Applications with the greatest requirement for false-alarm immunity include offshore helidecks and aircraft hangars in both military and commercial settings.
In most hangars, aircraft are positioned by tow, by contrast, on helidecks and in hardened aircraft shelters, aircraft manoeuvre under their own power.
The resulting exhaust plumes, thermal backgrounds and transients present specific challenges for optical flame detectors.
This article discusses why some flame detectors experience high false alarm rates in these settings and outlines how FGD addressed these challenges.
We also summarise a trial installation in Asia, where detectors were evaluated within an operational hardened aircraft shelter, to single and twin jet engine fighter aircraft.
Triple infrared (IR3) flame detectors are widely regarded as one of the most used optical flame detectors for hydrocarbon fires today.
These detectors utilise three sensors, each sensitive to a different infrared (IR) wavelength.
The IR radiation from a typical hydrocarbon fire is most intense around 4.5 microns, see figure 1 and many detector manufacturers select this wavelength or a band very close to it, to be employed as one of the three sensors, some call this the “fire” channel.
The energy being monitored at this wavelength is due to the hot carbon dioxide which is produced as part of the combustion process when a hydrocarbon fuel burns.
A key advantage of this wavelength is that light from the sun is absorbed by the carbon dioxide in the atmosphere, making the detectors “solar blind.”
Juan Bonilla
The other two sensors monitor adjacent spectral bands (guard bands) for the presence of potential false alarms.
For radiation sources, such as heaters, lamps or sunlight, the intensity at 4.5 microns is not greater than that of the guard bands, which helps prevent false alarms.
If we now consider our application, where aircraft move under their own power, it is possible to see a significant challenge.
These aircraft use hydrocarbon-based aviation fuels, which when burned produce large amounts of carbon dioxide.
If helidecks or hardened aircraft shelters are protected with standard triple infrared detectors, they may experience numerous false alarms.
In fact, the U.S. DoD’s UFC 4-211-01 (dated 3rd April 2021) cautioned that, “no current commercial optical detector—including the Model X3301—[was] suitable for aircraft hangars … where aircraft engines are started/run or aircraft taxi into or out of the structures.”
FlameSpec CO2L (pronounced “cool”) was specifically engineered to solve the hangar problem the UFC identifies.
The detector delivers fast, reliable flame detection while remaining robust to engine start/idle exhaust, hot-start plumes, sun glare at open doors and other high-noise conditions typical of live flight-operations.
Put simply: the UFC statement was true then; but the document pre-dates the FlameSpec CO2L development.
FlameSpec CO2L is a specialised IR3 configuration for applications where exhaust (combustion) gases are known to cause false alarms in competing devices.
Juan Bonilla
The product was developed using the extensive data library of fires the standard FlameSpec IR3 detector has recorded.
In normal operation the FlameSpec IR3 acts as a mini spectrum analyser, recording both fire and non-fire events with a sample rate of 50 milliseconds of all three sensors.
This data is stored in solid-state memory within the detector. The data from these events was thoroughly analysed and the detector’s performance optimised to distinguish between the energy signals from a hot carbon dioxide plume and burning aviation fuel.
The FlameSpec CO2L detectors have been independently tested and approved by Factory Mutual (FM).
The response data to one of the detectors, as approved by FM to aviation fuels, with the detector set to extreme sensitivity, proves this.
When exposed to a small 1 x 1 ft pan of JP5 fuel at a distance of 164ft (50m), the detector responded in an average of 3.6 seconds.
When the JP5 pan size is 2 x 2 ft at a distance of 262ft (80m), the detectors average response time was 10.3 seconds.
Finally, when using a Kerosene pan size of 1 x 1 ft at a distance of 164ft (50m), the response of the detector was on average 2.5 seconds.
Hardened aircraft hangars (HAS) are specialised facilities designed to protect aircraft from external threats, including natural disasters, military attacks and environmental factors.
These structures are reinforced to withstand extreme conditions, ensuring the safety of both the aircraft and the personnel working within them.
Juan Bonilla
There are several types of HAS, each serving different operational needs.
One type houses aircraft that are towed or pushed back into the hangar. Another type is the “flow-through” style hangar, where aircraft move under their own power, in via one door and out via another.
This type of hangar is commonly used for operations that require quick turnaround times.
Both types of HAS are critical in military aviation, where the need for security and operational readiness is paramount, false alarms must be avoided at all costs.
In a HAS, false alarms can lead to a variety of specific problems that impact safety and operational efficiency, these may include:
Firefighting systems discharge extinguishing agents unnecessarily, leading to wasted resources, costly clean-up and possible damage to the aircraft they house.
Interruptions to regular operations, aircraft may need to be moved out of the shelter or equipment may have to be powered down, wasting valuable time and resources.
Prolonged downtime as maintenance teams need to investigate the incident.
Loss of system confidence.
Last year, FGD was approach by a government agency in Asia for information about the FlameSpec product line and our experiences within HAS.
The discussions addressed several topics, including the fact that even flame detectors using the same detection technologies can behave differently due to variations in their design.
We were fortunate that the FlameSpec CO2L detector was selected for testing with two different fighter jets in an operational HAS.
Juan Bonilla
Four FlameSpec CO2L detectors were provided, installed and commissioned by our partner. One flame detector was placed in each corner of the hangar.
Since the aircraft could approach from either door, two detectors would always be positioned to have full visibility of the aircraft engine(s) whilst the plane was moving or stationary with the engine running “ground idle.”
Testing was conducted it what we considered to be the hardest HAS challenge, a flow-through style hangar. The tests were conducted in three rounds with both single and dual jet engine fighter aircraft.
Round 1 – Aircraft reached the hangar door and moved slowly under its own power to its parking position, after some seconds being parked the aircraft moved out of the other door.
Round 2 – Essentially a repeat of Round 1.
Round 3 – Operational simulation: The aircraft moved to its parking position and remained stationary with the engine running at “ground idle” for three minutes. The engine was then shut down and the hangar doors closed for the aircraft to be refuelled. Once the refuelling was complete, the aircraft was prepared for take-off, the engine was started and run at ground idle for twenty minutes before the aircraft moved out of the hangar.
Across the full test, FlameSpec CO2L recorded no false alarms, validating its resilience to hot carbon dioxide exhaust signatures and its stability in live flight operational environments; it has since been adopted at scale for multiple operational HAS.
In this article, we shed light on the real-world challenges optical flame detectors face when deployed near operational aircraft.
To tackle this issue head-on, FGD developed the FlameSpec CO2L, specifically designed to filter out hot carbon dioxide- a common culprit behind false alarms on helidecks and within hardened aircraft shelters (HAS).
Juan Bonilla
We also delved into the results of recent trials conducted in an active HAS environment, where the FlameSpec CO2L showcased exceptional performance, flawlessly navigating the complex conditions without a single event being recorded.
This success highlights the detector’s groundbreaking capability in high-stakes aviation settings.
While this article has mainly spotlighted HAS applications, it’s important to note that aircraft hangars come in diverse sizes, serve various functions, and accommodate different numbers of aircraft.
According to the NFPA 409 standard, hangars are classified into one of four group types to address these variables.
We’re excited to announce that the FlameSpec CO2L detector has recently achieved full compliance with the stringent requirements outlined in Annex C of this standard—underscoring its adaptability and reliability across a wide range of hangar environments.
