Transport aircraft fuel tank system design review, flammability reduction, and maintenance and inspection requirements

Aircraft Engineering and Aerospace Technology

ISSN: 0002-2667

Article publication date: 1 April 2000



(2000), "Transport aircraft fuel tank system design review, flammability reduction, and maintenance and inspection requirements", Aircraft Engineering and Aerospace Technology, Vol. 72 No. 2.



Emerald Group Publishing Limited

Copyright © 2000, MCB UP Limited

Transport aircraft fuel tank system design review, flammability reduction, and maintenance and inspection requirements

Transport aircraft fuel tank system design review, flammability reduction, and maintenance and inspection requirements

Keywords: Aircraft, Transport, Fuel systems, Design, Safety

The US FAA has issued a Notice of Proposed Rulemaking (NPRM) which would require design approval holders of certain turbine-powered transport category aircraft to submit substantiation to the FAA that the design of the fuel tank system of previously certificated aircraft precludes the existence of ignition sources within the aircraft fuel tanks. It would also require the affected design approval holders to develop specific fuel tank system maintenance and inspection instructions for any items in the fuel tank system that are determined to require repetitive inspections or maintenance, to assure the safety of the fuel tank system. In addition, the proposed rule would require certain operators of these aircraft to incorporate FAA-approved fuel tank system maintenance and inspection instructions into their current programmes. Three amendments to the airworthiness standards for transport category aircraft are also proposed. The first would define new requirements, based on existing requirements, for demonstrating that ignition sources could not be present in fuel tanks when failure conditions are considered. The second would require future applicants for type certification to identify any safety critical maintenance actions and develop limitations to be placed in the instructions for continued airworthiness of the fuel tank system. The third would require means to minimize development of flammable vapours in fuel tanks, or means to prevent catastrophic damage if ignition does occur.

These actions are the result of information gathered from accident investigations and adverse service experience, which has show that unforeseen failure modes and lack of specific maintenance procedures on certain aircraft fuel tank systems may result in degradation of design safety features intended to preclude ignition of vapours within the fuel tank.

Background to the proposed rule

In July 1997, a 25-year old Boeing 747-200 series aircraft was involved in an inflight breakup after takeoff from JFK in New York, resulting in 230 fatalities. The accident investigation conducted by the National Transportation Safety Board (NTSB) indicated that the centre wing fuel tank exploded due to an unknown ignition source. The NTSB has issued recommendations intended to reduce heating of the fuel in the centre wing fuel tanks on the existing fleet of transport aircraft, reduce or eliminate operation with flammable vapours in the fuel tanks of new type certificated aircraft, and also to re-evaluate the fuel system design and maintenance practices on the fleet of transport aircraft. The accident investigation has now focused on mechanical failure as providing the energy source that ignited the fuel vapour inside the tank. The accident has prompted the FAA to examine the underlying safety issues surrounding fuel tank explosions, the adequacy of the existing regulations, the service history of aircraft certificated to these regulations, and existing fuel tank system maintenance practices.

Flammability characteristics have received detailed investigations as well as regulations/certification of designs that preclude the presence of ignition sources within the aircraft fuel tanks. Historically, manufacturers have been required to provide maintenance related information for fuel tank systems in the same manner as for other systems. Also, over the years, many design changes have been introduced into fuel tank systems that may affect their safety and there are several ways in which major design changes have been approved. In addition, aircraft operators are required to have extensive maintenance or inspection programmes that include provisions relating to fuel tank systems.

The FAA has examined the service history of transport aircraft and performed an analysis of the history of fuel tank explosions on these aircraft. While there were a significant number of fuel tank fires and explosions that occurred during the 1960s and 1970s on several aircraft types, in most cases, these were found to be related to design practices, maintenance actions or improper modification of fuel pumps. Some of the events were caused by lightning strikes. In most cases, an extensive design review was conducted to identify possible ignition sources and actions were taken that were intended to prevent similar occurrences. However, recent fuel tank system related accidents have occurred in spite of these efforts.

In May 1990, the centre wing fuel tank of a Boeing 737-300 exploded while the aircraft was on the ground at the airport at Manila, Philippines. The aircraft was less than one year old. In the accident the fuel-air vapours in the centre wing tank exploded as the aircraft was being pushed back from a terminal gate prior to flight. The accident resulted in eight fatalities and injuries to an additional 30 people. Accident investigators considered a plausible scenario in which damaged wiring located outside the fuel tank may have created a short between 115 volt aircraft system wires and 28 volt wires to a fuel tank level switch. This, in combination with a possibly defective fuel lever float switch, was investigated as a possible source of ignition. However, a definitive ignition source was never confirmed during the accident investigation.

This unexplained accident occurred on a newer aircraft, in contrast to the July 1997 accident which occurred on an older Boeing 747 aircraft that was approaching the end of its initial design life. These two accidents indicate that the development of an ignition source inside the fuel tank may be related to both the design and maintenance of the fuel tank systems. Since the 1997 accident the FAA, NTSB and aviation industry have been reviewing the design features and service history of the Boeing 747 and certain other transport models. Based upon its review, the NTSB has issued recommendations to the FAA intended to reduce the exposure to operation with flammable vapours in fuel tanks and address possible degradation of the original type certificated fuel tank system designs on transport aircraft.

These recommendations include reduced flammability exposure, both long-term design modifications and near-term operational. Also, ignition source reduction and service history are considered. Particular emphasis is on Boeing 747s and on other aircraft with fuel quantity indication systems (FQIS), wire installations that are co-routed with wires that may be powered. The maximum extent of physical separation and electrical shielding of FQIS wires is required. Service histories have also been reviewed and the findings indicate ageing aircraft related phenomena. Accident investigation has identified ageing of fuel tank system components, contamination, corrosion of components and copper-sulphur deposits on components as possible conditions that could contribute to development of ignition sources within the fuel tanks. Various kinds of debris have been found inside fuel tanks. Wear or chafing of electrical power wires routed in conduits that are located inside fuel tanks can result in arcing through the conduits.

Inspection of wiring to fuel pumps on Boeing 737 aircraft with over 35,000 flight hours have shown significant wear to the insulation of wires inside conduits that are located in fuel tanks and inspections are now required. Corrosion on bonding jumper wires has been found on one A300 and the manufacturer has developed a one-time-inspection service for all its aircraft to ascertain the extent of of the copper-sulphur deposits and to ensure that the level of jumper wire damage found on the one A300 aircraft is not widespread.

Experience of unanticipated failure conditions has been addressed by the issue of various ADs since 1997 for Boeing 747-100, -200 and -300 aircraft and 737-100, -200, -300, -400, and -500 aircraft.

Maintenance practices and deficiencies

A review has also been undertaken of the design features and service history of the Boeing 747 and other aircraft models as well as the current fuel tank system maintenance practices for these aircraft. It is noted that, typically, inspections of fuel tank system components normally do not provide information regarding the continued serviceability of components within the system, unless the visual inspection indicates a potential problem area.

A list of deficiencies has been made, featuring design features, malfunctions, failures, and maintenance related actions that have been identified through service experience to result in a degradation of the safety features of aircraft fuel tank systems. These anomalies occurred on in-service transport category aircraft contrary to the intent of regulations and policies intended to preclude the development of ignition sources within aircraft fuel tank systems. Included are a substantial number of incidents relating to pumps, as well as wiring to pumps in conduits located inside fuel tanks and fuel pump connectors. FQIS wiring and FQIS probes have also been identified in incidents, as well as the condition of bonding straps and the possibilities relating to electrostatic charge.

Proposed special federal aviation regulation (SFAR)

The FAA considers that an SFAR provides a means to establish clear expectations and standards as well as a timeframe within which the design approval holders and the public can be confident that fuel tank safety issues on the affected aircraft will be uniformly examined. The proposed rulemaking is intended to ensure that the design approval holder completes a comprehensive assessment of the fuel tank system and develops any required inspections, maintenance instructions, or modifications.

The proposed SFAR would require the design approval holder to perform a safety review of the fuel tank system to show that fuel tank fires or explosions will not occur on aircraft of the approved design. The proposed rule would apply to holders of TCs and STCs for modifications that affect the fuel tank systems of turbine-powered category aircraft for which the TC was issued after January 1, 1958 and the aircraft has a maximum type certificated passenger capacity of 30 or more, or a maximum type certificated payload capacity of 7,500lbs or more. The FAA has determined that turbine-powered aircraft, regardless of whether they are turboprops or turbojets, should be subject to the rule, because the potential for ignition sources in fuel tank systems is unrelated to the engine design.

A 12-month compliance time is proposed from the effective date of the final rule, or within 12 months after the issue of a certificate for which application was filed before the effective date of this SFAR, whichever is later, for design approval holders to conduct the safety review and develop the compliance documentation and any required maintenance and inspection instructions. It is also proposed that affected operators incorporate FAA-approved fuel tank system maintenance and inspection instructions in their programme within 18 months of the effective data of the proposed rule.

In consideration of the foregoing and a considerable amount of additional data and guidance material, the FAA proposes to amend parts 21, 25, 91, 121, 125 and 129 of Title 14, CFR.

ReferenceNPRM 14 CFR,