If you’ve ever been lucky enough to see the 21 B-2 Spirit (a.k.a. the stealth bomber) overhead or experience the thrilling roar of fighter jets performing a military flyover, you can begin to imagine the power that is housed every day in our country’s military aircraft hangars.
According to one study, the US military maintains over 10,000 aircraft – that’s the largest fleet of military aircraft in the world. Not only is our fleet sizable – it’s also extremely valuable. A single F-35A Lightning II costs the military about $102 million – but that’s a bargain compared to the nearly $150 million fly-away cost of an F-22. And each of the military’s stealth bombers cost more than $1 billion.
Those awe-inspiring aircraft, however, offer only a part of the picture. Depending on its size, a single military hangar can cost millions to build. Moreover, those hangars house costly military equipment, store hundreds of gallons of jet fuel and other hydrocarbons, and most importantly are often occupied by our nation’s most elite and highly-trained military personnel. With so much at stake, it is critical that military hangars have an effective system in place to protect their assets from inherent fire risks.
An aircraft exposed to fire can sustain damage in less than a minute, but the high expansion foam systems used to protect aircraft in the event of a hangar fire can take more than two minutes to fill the hangar and suffocate flames. This race against time makes it vital that a hangar can rely on the accuracy of its flame detection system and the subsequent activation of its suppression system – with minimal false alarms.
Standards for Hangars
National Fire Protection Association (NFPA) 409 Standard on Aircraft Hangars, in conjunction with other standards, provides guidelines for the optimal design of hangar fire detection and suppression systems. The multiple codes governing fire protection within hangars is complicated, but here it is in a nutshell:
- NFPA 409 categorizes hangars into four groups based on their size, the height of access doors, and construction materials.
- Selected fire detection systems must be appropriate for the category of hangar and the activities and applications performed within the hangar.
- The layout of detectors must be chosen based on a Geographic Coverage Assessment (GCA) conducted by qualified individuals. GCAs, sometimes referred to as “hazard mapping,” demonstrate that a fire protection system has been properly designed and provide ample documentation of the detector layout and expected performance.
- Hangars are also required to use either ultraviolet/infrared or multi-frequency infrared flame detectors proven to be capable of detecting a fully developed 10ft by 10ft (3m by 3m) jet fuel fire from roughly 150 ft. (45m) away – and a fire under any aircraft in the hangar must be detectable by at least one optical flame detector.
- Finally, a fire must be confirmed by a second flame detector or traditional fire protection systems before foam suppression systems automatically activate.
False Alarms Plague Hangars
Traditional ceiling-mounted fire detectors can take several minutes to detect a fire and activate an alarm inside a hangar. The structure’s high ceilings and potential for thermal stratification thwart the limited capabilities of traditional radiant-based detectors. That is why optical flame detectors have been employed in both civilian and military hangars for years.
There are four primary optical flame sensing technologies: ultraviolet (UV), ultraviolet/infrared (UV/IR), multi-spectrum infrared (MSIR), and visual flame imaging. Optical detectors, with wide horizontal and vertical fields of view and detection ranges of 130ft (40m) or more, can detect small fires over a large section of a hangar in 10 seconds or less.
Despite their advantages, many optical flame detectors are prone to false alarms that activate a hangar’s flame suppression systems. This can have expensive and even dangerous consequences. In 2015, an investigative piece by the Washington Post uncovered several incidents in which false alarms had activated suppression systems, leading to a release of foam within military aircraft hangars. In one incident, a welder set off a fire suppression system and submerged three aircraft in foam. In another incident, six Blackhawk helicopters were covered in foam after fire security personnel caused an accidental activation. And in 2014, an unnecessary foam discharge killed a maintenance worker at Elgin Air Force Base.
Every time a suppression system is activated in response to a false alarm, personnel are placed at risk, money and resources are wasted, and operational readiness is impaired. Therefore, it is critical that the military install reliable flame detectors that are highly immune to false alarms and spurious suppression system activations.
Under current Unified Facilities Criteria (UFC) standards, the Air Force and Navy are required to use a multi-spectrum infrared (MSIR) flame detector, for fire detection. Unfortunately, these detectors are easily fooled by the multitude of false alarm stimuli that exist in and around a hangar. Visual flame detectors – more specifically intelligent visual flame detectors (iVFDs) – are better suited for the challenges related to aircraft hangars. That’s because unlike traditional radiant-based detectors, iVFDs are not prone to false alarms. Moreover, a smaller number of iVFD detectors are required to fully cover a hangar’s large footprint.
Challenges of Optical Flame Detection Technologies Relative to False Alarms
Optical flame detectors are designed to detect the absorption of light at specific wavelengths – this is what allows them to discriminate between flames and false alarm sources. However, a hangar’s surrounding environment and the day-to-day activities can easily fool optical detectors and trip a false alarm.
For instance, aircraft hangars have oversized doors that often remain open during operating hours to enable easy ingress/egress for personnel and aircraft. Open doors mean fire detectors have to contend with sunlight. Sunlight is more likely to trigger false alarms with UV, UV/IR and single IR units, but some MSIR detectors are also impaired by it. In fact, when bright sunlight is within the detector’s field of view (FOV), the MSIR’s range is significantly shortened and the unit may not detect or activate a response to a genuine fire beyond that reduced range. Visual flame detectors are not prone to trip an alarm when the sun is within view, nor is their effectiveness impacted by sunlight.
Snow surfaces can reflect additional sunlight and desensitize radiation toward detectors. Fog can also enter hangars through doors and absorb the IR radiation that MSIR detectors depend on, rendering them largely impractical. Visual detectors can see through much heavier fog, rain or snow than MSIR detectors.
Single-frequency IR detectors that only use the 4.2-4.6 micron frequency range to detect hot CO2 can be easily tricked by engine or generator exhaust. This is a significant problem when jet engines and maintenance equipment motors are frequently running inside the hangar. MSIR detectors can avoid falsely tripping an alarm by using reference, or “guard,” frequencies to distinguish genuine flames from black-body radiation. MSIR detectors usually use two guard bands to distinguish the sharp narrow peak of a fire from the broad emission spectrum of a black-body radiator. However, while a “false trip” should not occur, the detector suffers significant desensitization in the presence of a black-body radiator.
The high levels of background radiation make it harder to distinguish the spike in the 4.4 micron range from all the radiant noise, so a MSIR detector will take longer to detect a flame or may require the fire to grow larger before detection occurs. This costs time, and every second is critical when trying to protect aircraft in hangars or fuel storage tanks in fuel depots. Visual flame detectors, by contrast, do not use the 4.4 micron frequency and are not fooled or impaired by black body radiators or hot engine exhaust. The size of the heat signature from the large volume of gas leaving the jets can make the engine exhaust detectable well outside the normal advertised range of a MSIR detector. Even engine exhaust from jets outside the hangar can trigger false alarms or cause desensitization in IR or MSIR detectors.
MSIR units specified under UFC standards, attempt to combat this issue by having a detector operate in “hangar mode” – this means the detector has a built-in time delay before it sounds an alarm. Why pay for a high-performance detection system that needs to be handicapped with built-in delays just to prevent false alarms? Instead, why not invest in and install a visual detector that’s less prone to false alarms without the need for the time delay?
Advantages of Intelligent Visual Flame Detectors
An intelligent visual flame detector is essentially a camera with built-in artificial intelligence or AI, so it continuously scans and analyzes a video feed to instantly identify fires. Some models can also record and transmit video with overlaid boxes that indicate where the fire is thought to exist. Remote viewing of the video transmission allows personnel to quickly confirm a fire in the event of an alarm or activation of suppression systems.
If there were an actual fire, video recordings can later help investigators identify the precise point of origin and potential cause. This is particularly useful given that many fires destroy any definitive evidence. When there is an undetermined cause or lack of conclusive evidence, the result is a so-called “black-hole” fire. Such fires are particularly frustrating because they contribute little to future fire prevention. Not only do iVFDs supply video that helps minimize black-hole fires, but the video can also help determine the cause or origin of false alarms to prevent future false activations.
Some systems harness cameras to radiant-based detectors to provide video capabilities – but the detectors and cameras are still two separate devices, so they are often mounted at different locations and typically maintain separate connections to a distributed control system (DCS). That means more devices need to be tested, diagnosed and repaired – which leads to higher cost in the long run.
The distinctions between iVFDs and MSIR detectors are shown in a recent model prepared by Dräger, an international manufacturer of medical and safety technology. This analysis compared the performance of the MSIR detector currently specified for use in the US Military’s UFC, to that achieved by the Dräger Flame 3000 and Flame 5000. Available upon request.
In summary, Dräger’s iVFDs offered wider fields of view and longer effective ranges than the
Optical flame detectors have been proven to be invaluable in protecting the aircraft and equipment inside military hangars. Multi-spectrum infrared detectors currently dominate aircraft hangar installations and military hangar systems. However, MSIR detectors no longer provide the most advanced or most reliable technology available for the application. The hangar environment is so problematic for MSIR units that they must operate in “hangar mode” to help reduce the likelihood of tripping false alarms.
Dräger’s intelligent visual flame detectors can achieve accurate and effective detection at longer ranges with fewer false alarms than MSIR units. That means iVFDs can ultimately provide more effective coverage to larger hangars with fewer detectors – which can significantly lower installation and maintenance costs. In the end, flame detection is about safety of personnel and protection of assets. iVFDs can deliver the capability and reliability to safeguard the people and aircraft that safeguard us.
David Mayfield is the Strategic Marketing Manager, US Defense and Security for Dräger. He has more than 10 years of experience in safety. David was formerly director of marketing for healthcare & recycling services of Waste Management, senior VP of sales and marketing of Sharps Compliance, and national sales director for Valeant Pharmaceuticals’ neuroscience division.