AAR MRO Services supports airline operators with everything from maintenance inspections and equipment upgrades to airframe painting and heavy maintenance for all major aircraft in service. The largest MRO operator of its type in the Americas, AAR recently opened the company’s largest facility, located at the Chicago Rockford International Airport.

Here, each of two 10-story hangar bays can accommodate hundreds of ‘small’ aircraft, two Boeing 787s or even an Airbus A380, the largest commercial aircraft in production today. Also in each of the two bays are 10 Det-Tronics optical flame detectors that function as the critical sensors for the AAR hangar’s fire protection system.

Heavy Maintenance Inspections

Aircraft spend anywhere from three days to two months in AAR’s hangars

Russel Daubert, AAR Rockford’s Facility Manager, and Chris Wolf, Director of Maintenance, have overseen the 24-hour operations at the MRO hangar since it opened in late 2016. AAR serves multiple airline customers, and like any MRO facility, Wolf says their goal is to “get lines that are current, which means an airline operator will continually bring in one plane after another to keep their fleet operating safely.”

Daubert adds, “The biggest portion of our work is airframe overhauls and heavy maintenance inspections, and depending on the aircraft, we can have up to 225 aircraft in each hangar at one time plus 50 to 60 crew.” Aircraft spend anywhere from three days to two months in AAR’s hangars. Fire protection in MRO hangars must be able to handle the challenges associated with servicing aircraft.

Conventionally Constructed Fire Hangars

According to Wolf, aircraft bring inherent fire hazards to MRO facilities. “These aircraft come in with 70,000 to 80,000 pounds of fuel,” Wolf says. “Add the oxygen tanks on board for passenger and crew safety, plus the possible sparks from electrical equipment or other sources, and you have all the ingredients needed for fire.” Maintenance also involves painting aircraft in the hangars, which can result in the circulation of highly flammable paint plumes under and around wings and fuselage.

Hangars in these groups usually require both sprinklers and foam for fire protection

Fire protection standards specific to aircraft hangars are spelled out in the National Fire Protection Association’s NFPA® 409 Standard on Aircraft Hangars. This document classifies hangars by size and construction type; conventionally constructed fire hangars with fire areas of 40,000 sq. ft. or less are classified in Groups I, II and III. Hangars in these groups usually require both sprinklers and foam for fire protection.

High Expansion Foam Suppression System

AAR’s Rockford facility is unique both for its immense size and for its construction method, a fabric tension membrane over steel trusses. The 2-inch thick insulated material meets NFPA 701 and ASTM E-84 standards for flame retardancy, fire safety advantages that led the NFPA to decide hangars covered in this fabric would fall in a Group IV classification. Group IV hangars can have an unlimited fire area and need only a low- or high expansion foam suppression system.

One of the goals of MRO service providers is to provide fast turns of the planes entrusted to them by airline operators. To support this objective, a hangar fire detection system must have two very important capabilities: quickly detect the presence of flames, and reject false alarms (generated by welding, engine start-ups, etc.) that could unnecessarily initiate suppression systems, interrupt operations and potentially lead to significant aircraft damage.

Fire Protection System

The project manager for the Chicago Rockford hangar expansion looked to local fire protection contractor

The solution for hundreds of hangars in the past 10 years – from military bases to commercial hangars and MRO facilities – has been to deploy optical flame detectors from Det-Tronics. When it was time to specify the fire protection system for the mammoth hangar bays, the project manager for the Chicago Rockford hangar expansion looked to local fire protection contractor, Absolute Fire Protection, Inc., to handle the fire protection system.

In turn, John Danis of Absolute called in 3S Incorporated, a Harrison, Ohio firm that specializes in industrial and special hazard systems, to design the detection and foam suppression part of the fire protection system. Because of the size and scope of the Rockford hangar, 3S and Absolute, along with other design and building partners, worked for nearly three years to take the project from initial planning to construction.

Multispectrum Infrared Flame Detectors

During that time, a construction engineer had calculated it would take no less than 84 detectors per hangar bay to monitor the facilities for fire. Aaron Hinkle, sales engineer at 3S, disagreed. “I realized that was far more than necessary, if we just picked the right product for the job,” he says.

All the alarms contractor had to do was install four detectors on each side wall"

Hinkle had worked with the Det-Tronics X3301 Multispectrum Infrared (IR) flame detectors on previous hangar projects, and he knew the units possessed the optical power, field-of-view capacity and speed to do what was required. In consultation with Det-Tronics applications engineers, Hinkle came to the conclusion that, “Because of the X3301’s performance attributes, each 119,000-square-foot hangar could be covered with just 10 detectors from Det-Tronics. Using just ten detectors per bay greatly simplified the work,” Hinkle explains. “All the alarms contractor had to do was install four detectors on each side wall and two on the back wall. The front wall is the giant door that opens up to allow the craft to enter and exit.”

False Alarm Rejection

There were considerable cost savings in equipment and related hardware, as well as labor savings due both to the small number of units to be installed and the fact that the X3301 detectors could be placed at a much easier-to-reach height of just 8 to 10 feet off the floor rather than near the top of the 10-story hangar bays.

To maximize false alarm rejection, X3301 flame detectors are programmed to run in Det-Tronics® Hangar Mode™, an option that incorporates a delay mechanism. The mechanism extends the processing time to react to fires, letting the detector distinguish between an actual fire and an event like a short duration auxiliary power unit startup. The operation mode has no effect on detection ranges or field of view, but can prevent an innocent action (such as a crew firing up gas heaters to stay warm) from resulting in an unwanted foam dump.

Foam Suppression System

AAR’s foam suppression system has gone off only once, and that was intentional

An impressive demonstration to date, AAR’s foam suppression system has gone off only once, and that was intentional. To certify that the new system was working properly, Absolute, 3S and other suppliers commissioned the overall protection system by simulating an actual fire suppression event. They recorded it on video, and it’s a stunning sight. Daubert, facility manager for the AAR hangars, was delighted when he saw the video.

Within seconds of being triggered, foam erupts from dispensers in the ceiling. In no time, it has put a layer on every inch of the hangar’s floor. Within 3-and-a-half minutes, the foam has stacked up to a 10-foot depth, smothering any possible fire.”

I had never seen a system of that magnitude before,” Daubert continues. “Seeing just how fast we could stop a fire from spreading and put it out was pretty impressive. Thanks to the Det-Tronics detectors and the system’s other components, it’s obvious our hangar is well protected from the dangers of fire.”

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