Aircraft fire testing
Often, on-site test labs are fully equipped and qualified to perform multiple types of aircraft fire testing required to demonstrate compliance with aircraft flammability regulations.
There is frequently confusion between the different categories of fire testing basically defined as follows
• Fireproof is the capacity to withstand the heat associated with fire to the highest level, to withstand at least as well as steel in dimensions appropriate for the purpose for which they are used.
• Fire resistant is the capacity to withstand the heat associated with fire at least as well as aluminium alloy in dimensions appropriate for the purpose for which they are used.
• Flame resistant means not susceptible to combustion to the point of propagating a flame, beyond safe limits, after the ignition source is removed.
• Flash resistant means not susceptible to burning violently when ignited.
These are general terms used to define tests. Some of these definitions allow for variations. For example, flame resistant refers to safe limits.
These limits vary depending on the material, or where and how a material is used, and this is reflected in the regulations, for example, aircraft interiors fire performance FAA 14 CFR:Part 25 or Resistance to Fire in Designated Zones ISO 2685:1998. Some tests are quite specific.
As steel is considered to be fireproof and aluminium to be fire resistant, unfinished metal parts could already be assumed to meet the flame resistance and flash resistance tests. However, parts containing alloys for example, do require testing.
Companies who do this are approved to perform the following types of fire testing:
• Vertical and horizontal Bunsen burner tests
• 45 degree Bunsen burner test
• 60 degree Bunsen burner tests for wires and cables
• Oil burner tests for aircraft seat cushions
• Fire containment testing for waste compartments
Fire testing of fuselage burnthrough resistance
Fire testing has shown us that burning aviation fuel can penetrate into an aircraft cabin through air return grilles, seams, joints or window reveals. Even aluminium sidewall panels offer minimal burnnthrough resistance. FAA researchers are focusing on the thermal acoustical insulation as the most potentially effective and practical means of achieving a burnthrough barrier.
Airborne equipment environmental testing.
All materials used in an aircraft cabin or cargo compartment must meet the applicable flammability requirements for that aircraft type. Such materials include, but are not limited to, interior panelling, floor coverings, seat cushions and upholstery, seat belts, curtains, decorative furnishings, padding, galley structure and furnishings, transparencies, stowage and baggage compartment structures, thermoformed parts, cargo liners, and insulation materials.
When repairing or replacing the interior material in an aircraft, compliance with the applicable material flammability requirements must be certified.
The organisation carrying out the repair or refurbishment needs to ensure compliance with the appropriate standards for the aircraft being refurbished.
Minimum requirements for aircraft are dependent on the airworthiness category of the aircraft. Standards are periodically revised due to inadequate service history or new technologies. EASA FAR & JAR all provide certification standards.
Whilst these standards are mostly harmonised, not all flammability requirements are the same and harmonising of changes to standards have occurred at different dates.
The certification basis of an aircraft is defined in the Type Certificate Data Sheet (TCDS).
Occasionally retroactive action is warranted to correct identified unsafe conditions in the flammability standard of the aircraft fleet. This retroactive action can be aircraft type specific but for flammability issues tends to be more generalised and applicable by aircraft weight or passenger capacity.
For new aircraft types since 1984, there has been a steady increase in the number of tests required for various aircraft interior components. These address specific tests for critical areas of the cabin and are in addition to the general Bunsen burner test for most cabin materials. There are additional tests for seat cushions, ceiling and wall panels, cargo compartment liners, and insulation materials. This increase in the number of tests will continue with new standards for electrical wiring and air conditioning ducts likely in the future.
Additional to these new flammability standards for new aircraft designs, the same requirements were made retroactive for some existing aircraft, mainly based on passenger capacity. The outcome of this action is many flammability requirements for older transport category designs are no longer entirely defined by their certification basis. Specific tests include:
• Oil burner tests for seat cushions
• Radiant heat release and smoke criteria for interior ceiling and side-wall panels
• Burnthrough criteria for cargo compartment liners
• Flame propagation of thermal/acoustic materials
• Flame penetration of thermal/acoustic materials
Cabin material in aircraft must meet the applicable flammability standard. There are three ways to ensure this:
1. Carry out all component replacements or repairs in accordance with approved material and data (e.g. aircraft manufacturer’s parts and instructions, STCs, etc),
2. Obtain material which has been tested to the appropriate standard from an approved supplier, or
3. Obtain commercial material and organise samples to be tested for their flammability.
An aviation materials supplier should supply batch test certificates for the materials, ensuring that these certificates cover the required flammability standard adequately.
Fire retardant chemicals
If fire retardant chemicals are used to provide the required flammability properties of the material, the long term performance must be taken into consideration to ensure the material in service continues to meet the certification standard.
Fire retardant performance deteriorates with wear and, particularly, cleaning. Therefore, the service life of material treated with the fire retardant must be established and appropriate maintenance instructions provided to ensure continuing airworthiness of the cabin interior.
An aircraft which has been refurbished with material treated with fire retardant may no longer meet the flammability requirements after time in service. If a service life or instructions for reapplication of the retardant have not been established, then the effectiveness of the retardant should be treated with suspicion.
Even adhesives and laminates used in repairs require flammability tests assembled in a representative manner. Amongst other issues, research has shown that the increase in heat released from the addition of a new surface finish or laminate is variable and dependant on the underlying substrate material, thus showing an indirect relationship between the flammability of individual materials and the laminated assembly.
However, families of materials with similar construction methods but varying thickness may only require testing of the critical configurations. The FAA is developing methods to qualify adhesives independently.
Aircraft seat covers flammability testing
Seat covers are substantial in size, and have the potential to increase the fuel load and ignitability of the interior. They must therefore be an approved modification that meets the requirements of the certification basis of the aircraft to which they will be fitted.
There are certain items of an operational nature that are not required to undergo a flammability test. Blankets, pillows, headrest protectors, and items that are not part of the aircraft type design are not required to meet the flammability certification standard. However, incidents have occurred with fires starting and propagating in airline comfort blankets. The FAA have a specifically designed flammability test for blankets that is recommended.
A recent problem for the major aircraft manufacturers occured when a seat supplier was accused of falsifying records on its flammability tests, an issue that Airbus and Boeing investigated and believed was rectified.
The deception delayed aircraft deliveries.