Transcript
Fuel Tank Safety (FTS) / Airworthiness Limitation (AWL) / Critical Design Configuration Control Limitation (CDCCL)
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BACKGROUND
TWA B747-131 Fligth-800 accident, involving Centre Wing Tank (CWT) explosions
Investigations revealed penetration of 115 Vac electrical energy into the center fuel tank due to chaffed Fuel Quantity Indicating System (FQIS) wiring
Together, with the inadequate electrical bonding of fuel system Components, separation of fuel system wiring, and fuel vapors, resulted in loss of the passenger lives and the airplane.
Fuel tank explosions can only occur when an ignition source is in contact with flammable fuel vapor.
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BACKGROUND
TWA B747-131 Fligth-800 accident, involving Centre Wing Tank (CWT) explosions
Investigations revealed penetration of 115 Vac electrical energy into the center fuel tank due to chaffed Fuel Quantity Indicating System (FQIS) wiring
Together, with the inadequate electrical bonding of fuel system Components, separation of fuel system wiring, and fuel vapors, resulted in loss of the passenger lives and the airplane.
Fuel tank explosions can only occur when an ignition source is in contact with flammable fuel vapor.
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FUEL TANK EXPOLSION INCIDENT March 2001, Thai Airways 737
CWT fuel pumps left running on when dry. Electrical ARC / fault resulted resulted in Centre Tank Tank explosion explosion
airplane was being prepared for a departure to Chiang Mai
149 passengers were waiting to board
The fire erupted into the cabin, killing one cabin crew member and injuring 6 others
The fire was put out in 60 minutes
The airplane was destroyed
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FUEL TANK EXPOLSION INCIDENTS China Air 737-800
While retracting slats during taxi, one loosened slat fairing bolt penetrated into the CWT resulting CWT explosion.
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FUEL TANK EXPOLSION INCIDENTS Philippines Airlines B737 1990
CWT explosion during push back
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COMMON FEATURES OF SIGNIFICANT ACCIDENTS/INCIDENT 1990 - B-737-300 Manila, Philippines Almost new airplane
1996 - B-747, TWA 800, JFK,USA 25 year old airplane
2001 - B-737-400 Bangkok, Thailand 10 year old airplane
Air Conditioning Packs running Air Conditioning Packs running Air Conditioning Packs running during Ground Operations during Ground Operations during Ground Operations Empty CWT explosion during pushback from gate CWT pumps operating at time Of Explosion 8 fatalities Jet-A fuel, approx. 95 degree F ambient temp.(35’C) NO IGNITION SOURCE IDENTIFIED 12/1/2013
Empty CWT explosion During climb CWT pumps operating at time of Explosion 230 fatalities
Empty CWT explosion minutes after refueling CWT pumps operating at time of explosion 1 fatality
Jet-A fuel, approx. 120 degree Jet-A1 fuel, approx. 97 degree F F tank temp.(49’C) ambient temp.(36’C) NO IGNITION SOURCE NO IGNITION SOURCE IDENTIFIED IDENTIFIED 8
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THE “FIRE” TRIANGLE Fire is a chemical process requires three things: OXYGEN, HEAT and FUEL Without one of these elements, a fire cannot start or continue. The process in a fire is called oxidation. A fire hazard can be minimized by eliminating any one side of the fire triangle
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REGULATORY ACTION/ REQUIREMENTS
Special Federal Aviation Regulation 88 (SFAR 88), advising airplane manufacturers to carry out a design review of fuel tank system to avoid entry of any ignition source into the fuel tanks in compliance of FAR 25-901 & 25.981 (a) and (b).
National Aviation Authorities (NAA) using NT/POL/25/12, JAR 25.901 / 25.1309 & Technical Guidance Leaflet (TGL) 47.
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SFAR No. 88--Fuel Tank System Fault Tolerance Evaluation Requirements
Applicability
the holders of type certificates, and supplemental type certificates that may affect the airplane fuel tank system, for turbine-powered transport category airplanes, provided the type certificate was issued after January 1, 1958, and the airplane has either a maximum type certificated passenger capacity of 30 or more, or a maximum type certificated payload capacity of 7,500 pounds or more. This SFAR also applies to applicants for type certificates, amendments to a type certificate, and supplemental type certificates affecting the fuel tank systems for those airplanes identified above, if the application was filed before June 6, 2001, the effective date of this SFAR, and the certificate was not issued before June 6, 2001.
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SFAR No. 88--Fuel Tank System Fault Tolerance Evaluation Requirements
Compliance (a) Conduct a safety review of the airplane fuel tank system to determine that the design meets the requirements of §§ 25.901 and 25.981(a) and (b). (b) Develop all maintenance and inspection instructions (c) Submit a report for approval to the FAA Airplane Certification Office (ACO)
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FAR §25.981 Fuel tank ignition prevention a.
“No ignition source may be present at each point in the fuel tank or fuel tank system where catastrophic failure could occur due to ignition of fuel or vapors.”
b.
Critical Design Configuration Control Limitations (CDCCL), inspections, or other procedures must be established, as necessary, to prevent development of ignition sources within the fuel tank system and must be included in the Airworthiness Limitations section of the Instructions for Continued Airworthiness.
c. The fuel tank installation must include either: (1) Means to minimize the development of flammable vapors in the fuel tanks (in the context of this rule, “minimize'' means to incorporate practicable design methods to reduce the likelihood of flammable vapors); or (2) Means to mitigate the effects of an ignition of fuel vapors within fuel tanks such that no damage caused by an ignition will prevent continued safe flight and landing.”
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THE CONCEPT OF IGNITION SOURCE PREVENTION
Fuel tank explosions can only occur when an ignition source is in contact with flammable fuel vapor.
Fuel vapors become flammable when they are mixed in certain proportions with oxygen, and are under specific temperature and pressure conditions.
some fuel tanks have a significantly higher exposure to flammable vapors than other tanks.
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THE CONCEPT OF IGNITION SOURCE PREVENTION
First Line of Defense to prevent recurrence of such events, Airplane manufacturer’s reviewed the airplane fuel system designs to meet the above requirements.
a number of service Bulletins (SBs) /Production Modifications (Production Revision Record: PRR) have been introduced for incorporation on Production and In-Service airplanes, trying to build the airplane fuel system design immune to such occurrences.
FAA legislation requires that airplane types that have fuel tanks, which exceed the average flammability exposure level (> 7%), specified in the rule shall be fitted with either a Flammability Reduction System (FRS), or an Ignition Mitigation Means (IMM).
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THE CONCEPT OF IGNITION SOURCE PREVENTION
EASA has issued a Safety Information Bulletin (SIB 2010-10) which mandates the installation of a FRS on production airplane subject to high flammability exposure and manufactured after 31st December 2011.
EASA will publish a Notice of Proposed Amendment (NPA) by mid 2011 to address the in- service airplane retrofit.
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THE CONCEPT OF IGNITION SOURCE PREVENTION Compliance Target In relation to the affected airplane, fifty percent (50%) of each operator's fleet of airplane are to be completed within 4 years of release of the Service Bulletins i.e. by 2014. One hundred percent (100%) of each operator's fleet of airplane are to be completed within 7 years after the release of the Service Bulletins i.e. by 2017. NOTE: Portable gas detectors are used to monitor Oxygen and flammable vapor concentrations within the fuel tanks. Oxygen concentrations should be between 19.5 and 23.5 percent. Level below 19.5 percent is considered Oxygen deficient, while levels above 23.5 percent will significantly increase the risk of fire and/or explosion. Monitoring these figures is critical to the safety of the maintenance crew.
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A Flammability Reduction System (FRS)
FRS is a system that is designed to prevent fuel vapors from being in a flammable condition
A
practical Flammability Reduction System (FRS) will reduce fuel tank oxygen content to below the level that will support combustion (approach being adopted by Airbus).
Boeing has developed Nitrogen Generation System (NGS) to satisfy FRS requirement.
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An Ignition Mitigation Means (IMM)
IMM is a system that detects an ignition source and releases a fire suppressant to prevent the development of a fire.
Such a technology is used on small military airplane but is not considered sufficiently robust for use in large commercial airplane and as such is not being pursued by Airbus or Boeing.
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CHANGES TO “INSTRUCTIONS FOR CONTINUING AIRWORTHINESS” (ICA)
Second Line of Defense of the FTS regulations account for this situation and requires updating of Instructions for Continued Airworthiness (ICA) / Airplane Maintenance Program for all the type certificated airplane to avoid any likelihood of compromise / violation of the fuel system design criteria during any scheduled and / or unscheduled maintenance. This requirement ensures proper documentation of any maintenance action performed on the airplane and / or its components for safety, reliability and maintainability.
AWL
/ CDCCL tasks were introduced by airplane manufacturers in Maintenance Planning Documents AND have already been incorporated into the PIA’s fleet maintenance program.
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CHANGES TO “INSTRUCTIONS FOR CONTINUING AIRWORTHINESS” (ICA)
Boeing MPD section-9 is related to Airworthiness Limitations (AWL) which describes the Fuel Airworthiness Limitation Items (ALIs) and the associated CDCCL tasks.
Airbus
/ ATR publishes “Airworthiness Limitations” (ALS) section -5 which is related to Fuel Airworthiness Limitations (FAL). These requirements, together with the Life Limits / Monitored Parts, structural Airworthiness Limitation Items (ALI) and systems Certification Maintenance Requirements (CMR) comprise the Airworthiness Limitation Section which satisfies the regulatory requirements.
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Airworthiness Limitation Items (ALIs) ALIs
identify inspection tasks related to fuel tank ignition source prevention which must be done to maintain the design level of safety for the operational life of the airplane.
These ALIs are mandatory and cannot be changed or deleted without the approval of applicable regulatory agency.
Strict adherence to methods, techniques and practices as prescribed is required to ensure the ALI is complied with.
Any
use of methods, techniques or practices not contained in these ALIs must be approved by the applicable regulatory agency.
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Critical Design Configuration Control Limitations (CDCCLs)
CDCCLs are a means of identifying certain design configuration features intended to preclude a fuel tank ignition source for the operational life of the airplane.
CDCCLs are mandatory and cannot be changed or deleted without the approval of the applicable regulatory agency.
“A critical fuel tank ignition source prevention feature may exist in the fuel system and its related installation or in systems that, if a failure condition were to develop, could interact with the fuel system in such a way that an unsafe condition would develop without this limitation.”
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Critical Design Configuration Control Limitations (CDCCLs)
Strict adherence to configuration, methods, techniques, and practices as prescribed is required to ensure compliance with the CDCCL.
Any
use of parts, ,methods, techniques or practices not contained in the applicable CDCCL must be approved by the FAA office that is responsible for the airplane model Type Certificate, or applicable regulatory agency.
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Certification Maintenance Requirements (CMRs)
A CMR is a required periodic task, established during the design certification of the airplane as an operating limitation of the type certificate.
CMRs usually result from a formal, numerical analysis conducted to show compliance with catastrophic and hazardous failure conditions.
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Sample of AWL / FAL for B777
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Sample of AWL / FAL for A310 CDCCL
Requirement
Reference
Applicability
1
Air gap between a fuel quantity indicating probes and the airplane structure.
Critical to prevent spark generation during lightning strike conditions and must not be compromised The gap stated in the relevant maintenance procedure in the AMM must be achieved during installation of a fuel quantity probe.
AMM 28-42-31 pb 401 28-42-32 pb 401 28-42-33 pb 401
ALL ALL ALL
2
Separation of fuel quantity and level indicating system wiring from other wiring.
To prevent cross coupling of high voltage transients and thereby keep unsafe ignition energies from inside the tank it is required that operators obey the wire routing and separation standards given in the Electrical Standard Practices Manual (ESPM).
ESPM 20-10-00 20-33-20 20-33-21 20-33-22
ALL ALL ALL ALL
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Sample of AWL / FAL for A310 3
CDCCL
Requirement
Reference
Applicability
Direct bonding on items of equipment inside a fuel tank.
Direct bonding is critical to prevent spark generation during component failure and lightning strike conditions and must not be compromised Direct bonding in fuel tanks must be carried out during component installation to the appropriate bonding method and standard stated within the installation procedure in the AMM.
AMM 28-00-00 pb 401 28-11-21 pb 401 28-11-22 pb 401 28-11-24 pb 401 28-11-32 pb 401 28-12-21 pb 401 28-12-33 pb 401 28-12-34 pb 401 28-21-13 pb 401 28-21-21 pb 401 28-24-31 pb 401 28-25-15 pb 401 28-27-12 pb 401 28-43-21 pb 401 28-46-21 pb 401
ALL ALL ALL ALL A310-300 ALL A310-300 A310-300 ALL ALL ALL ALL A310-300 ALL ALL
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Sample of AWL / FAL for A310 4
CDCCL
Requirement
Reference
Applicability
Safety critical features of fuel pumps
These features must be maintained throughout the full life of the fuel pump to avoid the possibility of generation of an ignition source by overheating or sparks caused by arcing, friction etc. Repair and overhaul of fuel pumps must be carried out in accordance with the equipment manufacturer’s maintenance instructions or other maintenance instructions acceptable to the certifying authority.
CMM 28 09 01 28 24 11
ALL ALL
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MANAGEMENT / CONTROL OF CDCCL
Refer Doc ref # PIA/CDCCL/SFAR88 for detailed procedure for the Management & Control of CDCCL in PIA.
It is the responsibility of certifying staff / maintenance personnel to ensure that the above stated CDCCL configurations are maintained to reduce the risk of any ignition source penetration into the fuel tank (s).
If any of these CDCCL is compromised, this may lead to wire chaffing, fault current (short circuit), or overheating of the CWT fuel pumps.
This in turn may result in fuel tank explosion.
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MANAGEMENT / CONTROL OF CDCCL
Dry running of the fuel pumps should not be allowed as it has already resulted in catastrophic incidents.
This requirement has been addressed both by Airbus & Boeing by introduction of new logic circuitry to avoid dry running on CWT fuel pumps.
Introduction of Ground Fault Interrupter (GFI) relays also ensures NO overheating / fault current in the fuel pump circuitry.
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MANAGEMENT / CONTROL OF CDCCL
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OVERHAUL SHOP CONTROL FOR CDCCL
Fuel pumps CMM have also been revised in compliance of CDCCL requirements and provides specific instructions / procedures.
Tools, kits, jigs, materials, test equipment required for servicing, repair or overhauling of a fuel system component should be used accordingly.
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AD/ SB Control for CDCCL
AD/SB evaluation check list was updated to include a clause to check whether the literature under evaluation affects CDCCL or not.
Certifying staff / maintenance personnel should assess the affect of the current activity on any of the AWL/CDCCL, and if, it is confirmed that a CDCCL / AWL is directly or indirectly affected, work instructions provided by the AD / SB or task card should be revised / corrected in coordination with Airworthiness Management Division to preclude any chance of CDCCL violation.
A data base will be updated to have a record of all the ADs, SBs, and ECO which have an effect on any of the CDCCL requirements.
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AD/ SB Control for CDCCL Scheduled / unscheduled repair schemes control
Any scheduled /un-scheduled locally prepared repair scheme should be examined for effect on any of the CDCCL item(s).
Same level of understanding is required by shop personnel involved in the servicing / repair / overhaul of Fuel pumps, FQI Tank unit, Fuel Quantity Indicating Computer (FQIC), Fuel Quantity Processor Unit (FQPU), harnesses.
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AD/ SB Control for CDCCL Maintenance Program Control for CDCCL
Inspections/ tasks introduced through Locally generated task cards as per requirements of an AD, inspection SB, or any other local operational /maintenance requirement also requires assessment whether it affects the CDCCL or not.
PIA fleet Maintenance program should be scrutinized on a regular basis upon receipt of MPD revision, to sort out the newly introduced tasks for having an impact on the CDCCL.
This exercise should be continued in future as well.
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MAINTENANCE DATA - MANUAL CONSULTATION
Use of updated / latest maintenance data is of utmost importance for maintaining FTS design criteria / CDCCL.
Air Transport Association of America (ATA) Chapter 47 has been assigned for FRS / NGS in concerned airplanes Airplane Maintenance Manuals (AMM).
CDCCL warning notes have also been introduced in AMM and Components Maintenance Manuals (CMM).
All certifying staff / maintenance personnel are required to consult concerned AMM /CMM (as applicable).
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