BY FRANÇOIS JACQUET and RON GOULD
It was supposed to be a one-time experiment, an interesting way of disposing of a retired research airframe. In a four-day workshop named the “Fast Fracture Test,” the National Research Council of Canada (NRC) and Ottawa MacDonald-Cartier International Airport Authority (OMCIAA) expanded their collaboration and understanding of issues related to aircraft firefighting.
In May 2007, with the support of Canadian Bomb Data Centre officers of the Royal Canadian Mounted Police (RCMP), an explosive device was detonated among the luggage in the partially pressurized and instrumented aft cargo hold. The object was to overpressurize and fracture the structure and then collect the structural evidence for forensic and metallurgical examination.
The OMCIAA, RCMP, and NRC had collaborated previously by establishing another Boeing 727 (B727) as a full feature training aid positioned adjacent to the airport firehall. Not many fire departments have a real aircraft for training and, understandably, they were unwilling to have theirs blown up.
Since being withdrawn from use as a cargo-only transport, the NRC airframe had been stripped internally to permit fatigue and corrosion damage inspections and aging wiring studies. The fuselage had also been partially dismantled, which resulted in the constraint that only the aft main deck and cargo hold could be pressurized. Attendance by OMCIAA fire personnel, supported by the Ottawa Fire Service (OFS), was essential to ensure the survival of the fractured structure of interest should there be a postblast fire. Months of planning, blast calculations, security arrangements, and equipment set-up resulted in a misfire. The device did not fully detonate, but it did initiate a fire involving the luggage (photo 1).
|(1) National Research Council of Canada (NRC) test aircraft. (Photos courtesy of the National Research Council of Canada.)|
1 The unregulated materials comprising the luggage and its contents can negate the controlled flammability and smoke regulations mandated to the materials employed in the construction of the aircraft.
The entry doors, hatches, and windows of the pressurized zone had received additional sealant to aid with the pressurization. After the cargo had been loaded, the aft cargo door was also sealed. Because this door was borrowed from another aircraft and had to be returned undamaged, this could not be the point of entry to combat the fire.
Fire had first been detected in the remote command post from a camera view of the main-deck area and was reported by radio to the incident commander (IC) when flames and smoke were observed rising above the floor along the side wall (air distribution ducts removed).
To ensure the safety of the firefighters, the range safety officer communicated directly with the IC to establish that the explosives had been spent before the airport and city firefighters went into action. The IC established four sectors: interior, exterior, backup, and water supply. The exterior team, based on the constraint to not damage the cargo door, was required to penetrate through the aft cargo fuselage structure on the opposite side (port) using a piercing nozzle. A water supply was established using an OFS tanker. Because the main-deck area of the aircraft had been stripped of insulation and interior panels, the fire had little fuel available to progress upward. Access to the interior was made using the rear air stairs, a feature of the B727 but not common in other commercial transport. An attack line (1¾-inch) flowing AFFF 3 percent was used to stop the vertical spread of the fire into the cabin. The fire was fought through the test-related hatch opened in the main-deck floor. An engine company from the OFS provided a backup hoseline.
The fire was contained within five minutes. The interior team cross-ventilated the cabin by opening the rear air stair door, entering onto the main deck, and proceeding forward to open the over-wing exits. The sealant added to pressure seal the over-wing exits, and the reduced visibility increased the difficulty of this task.
In less than 45 seconds, the interior visibility had been reduced to nil. The experimental setup did not provide for emergency floor lighting that might be available in a real incident.
2 Although the cargo configuration of the main deck obviated the hazard of negotiating between seat sets, the floor in the area of the over-wing exits was incomplete, and some firefighters had difficulty negotiating the exposed seat tracks on top of the center fuel tank in the smoke-filled interior. The convenient presence of the rear air stair led the interior team to select this entry route. Had the fire been more intense or lasted longer, they may have been traversing a compromised floor system. Firefighters should not engage in interior aircraft firefighting if outside fires threatening the integrity of the fuselage are not contained. The more typical over-wing approach to the wing exits would have avoided this. In a later classroom session, the firefighters were shown samples of floor panel structures, were given an explanation of their adhesively bonded construction, and were able to examine damaged samples. Aircraft cabin floors should not be breached, since various systems run through them (electric cables, fuel distribution lines, and high-pressure hydraulic lines, for example). Firefighters should know about the locations of these systems throughout the structure prior to breaching or piercing.
On the following day, supporting bomb technicians and police forensic laboratory personnel were to conduct the postblast investigation. Again, the cargo door issue presented an unanticipated complication. An access hatch had been cut through the main-deck floor into the cargo hold to permit installing the explosive device at the last minute before pressurization. Initial access to the cargo hold would also be through this hatch until an area behind the cargo door could be cleared of luggage and blast-protection panels. The smoke composition of the burned luggage was found to have persisted at a level too toxic to permit entry without self-contained breathing apparatus (SCBA). To resolve this, the test pressurization compressor system was operated until air sampling proved the atmosphere was safe for breathing (photo 2).
|(2) During the postblast investigation, supporting bomb technicians and police forensic laboratory personnel wore self-contained breathing apparatus until air sampling confirmed that the toxic smoke from the burned luggage in the cargo hold was dissipated and the air was safe for breathing. Note the missing floor panels near the emergency exits.|
Following the workshop, the aircraft was moved to a fenced compound to be scrapped. As airliner incidents are not confined to the airport boundaries, both airport and city firefighters were invited to participate in dismembering the airframe by applying their tools and techniques.
3 Hydraulic spreader-cutters are not effective for creating or enlarging a hole in a tubular aircraft fuselage. The materials and cylindrical shape cause the skin to tear or fracture before displacement, and it takes a long time to create an opening of useable size.
4 Metal-cutting chain saws do penetrate the fuselage skin, but they were found to snag almost immediately on the underlying stringers, resulting in broken chains.
5 Two types of piercing nozzles were available. The lightweight aluminum nozzle was more difficult to drive than the heavier steel unit because the lighter aluminum unit would bounce off the fuselage when struck. Firefighters need opportunities to observe typical system installations on the inside surfaces of the fuselage to gain an appreciation for where holes can be forced without encountering significant structural impediments to penetration. The windows between heavy frames and the rivet rows of longitudinal stringers can be used as exterior visual aids in guiding placement between frames and stringers.
6 If a door will open, use it. If the fuselage is breached, use that opening. Cut a hole only as a last resort. The quickest method is to use a gas-operated cutoff saw, but there are a multitude of steel fasteners in an aluminum aircraft, and sparks may result. The diameter of the cutting wheel should be large enough to permit the cutting of the fuselage skin, stringer, and interior lining in one action. Angle the cuts on the curved structure to ensure that the section can be removed outward without jamming. Diamond-coated and carbide-toothed metal wheels remain their full diameter, whereas abrasive discs wear down. Abrasive discs are cheaper and can be used for training, but the saw should be mounted with the metal wheel for immediate emergency use. A 14-inch-diameter wheel is the minimum size recommended. Firefighters should be trained to use gravity, when possible, to assist in holding and cutting with a saw; they should not push forward but instead pull toward the operator to use the blade’s rotation to keep the saw blade fully engaged in the structure.
7 Cabin side windows are unique in that they are one of the few applications of a transparency material termed “stretched acrylic.” These transparencies are easily shattered and result in hazardous piercing and cutting shards. These outer transparencies and the as-cast (Plexiglas®) or polycarbonate inner pane are easily cut with a gas cutoff saw. These transparencies are often only clamped in place and can be quickly cleared from the opening once they are cut or shattered.
8 Stretched acrylic cabin side windows are formed by heat treating and mechanically forming billets of as-cast acrylic. When exposed to moderate heating, these transparencies will relax to their prestretched dimensions, and if only “clamped” into their support frames, the shrinkage will cause them to fall out of position and permit direct flame access to the interior. When exposed to exterior pool fires, these transparencies ignite early, shrink from the heating, and do not self-extinguish.
9 Some military transport and fighter aircraft have plastic cockpit windshields that can be cut, but large transport commercial airliners have multilayer glass cockpit windshields that cannot be cut. These windshields are certified to withstand the impact of a four-pound bird with the aircraft traveling at 350 knots (402 miles per hour). They are very tough, and multiple impacts with a sledge or pick-ax will not penetrate these transparencies. On some aircraft, a windshield on either side of the cockpit can be opened from the inside or the outside. If this opening mechanism is jammed, do not attempt to break the window, as it is constructed in the same manner as that used for the forward-facing windshields (photo 3).
|(3) The glass-laminate cockpit windshield after multiple impacts.|
10 Passenger aircraft can be fitted with high-pressure bottled breathing oxygen supply networks for the passengers, or the cabin may contain hundreds of small chemical oxygen generators. These generators can pose problems for firefighters: If initiated, they supply oxygen to a fire for upward of 40 minutes. If they are displaced in a crash and come in close contact with flammables, their outer case can achieve 650ºF. If split open, their inner casing can reach a temperature of 1,200ºF. These generators, once started, cannot be turned off (photo 4).
|(4) The heat’s effect on nonmetalized PET Mylar® thermal acoustical insulation blanket material.|
11 Composite construction will change the firefighters’ approach to aircraft emergency response: Broken carbon fibers present a unique personal penetration hazard. Burning epoxies add to the complexity of the on-site and downwind considerations for inhaling toxic gases and fiber filaments. Applying water to the exterior to lower the interior gas temperature of a structure is not effective on carbon composites; they transfer heat poorly. Unlike the aluminum structure that melts and flows away, composite structures will burn until the epoxy is consumed, and then the fiber structure will collapse, making moving through and investigating the scene more difficult. Firefighters may be charged with the postfire remedial action of applying a “fixant” to lock down loose broken fibers, since they are on scene at the appropriate time and are dressed for the task (photo 5).
(5) Broken carbon fibers present hazards for firefighters.
We did not dispose of the test aircraft completely. We retained the following: a section of the lower hull as a training aid for piercing nozzle exercises, the over-wing area of the upper fuselage containing the emergency exits to permit training with these doors, and the undercarriage to demonstrate the location of fusible plugs in the wheel rims and to train for safe approach angles when responding to overheated brakes and burning tires.
The collaboration between the OMCIAA and the NRC continues with classroom information sessions and field trials on topics of mutual interest. Whenever possible, OFS personnel are invited to attend. The NRC holds a large collection of impact- and fire-damaged metal and composite structures that are used to illustrate some of the physical and environmental hazards presented to firefighters in the debris field surrounding a crashed, burning, or exploded aircraft. OMCIAA firefighters have the tools and training to make the field trials realistic. The NRC and the OMCIAA have most recently entered into field trials on composite structures that involve cutting, burning, and applying fixant materials at extremely low temperatures.
The lessons learned and failure of the first destructive test resulted in a second workshop, named “Kaboom 2,” held in September 2009 on another NRC test airframe. This test succeeded in generating a critical length fuselage fracture. Although the OMCIAA firefighters were staged and ready to respond, there was no fire. This second airframe is being held in anticipation of the arrival of a new Rosenbauer Panther® 6 × 6 fitted with a high-reach extendable turret and a piercing nozzle. Knowledge of the construction and substructures will be even more important to successful penetration when operating under remote control. Once again, the OMCIAA training aircraft was not selected for this task, as the OMCIAA will not allow holes to be put in its aircraft by any means.
FRANÇOIS JACQUET has been a member of the Ottawa McDonald-Cartier International Airport Fire Department (Ontario) for 14 years and is the fire chief. He began his career 20 years ago with the Yellowknife Fire Department (Northwest Territories). He has a bachelor’s and a master’s degree in economics from Laval University.
RON GOULD graduated as a mechanical technologist from Algonquin College (Ottawa) and has been a technical officer with the National Research Council of Canada for 36 years. Most of his activities have been related to full-scale and component specimen fabrication, nondestructive inspection, and materials testing and failure analysis of metallic and composite aerospace structures. He is the custodian of the Aircraft Specimen Library and was project manager on two recent full-scale destructive tests.