3.1.2 FUEL FLAMMABILITY IN TWA 8004
3.2 ELECTRICAL COMPONENT FAILURES5
4.1.1 C-17 OBIGGS NITROGEN INERTING SYSTEM6
4.4 VENTED AIR GAPS AND INSULATION7
On the 17th of July, 1996, 13 minutes in it’s flight, Trans World Airlines Flight 800 (TWA 800) crashed into the Atlantic Ocean. The investigation by the National Transportation Safety Board (NTSB) shows that the aircraft exploded within the Central Wing Fuel Tank (CWT).
Unfortunately, NTSB was unable to locate the source of ignition, but several theories of explosion where explained within this report. These theories include probability of the fuel flammability within the conditions before the aircraft’s explosion and the Failures of the electrical components
The solutions to the explosion theories include Nitrogen inerting, Jet fuel alternative, installation of foams and vented air gaps.
The report recommends using the JP-5 fuel alternative instead of Jet A (fuel used in TWA800). This option was more favourable than the other solutions to the constraints of Boeing’s time and budget.
Boeing Ltd. has initiated a project that will improve the design aircraft. This design will provide a safer and more comfortable flight. In conjunction with this project, Batchelor Consultancy Pty. Ltd. has been commissioned by Boeing Ltd. to examine the reason and of failure of Trans World Airlines Flight 800 (TWA 800) and provide suitable recommendation to increase the safety of future flights of Boeing. This report will cover all the requirements that are stated in the Safety Modification Legislation AIR142.
The matters to be considered while writing the report will include:
?Review the circumstances surrounding the accident to establish the probable cause/s of the accident.
? Provide the best recommendation that will increase the maximum safety to Boeing’s customer as well as their employees. Recommendations should also take into considerations the cost and time of conducting the stated recommendations.
On July 17, 1996, about 8:19 p.m. eastern US time, a Boeing 747-131, operated as Trans World Airlines Flight 800 (TWA 800) took off and was bound for Charles De Gaulle International Airport (Paris, France). TWA 800 was heading northeasterly, roughly paralleled the southeastern coastline of Long Island (Phillips, 1996, p27).
The weather at John F. Kennedy International Airport was clear with 25 mi. Visibility, temperature was 71F and winds from the southwest at 4 kt.
At about 8:28 p.m., when the aircraft at an altitude of 10,000 ft. the aircraft accelerated from 250 kt. To 300 kt. which indicated the airspeed was for the en route climb segment. After 13 minutes into the flight, air controllers lost radar contact with the Boeing 747-131. The Center Wing Tank (CWT) had exploded eight miles south of East Moriches, New York, at an altitude of 13,700 ft.
After the explosion, NTSB was commissioned by the U.S. government to investigate into the cause of the explosion and recommend the Federal Aviation Administration to develop and implement designs or operational changes to increase the safety of transport category aircraft. During the NTSB’s investigation, Seven main types of evidences where used to find the cause of the explosion. These include (NTSB hearing summary 1997):
?Radar – The short flight of TWA800 was tracked by radar from two stations. The radar also recorded the trajectory path of the numerous pieces of the aircraft’s wreckage.
?Cockpit Voice recorder (CVR) – The device, which records the voice from the cockpit, captured a high energy signal that was consistent with an explosion in the fuel tank, whose sound was transmitted to the cockpit area microphone.
?Flight data recorder (FDR) – The device records 18 aspects of the flight, such as altitude and heading.
?Witnesses to the Explosion – 200 witnesses that were likely to have spotted the burning aircraft after it had begun to break up.
?Reconstruction of the debris – Portion of the airplane have been reconstructed, including the CWT, the passenger cabin above the CWT, and the air conditioning packs and associated ducting beneath the CWT. The reconstruction shows outward deformation of the CWT walls and deformation of the internal components of the tank that are consistent with an explosion originating within the tank. A close scientific examination and analysis of almost 200 holes, slits, punctures and penetrations.
?Flight and miscellaneous tests – Cooling of the air conditioning pack bay and temperature of fuel tanks are measured in flights similar to that of TWA800.
In July 1997, a hearing of the NTSB’s investigation was conducted (NTSB hearing summary 1997). NTSB officials provided vital information regarding the flight path and the sequence of the aircraft’s explosion. The NTSB showed and concluded that the aircraft had an explosion came from around the Center Wing Fuel Tank (CWT), but was unable to find the source of ignition that caused the explosion. They also proved that TWA flight 800 was not downed by a missile or by a bomb explosion.
According to the NTSB’s investigators (NTSB hearing summary 1997), the explosion of TWA 800 was divided into three-stage sequence. The explosion was the first stage. The stage shows that the explosion released wreckage from in and around the center fuel tank. Secondly, the separation of the forward fuselage from the rest of the aircraft. Finally, the continuing flight and eventual breakup and descent of the remainder of the aircraft.
Based on the reconstruction of the debris (McKenna, 1997, p32), NTSB investigators said they believe the force of the explosion in the Center Wing Fuel Tank (CWT) pushed the No.3 spanwise beam, which forms the front wall of the tank, into the front spar ahead of it (see figure 1). The front spar broke free of the wing’s upper skin and rotated down into the forward cargo bay.
Cracks propagated from the sides of the spar into the pressure bulkhead below it, then into the aircraft’s belly skin. The skin cracked forward longitudinally for about 200in, then each crack turned in toward the aircraft centerline until they intersected.
Figure 1 The Center Wing Fuel Tank Area (source: Dornheim, 1997, p56)
The 747’s wing rear spar forms the aft wall of the center tank. Three span-wise beams pass laterally through the tank. The No. 1 of these span-wise beams is between the rear spar and the wing’s mid-spar. Ahead of the mid-spar are the No. 2 and No. 3 s
span-wise beams, the latter forming the front wall of the tank. Between the No. 3 span-wise beam and the front spar is the dry spar, which contains no fuel.
With the skin and some stringers and frames fractured, this belly skin was pushed down into the slipstream by cabin pressurisation and perhaps the overpressure from the center tank blast. It then tore free, taking with it a 13.5-ft long section of the aircraft’s keel beam. The skin to the right and left of the belly section then peeled back and free of the aircraft. About four seconds after the blast, a 70-ft section of the aircraft, from just in front of the tank to the nose, rotated to the right and down, then tore free.
The rear fuselage, wings attached, are likely rolled about 50 degrees to the left and continued climbing, then rolled right until it was inverted and in a dive.
Compression-buckling on the upper skin of the center wing tank and tension failures on the bottom skin indicate the wings failed upward, with the left wing breaking free and the right remaining attached to a large section of the center fuselage. The intact mid-spar and rear spar were strong enough to hold the wing together until the aircraft was well into the dive.
After about 50 to 90 seconds succeeding the explosion, most of the debris, resulting from the explosion, were in the water.
Due to the inability of the NTSB investigators to find the source of the ignition in the CWT and exploded TWA flight 800, several theories have been put forward by NTSB and the aircraft industries. These theories include the jet fuel flammability and the failure of some electrical components onboard of TWA 800.
The characteristics of Jet Fuel provide us with an understanding of how the temperature and conditions surrounding may effect the CWT explosion of TWA flight 800. The main terms used to describe jet fuels characteristics include (DornHeim, 1997, p62):
?Fuel/air ratio, which is the volumetric ratio of fuel vapors to air.
?Lower flammability limit (LFL). This is the minimum fuel/air ratio required for a flame to spread.
?Upper flammability limit (UFL), which is the maximum fuel/air ratio that sustains flame propagation.
?Flash point. The lowest temperatures of a liquid when a test flame will cause vapors near he surface to momentarily ignite, or flash.
?Autoignition temperature. This is the lowest temperature of a vessel at which injected fuel vapor will spontaneously ignite.
The kerosene-type Jet A (DornHeim, 1997, p57), the fuel that was used in TWA flight 800, have a flash point above 100 F. They have low vapor pressure and are popularly thought to be hard to ignite, but research decades ago showed the fumes could be ignited well below the flash point. The specification of Jet A includes a 100F minimum flash point. The autoignition temperature is not specified but is around 450F.
As stated by DornHeim (1997), the LFL drops as altitude increases because the lower air pressure makes the mixture richer. During a climb, the fuel will become more flammable. Also, oxygen dissolved in the fuel comes out of solution before other gases, causing oxygen enrichment of the ullage, which increases ignitability.
NTSB researchers briefed investigators on preliminary test results that Jet A (the fuel type used by TWA800) fuel is far more flammable than aircraft manufacturers and operators had previously believed particularly when it is heated to high temperatures (McKenna, 15/12/97,P32). A spokesman from California Institute of Technology (CalTech) said his tests indicated that roughly 50F increase in the temperature of Jet A can drop the minimum temperature required to ignite its vapors by a factor of three.
NTSB officials said flight tests conducted in July 1997 indicated that the temperature inside the center fuel tank of TWA 800 may have been as high as 128F. Earlier, safety board research had indicated that the temperature in the tank was less then 96F. The data collected from the flight test combined with the CalTech research, would seem to indicate that the risk of explosion in the tank was substantially higher than conventional industry thinking would indicate.
Before the disastrous flight of TWA 800, all three air conditioning packs were operated on the ground for about 2 hours to generate heat beneath the CWT (Daschle, 1996, p4). In the flight tests, the fuel-air mixture in the CWT ullage was stabilised at a temperature below the LEL on the ground. However, as the aircraft climbed, the atmospheric pressure decreased (the LEL decreases with decreasing atmospheric pressure) reducing the LEL temperature and allowing an explosive fuel-air mixture to exist in the tank ullage.
NTSB research has uncovered minor failures and degradations of electrical hardware that has led some participants in the accident investigation to question whether more should be done to identify, track and correct such problems in aircraft systems.
As pointed out in the NTSB hearing, the failures of electrical components could include the following (NTSB hearing summary 1997):
?A possible short circuit to the fuel quantity indication system (FQIS) wiring, outside the fuel tank, combined with latent failures not apparent during operation of the plane, or copper sulfide deposits on FQIS components in the fuel tank.
?Energy induced into the FQIS combined with latent failures, foreign materials or copper sulfide deposits in the fuel tank.
?Damage to wiring above the forward cargo compartment. In the flight of a different aircraft, unrelated to the accident investigation has found that a cargo container may have struck the wiring in this area and created a short circuit. In wreckage recovered from the accident aircraft, a portion of that wring is missing.
?Possible short-circuit in other parts of FQIS wiring, some of which has not been recovered from flight 800.
?Short circuits in the power cable that could have ignited the fuel vapors in the partially filled center fuel tank.
As stated in the NTSB summary, before an explosion can occur, three elements are needed, these include:
To prevent any explosion from occurring, one of these elements, as stated above, must be removed. The following solution, that include nitrogen inerting, foam installation, jet fuel alternative and vented air gaps, take this into account.
Without oxygen in the fuel-air mixture, the fuel tank ullage could not ignite, regardless of temperature of ignition considerations.The military (Daschle, 1996, p4) has prevented fuel tank ignition in some aircraft through the creation of a nitrogen-enriched atmosphere (nitrogen-inerting) in fuel tank ullage, thereby creating an oxygen-deficient fuel air mixture that will not ignite Nitrogen-inerting has been accomplished several ways
?Adding nitrogen to fuel tank(s) from a ground source before flight;
?Discharging onboard supplies of compressed or liquefied nitrogen in flight;
?The use of on-board inert gas generation systems that separate air into nitrogen and oxygen.
Nitrogen-inerting, using a ground source of nitrogen, might prevent explosions such as that occurred to the TWA800 aircraft. But Nitrogen-inerting may not prevent an explosion after the fuel tanks have been emptied during flight through fuel consumption, or when ullage is exposed to warmer air as an aircraft descends.
The development and installation of such systems are expensive and may be impractical because of system weight and maintenance requirements in some airplanes (Velotti, 1997, p60). However, since these studies were conducted, advances in technology for separating nitrogen from air and instances of tank ignition may now make it possible to show that inerting of fuel tanks is cost beneficial.
C-17 OBIGGS system is an example used by McDonnell Douglas that use an on-board inert gas generation systems that separate air into nitrogen and oxygen.
4.1.1 C-17 OBIGGS NITROGEN INERTING SYSTEM
The aircraft company of McDonnell Douglas (Dornheim,, 1997, p63) has designed and built an on-board system for the C-17 (OBIGGS) which generates nitrogen and uses the gas to inert fuel tanks. The OBIGGS system bleeds air from inboard engines, separates oxygen from the nitrogen, stores the nitrogen and, dispenses it, to inert the tanks as needed. The system is designed to keep the ullage oxygen fraction below 9% by volume, less than the 9.5% – 12 % minimum required for an explosion. It weighs 2,000 lb. and costs approximately $1.5 million per unit. The space of the aircraft that the system takes up is approximately 80 cu. ft. of space on the aircraft.
Obiggs has no effect on tank capacity, and its bleed air requirements are negligible
The safety foam is a sponge-like material constructed of interconnecting strands (Ott, 1997, p65). In use in land-based aircraft since Vietnam, foams have proved to be an effective protection against such ignition sources as hostile groundfire, lightning, static discharge and electrical shorts.
The foam acts like a three-dimensional fire screen and functions in several ways. The foam absorbs heat and removes energy from the combustion process. It mechanically interferes with the compression wave of an explosion, making the vapors lean in the ullage and less vulnerable to explosions.
Boeings fuel systems specialists have been negative regarding the use of safety foam in commercial aircraft (McKenna, 1997, p33). Even though the foam has been improved to minimise the weight, the foam carries a weight penalty and, when inserted in fuel cells, limits tank capacity, an important factor in long-range operations. The extra weight of foam that every aircraft must curry would average at 1,200 to 1,400 lb. This extra weight equals as many as eight passengers with baggage. Another problem cited by Pa.-based Foamex International Inc, is the lingering problem with static discharge in cold weather operations.
A representative from the Fuel Systems Safety Program, said (McKenna, 1997, p33), an alternative to increasing fuel system safety is the development of a JP-5 like fuel. Used primarily by the U.S Navy, JP-5 is refined with a higher flammability level and less volatile than that of Jet A, making it safer for use in the hot environment of aircraft.
The cost of the JP-5 fuel compared with the cost to the current fuel shows that they are not much different.
A reduction in the potential for fuel tank explosions could be attained by reducing the heat transfer to fuel tanks from sources such as hot air ducts and air conditioning packs that are now located under or near fuel tanks in some transport-category aircraft (Daschle, 1996, p6). This may be achieved by installing additional insulation between such heat sources and fuel tanks that must be colorated with heat-generating equipment such as hot air ducting and air conditioning packs.
Boeing (McKenna, 1997, p34) also is investigating modifying 747s with a vented air gap to direct ram air between the air-conditioning packs and the bottom of the fuel tank. This might provide a cooling flow as well as some insulation effect. Another possibility is directing ram air into the center tank to sweep explosive vapors overboard. But that could have significant environmental considerations, since it would involve dumping hydrocarbons in the atmosphere
Safety has always been the main concern of the aircraft industries will always be until aircrafts are designed so that their fuel tanks are safeguarded against conditions that could trigger an explosion. From. After what have happened to Trans World Airlines Flight, we are able to understand ways to reduce Fuel tank explosion. We have found that both fuel flammability and faulty electrical components can have serious consequences, but now we have learnt and will try to capatalise on it
Significant consideration should be given to the development of airplane design modifications, such as nitrogen-inerting systems and the addition of insulation between heat-generating equipment and fuel tanks to increase the safety of both passengers and crewmembers. Appropriate modifications should apply to newly certified airplanes and where feasible, to existing airplanes.
We must look at developing a method that include inert gas generators and to keep fuel cool and out of explosive range. An increased level of safety must justify new protective features with minimum added complexity, weight and operational constraints.
By taking these point under consideration and reviewing the possible solution to this problem, we recommended Boeing or any other aircraft industries to take the JP-5 Jet Fuel into the design of aircraft. We have chosen this solution instead of the other four solutions, because it does not provide any deficiency to the fuel capacity, nor would it cost as much as the other solutions. But, the main reason was that, it is the most efficient out of the four.
We recognises that such design modifications take time to implement and believes that in the interim, operational changes are needed to reduce the likelihood of the development of explosive mixtures in fuel tanks. Therefore we recommend Boeing to use insulations around any heat-generating equipment. This solution should be in use until the design and manufacturing of the JP-5 technology has finished.
Daschle, L., ( 1996), “Safety Recommendation”, National Transportation Safety Board, Dec 13,pp. 1-7.
Dornheim, M., (1997), “ Industry Grapples With Fuel Tank Safety”, Aviation Week & Space Technology, July 7, pp. 56-57.
Dornheim, M., (1997), “Fuel Ignites Differently in Aircraft, Lab Environments”, Aviation Week & Space Technology, July 7, pp. 61-63
McKenna, J., (1997), “TWA Probe Targets Aging Aircraft Systems”, Aviation Week & Space Technology, Dec 15, pp. 30-32
McKenna, J., (1997), “ Boeing Eyes Fuel Change To Increase Tank Safety”, Aviation Week & Space Technology, Dec 15, pp. 33-34.
National Transportation Safety Board, (1997). “NTSB hearing summary”, NTSB .
Ott, J, (1997), “Methods to Protect Fuel Tanks Explored”, Aviation Week & Space Technology, Feb 17, pp.65-67.
Phillips, E., (1996), “ TWA Probe Advances, But No Cause Found”, Aviation Week & Space Technology, July 29, pp. 26-28.
Velocci, A., (1997), “Airline Industry Takes Fresh Look at Inerting”, Aviation Week & Space Technology, July 14, pp. 60-61.
?F = (9/5) ?C + 32?
?C = (5/9)(?F – 32?)
Cockpit Voice Recorder (CVR) source: (AV&ST, 15/12/97,p31)
The National Transportation Safety Board was able to recover the Fairchild Model A – 100 cockpit voice recorder from TWA Flight 800’s wreckage. It yielded more than 31 min. of good quality audio, despite an extremely distorted exterior and a wet tape. The recording started at 7:59:40 p.m. EDT while the aircraft was at the departure gate, and stopped when electricity was interrupted at 8:31:12 p.m.
The latter portion of the NTSM transcript is reproduced here, starting while the aircraft is climbing through 8,000 ft. There did not appear to be anything relevant to the accident before that.
The crew refers to cross feed at 20:25:59, and notes unusual fuel flow readings on the No. 4 engine at 2029:15
RDO: Radio transmission from accident aircraft.
CAM: Cockpit Area Microphone sound or source:
1. Voice identified as Captain (left seat).
2. Voice identified as First Officer (right seat).
3. Voice identified as Second Officer.
4. Voice identified as Instructor Flight Engineer.
CTR: Boston ARTCC Controller (center)
FIC: TWA Flight Information Controller.
*Unintelligible word; () Questionable text; (()) Editorial insertion
Note: All times are expressed in Eastern Daylight Saving time. Only radio transmissions to and from the accident aircraft were transcribed.
2024:41.7 RDO-2 Now York center TWA’s lifeguard eight hundred heavy eight thousand two hundred climbing one one thousand.
2024:48 CTR Twa eight hundred Boston center roger climb and maintain one three thousand.
2024:53.4 RDO-2 TWA’s eight hundred heavy climb and maintain one three thousand.
2024:57 CAM-1 climb and maintain one three thousand.
2325:34.5 RDO-2 TWA’s eight hundred heavy ah about two thousand feet a minute here until accelerating out of ten thousand.
2025:41 CTR roger sir climb and maintain flight level one niner zero and expedite through fifteen.
2025:47.1 RDO-2 TWA’s eight hundred heavy climb and maintain one niner zero and expedite through one five thousand.
2025:53 CAM-1 climb to one nine zero expedite through one five thousand.
2025:57 CAM-3 pressurization checks.
2025:59 CAM-3 (takeoff) thrust go on cross feed?
2026:07 CAM-3 I’ll leave that on for just a little bit.
2026:24 CTR TWA eight hundred amend the altitude maintain ah one three thousand thirteen thousand only for now.
2026:30.3 RDO-2 TWA’s eight hundred heavy oday stop climb at one three thousand.
2026:35 CAM-1 stop climb at one three thousand.
2027:47 CAM ((sound of altitude alert tone)).
2028:13 CTR TWA eight hundred you have traffic at one o’clock and ah seven miles south bound a thousand foot above you he’s ah Beech nineteen hundred.
2028:20.6 RDO-2 TWA’s ah eight hundred heavy ah no contact.
2028:22.5 RDO-3 FIC TWA eight hundred.
2028:25.7 RDO-3 Eight hundred with an off report ah plane number one seven one one nine we’re out at zero zero zero two, and we’re off at zero zero one nine , fuel one seven nine decimal zero, estimating Charles de Gaulle at zero six two eight.
2028:42 FIC TWA eight eight hundred got it all.
2029:15 CAM-1 look at that crazy fuel flow indicator there on number four.
2029:35 CAM-1 some where in here I better trim this thing (in/up)
2029:39 CAM-1 some place in here I better find out where this thing’s trimmed.
2030:15 CTR TWA eight hundred climb and maintain one five thousand.
2030:19.2 RDO-2 TWA’s eight hundred heavy climb and maintain one five thousand leaving one three thousand
2030:28 CAM-1 climb to one five thousand.
2030:42 CAM ((sound similar to a mechanical movement in cockpit)).
2031:05 CAM ((sounds similar to recording tape damage noise)).