Fuselage Airflow Concept Could Aid in Airborne Fire-Fighting

A fuselage air circulation system in development could significantly improve cabin air quality, help combat corrosion, and provide an additional margin of safety should a fire break out in the inaccessible spaces between the passenger cabin and the outer metal skin of the airplane.

To be marketed as JET-AIR, the system is under development by Air Data Inc., a Montreal, Canada-based company that produces communications control units, infrared (IR) fire detectors and other components for aircraft.

Indeed, while its developers envisioned the system as a boon to passenger comfort by providing humidified air with a corollary role in helping to prevent smoke and fire ingress into the cabin, its more important role will be to help prevent smoke and fire ingress into the cabin. The design would offer even greater capability if installed in combination with an array of IR detectors. According to an independent review, the new ventilation concept, if augmented by IR fire detection devices, could mark an "awesome" leap forward for coping with in-flight fire. An ETOPS (extended operations) aircraft so equipped would be a much safer proposition. Aircraft presently approved for ETOPS flights have negligible capability for coping with fire in inaccessible spaces, save for the fire detection and suppression systems in the belly holds.

The main idea behind the concept is to divide fuselage air circulation into two separate flows of air. One flow goes into the cabin. The other goes into the envelope, or the space between the cabin liner and the fuselage. For normal operations, piccolo tubing in the envelope provides a controlled flow of air into the envelope. It is at a slightly higher pressure than air going to the cabin. By this means, the air supplied to the cabin can be sterilized and humidified, providing a more healthy and comfortable environment for the passengers. The very slightly higher pressure in the envelope prevents this humidified cabin air from entering the envelope, especially near the ceiling, thereby preventing the buildup of water condensation. This condensation can cause water to drip during flight, a phenomenon sometimes known as "rain in the plane." In existing designs, the water accumulations are breeding grounds for germs, can degrade wiring insulation and electrical systems, and eventually will contaminate many of the thermal acoustic insulation batts. Condensation is the primary cause of in-fuselage corrosion and microbial blooms within air conditioning ductwork. This conceptual circulation system provides germ-free humidified air to occupants, and a flow of dry air into the envelope. An electronic control unit (ECU) automatically adjusts cabin and envelope airflows to maintain positive pressure in the envelope through all phases of flight.

Some of the air in the envelope suffuses into the cabin through special joints in the sidewall panels. Passing some of the air through the envelope first, and then into the cabin through the liner joints improves cabin air quality through absorption and filtering of such contaminants as ozone, oil aerosols, and volatile organic compounds (VOCs). The VOCs can come from oil-polluted duct linings, from anticorrosion treatment of the airplane, from mold in the envelope, and from ground exhaust ingestion.

Cabin humidity control is a huge factor in fatigue, and the low humidity in most airliner cabins means passengers must maintain a high fluid intake to avoid dehydration and headaches. However, the dehumidification is necessary to control the "rain in the plane." The ability to cocoon the passengers and crew in a humidity controlled climate while keeping the air between the cabin lining and fuselage skin dry marks a major step forward. Offsetting these potential advantages are system complexity and weight. Much of the apparent complexity stems from the ducting, flow blockers and piccolo tubes. These are static items. Once installed, they have no moving parts. Jean-Pierre Lepage, president of Air Data, observed that the valving and ECU have a mean time between failure (MTBF) of 100,000 hours. To be sure, the JET-AIR system would add some weight, but airliners already are paying a penalty by flying airplanes laden with waterlogged insulation batts. Estimates of the added water range from 80 to 1,000 pounds.

The ability to incorporate bacteriological control within the ducting marks a leap ahead in terms of public health. At present, no airliners are equipped to exercise disease control, per the spread of drug-resistant tuberculosis or, more recently, per the SARS epidemic, where it was proven that some passengers were the trans-oceanic "Typhoid Marys of SARS." (See ASW, Jan. 12)

In the event of smoke and/or fire in the envelope, the airflow into the envelope can be reversed. Now the piccolo tubes and the central conduit can facilitate the evacuation of smoke and toxic gases, preventing their entry into the occupied cabin. In addition, when the flow is reversed, a more direct means than the floor vents is available through the panel joints to evacuate smoke and contaminants from the cabin.

Lepage said the concept of pressurizing and depressurizing the envelope as a means of controlling and evacuating smoke in the cabin emerged in research dating back to the mid- 1990s. The dual-airflow concept has been patented, and final system design is expected to be completed in the first quarter of 2005. A prototype system was successfully tested on a Boeing [BA] B737 cabin section in the late 1990s.

Development is now focused on refining the concept into a production-ready system that can be installed on new aircraft or retrofitted onto existing aircraft, Lepage said. For smoke evacuation, larger conduits may be necessary for some widebody aircraft, but they can be run under the floor rather than in the "attic" space over the cabin, he suggested.

Lepage and his colleague, Air Data Vice President Louis Labreche, asserted the system offers a number of attributes in either mode of operation, when it employs positive pressure or negative pressure.

The envelope isolation and flow blockers appear to be keys to system performance. The system offers a means to reduce toxicity in the event of an underway fire in the cockpit or cabin (although this potential capability may also depend upon having an independent backup power supply). Regarding power requirements in an emergency situation, Lepage said, "The electronic control of the system is low power and could be connected to the aircraft essential bus, or it could alternatively have its own power backup system."

"As far as de-pressurization of the envelope during a fire incident, for moderate or high altitude, the outside atmospheric low pressure would be used directly, and for low altitude, a static venturi air pump which requires no electrical power would be used," Lepage told ASW.

The flow blockers behind the cabin lining can help slow the spread of a hidden fire. It seems possible to take the concept a step further. If augmented by an array of IR fire detection devices, the system would mark a major improvement regarding in-flight firefighting capability. Consider installing IR fire detectors behind the liners, and incorporating them into a "mapping cockpit display" for determination of any fire (electrical in particular) and its behavior behind the cabin linings (smoldering, suppressed, or progressing). IR sensors could be positioned such that relative temperatures sensed would allow a "fix" on the fire (much the way sonobuoys allow determination of a submarine's position by comparison of their detected sound amplitudes). Such a capability would be superior to the Swissair "Modification Plus" program video cameras and their limited deployment, limited field of view, and their reliance on the pilot picking up the smoke on the cockpit video monitor (see ASW, July 30, 2001). Indeed, once the pilot sees smoke, he has the problem of determining its origin, since underfloor smoke can rise into the crown ceiling area just like a thermal rises. Without relative indications of local temperatures from IR detectors, the pilots cannot pinpoint a fire's exact location (the SwissairFlight 111 conundrum). The advantage of IR mapping inside the pressure hull is that it gives the crew information that escaping smoke cannot (because of airflow ventilation behind cabin linings) - the position of the hottest area.

IR detectors could be networked to map out a fire locale for a cockpit display (cathode ray tube or flat panel) of the location, and the rate that fire anywhere on the airplane was spreading, or not spreading. Indeed, given that an ETOPS airplane must be able to suppress a fire until it can land (up to 195 minutes), such information would be critical. It could reassure the pilots that the fire was not spreading, i.e., it was truly suppressed. Conversely, pilots would be much better informed if the fire is spreading and if a ditching is inevitable.

The IR mapping concept suggests a marketing moniker. A catchy and memorable name would, in the course of time, become inextricably linked with the concept.

If the JET-AIR system featured parallel piping that could carry nitrogen enriched air (NEA) to areas behind the cabin linings and lavatories (i.e., to hot spots where Halon suppression could not be used in the concentration required in the cargo holds) the system would offer another advantage at negligible penalty in weight (especially if fuel tank safety systems are producing NEA, which could be switched to the envelope piping in the event of a fire). To be sure, fire suppressants such as Halon can be injected into the envelope to extinguish a fire or an incipient fire (smoldering), but on-board supplies of Halon are limited, whereas a steady supply of NEA can be produced by the fuel tank inerting system.

The JET-AIR system also seems to offer as a byproduct of its better filtration a means of minimizing accumulations of dust behind cabin linings. Less dust is less fire-hazard tinder.

Together with fire-resistant thermal acoustic insulation blankets, any such airplane should be relatively fireproof (to the extent that an electrical fire could be IR-mapped and displayed). This notion of fireproofing was raised explicitly by the Transportation Safety Board (TSB) of Canada in the course of its investigation into the recent mother of in-flight fire disasters, the crash of Swissair Flight 111 (see ASW, Dec. 11, 2000 and Sept. 10, 2001). TSB officials have called for an integrated, strategic approach to in-flight fire prevention and fire fighting. The JET-AIR concept, in concert with IR mapping and the NEA influx, suggests one form that vision could take. Labreche, e-mail llabreche@airdata.ca. For more information on the company and its range of aviation products, see www.airdata.ca