Printed headline: Flammability Testing
Fires onboard aircraft mercifully are extremely rare, but if one does occur, it can lead to the loss of that aircraft and passengers and crew in relatively short order.
There is a general view in the industry that in the event of a major fire onboard, the crew has an average of 17 min. to land the aircraft safely. During that time, it is critical that the fire is contained so it does not spread through the cabin or other areas—and that is where flammability and fire-resistance testing comes in.
Flammability testing of many parts of the aircraft is required, from interior panels and seats in the cabin to internal components and systems such as the auxiliary power unit (APU). NTS, based in Anaheim, California, confirms it assesses an enormous variety of aircraft systems and components for resistance to fire. The company typically will perform flammability and fire-resistance tests on “everything that is on the interior or exterior of the aircraft,” explains Bill Arnold, program manager at the company’s lab in Fullerton, California. He says NTS laboratories must “recreate life-like environments” to ensure new products will withstand potential onboard fires. “This includes every section from inside to outside, even under conditions such as vibration,” he says.
The rationale behind flammability testing is to ensure an aircraft can cope in the short-term with an onboard fire, giving the pilots time to take evasive action to protect passengers and crew. “Flammability testing is critical to safety,” explains Jarod Triplett, president of Illinois aircraft interiors specialist Skandia, which has an FAA-recognized flammability test laboratory. “The intention is to use materials that are so flame-resistant that if an inflight fire occurs, there is enough time to land the plane and evacuate all passengers and crew before the aircraft cabin flashes over.”
John Moylan, general manager at Element Materials Technology in Los Angeles, which specializes in testing aircraft interiors, agrees the aim of testing is to do “everything we can to give people time to get off the aircraft before a fire event becomes serious.” The FAA has prescribed a series of different flammability tests for the materials that go into aircraft interiors, he notes.
The most common types are ignition tests—12-sec. or 60-sec. vertical ignition—in which a Bunsen burner is applied to samples of materials. The items to be tested are suspended vertically or horizontally. The duration of the burn and the rate at which the material burns once the burner is removed are measured, as well as the amount of smoke generated. Toxic gases within the smoke also are measured and assessed.
The FAA has criteria based on the burn time and length that determine whether materials can be used onboard. “Often in fire events, it is the smoke that kills people, not the flames, so the toxicity of the smoke is a very important factor,” Moylan explains. Smoke is also a factor when it comes to cabin visibility, which has an effect when an aircraft is being evacuated. At Element, the “heat-release rate”—the amount of energy that materials release when they are burned—also is measured. This is effectively a measure of the fuel that is available for a fire, and is another area covered by FAA regulations.
There tend to be many commonly used materials within aerospace interiors where the flammability properties are known, but the introduction of newer materials requires new tests, particularly in the business jet market. “People may want to use more exotic materials. We do a lot of testing of materials for luxury and business-jet interiors as a result,” Moylan says. If a material fails an ignition test, it cannot be used on an FAA-certified aircraft.
At Skandia, testing is performed on all interior components as standalone items. Additionally, tests are required on “as-installed configurations,” says Triplett. For example, if two materials are adhered in the cabin, they must be fire-tested separately and as a combination. Much like at Element and NTS, Skandia tests measure the length of the burn of the material and the duration after the fire source is removed from the specimen. For example, cushions undergo a flammability test measuring the length of the burn across the specimen and the percentage of weight loss during the testing period.
Heat-release tests will apply a flame and then measure the amount of heat given off by a material, while smoke-density tests measure what is emitted from the test sample. “Airbus and Boeing both have standards for acceptable levels of toxic off-gassing from materials,” Triplett explains.
Facilities at Skandia include two Bunsen-burner test chambers, three oil-burn laboratories, separate test apparatus and an 800-ft.2 conditioning room. “Ultimately, all items must show a certain level of flame resistance,” Triplett says. “But the standards and test requirements vary, based upon the type of the operation of the aircraft, number of passengers and where the materials are being installed.”
NTS also carries out a wide range of flammability tests in its dedicated laboratories. These include evaluations for ignition, fire resistance and fireproofing. Flame-spread tests on materials might take 5-15 min. The material passes the test if no backside ignition, burn-through or sustained flame is observed. “The main objective is that the material will self-extinguish immediately or soon after the flame is removed,” Arnold says.
At NTS’ Fullerton laboratory, lengthier fire-resistance and fireproofing tests use much larger kerosene burners with nozzles set up to burn at 2,000F and at a minimum of 4,500 Btu per hour. These burners are used to assess the fire resistance of larger items such as aircraft engines and firewalls. A fire-resistance test takes 5 min., and a fireproofing evaluation lasts 15 min. “The aim is always to give the pilot time to shut down the engine if necessary and engage the extinguisher systems,” Arnold says.
The objective of these more intense tests can be to ensure there is no burn-through on a firewall, for example. “Backside ignition-testing” measures the likelihood of a firewall becoming hot enough on its backside to set off a blaze elsewhere, he notes. “We provide the fire. The objective is that the item under test doesn’t add to that hazard. You don’t want it to leak, burn through or experience backside ignition.” Samples under test include not just firewalls but mechanical components such as actuators, seals and valves or new types of composite materials.
If fire-resistance testing identifies a problem, a customer may change the design or the material. Composites and silicone, aluminum and stainless steel materials are among those tested at Fullerton. “There’s a wide variety of materials we test,” says Arnold.
The standards for fire resistance are only likely to become more stringent in time. Engineers at NTS are working closely with the FAA to upgrade flammability and fireproof test regulations, they say.
The aim is to standardize the regulations and bring them in line with ISO 2685, the European equivalent. The European Aviation Safety Agency also is helping the FAA to rewrite the specifications, NTS says.
Types of Aircraft Fires
- Engine Fire - An engine fire normally is detected and contained by the aircraft fire-detection and suppression systems. In certain circumstances (e.g., an engine explosion), the nature of the fire is such that onboard systems may not be able to contain the flames, and they may spread to the wing and/or fuselage. Where an engine fire has been contained, there is still a risk that it may reignite, so it is advisable for the crew to land the aircraft as soon as possible and allow fire crews to visually examination the engine.
- Cabin Fire - A fire within the cabin usually will be detected early and contained by the crew using onboard firefighting equipment. As with an engine blaze, it is still advisable to land the aircraft as soon as possible and carry out a detailed examination of the cause and any damage.
- Hidden Fire - A hidden fire may be detected by onboard detection systems or by the crew or passengers noticing smoke or fumes, a hot spot on a wall or floor, or by unusual electrical malfunctions, particularly when the systems are unrelated. This is the most dangerous type of fire because hidden ones are difficult to locate and access. The time delay may allow the fire to take hold. A hidden fire initially may be difficult to confirm, and the crew may be slow to initiate an emergency landing. The consequence of such a delay may be that the fire becomes non-survivable before the aircraft can be landed.
Source: Flight Safety Foundation Skybrary