Fuel tank enhancements as a means to decrease risk of fuel tank explosion on commercial passenger aircraft


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Fuel tank enhancements as a means to decrease risk of fuel tank explosion on commercial passenger aircraft

  1. 1. FUEL TANK ENHANCEMENTS AS A MEANS TO DECREASE RISK OF FUEL TANK EXPLOSION ON COMMERCIAL PASSENGER AIRCRAFT by Mersie Amha Melke An Aviation and Aerospace Accident Investigation and Analysis Research Proposal Submitted to the Extended Campus in Partial Fulfillment of the Requirements of Master of Aeronautical Science ASCI 615 Embry-Riddle Aeronautical University Worldwide Online November 2009
  2. 2. ABSTRACT Researcher: Mersie Amha Melke Title: Fuel tank enhancements as a means to decrease risk of fuel tank explosion on commercial passenger aircraft Institution: Embry-Riddle Aeronautical University Degree: Master of Aeronautical Science Year: 2009 On July 21, 2009, the FAA issued a final rule to mandate a performance based set of requirements on all commercial aircraft fuel systems that have more than 30 seats or 7500 pounds (Lbs) payload and produced after January 1, 1992. The rule bases on experience of fuel tanks prone to ignition and/or combustion that subsequently exploded. This research paper shall address the pattern of accidents before the making of the rule and the validity of the rule in addressing an evident safety threat. The paper uses accidents from the cabin safety research technical group (CSRTG) to carry out the analysis. ii
  3. 3. INTRODUCTION Background of the Problem Fire is essentially an oxidation reaction and requires a combustible material, oxidizer, ignition and enough heat or energy to sustain the reaction (Wood & Sweginnis, 2006). The flammable liquid fluids like fuel and hydraulic fluid used on aircraft do not burn as liquids but rather as vapors. Since the combustion takes place in the vapors (gaseous form) above the surface of the liquid or in a mist (suspended liquid droplets) formed by a liquid, the key characteristic of a liquid in analyzing flammability is its tendency to form flammable vapor. Liquids form flammable vapor or mist at their flash point (Wood & Sweginnis, 2006). Consequently the empty portion of the fuel tank above the fuel known as the ullage consist of combustible fuel vapor or mist depending on the contemporary temperature and pressure of the tank. Atmospheric air is 20% oxygen by volume and therefore supports fire. However, this percentage, even though it stays constant with rising altitude, the amount of oxygen available decreases due to the smaller amount of air molecules available. Consequently, flammability calls for a
  4. 4. controlled environment where there is enough oxygen to sustain combustion as is the case with aircraft engines. Ignition source must first raise the temperature of the combustible vapors (materials) in its immediate vicinity to the ignition temperature of the material for a fire to start. Concurrent with this, in order to have a continued combustion, emission of heat or energy to sustain the reaction is necessary either by the ignition source or by the reaction itself. Statement of the Problem On July 21, 2008, the FAA issued a final rule mandating a performance-based set of requirements that set acceptable flammability exposure values in tanks most prone to explosion or require the installation of an ignition mitigation means in an affected fuel tank. This rule is effective on all 14 C.F.R. Part 121 certified Boeing and Airbus aircraft manufactured after January 1, 1992 (“FAA,” 2008). In addition, it had initiated from findings of previous accidents with a probable cause identified as a risky fuel tank installation prone to explosion. The NTSB has been pushing for this rule in its accident reports linked with fuel tank explosions and had recommended the enhancement of fuel tanks to arrest fuel vapor ignition on all aircraft including cargo airplanes 2
  5. 5. (Fiorino, 2005). However, cargo aircraft are not in the scope of the final rule issued by the FAA. This research paper shall address the trend of the accidents that have similar backgrounds as the one given above to understand the reason behind the final rule. In doing so, the paper will identify the common installations of the fuel tank that were prone to explosion. Consequently, the paper intends to answer the question whether the final rule issued by the FAA on fuel tank enhancement is addressing an evident safety threat. REVIEW OF RELEVANT LITERATURE AND RESEARCH Selection Process for the Analyzed Accidents The cabin safety research technical group (CSRTG) keeps a database of survivable accidents. This research paper extracts information from the database that has a compilation of accident reports between 1966 and 2008. The extraction focuses on accidents related to fuel tank explosions to visualize the reason behind FAA’s final rule from the perspective of the extracted data. Primarily the database search involved using the search phrase ‘Fuel tank explosion’. This search came up with three results as shown in Appendix A. Consequently, in order to accomplish a comprehensive search of the database, the author of this paper did a second search with the 3
  6. 6. search phrase ‘explosion’ and a keyword identifier of ‘FIRE – FLAMMABILITY OF FUEL’. The second search came up with 66 results. A review of these results showed that, accident reports with the word ‘explosion’ despite its contextual meaning in relation with fuel tanks had also been included in the results. Therefore, the author of this paper reviewed each accident report summary of the 66 results to collect those, which may have a relation with fuel tank explosions. As a result, six accidents showed relation with their fuel tanks exploding. Consequently, the author chose these six accidents together with the ones listed in appendix A. Appendix B lists the 6 accidents. Subsequently, appendix C includes a summary of the relevant data as determined by the investigation done on the nine accidents identified in appendix A and B. Observations on the Summarized Data Review of appendix C shows that the three accidents indexed in appendix A, all have their initial probable cause linked with fuel tank explosion. This means, uncontrolled ignition of the aircraft’s fuel tank caused these accidents. A common occurrence in these three accidents is the presence of flammable fuel vapor, a heat source in the form of, for instance, hot air-conditioning 4
  7. 7. system and ample oxygen to facilitate the combustion. In addition, the accident in Manila, Philippines involving a B737-400(indexed in appendix B), also involves the fuel tank exploding as an initial cause for the accident. In the Philippines accident, hot engine fuel pumps in an almost empty center wing tank (CWT) were the cause of ignition (“Accident Database,” 2007). In contrast, the accidents indexed in appendix B have their fuel tanks exploding after a previous event or chain of events that occurred during the accident. Here, one has to note that, the explosion had inhibited the survivability of the accidents and had there been means to avoid the explosion installed on the aircraft, less fatalities and injuries would have been recorded. Significant points worth mentioning from the accidents indexed in appendix C is the range of fuel types involved in an explosion. Both JP-4 fuel and Jet-A fuel types had been involved, with the former being more flammable (Wood and Sweginnis, 2006). Consequently, one can assert that both fuel types will cause explosions without extra protection of their respective tanks. One can also deduce another significant observation from the recommendations of the Convair 580 accident. The NTSB had asked as early as the 1970s that the FAA initiate 5
  8. 8. action to incorporate in its airworthiness requirements, a provision for fire safety devices on aircraft fuel systems, which will be effective in the prevention and control of both in-flight and post-crash fuel system fires and explosions (“Accident Database,” 2007). Furthermore, the NTSB recommended that rulemaking action in this matter specifically apply to future passenger-carrying aircraft in the transport category with consideration to an adaptation of all other passenger-carrying aircraft then in service (“Accident Database,” 2007). In summary, whether the explosion of the fuel tank caused the accident or the chain of events preceded the fuel tanks explosion, 3 factors standout as common entities in all the accidents. These are the presence of flammable vapor in the tank, which exploded, an ignition source in the form of sustainable heat energy from the surrounding environment and ample oxygen to continue the combustion process. Therefore, working to exclude one of these three is necessary to discontinue the occurrence of accidents with tank explosions. FAA’s final rule (2008) excludes aircraft used in 14 C.F.R. Part 135 operations. However, despite the issue of size addressed by FAR part 135, FAA’s final rule is effective on all new aircraft affected by 14 C.F.R. Part 6
  9. 9. 25.981, which is a common coverage clause for both large and small aircraft. This emphasizes the FAA’s intention to exclude fuel tank designs prone to explosion from future Part 135 aircraft and underscoring the absence of them in current Part 135 aircraft. However, this assertion is not in agreement with two accidents identified in appendix B, Convair580 and DHC-8, both of which are Part 135 aircraft. Regarding the exclusion of aircraft produced before January 1, 1992, the final rule asserts that such aircraft will be retired before the accomplishment threshold of the final rule (“FAA,” 2008). In fact, the FAA provides provision for such aircraft to change them to cargo operations under the premise that cargo aircraft are less prone to fuel tank explosion due to their lesser usage cycle and absence of heating devices like hot air- conditioning units as compared to commercial passenger aircraft (“FAA,” 2008). Consequently, it would be safe to assert that the FAA accepts inherent risks on cargo aircraft based on the statistical concept of less exposed fuel tanks being equivalent with less prone ones. CONCLUSIONS Considering the array of accidents summarized in appendix C, one can assert that FAA’s final rule on fuel tank enhancements does address an evident problem in 7
  10. 10. contemporary and previous aircraft. However, the amount of time taken to issue this rule shows a gap. This gap is between the safety hazard identifying entities like the NTSB and the entities responsible for ensuring the elimination of these hazards, the FAA. The time of recommendation for the Convair accident in New Haven, U.S.A as compared to the time the final rule came out bares evidence to this assertion. In addition, the pattern of fuel tank related anomalies observed over the period of time covered by this paper shows that a variety of aircraft and fuel type (covered in appendix A, B and C) are prone to the problem. However, the FAA’s final rule provides provisions in order to accommodate cost issues concurrent with enhancing fuel tanks against such threats instead of mandating it as a whole. Therefore, one can say that the FAA’s final rule does address an evident fuel tank problem. At the same time, this rule accommodates provisions that ignore some fuel tank installations it primarily intends to abolish based on statistical improbabilities that it will not occur on them. 8
  11. 11. REFERENCES Accidents Database – Accidents Database: 2007, September 19 (Version 5.1) [Data file]. R.G.W Cherry & Associates Limited on behalf of the International Cabin Safety Research Technical group. Fiorino, F. (2005, July 11). NTSB urges fuel tank action; Investigators press for FAA rule to require retrofit of transport fleet with fuel-inerting systems. Aviation Week and Space Technology, 2, 43. Federal Aviation Administration (2008, July 21). Reduction of Fuel Tank Flammability in Transport Category Airplanes Final Rule, 14 CFR Parts 25, 26, 121 et al. Wood, R.H. & Sweginnis, R.W. (2006). Aircraft accident investigation. Casper, WY: Endeavor books. 9
  12. 12. APPENDIX A Commercial aircraft accidents identified with search phrase - fuel tank explosion. Note. Table prepared by author, data obtained from “CSRTG Accidents Database,” by R.G.W Cherry & Associates Limited, 2007. Item no. Year Place of accident Aircraft 1 May 04, 2006 Bangalore, India B727-200 2 March 03, 2001 Bangkok, Thailand B737-400 3 July 17, 1996 Long Island, U.S.A. B747-131 APPENDIX B
  13. 13. Commercial aircraft accidents identified with search phrase ‘explosion’ and keyword ‘FIRE – FLAMMABILITY OF FUEL’. Note. Table prepared by author, data obtained from “CSRTG Accidents Database,” by R.G.W Cherry & Associates Limited, 2007. Item no. Year Place Aircraft 1 June 07, 1971 New Haven, U.S.A CV580 2 November 20, 1974 Nairobi, Kenya B747 3 May 09, 1976 Madrid, Spain B747 4 April 15, 1988 Seattle, U.S.A DHC-8 5 February 14, 1990 Bangalore, India A320 6 May 11, 1990 Manila, Philippines B737-300 APPENDIX C Summary of the Accidents Indexed in Appendix A and B Trans World Airlines flight 800 was on a scheduled flight on July 17, 1996. The airplane, a Boeing 747, ii
  14. 14. climbing near 13,800 feet, exploded in-flight at its center wing fuel tank approximately 13 minutes after takeoff, resulting in loss of structural integrity of the aircraft in-flight. Prior to dispatch of the airplane, the air- conditioning air cycle machines, located under the center wing tank, had been operating for up to 2 hours. The center wing tank estimated fuel temperatures were 113-115°F. At the altitude and temperatures of the event, the fuel tank air/vapor mixtures were flammable. The fuel type was Jet A. On March 3, 2001, Thai Airways flight 114, a Boeing 737-400, destroyed while parked at gate 62 of Bangkok international Airport, Thailand, is another example. The authorities responsible determined that the probable cause of this accident was an explosion of the center wing tank (CWT) resulting from ignition of the flammable fuel/air mixture in the tank. The source of ignition energy for the explosion could not be determined with certainty, but the most likely source was an explosion originating at the CWT pump because of running the pump in the presence of metal shavings and a fuel/air mixture. On May 4, 2006, Transmile Airlines B-727 airplane, suffered a fuel tank explosion in Bangalore, India. The accident occurred while the airplane was waiting towing for a return flight. The evidence indicates that an explosion iii
  15. 15. in the left wing fuel tank destroyed the structural integrity of the wing. Investigators found evidence of damaged wiring and electrical arcing within the left wing fuel tank in an aluminum conduit tube that carried 115V AC electrical power to the fuel pump. Previous modifications of the accident airplane in accordance with an FAA Airworthiness Directive (AD) to avoid arcing were ineffective. The investigation into this wing fuel tank explosion is ongoing. On 7-Jun-1971, an Allegheny Airlines CV580 registered as N5832 was attempting an instrument approach to Tweed-New Haven Airport, New Haven, Connecticut. In poor forward visibility, the aircraft struck three beach cottages located on the northern shore of Long Island Sound. An intense fire ensued immediately upon initial impact and continued to burn to the point of near total destruction of the upper portion of the fuselage and cabin area of the aircraft. Because of the ensuing investigation, the NTSB recommended that the FAA initiate action to incorporate in its airworthiness requirements, a provision for fuel system fire safety devices, which will be effective in the prevention and control of both in-flight and post-crash fuel system fires and explosions. Furthermore, the NTSB recommended that rulemaking action in this matter iv
  16. 16. specifically apply to future passenger-carrying aircraft in the transport category with consideration to an adaptation of all other passenger-carrying aircraft then in service. On 20-Nov-1974, a Lufthansa B747-130 was taking off from Nairobi Airport, Kenya. The aircraft lost altitude and the rear fuselage made contact with the ground. The impact and a subsequent fire destroyed the aircraft. Once the aircraft was stationary, the fire spread rapidly and the inner left wing exploded. The centre wing fuel tank did not contain fuel; however, it exploded probably due to heated gases resulting from the surrounding fire. This added to the damage and fire intensity in the forward freight bay area. On 09-May-1976, an Imperial Iranian Air Force Boeing 747-131 was flying from Teheran, Iran, to McGuire Air Force Base, New Jersey, with en route stop at Madrid, Spain. During descent for the approach at 6,000 feet, the airplane struck by lightning exploded and separation of the left wing followed causing loss of control. The fuel onboard was a mixture of 58% JP-4 and 42% Jet A type. Evidence found included those of lightning strike, pitting and local burn areas typical of lightning attachment on the left wing tip and on the vertical fin at the VOR antenna. After analyzing v
  17. 17. all of the available evidence, it is concluded that the most probable sequence of events, which culminated with multiple structural failures, and separation of the wing began with an ignition of the fuel vapors in the No. 1 fuel tank. The damage to the structure in the area of the tank provided position indications of an explosion. On 15-Apr-1988, a Horizon Air DHC-8-102 registered as N819PH was making a precautionary landing at Seattle-Tacoma international airport. After lowering the landing gear on final approach, a massive fire broke out in the right engine nacelle. After the first officer shut down the engine, the captain proceeded to land the aircraft; however, shortly after touchdown, the crew realized that almost all directional control and braking capability was lost. The aircraft departed the paved surface of the runway and crashed. One of the probable causes of this accident was the loss of the right engine center access panels from a fuel explosion that negated the fire suppression system and allowed hydraulic line burn-through that in turn caused a total loss of aircraft control on the ground. On 14-Feb-1990, an Indian Airlines A320-231 was approaching Bangalore Airport, India. During the final approach, the aircraft descended below the normal approach path and its wheels contacted ground in a golf course area vi
  18. 18. about 2300 feet short of the runway and crashed into an embankment at the boundary of the golf course. The aircraft thereafter hopped over a ditch and a road adjacent to it and landed on an area outside the boundary wall of the airport. Destruction of the aircraft was due to impact with ground and subsequent fire. Inspection of the wreckage revealed a hole in the forward spar apparently caused by some force from inside the right wing tank indicative of a post crash explosion from inside the tank. On 11-May-1990, a Boeing 737-300 exploded in Manila, Philippines, while towing it from its stand to an engine start area. During the pre-start sequence ignition of the fuel vapor in the empty centre wing tank started. The resulting explosion ripped the floor open and upwards into the cabin and a fireball erupted into the cabin. The investigation found no evidence of a bomb, an incendiary device, or sabotage. The investigation has yet to reveal the exact ignition source. At the time of the accident, all the fuel boost pumps were in the “on” position. The center fuel tank was at minimal fuel level since 9 March 1990. During the pushback of the airplane the center fuel tank low-pressure light illuminated, indicating the absence of all usable fuel from the center fuel tank. Laboratory examination of the fuel samples from the airplane and fuel vii
  19. 19. storage tanks indicates that the fuel vapor in the center tank would have had a flash point of between 112 - 117°F. The ambient temperature at the time of the accident was 95°F. The fuel was approximately at 115°F based on samples of fuel drawn from other similar airplanes following the incident. viii