Engine and APU Fire Extinguishing (FIREX) Test On a Part 25 Aircraft
As a company we prepare Flight Test Guide to use as guidance and recommendations for engineers involved in planning and conducting aircraft development and certification testing. One example is the Engine and APU Fire Extinguishing (FIREX) tests on a Part 25 category aircraft. Our Guides contain test requirements as well as applicable regulations which are reviewed together with lessons learned from previous test experience.
Introduction
Firex may be defined as any fire extinguishing or suppression system that consists of the extinguishing agent container (vessel) and all attached components. There are generally three, cartridge-activated, firex systems onboard commercial aircraft. These systems provide fire extinguishment or suppression for engines, auxiliary power unit (APU), and cargo compartment. The extinguishing agent is contained in a pressurized vessel that is sealed with a precision burst disc. A discharge head, containing a cartridge and strainer, is attached to the pressure vessel. This flight test guide will focus on the description of the certification tests related to engine and APU firex systems.
Certification for aircraft in-built Engine and APU FIREX systems must demonstrate that these fire extinguishing systems are capable of functioning properly over the temperature and pressure range likely to be experienced in the compartment in which the extinguishing bottle is stored.
To be deemed acceptable with respect to the Federal Aviation Regulations (see section 3), a Halon 1301 suppression system must effectively distribute enough quantity of agent within an engine nacelle or APU compartment. The current level of safety has been historically defined as a quantity of Halon 1301 providing a volumetric concentration of 6% for a duration of 0.5 seconds (Figure 1) throughout the protected zone within the nacelle (or APU compartment).
Figure 1 - Certification criteria based on the distribution of volumetric concentration of Halon 1301
Work is occurring which will lead to the eventual replacement of Halon 1301 as the primary fire suppressant in an engine nacelle (reach out for more info on the replacement of Halon 1301, the suppression system development and certification processes offer an opportunity to reduce the use of Halon 1301 by using a simulating chemical in its place).
Certification requirements
a) 14 CFR 25.851 - Fire extinguishers.
(b) Built-in fire extinguishers. If a built-in fire extinguisher is provided: (1) Each built-in fire extinguishing system must be installed so that (i) No extinguishing agent likely to enter personnel compartments will be hazardous to the occupants; and (ii) No discharge of the extinguisher can cause structural damage. (2) The capacity of each required built-in fire extinguishing system must be adequate for any fire likely to occur in the compartment where used, considering the volume of the compartment and the ventilation rate. The capacity of each system is adequate if there is sufficient quantity of agent to extinguish the fire or suppress the fire anywhere baggage or cargo is placed within the cargo compartment for the duration required to land and evacuate the airplane.
b) 14 CFR 25.1195 - Fire extinguishing systems.
(a) Except for combustor, turbine, and tail pipe sections of turbine engine installations that contain lines or components carrying flammable fluids or gases for which it is shown that a fire originating in these sections can be controlled, there must be a fire extinguisher system serving each designated fire zone.
(b) The fire extinguishing system, the quantity of the extinguishing agent, the rate of discharge, and the discharge distribution must be adequate to extinguish fires. It must be shown by either actual or simulated flights tests that under critical airflow conditions in flight the discharge of the extinguishing agent in each designated fire zone specified in paragraph (a) of this section will provide an agent concentration capable of extinguishing fires in that zone and of minimizing the probability of re-ignition. An individual “one-shot” system may be used for auxiliary power units, fuel burning heaters, and other combustion equipment. For each other designated fire zone, two discharges must be provided each of which produces adequate agent concentration.
(c) The fire extinguishing system for a nacelle must be able to simultaneously protect each zone of the nacelle for which protection is provided.
c) 14 CFR 25.1197 - Fire extinguishing agents.
(a) Fire extinguishing agents must (1) Be capable of extinguishing flames emanating from any burning of fluids or other combustible materials in the area protected by the fire extinguishing system; and (2) Have thermal stability over the temperature range likely to be experienced in the compartment in which they are stored.
(b) If any toxic extinguishing agent is used, provisions must be made to prevent harmful concentrations of fluid or fluid vapors (from leakage during normal operation of the airplane or as a result of discharging the fire extinguisher on the ground or in flight) from entering any personnel compartment, even though a defect may exist in the extinguishing system. This must be shown by test except for built-in carbon dioxide fuselage compartment fire extinguishing systems for which (1) Five pounds or less of carbon dioxide will be discharged, under established fire control procedures, into any fuselage compartment; or (2) There is protective breathing equipment for each flight crewmember on flight deck duty.
d) 14 CFR 25.1199 - Extinguishing agent containers.
(a) Each extinguishing agent container must have a pressure relief to prevent bursting of the container by excessive internal pressures.
(b) The discharge end of each discharge line from a pressure relief connection must be located so that discharge of the fire extinguishing agent would not damage the airplane. The line must also be located or protected to prevent clogging caused by ice or other foreign matter.
(c) There must be a means for each fire extinguishing agent container to indicate that the container has discharged or that the charging pressure is below the established minimum necessary for proper functioning.
(d) The temperature of each container must be maintained, under intended operating conditions, to prevent the pressure in the container from (1) Falling below that necessary to provide an adequate rate of discharge; or (2) Rising high enough to cause premature discharge.
(e) If a pyrotechnic capsule is used to discharge the extinguishing agent, each container must be installed so that temperature conditions will not cause hazardous deterioration of the pyrotechnic capsule.
e) 14 CFR 25.1201 - Fire extinguishing system materials.
(a) No material in any fire extinguishing system may react chemically with any extinguishing agent so as to create a hazard.
(b) Each system component in an engine compartment must be fireproof.
3.2 Advisory Circular(s)
a) AC 20-100
i) The purpose of this Advisory Circular is to describe the installation and use of a model GA-2A fire extinguisher agent concentration recorder to determine the distribution and concentration of fire-extinguishing agents when discharged in an aircraft; powerplant compartment.
b) AC 20-144A
i) This advisory circular (AC) provides acceptable means for showing compliance of Electrical Firing Cartridge Components in an Aircraft Fire Extinguishing or Suppression System with the requirements of title 14, Code of Federal Regulations (14 CFR) 23.1301, amendment 23-62 or earlier, 25.1301, 27.1301, and 29.1301, Function and installation, with regard to fire extinguishing or suppression systems that contain electrical firing cartridge components.
c) AC 33-17-1A
i) This advisory circular (AC) provides definitions, guidance, and acceptable methods, but not the only methods, that may be used to demonstrate compliance with the engine fire protection requirements of Title 14 Code of Federal Regulations (14 CPR 33.17). The guidance provided in this AC supersedes information contained in AC 33-2B titled "Aircraft Engine Type Certification Handbook" (Chapter 3, Section 22, titled "Section 33.17, Fire Prevention").
If you are interested in knowing more about Firex testing, please reach out to ES3AERO!