Detect and Suppress

Existing fire-protection regulations for cargo airplanes are inadequate, and action is needed to improve the detection and suppression of blazes in cargo containers, the U.S. National Transportation Safety Board (NTSB) says.

The agency cited information gathered in its recent cargo container fire study and the investigations of three in-flight cargo airplane fires — a February 2006 fire on a United Parcel Service (UPS) McDonnell Douglas DC-8-71F; the fatal September 2010 crash of a UPS Boeing 747-400F; and the fatal July 2011 crash of an Asiana Cargo 747-400F (“In-Flight Fires”) — in issuing three safety recommendations in late November to the U.S. Federal Aviation Administration (FAA).

“These fires quickly grew out of control, leaving the crew with little time to get the aircraft on the ground,” NTSB Chairman Deborah A.P. Hersman said. “Detection, suppression and containment systems can give crews more time and more options. The current approach is not safe enough.”

The NTSB’s recommendations call on the FAA to:

“Develop fire detection system performance requirements for the early detection of fires originating within cargo containers and pallets and, once developed, implement the new requirements;

“Ensure that cargo container construction materials meet the same flammability requirements as all other cargo compartment materials in accordance with [U.S. Federal Aviation Regulations (FARs) Part 25.855]; and,

“Require the installation and use of active fire suppression systems in all aircraft cargo compartments or containers, or both, such that fires are not allowed to develop.”

Cargo aircraft currently are subject to the same FAA fire-protection regulations that govern all transport category aircraft, the NTSB said.

“Although these regulations limit the flammability of construction materials used in cargo compartments and also specify minimum fire resistance requirements for cargo compartment liners, there is limited regulation concerning fire protection associated with cargo containers,” the NTSB said.

For example, the agency noted that materials selected for the construction of cargo containers undergo a horizontal Bunsen burner test, “which does not prevent the use of highly combustible materials.”

In addition, “the effect of the use of containers and pallets to contain cargo is not factored into the current overall fire protection strategy or certification process,” the NTSB said, noting that the certification process is conducted using empty cargo compartments.

In a letter to then-Acting FAA Administrator Michael Huerta that accompanied the safety recommendations, the NTSB discussed the findings of accident investigations and a series of tests conducted in August 2011 to develop a better understanding of cargo container fires and the most appropriate prevention strategies.¹

The tests — designed in part to examine the burning characteristics of cargo container fires — prompted researchers to conclude that “container design has a significant effect on the time it takes for an internal fire to become detectable to a smoke detector outside the container” and that “container construction materials have a significant effect on the total fire load² and energy release rate of a cargo fire,” the NTSB said.

In the two accidents in 2010 and 2011, investigators found “a relatively short interval between the time a fire warning indication was delivered to the flight crew and the onset of flight control and aircraft system failures,” the NTSB said. In the fatal UPS crash, about 2 minutes 30 seconds elapsed between the first fire warning and the loss of some aircraft systems; timing information has not been released in the ongoing Asiana investigation, the NTSB said.

The NTSB’s report on the 2011 tests, published in a report in March 2012, concluded that “the time it takes for a fire detection system to detect a fire originating within a cargo container may easily exceed the one-minute time frame specified in … Part 25.858(a)” and that “the growth rate of container fires after they become detectable by the aircraft’s smoke detection system can be extremely fast, precluding any mitigating action and resulting in an overwhelming fire.”

In tests of cargo containers, the NTSB found that the time between fire initiation and fire detection ranged from 2 minutes 30 seconds to 18 minutes 30 seconds — longer than the one-minute detection time currently required.

“The fires grew very large, capable of causing significant damage to an aircraft, shortly after becoming a detectable fire,” the report said. “The NTSB is concerned that, when fires inside containers become detectable to the aircraft’s smoke-detection system, there is little time until the fires reach levels that can compromise the integrity of the cargo compartment and then threaten the structure and systems of the aircraft. …

“If the fire were to be detected while generating smoke inside the container, valuable time would be gained for alerting flight crews and mitigating the effects of the fire.”

Because existing regulations dealing with flammability limits are “very limited” for cargo container materials, those materials can significantly increase the fire load within a cargo compartment, the NTSB said.

For example, the agency cited collapsible containers made from corrugated polypropylene as “significant contributors” to fire intensity.

Fire Suppression

Most current practices base fire suppression in main deck cargo compartments on passive suppression systems, such as the use of fire-resistant materials and oxygen deprivation. Because the compartments are so large, however, fires can become very large before oxygen deprivation slows their growth, the NTSB said.

In the 2006 UPS blaze, the agency said, “the aircraft did not achieve depressurization [which aids in suppressing flames] until after system failures and flight control issues began to occur.”

Tests by FAA researchers have found that, although depressurization contributes to fire suppression, when an aircraft descends to a more oxygen-rich environment, the fire again begins to grow.

“Hence, experience from the UPS [Dubai] accident, as well as FAA experiments, suggest that passive fire suppression in large cargo compartments due to oxygen deprivation may not be effective,” the NTSB said.

The agency noted that, in 2007, as a result of its investigation of the 2006 fire, it had recommended that the FAA require fire-suppression systems for the cargo compartments of all FARs Part 121 cargo airplanes. The NTSB reported that the FAA’s response had been that the cost of installing “compartment-flooding fire-suppression systems, as those used in Class C cargo compartments,³ was not justified for the main deck cargo compartments of aircraft of any weight.”

However, the NTSB said that the fires in 2010 and 2011 “continue to demonstrate the critical need to suppress cargo fires.”

As an alternative to the compartment-­flooding system evaluated by the FAA, the NTSB suggested alternatives, including the “aircraft-based system” used by FedEx and in-container suppression systems being developed by the industry.

‘Multi-Layered Approach’

The NTSB’s issuance of the safety recommendations coincided with an announcement by UPS that it had developed a “multi-layered approach consisting of matched solutions that include checklists, training and new technologies” to mitigate in-flight cargo fires.

Among those new technologies are fire-resistant fiber-reinforced plastic containers, experimental fire-suppression units that “smother a fire with potassium aerosol powder and can save 95 percent of packages in the container” and fire-containment covers for palletized cargo.

The approach was developed by a UPS/Independent Pilots Association task force that had identified as its first step “increasing the time a crew had to manage a smoke or fire event,” said Capt. Bob Brown, a task force member.

The group’s goal was to contain a fire inside a unit load device (ULD) for four hours. In a test in October, a ULD containing 215 packages, including “20 working laptops with batteries, 50 working cell phones with batteries and 300 bulk-shipped lithium ion batteries, was set on fire by six lithium ion batteries,” Brown wrote in Leading Edge, the UPS flight operations and safety magazine.⁴ “Although temperatures reached as high as 1,200 degrees [F (649 degrees C)], the fire was suppressed for four hours and 95 percent of the packages were undamaged. Even the laptops worked.”

UPS said that it also is installing quick-donning integrated oxygen masks and smoke goggles in all aircraft, and the VisionSafe Corp. Emergency Vision Assurance System (EVAS), designed to displace smoke in a pilot’s vision path to allow him or her to see basic flight instruments and the flight path, as well as emergency checklists and navigation charts.⁵

Automatic Suppression Systems

FedEx Express began installing on-board automatic fire-suppression systems in its aircraft in 2009, the same year it won the FSF-Honeywell Bendix Trophy for Aviation Safety for developing the devices (ASW, 11/09, p. 39).

The system incorporated infrared heat sensors, foaming-agent generators and an overhead cargo-container injector. If the sensors detect heat in a cargo container, the overhead fire-suppression equipment activates, piercing the container and injecting argon foam. At the same time, the crew is alerted.

Notes

1. NTSB. Materials Laboratory Study Report No. 12-019. March 21, 2012. In addition to addressing the burning characteristics of container fires, the study also examined the fire-load contribution of lithium and lithium-ion batteries. The NTSB noted that the involvement of these types of batteries “has come into question” in both the 2006 fire and the 2010 fire.
2. Fire load is defined by the NTSB as “the amount of combustible material that can become involved in a fire.”
3. Class C cargo compartments have smoke or fire detector systems that provide warnings on the flight deck; built-in, pilot-controlled, fire-suppression systems; methods of excluding hazardous amounts of smoke from any occupied portions of the airplane; and methods of controlled compartment ventilation.
4. Brown, Bob. “UPS/IPA Safety Task Force Pioneers Advancements in Aviation Safety.” Leading Edge (Fall 2012): 2.
5. VisionSafe Corp. EVAS.