Air Canada AC116 - 767-300  
Cargo Hold Fire 
-  Safety Recommendations

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AVIATION SAFETY RECOMMENDATIONS

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Transportation Safety Board of Canada

TRANSPORTATION SAFETY BOARD OF CANADA
AVIATION SAFETY RECOMMENDATIONS

DATE ISSUED: 14 November 2002

FORWARDED TO:

The Honourable David Michael Collenette, P.C., M.P.
Minister of Transport

Ms. Marion Blakey
Administrator, Federal Aviation Administration

SUBJECT: Water Line Ribbon Heater Fires and Contaminated Thermal Acoustic Insulation Blankets

Background

On 13 May 2002, Air Canada Flight 116, a Boeing 767-300, registration C-GHML, with eight crew members and 177 passengers on board, was arriving at Toronto Lester B. Pearson International Airport on a flight from Vancouver, British Columbia. At about 1732 EDT, while on final approach, the flight crew received an aft cargo bay fire warning. The flight crew followed checklist procedures, activated the cargo bay fire extinguishers and declared an emergency. Although the fire indication went out approximately 50 seconds after activation of the fire extinguishers, a slight smell of smoke continued to be noticed by the cabin crew and flight crew. Flight 116 landed and stopped on the runway to allow a preliminary examination of the aircraft by airport firefighters. Firefighters, using infrared sensing equipment, did not detect any sign of heat from the fire. The aircraft was taxied to the terminal, but stopped approximately 40 feet back from the gate to allow firefighters to open the aft cargo compartment for a detailed inspection. When the cargo door was opened, a significant amount of smoke was observed. Firefighters entered the cargo compartment and confirmed that the fire had been extinguished. The emergency situation was secured and passengers were deplaned using portable stairs. A Transportation Safety Board of Canada (TSB) investigation (A02O0123) is ongoing.

 

To date, the investigation has determined that an Electrofilm « brand heater ribbon, used to prevent the potable water drain/supply line from freezing, failed and exhibited signs of overheating and arcing in the vicinity of station 1395. The heater ribbon, which was spiral wrapped around the water line, burned through both the protective tape used to hold the heater ribbon in place and the Rubatex « foam thermal insulation material wrapped on top of the protective tape, igniting the non-metalized Polyethylene Teraphthalate (PET)-covering (Mylar«)of the thermal acoustic insulation blanket mounted on the
vertical web of the floor beam. The fire then spread to the PET insulation blanket covering on the bottom of the pressurized hull and ignited debris in the non-fully enclosed floor area of the aft cargo compartment. The fire became self-propagating, burning its way forward, inboard and outboard, spreading approximately 46 centimetres (18 inches) up the right side wall of the aircraft before it was extinguished by halon from the fire extinguishing system. Heat from the fire was intense enough to burn holes through the aluminum web of a floor beam and significantly distort the top cap (chord) of the beam structure. Floor beam damage. Floor beam damage.

 

Heater ribbons are used on the Boeing 767 and other aircraft to prevent water lines from freezing. They generally consist of a number of heating elements encased in an insulating material such as vinyl, rubber (Ethylene Propylene Diene Terpolymer, EPDM) or silicone rubber. Typically, a heater ribbon system is thermostatically controlled with power being applied, both on the ground and in the air, when the water temperature in the line approaches freezing. In C-GHML, the aft cargo compartment ribbon heater control thermostat was located 85 inches aft of station 1395, below the bulk cargo floor. The thermostat was set to turn the ribbon heaters on when the ambient air temperature reached 50 F and off at 60 F. Although the exact failure mechanism of the heater ribbon on the occurrence aircraft is still under investigation, examination of other failed heater ribbons suggests that an internal short or arcing event between two of the elements occurred.

 

Discolouration, consistent with overheating while in service, was observed on some intact heater ribbons on C-GHML and on numerous other aircraft examined by investigators and Air Canada maintenance personnel. Overheating can cause the heater ribbon insulating material to degrade. This degradation may contribute, in some instances, to an internal short or arcing event between two of the heating elements, which can cause the Heater Ribbon Installation. Heater Ribbon Installation.
insulating material between the elements to carbonise and/or burn. A reduction of material separating the elements could allow them to migrate towards each other and arc again.

 

 

TSB has identified safety deficiencies related to the potential for heater ribbon installations to start a fire, and the potential for contaminated thermal acoustic insulation blankets and debris in the vicinity of heater ribbons to propagate a fire. Failed Heater Ribbon Failed Heater Ribbon.

 

Potential for Water Line Ribbon Heater Installation Failures to Start a Fire

In addition to the fire damage associated with the occurrence flight, TSB investigators found burned Cox and Company heater ribbon, protective tape and Rubatex « foam insulation behind the aft wall of the aft cargo area on the occurrence aircraft. Although a self-propagating fire had not occurred, the area had become hot enough to burn through the insulation wrap and a nearby plastic clamp.

Another Air Canada Boeing 767-300 was examined by TSB investigators, and burned Cox and Company heater ribbon was found in the vicinity of station 1395. In this case, the water line was wrapped in a "jacket" that consisted of an insulation blanket covered on one side with PET and on the other side with Bradley BF-6620, a polymer coated fabric. The "jacket" was held in place with a "hook and loop" fastener system similar to Velcro «. The failure had burned a hole through the "jacket". Further examination of the same aircraft revealed another overheated Cox and Company heater ribbon behind a side wall panel in the aft cargo compartment.

Air Canada conducted an inspection of its fleet of fifty-five Boeing 767-200 and 767-300 aircraft. Numerous occurrences of overheated / burned heater ribbons were found in both visually accessible areas and "hidden" areas, such as behind wall and floor panels. Thirty of the aircraft were found to have defective heater ribbons (including both Cox and Company and Electrofilm « brand), resulting in 66 ribbons being either removed or de-activated.

Between June 1985 and June 2002, operators of Boeing aircraft made a total of 67 reports to Boeing of heater ribbon failures where thermal degradation was evident. Charred insulation material was identified in many of the reports. Structural damage from fire had occurred in at least two cases. In some instances, water had leaked from melted water lines which, in one case, led to failure of an engine indication and crew alerting system (EICAS) computer.

Service Difficulty Reports USA 1999042300717, USA 1988040800197, AUS 19990967, and AUS 19991248 all report burned heater ribbons. Two of the reports involved Boeing 767s and two involved Boeing 747s. There have been no previous TSB safety communications concerning heater ribbons, and heater ribbons have not been addressed in TSB occurrence reports or in the TSB occurrence database.

The above information concerning heater ribbon failures supports the existence of an unsafe condition relating to the potential for water line heater ribbon installations to provide a source of ignition, combined with the availability of flammable materials in sufficiently close proximity to the ignition source to ignite.

Heater ribbons are used extensively in transport category aircraft, including Boeing 707, 727, 737, 747, 757 and 767 series and Boeing (Douglas) DC-9, DC-10, and MD-11 aircraft. In-line water heaters and water lines that contain integral heating elements are more prevalent on newer generation aircraft. Although historical data is limited, the possible failure modes of in-line and integral water heaters are considered to present a much lower threat of ignition than those associated with external heater ribbons.

Ribbon heaters manufactured for use in aircraft must comply with the requirements of US Federal Aviation Regulations (FAR) 25.853 and Appendix F to Part 25. These FARs specify the flammability characteristics of materials used in transport category aircraft. The ribbon heaters installed in C-GHML met these requirements; however, the FAR requirements may not adequately address the issue of fires associated with failures within specific heater ribbon installations.

The heater ribbon was protected by a circuit breaker (CB); however, the CB did not open. Not all electrical fault situations will cause a CB to open. The CB is designed to protect the circuit when the temperature and time duration characteristics of the over-current condition exceed the CB's design limits. Subsequent to the cargo bay fire, Air Canada took immediate action to reduce the risk of heater ribbon fires. An inspection of specified areas of the 767 aircraft fleet was conducted and defective heater ribbons in these areas were removed or de-activated. However, not all of the heater ribbons in the 767 aircraft were examined, nor were any heater ribbons examined on other aircraft types in Air Canada's fleet.

Air Canada amended their Boeing 767 Service Check (96 hour maximum interval) to include a requirement to remove all debris found below the floor level of both the forward and aft cargo compartments. They also enhanced their zonal General Visual Inspection (GVI) to ensure inspection of ribbon heaters during the scheduled 24-month "M" checks.

As a result of the fire on aircraft C-GHML, Boeing released Alert Service Bulletin (ASB) 767-30A0037 on 28 May 2002 to provide instructions and corrective action necessary to avoid a possible fire in the forward and aft cargo areas. The bulletin is applicable to all 767-200, 300 and 300F aircraft with non-fully enclosed cargo floors in the lower cargo areas. The bulletin called for operators to take the following actions with respect to visually accessible potable water and drain lines located under the cargo floor in the forward and aft cargo areas:

  • Remove all foreign object debris (FOD) found on, near or around the potable water and drain line.
  • Inspect all heater ribbons on the potable water and drain lines for excessive heat damage.
  • Inspect all heater ribbons on the potable water and drain lines for damaged or missing protective tape.
  • Replace heater ribbon and add protective tape, if necessary.

 

On 07 June 2002, the Federal Aviation Administration (FAA) issued Airworthiness Directive (AD) 2002-11-11 which reflected Boeing's ASB. The AD indicated that action associated with the AD is considered interim action until final action is identified, at which time the FAA may consider further rule-making. Action taken in accordance with the ASB and the AD should reduce the risk of fires associated with heater ribbons, but they do not adequately defend against risk in a number of areas:

  • They provide for a one time inspection only. Unsafe conditions that occur in the future could go undetected.
  • Action is not required to identify and remove damaged or failed heater ribbons that do not show signs of excessive heat damage. Consequently, the risk of these heater ribbons starting a fire will not be addressed by the ASB and AD. Similarly, the ASB and AD do not address the risks associated with heater ribbons that do not appear heat damaged but fail to pass a continuity check.
  • Action is not required to inspect heater ribbon that is covered in Rubatex « foam insulation or to inspect Rubatex « foam insulation that is covered in protective tape. Consequently, heat damage in these uninspected areas will not be found and a damaged heater ribbon could start a fire. The TSB investigation has found numerous examples of heat damaged heater ribbon and Rubatex « foam that were hidden from view.
  • Action is not required for non-visually accessible areas that contain heater ribbons. The TSB investigation found burnt heater ribbons in these areas. Consequently, risks associated with heater ribbon fires starting in these areas will not be addressed.
  • The required action is only applicable to 767-200, 300 and 300F aircraft. Because the unsafe condition exists on other types of aircraft, the risk of heater ribbon fires on these other types remains.
  • The ASB and AD caution against having less than one inch (2.54 centimetres) between the heater ribbon and any fuselage insulation. Although Rubatex « foam is commonly wrapped over the heater ribbons, there is no requirement to change existing heater ribbon installations. Consequently, the ASB and AD may not effectively reduce the risk of heater ribbon igniting Rubatex « foam.

 

On 15 July 2002, Transport Canada (TC) sent a letter to the Manager, Seattle Aircraft Certification Office FAA, expressing concern regarding the AD. The letter states:

Transport Canada understands that heater tapes are used in numerous other areas of the aircraft and is concerned that the subject AD does not address those areas. In addition, we are also concerned that the one time inspection and replacement of defective heater tapes (with new parts that are the same) called for by the subject AD will not eliminate the known ignition source because replacement heater tapes could fail in the same manner.

 

While TC expressed concern, they have not yet taken any independent action.

Widespread use of heater ribbons on transport category aircraft exposes the travelling public to the risks associated with heater ribbon fires. Recent actions taken to reduce these risks are not comprehensive and do not address the risk in the long term. Consequently there remains inadequate defences against heater ribbon installations starting a fire, therefore the Board recommends that:

The Department of Transport take action to reduce the short term risk and eliminate the long term risk , of heater ribbon installation failures starting fires, and coordinate and encourage a similar response from other appropriate regulatory authorities.
A02-04

 

Potential for Contaminated Thermal Acoustic Insulation Blankets and Debris to Propagate a Fire

During inspection of the occurrence aircraft and other 767 aircraft, TSB found contaminated insulation blankets and debris in all cargo compartments with open floors. Also, a considerable amount of blanket contamination in the form of dust, dirt, and lint was found under and behind panels in areas that are not readily accessible without the removal of panels. Subsequent to the occurrence, Air Canada examined the open forward and aft cargo areas of its 767 aircraft. A general clean-up of debris found in these areas was carried out. This action however did not address the contaminated blankets.

 

Contaminated thermal acoustic insulation blankets, as well as debris, in the vicinity of heater ribbon, provided fuel for the fire. For this occurrence, debris consisted of paper, candy wrappers, Styrofoam packing peanuts, small polyethylene beads, and rubber powder from a power drive unit. Samples of the burnt PET covered insulation blankets were analysed for the presence of fire accelerants. An isoparaffin solvent was detected which bore a resemblance to a product with Debris and burned insulation blanket Debris and burned insulation blanket.
the brand name ISOPAR L. A somewhat similar product is marketed under the brand name SOLTROL. These products are used for parts cleaning and degreasing applications, as well as for solvents in inks, paints, and agrochemical formulations, such as pesticides. ISOPAR L is a clear, colourless, combustible liquid. The liquid readily forms a flammable mixture with a flash point of 66 C (150 F). This product may have originated from sources such as aircraft cargo, luggage, recent repair/maintenance activities, or from pesticide products (the occurrence aircraft had operated in South America and may have been exposed to pesticides in association with operations in a tropical environment). The presence of the ISOPAR L contaminant would create a significant heat release once ignited. The relatively high temperature, localized fire damage observed on the floor beam web of the occurrence aircraft is consistent with a post-fire effect from the ISOPAR L alone, and/or in combination with combustible debris. It is plausible that liquid(s) might have migrated into the fibreglass batting of the thermal acoustical insulation blankets through an opening, or tear in the outer cover, or might have wetted surfaces between the airframe and the exterior of the insulation blankets. An electrical heater ribbon arcing event is considered to be a potential ignition source for any of the combustible liquid(s) in question.

 

Contaminated thermal acoustic insulation blankets have fuelled aircraft fires on other occasions. A Lan Chile Airlines aircraft (Miami, Florida; B767-375ER; 28 January 2002) had a fire in the forward (lower) cargo compartment which was fuelled by contaminated insulation blankets. These insulation blankets showed significant signs of contamination and wear. A Delta Airlines aircraft (Goose Bay, Labrador; Lockheed L1011; 17 March 1991) had a fire under the cabin floor area on the aft left side of the aircraft. A factor contributing to the severity of the fire was the large accumulation of dust and lint on aircraft components including insulation blankets in the area. As evidenced by Swissair 111, a self-propagating fire on board a transport category aircraft can have catastrophic results.

It is likely that many large transport aircraft contain sufficient amounts of contaminated thermal acoustic insulation blankets, dust, lint, and debris to sustain a fire.

In 1991, following an occurrence involving a fire in a Lockheed L1011 (TSB report A91A0053 refers), TSB issued a Safety Advisory (A910106) concerning the fire hazard associated with lint accumulation. The advisory suggested that TC notify maintenance inspectors and operators of transport category aircraft of the fire hazard and require that maintenance procedures be amended as required to ensure inspection and cleaning of areas where lint and debris can accumulate. In response, TC issued Service Difficulty Advisory AV-92-04 on 10 April 1992, which, in part, recommended that whenever planned inspections allow, an inspection be carried out for accumulation of lint, dust and cabin debris, and that visible accumulations be cleaned out to remove the fire hazards presented by them. In the United States, the National Transportation Safety Board (NTSB) issued recommendations A-91-71 and A-91-72 to address the safety deficiencies identified in the TSB occurrence investigation. NTSB recommended that the FAA notify principal maintenance inspectors and operators of transport category aircraft of the fire hazard posed by accumulations of lint and other debris on wire bundles. They also recommended that FAA require that transport category aircraft manufacturers and airlines amend maintenance manuals as necessary to ensure thorough inspection and cleaning of areas where lint and other debris may accumulate and pose a potential fire hazard. In response to the recommendations, FAA issued an airworthiness inspector's handbook bulletin, entitled "Origin and Propagation of Inaccessible Aircraft Fire Under Inflight Airflow Conditions". The bulletin provides information on the potential safety hazard applicable to all transport category aircraft from the accumulation of lint and other debris on wire bundles. It also requests that principal maintenance inspectors disseminate this information to all operators of transport category aircraft and review their operators' maintenance programs to ensure that they include inspection of aircraft wiring and removal of contaminants, especially in accessible areas.

In March 1998, as a result of a fire in a cargo compartment of a 747-200 freighter, Boeing issued service letters for all its aircraft models (Multi-Model Service-Related Problem 25-0103). An investigation by Boeing revealed that the presence of corrosion inhibiting compounds may have contributed to the fire, and could have been the reason the fire was not self-extinguishing. The investigation also looked at the accumulation of dust, lint, and other debris on insulation blankets in the outboard sections of the passenger/cargo compartment, and concluded that it was conceivable that a large buildup of contaminants on these blankets could ignite as a result of a high temperature source. The service letter informed operators that applicable Boeing manuals would be revised to address the effects of corrosion-inhibiting compound and other materials on the flammability of aircraft insulation blankets. It also informed operators that Boeing would provide presentations on this subject at future airline conferences to increase airline awareness. Operators were advised to increase attention to periodic inspections and cleaning of aircraft during maintenance to avoid insulation blanket contamination, and to remove foreign materials.

The issue of flammability of non-contaminated insulation blanket material is also of significant concern. The TSB issued safety recommendations concerning flammability test criteria (A99-07 and A99-08), and material flammability standards (A01-02 to A01-04) in association with the Swissair 111 accident investigation (A98H0003). The recommendations, however, did not specifically address the fire hazards associated with contaminated insulation materials, dust, lint or debris.

On 28 September 2000, FAA issued Flight Standards Information Bulletin for Airworthiness (FSAW) 00-09, entitled Special Emphasis Inspection on Contamination of Thermal/Acoustic Insulation. As well, on 08 November 2001, TC published Maintenance Staff Instruction (MSI) #42, Procedures for the Inspection of Thermal/Acoustic Insulation During Heavy Maintenance Checks for Contamination, which reflected the FAA requirements of FSAW 00-09. Recognizing that the flammability of most materials can change if the materials are contaminated, and that contamination may be in the form of lint, dust, grease, etc., which can increase the material's susceptibility to ignition and flame propagation, the FSAW and MSI require specific action on the part of Principal Maintenance Inspectors (PMIs) with responsibility for 14 CFR parts 121 and 125 operators. PMIs should ensure that the operator has established procedures in their approved maintenance program for the inspection of contamination on thermal/acoustic insulation during heavy maintenance checks. If the operator discovers contamination of the insulation, the operator should take corrective action, i.e., cleaning or replacement of the insulation as appropriate. As evidenced by the recent Air Canada and Lan Chile 767 cargo area fires, blanket contamination has persisted.

TC raised the issue of contamination at the International Aircraft Materials Fire Test Working Group. A Task Group has been formed to address this issue.

Air Canada's campaign to clean up debris will reduce the risk of readily available fuel. However; it will not reduce the risk of contaminated insulation blankets fuelling fires. The risk will not be reduced in those areas of Air Canada's 767s that were not inspected or in other aircraft types in Air Canada's fleet. Also, the risk will not be reduced in the long term.

The cargo area fire detection and suppression system did prove effective in this occurrence. The primary function of the system is to protect against the spread of fires outside the cargo compartment by suppressing the fire and keeping it in the cargo compartment. It is designed to do this until the aircraft can be landed. The cargo liner provides the last line of defence. When the cargo area fire suppression system on the 767 is armed, electrical power is cut to the galleys and some systems in the cargo area, but not the heater ribbons. Failure of a heater ribbon could thus continue to provide a source of ignition.

Although ASB 767-30A0037 and associated AD 2002-11-11 reduce the risk that a heater ribbon fire will propagate, they are not comprehensive defences for a number of reasons:

  • They provide for a one time inspection only. Unsafe conditions that occur in the future could go undetected.
  • Action is not required for areas that contain heater ribbons that are not visually accessible. The TSB investigation of this occurrence indicates that areas that are not visually accessible often contain a considerable amount of dust, dirt, and lint.
  • Consequently, risks associated with heater ribbon fire will still exist for these areas.
  • The required action is only applicable to 767-200, 300 and 300F aircraft. Because the unsafe condition exists on other types of aircraft, the risk of fuelling heater ribbon fires on these other types remains.

 

The wide spread existence of contaminated thermal acoustic insulation materials and debris on transport category aircraft exposes the travelling public to the risk of a self propagating fire. Recent actions taken to reduce these risks are not comprehensive and do not adequately address risk in the long term. Consequently, there remains inadequate defences against contaminated insulation materials and debris propagating a fire, therefore the Board recommends that:

The Department of Transport take action to reduce the short term risk and eliminate the long term risk, of contaminated insulation materials and debris propagating fires, and coordinate and encourage a similar response from other appropriate regulatory authorities.
A02-05

 

As the investigation proceeds, should the Board identify additional safety deficiencies in need of urgent attention, it will make further safety recommendation.

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