A fire in an aircraft poses a significant threat to life; owing to the large quantities of highly flammable fuel and limited possibilities for escape, the threat is particularly high compared to other situations.
To reduce the fire risk for those on board aircraft, a combination of approaches is required:
- On-going training of flight crews in fire emergency procedures
- Use of mechanically resistant fuel tanks and modified fuel composition to limit the spillage of combustible liquid and formation of ignitable fuel-air mixture
- Fire containment with the use of fire resistant structures to provide protection from fires in the fuselage and other high risk areas
- Testing of electrical and oxygen systems to reduce the likelihood of system faults
- Installation of fire alarm systems and effective automatic extinguishing equipment to increase available safe escape time
- Ensuring emergency exit slides and ramps can withstand radiant heat
- Use of materials and composites which do not add to the fire risk in terms of heat, smoke and toxic fume production
With extensive use of polymers and composites in the construction of an aircraft, this article focuses on how to improve safety by ensuring such products have appropriate fire performance in terms of limited ignitibility, flammability and secondary fire effects.
How it began
In the past, fire safety and associated testing was an afterthought in the aviation industry, with the focus being passenger convenience. The first flammability regulations were adopted between 1940 and 1960 and applicable to aircraft with 50-150 seats. Fire testing was limited to vertical flame spread assessments and many in the fire testing industry lobbied for a more comprehensive approach to testing to prevent fatalities.
In 1983, Air Canada Flight 797, a DC-9 flying at 33,000ft on route from Dallas to Toronto brought to bear what many had feared. The first signs of trouble were the wisps of smoke wafting out of the rear lavatory. The fire spread within the walls and throughout the passenger compartment and soon involved the more combustible items within the aircraft such as the plastics and seat cushions. Thick black smoke filled the cabin and the plane began an emergency descent. The conditions intensified when the plane landed and the exit doors were opened, allowing for a fresh supply of oxygen (flashover occurred ~1 minute after the doors were opened). 23 of the 46 people onboard died.
The Federal Aviation Administration (FAA) subsequently mandated that aircraft lavatories be equipped with smoke detectors and automatic fire extinguishers.
Additionally, the authorities instated new regulations relating to the fire performance of aircraft materials. Within five years, seat cushions were retrofitted with fire-blocking layers. Aircraft built after 1988 have more flame-resistant interior materials as additional tests became mandatory. These regulations applied to transport category aircraft, i.e. 20 seats or greater.
How we assess materials today
Materials and composites used in the construction of transport category aircraft must comply with national and international regulations with most countries having adopted the U.S. Federal Aviation Regulations (FAR). These regulations consider that limiting flammability and smoke density will adequately control the toxic fume production. This said, aircraft manufacturers such as Airbus, Boeing and Bombardier, impose additional toxicity requirements. The flammability and smoke density test methods for a given application are largely standardised, based on the FAR 25.853 standard.
Bunsen burner tests
These tests are used to assess a material’s flammability using a small Bunsen burner flame with 38mm in length and at 843˚C. The tests apply to almost all parts in the cabin and materials are assessed using the vertical flame test. The flame is applied to the lower edge of the specimen for either 12s or 60s (dependent upon application). In order to be classified as ‘self-extinguishing’ the material must show limited flame spread and after flaming, and flaming droplets must extinguish within a given time.
In addition to being tested in the vertical orientation, liners for cargo compartments must be tested at a 45˚ angle; for larger aircraft this test is only conducted on the floor panels with a separate Kerosene burner test being conducted on the walls and ceilings. The flame is applied for 30s, and no flame penetration is allowed; additionally there must be limited after flame and glowing time.
Windows are tested in a horizontal orientation, with the flame impinging on the specimen for 15s. This test method is also used in the automotive industry. The parameter used for classification is the rate of flame spread across the specimen.
Finally, there is a 60˚ test for wires and cables, which is conducted in order to classify them as ‘self-extinguishing’. When exposed to the flame for 30s, there must be no wire breakage, limited flame spread and after flaming; flaming droplets must extinguish within a given time.
Kerosene burner tests
The kerosene burner seat test was introduced in 1984. In this test, the burner attacks the side of the seat for 2 minutes, which is intended to replicate a post-crash fire burning through the fuselage. The percentage weight loss must not exceed 10% and two of the three tested seats should have a burned length that doesn’t exceed 430mm. These requirements aim is to ensure the seat does not contribute significantly to the heat and smoke production.
For large aircraft, the wall and ceiling lining of the cargo compartment is assessed using a kerosene burner test (much more severe ignition source that the Bunsen burner). The wall and ceiling panels are replicated in the test, with the flame being applied for a period of 5 minutes. The panels must not be penetrated during the test, and peak temperature 102mm above the panel must not exceed 204˚C.
Smoke development test
These tests are used to access the density of the smoke produced when the specimen ignites. The test is conducted in a 0.5m³ chamber where the specimen is exposed to both radiant and direct flame heat sources and the obscuration of a white light beam is used to measure smoke density.
The toxicity test is generally conducted in conjunction with the smoke density test. These toxicity tests, which have generally been developed by aircraft manufacturers, tend to utilise different test procedures. The results from one test cannot be automatically equated to another.
Rate of Heat Release test
These tests are used to measure the speed of heat evolution for materials under controlled conditions. Typically, The Ohio State University (OSU) heat release rate apparatus is used.
Vertically-oriented test specimens are placed within an insulated test chamber and exposed to a radiant heat and pilot flame insult. As specimens burn, temperature rise is measured and calculations are made to express energy produced per unit area and power produced by unit area.
How to assess materials in the future
Whilst product performance assessments employed today significantly reduce the fire risk, there is still room to improve.
In the last 10 years, over 3000 fire incidents have been recorded with around 40 being fatal [FAA]. With the expected growth in air travel, fire fatalities are said to increase by four percent annually. Consequently, we aim to further improve fire safety.
Between 2011 and 2014, the European Commission funded the project ‘AircraftFire’ which aimed to increase passenger survivability during major fire scenarios, particularly in new generation aircraft, e.g. A350 or B787. The project produced simulations of fire events, which input fire test data into models and combined them with the results of the project’s evacuation time modeling and fire detection / suppression review to enable regulators to decide if / how current fire safety practices need to be adapted.
Due to the international nature of aviation, greater attention must be paid to the harmonisation of aircraft material fire testing standards. Although most fire tests for aviation materials are the same through different jurisdictions, aircraft manufacturers police their own projects and impose additional fire tests they believe relevant. Although not a regulatory requirement, we see a variety of different toxicity tests conducted and a practice that would now be hard to stop. The toxicity test would benefit from harmonisation, plus modernisation more in-line with that taken in the European rail and internal marine industries.
Additionally, with the continuing use of private planes and commuter category aircraft, i.e. less than 20 seats, one would statistically expect greater fire incidents. Those planes are not required to meet the same flammability requirements as the transport category aircraft and although loss of life would be limited, efforts should also be placed on improving and testing the materials used in smaller aircraft.
Regulations are in place, which mandate the assessment of products fire performance. For each product application, most of the mandatory tests required are standardised across the global aviation industry. However, toxicity tests specified by aircraft manufacturers are not standardised. The tests and criteria are specific to the product application. Work is ongoing to develop the material fire test assessments. Exova can provide advice on the test methods and criteria that apply on a case by case basis.
For more information, go to www.warringtonfire.com