1.PURPOSE. This Acceptable Means of Compliance (AMC) provides information and guidance concerning a means, but not the only means, of compliance with CS 25.1189 which pertains to the shut-off of flammable fluids for fire zones of Transport Category Aeroplanes. Accordingly, this material is neither mandatory nor regulatory in nature and does not constitute a regulation. In lieu of following this method, the applicant may elect to establish an alternate method of compliance that is acceptable to the Agency for complying with the requirements of the CS-25 paragraphs listed below.
2.SCOPE. This AMC provides guidance for a means of showing compliance with regulations applicable to flammable fluid shut-off capability in Transport Category Airplanes. This guidance applies to new designs as well as modifications such as the installation of new engines or APU's or modifications of existing designs that would affect compliance to the requirements for flammable fluid shut-off means to a fire zone.
3.RELATED CERTIFICATION SPECIFICATIONS. CS 25.863, CS 25.1181, CS 25.1182, CS 25.1189, CS 25J1189.
4.OBJECTIVE This advisory material provides guidelines for determining hazardous quantity of flammable fluids: A. With respect to the requirement CS 25.1189(a) that each fire zone must have a means to shut-off or otherwise prevent hazardous quantities of flammable fluids from flow into, within, or through the fire zone. B. With respect to the requirement of CS 25.1189(e) that no hazardous quantity of flammable fluid may drain into any designated fire zone following shut-off.
5.BACKGROUND. Guidance is required because of different and sometimes inconsistent interpretation of what hazardous quantity means. Service History: The fire zone fire safety service history of CS-25 turbine engine aircraft has been very good, especially considering the potential hazards involved. This is attributed to the multi-faceted fire protection means required by CS-25. While it is not generally possible to define the contribution of each individual fire protection means, such as flammable fluid shut-off means, it is noted that the relatively few serious accidents that have occurred often involve initiating events such as engine separation or rotor non-containment, which can potentially negate some fire protection means, and in which flammable fluid shut-off means represent an important, or possibly sole, backup. Previous incidents have shown that hydraulic system leaks have fuelled fires, especially when fluid mist is produced at high pressure due to small (pinhole) leaks. This type of leakage can be of considerable duration, even with a limited quantity of flammable fluid at the source.
6.DEFINITIONS. A. Hazardous Quantity: An amount which could sustain a fire of sufficient severity and duration so as to result in a hazardous condition. B. Hazardous Condition: Failure Conditions which would reduce the capability of the aeroplane or the ability of the crew to cope with adverse operating conditions to the extent that there would be:
(i)A large reduction in safety margins or functional capabilities;
(ii)Physical distress or higher workload such that the flight crew cannot be relied upon to perform their tasks accurately or completely; or
(iii)Serious or fatal injury to a relatively small number of the occupants;
(iv)For the purposes of this AMC, and specifically with respect to fire zone fires, any condition which could breach or exceed the fire zone integrity requirements or structural fireproofness requirements of CS-25. C. Flammable Fluid. Flammable, with respect to a fluid or gas, means susceptible to igniting readily or to exploding. For the purpose of this AMC igniting readily includes ignition and burning when introduced into an existing flame, and includes fluids such as fuels, hydraulic fluid (including phosphate ester based fluids), oils, and deicing fluids.
7.COMPLIANCE METHODOLOGY: The quantity of flammable fluid which is hazardous may vary with fire zone size and design, fluid characteristics, different fire scenarios, and other factors. Since one of these factors is the presence or absence of flammable fluid shut-off means, the requirements of CS 25.1189(a) and CS 25.1189(e) are discussed separately below.
7.1 Shut-off Means Requirements (CS 25.1189(a)) Compliance with CS 25.1189(a) has been typically been shown by installation of shut-off means for flammable fluids that could contribute to the hazards associated with an engine fire, except for lines fittings, and components forming an integral part of an engine and/or fireproof oil system components, which are not required to have a shut-off means per CS 25.1189(a)(1) and (a)(2). Flammable fluids that have been considered include fuel supplied to the engine/APU, fuel that may enter the fire zone from engine recirculation systems and hydraulic fluids entering the fire zone. Oil that may be supplied from outside the fire zone, deicing fluid, and other fluids would require similar consideration, however these are not typically incorporated in modern CS-25 aircraft engine installations. Although shut-off means are typically incorporated, CS 25.1189(a) allows the option of otherwise preventing flow of hazardous quantities of flammable fluids. A shut-off means is, therefore, not required if no possible scenario will result in the flow of hazardous quantities of flammable fluid. Factors to be considered in determination of whether this compliance means is acceptable include the following: A. Considerations 1) Leakage rates and characteristics, including massive leakage caused by component failure or fire damage, and slow leakage, which may be a spray or mist if the source is under pressure, caused by failures such as cracks or pinholes. 2) The amount of fluid in the system that is subject to leakage. 3) Combining A.1), and A.2), the range of potential duration of leakage. 4) Scenarios in which the analysed system leakage is subject to ignition and is the initial fire source. 5) Scenarios in which the initial fire source is a different system, and fire damage to the analysed system can result in leakage which contributes to the magnitude or duration of the fire. B. Compliance Considering the above factors and service experience of oil systems without shut-off means, it is acceptable to not install a shut-off means for specific systems which contain flammable fluid if the following conditions are met: 1) All components of the analysed system within the fire zone are fireproof, and 2) The quantity of fluid which can flow into the fire zone is not greater than the fluid quantity of the engine or APU oil system for an engine or APU fire zone, and 3) Accomplishment of AFM Emergency Procedures will preclude continuation of a pressurized spray or mist. The meeting of conditions (1)-(3) are considered acceptable in precluding a hazardous quantity of flammable fluids from flowing into, within or through any designated fire zone.
7.2 Drainage Following Shut-off Requirements (CS 25.1189(e)) Following shut-off, flammable fluid will be contained within the components and plumbing in the fire zone, and usually within plumbing between the firewall and shut-off means. This is due to other requirements which affect the location of the shut-off means and, therefore, the amount of fluid between the shut-off means and the firewall that may drain into the fire zone following shut-off. These include the requirement to protect the shut-off means from a fire zone fire (CS 25.1189(d)), a powerplant or engine mount structural failure (CS 25.1189(g)), and engine rotor failure (CS 25.903(d)(1)). An analysis is required for each individual flammable fluid system to determine that the total amount is not hazardous. The analysis should consider the aircraft attitudes expected to be encountered during continued flight following shut-off, which may include emergency descent attitudes, but would not be expected to include climb attitudes steeper than those associated with one engine inoperative flight at V2. If the analysed system traverses more than one fire zone, each fire zone should be analysed separately for the maximum fluid volume which can drain into that fire zone. Credit should not be taken for fire extinguishing provisions. The following are alternate criteria for hazardous quantities of flammable fluid for this condition: A) A volume not exceeding 0·95 litre (1 US quarts) is not hazardous, or B) An amount shown not to be hazardous by analysis considering the factors listed in 7.1.A above. Additional factors relevant to this condition following shut-off are reduction in pressurized spray or mist due to reduction or absence of system pressure, and the possibility of rapid leakage or drainage due to either an initial leak or fire damage of plumbing and components, such as aluminium components or non-metallic hoses, following the required fire resistance period. Hazard assessment of such rapid leakage and drainage may include airflow ventilation limitation of fire intensity, and fire duration limitation through fire zone drainage. The analysis may consider that volume which is capable of being drained from the nacelle within a suitable period is not hazardous. The suitable period should be such that fluid leakage into the fire zone will not aggravate a fire beyond a fifteen minute period from its initiation. A five minute period may be suitable when considering fire resistant components and plumbing for which leakage due to fire damage will not occur during the first five minute period and may not occur immediately thereafter.
[Amdt 25/1]