outside the CWT was most likely the source of ignition by allowing the excessive voltage through the electrical wires associated with the indicators of fuel tank (Zinner, 2013).
Figure 1. Wiring Configuration of Boeing-747
The FAA issued SFAR (Special Federal Regulation of Aviation) that requires the airplane to be re-investigated to prevent the ignition. Newer standards have been reviewed by these regulations and information of fuel tank examination is gained rather than the prior standards that were existing at the time of certification of airplane was taking place. The Special Federal Aviation Regulation also requires safety measure to be taken regularly such as cleansing of silver sulphide depositing from the GQIS wires (Zinner, 2013). It was also required for the aircraft wiring to undergo regular maintenance. It is also recommended by the FAA that the maintenance personnel should be provided improved training procedures. Since some of the risks found by NTSB investigators involved in the similar Jets such as metal shaving in the bays of wire was viewed as common but the possible threats keep arising even after the review of SFAR. The FAA carries on monitoring of designs of fuel tank which keep resulting in further directives of airworthiness (Sanders, 2013).
During the investigation period, NTSB made it clearer that sole dependence on the prevention of ignition was an insufficient means of escaping from a CWT explosion (Hassen, et al. 2013). The CWT is required to be incombustible itself somehow as an additional protection layer. This has been accomplished by the military in the combat jets by inerting process that replaces oxygen with nitrogen in the tanks of fuel, reducing the proportion of oxygen in the fuel tank from 21% to less than 10%. These systems are considered not much important in weight and price by the industries of commercial aircrafts. Though, the commercial airplanes did not require the inerting processes normally used by the military, therefore a relatively lightweight and basic flammability reduction system was developed by FAA from advanced inerting technologies (Lipschultz, 2018). It was also issued by the FAA in 2008 that retrofitting of airplanes should be done within the period of 10 years in order to reduce the flammability of the fuel tank in the airplanes like Boeing. Several methods could have been utilised to reduce the probability of ignition that includes use of materials that prevent ignition such as poly-urethane foam fill and systems that replaces oxygen with inert nitrogen (Smail, et al. 2014). The fault could have been prevented following the risk assessment method according to the recommendations that is provided below in the v-diagram.
Figure 2. Assessment to Possible Prevention of Hazards
As a result of final investigation report of TWA flight 800, the recommendation provided by the National Safety Boards to the Federal Aviation Administration includes examination of the design, review of the design specifications, and implementation of developed actions to reduce the risks of ignition. It was suggested to the manufacturers of commercial airlines to investigate design practices with regard to the bonding of instruments inside fuel tanks and need to change the practices in order to mitigate the risks of ignition (Smail, et al. 2014). The design specification of the airplane should be reviewed for the wiring system of all airplanes certified in US. It includes identification of the systems that are life-threatening and revisions to make sure the separation of those systems and wiring. Whether the adequate development is being incurred in the respective areas or not, the FAA is required to provide briefing to the Safety Boards on the efforts being taken to address the issues determined in the Aging Transport Non-Structural System Plan (Lipschultz, 2018). The recommendations of Aging Transport System Rulemaking Advisory Committee addresses the issues determined through the rulemaking regardless of the scope (Nyhan et al. 2016). It includes the development of the training and maintenance in order to make sure the identification of possible safety hazards of wiring systems and the requirement of better documentation and report making processes of faulty wiring systems. Moreover, the requirement of the incorporation of the innovative technologies such as equipment of automated wire test and arc-fault circuit breakers (Hassen, et al. 2013).
By having a look at the recommendation provided after the incident, it is somehow clearer that in which way further incidents like the TWA-747 airplane crash can be avoided. The possible issues that would have occurred in the flight are addressed well according to the determined problems (Nyhan et al. 2016). Only a problem that is missing in the final reports is the clarity of decision of problem that caused the failure. It is because tests taken on the airplane fuels tanks do not support the decision. The maximum possible energy eliminated from the highest level of voltage through the wiring system was still lower than the least level of energy that could ignite or raise the temperature up to the ignition point of the air/fuel mixture (Lipschultz, 2018). However, it was concluded on the basis of voice recorded that directs to the problems raised in the indicator of fuel tank of the engine number four that could have resulted from the short circuit.
It was concluded by NTSB that even though the voltage in the indicators of FQIS is reduced to minimal level, a short circuit could have made the damage. It cannot be occurred certainly, but this proved to be most likely that it would have happened as suggested by the recordings among all of the considered scenarios. The FAA was urged by the NTSB to review the specifications of design and to review the wiring systems along with the improved training of maintenance. The NTSB repeated the previous recommendation as well which urges the use of inerting system in order to flammability.
References
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