A Brief Introduction to the APU Fire Detection and Extinguishing System

On aircraft, few threats demand quicker detection and response than onboard fires. The Auxiliary Power Unit (APU) is one system that demands special attention in this regard, its enclosed design, high operational temperature, and proximity to other critical systems classifying it as a risky zone. Although it is a supplementary power source and not as large as the main engines, high-precision fire detection and extinguishing measures must be in place to avoid structural damage or harm to passengers and crew. This blog will cover all the basics of an Auxiliary Power Unit (APU)’s fire detection and extinguishing system, walking you through its core components and typical operation.

Main Components of An APU’s Fire Detection System

Fire Detection Components

• Overheat Detectors: These devices trigger an electrical signal or alert when the surrounding temperature exceeds a predetermined threshold, providing early warning of potential fire conditions.
• Rate-of-Temperature-Rise Detectors: Also known as rate-of-rise (ROR) heat detectors, these respond to a rapid increase in ambient temperature over a short period that can indicate the sudden onset of a fire.
• Flame Detectors: Flame detectors are capable of identifying the presence of flames through specific wavelengths of infrared (IR) or ultraviolet (UV) radiation emitted by combustion.
• Linear Heat Detection Sensors: In some aircraft designs, linear heat detectors are installed along the APU compartment to provide continuous temperature monitoring over a large area.

Fire Extinguishing Components

• Fire Extinguishing Bottle: A dedicated fire extinguisher is mounted near the APU compartment, usually containing a halon-based agent like Halon 1301.
• Discharge Nozzles: Connected to the bottle via high-pressure tubing and mounted at key points within the APU compartment, these nozzles are precisely oriented to direct the flow of the extinguishing agent toward high-risk areas.
• The Squib: A squib is a small, electrically actuated pyrotechnic device installed on the fire extinguishing bottle that creates a brief explosive charge when triggered, rupturing a frangible disc that seals it.
• Fire Extinguishing Control Valve: In some aircraft configurations, an electrically operated control valve may be installed to regulate the flow of an extinguishing agent from the bottle to the nozzles in synchronization with squib detonation.
• Fire Control Panel: Located in the flight deck or cockpit, the fire control panel enables the flight crew to monitor the APU fire detection system, receive visual and aural alerts from sensors, and discharge the extinguishing agent.

How This System Responds to APU Fire Events

The APU fire protection system operates through two distinct phases to ensure timely response and containment.

1. Fire Detection and Warning

When a fire or overheat condition is detected within the APU compartment, the system responds immediately through the following actions:

• A fire warning is issued in the cockpit, typically including an aural alert and the illumination of a red warning light labeled “APU FIRE” to clearly indicate the affected zone.
• Depending on the aircraft, the APU is shut down either automatically upon detection or after manual action to avoid continued combustion.
• At the same time, a solenoid unlocks the APU fire switch, allowing the flight crew to pull it. Pulling the fire switch:
  • Shuts down the APU, if it is not already deactivated
  • Isolates the APU from its fuel supply, electrical systems, and bleed air lines
  • Arms the fire extinguishing circuit in preparation for discharge

2. Fire Suppression

If the fire persists after the APU has been shut down and isolated:

• The crew turns the armed fire switch to the discharge position, often labeled “DISCH,” holding it in place for at least one second to ensure full activation.
• This action sends an electrical signal to the squib, which releases the fire extinguishing agent into the compartment.
• A bottle discharge indicator light illuminates in the cockpit, confirming that extinguishing has been successfully deployed.

It is important to note that the fire bottle is often a single-shot system for the APU, meaning only one discharge is available. Therefore, both correct identification and timely activation by the crew are vital.

ASAP Aerospace: Your Source for Quality APU Fire Detection and Extinguishing System Parts

From early detection to rapid suppression, each element of this fire protection system is carefully curated to protect an aircraft’s safety when an APU overheats. As such, maintaining the integrity of such a system requires components that perform without compromise from a trusted distributor, such as ASAP Semiconductor. Through our website ASAP Aerospace, professionals can readily purchase high-quality fire detection system parts like squibs, sensors, and many other aircraft components.

With thousands of product options from reputable manufacturers and dedicated customer service, this website makes it easy to meet all operational requirements on one platform. Moreover, you never have to forgo timely fulfillment or competitive pricing to receive top-notch items. To see how else we can aid in your procurement needs, get in touch with a member of our staff over phone or email at any time.