(2) Compliance may be accomplished by a side-by-side arrangement of the controls to allow either separate or simultaneous control.
AC 27.1151. § 27.1151 (Amendment 27-33) ROTOR BRAKE CONTROLS.
a. Explanation.
(1) Amendment 27-33 added a new § 27.1151 that establishes requirements for rotor brake controls. Paragraph a is intended to require design features which, for all practicable purposes, prevent inadvertent brake application in flight even under conditions of reasonably expected crew error or confusion.
(2) Paragraph b requires warning devices to alert the crew if the brake has not been completely released.
b. Background. Inadvertent or undetected application of the rotor brake may be expected to result in excessive heat and fire in the rotor brake area. Rotor brake components are usually located integral with, or in close proximity to, rotor drive system components and, in some cases, close to critical hydraulic main rotor control system components. Fires in these areas would be extremely hazardous.
c. Method of Compliance.
(1) For paragraph (a), literal compliance can be achieved by lock-out devices sensitive to the higher RPM range of the main rotor or other flight parameters, hydraulic bypass or lock-out devices controlled by flyweight governor systems, or engines control position, etc.
The guard required by FAR 27.921 does not, in itself, provide compliance with this requirement. However, if careful evaluation of the overall control, including location, guard mechanism, control manipulation requirements, accessibility, etc., provides a high degree of assurance that inadvertent application will not occur, compliance may be assumed. Also, if brake application does occur, annunciation appears and no immediate hazard to flight operation exists, compliance may be assumed.
(2) Alerting devices supplied to comply with this rule should provide a signal at any time the rotor brake is engaged, including partial engagement. This means to alert the crew could be:
- A warning light indicating the mechanical control position, or the position of the brake for a power assisted system, or
- An unambiguous device warning the crew that the rotor brake is engaged (or partially engaged), or Page E - 121
- A locking device preventing the engine starting when the rotor brake control is not completely released.
AC 27.1163. § 27.1163 (Amendment 27-23) POWERPLANT ACCESSORIES.
a. Explanation.
(1) This section addresses the interface requirements for powerplant accessories which are mounted on the engine or rotor drive system components.
(2) Areas which should be addressed include structural loads imposed upon the engine case and isolation between the accessory and engine oil systems. Electrical equipment isolation from flammable fluids or vapors should be addressed as well as the effect of an accessory failure on the continued operation of the engine and drive system components.
b. Procedures.
(1) Accessories installed and certified by the engine manufacturer can be mounted on the engine without additional justification.
(2) Any accessory to be mounted on the engine, which was not certificated with the engine, and does not meet the engine installation design manual requirements should have a structural analysis showing the mounting of that accessory on the engine will not induce loads into the engine case which are higher than the original design loads.
(3) When the accessory is mounted and operating on the engine, it should not be possible to contaminate either the engine or accessory oil systems. This contamination can take the form of debris following a failure, airborne dirt or water, or any other substance that would impair proper operation of the engine or accessory.
Compliance with these requirements can be accomplished by a combination of test and analysis. The design interface should be such that when the equipment is operating, there are no high/low pressure differentials between the components which would induce fluid transfer between components resulting in a low fluid level in one component and an overfill condition in the other component. Where this potential exists, an analysis and/or test should be used to demonstrate compliance.
(4) Engine mounted accessories which are subject to arcing and sparking, must be isolated from all flammable fluids or vapors to minimize the probability of fire. This can be accomplished by isolating the electrical equipment from the flammable fumes or vapors or by isolating the flammable fumes or vapors from the potential ignition source.
Compliance can be shown by analysis.
(5) A failure mode and effect analysis should be submitted which shows that a failure of any engine mounted and driven accessory will not interfere with the continued Page E - 122
operation of the engine. If a hazard is created by the continued rotation of an engine driven accessory after a failure or malfunction, provisions to stop its rotation or eliminate the hazard must be provided. The effectiveness of this device should be demonstrated by test.
(6) The main transmission and rotor drive system should be protected from excessive torque loads and damage imposed upon them by accessory drives. One method which has been used is a torque limiting device; (i.e., shear section of main rotor driveshaft). The effectiveness of any protection device should be demonstrated by test.
Page E - 123
SUBPART E - POWERPLANT
POWERPLANT FIRE PROTECTION AC 27.1183. § 27.1183 (Amendment 27-20) LINES, FITTINGS, AND COMPONENTS.
a. Explanation. This section requires that any line, fitting or other component of a flammable fluid, fuel or flammable gas system which carries, conveys, or contains the fluid or gas in any area subject to engine fire conditions (i.e., a severe fire) must be at least fire resistant (reference § 1.1 for definition of fire resistant and see paragraph AC 27.859 which defines a severe fire). An exception is for flammable fluid tanks and supports which are part of and attached to the engine or are in a designated fire zone. These items are required to either be fireproof (see § 1.1 for definition of fireproof and see paragraph AC 27.859 which defines a severe fire) or to be enclosed by a fireproof shield, unless fire damage to any non-fireproof part (e.g., secondary line or valve support) will not cause leakage of a flammable gas, flammable fluid or otherwise prevent continued safe flight and landing of the rotorcraft. All such components must be shielded, located, otherwise protected or a combination to safeguard against the ignition of leaking flammable fluids or gases. Integral oil sumps of less than 25 quarts capacity on a reciprocating engine need not be fireproof or enclosed by a fireproof shield; however, they should be fire resistant. Most integral sumps in this category are, by natural design and material selection, fire resistant.
Exemptions to the preceding requirements are as follows:
(1) Lines, fittings and components already approved under Part 33 as part of the engine itself.
(2) Vent and drain lines (and their fittings) whose failure will not result in or add to an operational fire hazard. In addition, all flammable fluid drains and vents must discharge clear of the induction system air inlet and other obvious ignition hazards.
b. Procedures. A detailed review of the design should be conducted to identify and quantify all lines, fittings, and other components which carry flammable fluids and/or gases and are in areas subject to engine fire conditions such as engine compartments and other fire zones. Once these items are identified the design means of fire protection should be selected and validated, as necessary, during certification. For materials and devices that cannot be qualified as fireproof or fire resistant by similarity or by known material standards, testing to severe fire conditions (see paragraph AC 27.859 definition, AC 20-135, and AC 23-2 for detailed requirements) should be conducted on full-scale specimens or representative samples to establish their fireproof or fire resistance capabilities. Exceptions to these standards (as provided in the regulatory section) should be reviewed and approved/disapproved on a case-by-case basis during certification. Also, operational fire hazards from drains, vents, and other similar sources should be identified and eliminated during certification.
Page E - 124
AC 27.1185. § 27.1185 (Amendment 27-11) FLAMMABLE FLUIDS.
a. Explanation. This section requires that fuel, flammable fluid, or vapor tanks, reservoirs or collectors be sufficiently isolated from engines, engine compartments, and other designated fire zones so that hazardous heat transfer from these areas to fuel, flammable fluid, and vapor tanks, reservoirs, or collectors is prevented in either normal or emergency service.
b. Definitions.
(1) Fuel or Flammable Fluid Collector. Any device such as a large valve, accumulator, or pump that contains a significant amount of flammable fluid, fuel, or vapor (e.g., the volume equal to 10 ounces or more of fluid).
(2) Flammable Fluid or Vapor Tank. Any fuel, flammable, fluid, or vapor tank, reservoir, or collector.
(3) Sufficiently Isolated. Fuel, flammable fluids, or vapors in a tank, reservoir, or collector are insulated, removed, otherwise protected or a combination such that their worst case temperatures (the worst case measured or calculated surface temperature of their containers) in either normal or emergency service is always 50° F or more away from the autoignition temperature of the fuel, flammable fluid, or vapor in question.
(4) Minimum Autoignition Temperature. The temperature at a given vapor pressure at or above which liquid fuel or fuel vapor will self combust. When determining the minimum design value of autoignition temperature which will occur in either normal or emergency operations, the critical, in-service combination of vapor pressure and fuel temperature should be determined.
(5) Hazardous Heat Transfer. A total incident heat flux (a combination of conduction, convection, and radiation, as applicable) from or in an engine compartment or other designated fire zone, which would raise the temperature level of a flammable fluid or fuel, their vapors, or the surface temperature of their containers to within 50° F or less of the minimum in-service autoignition temperature. Typically, the most critical heat transfer case to be considered is emergency service where a severe fire (see definition) is assumed to occur in each engine compartment and each designated fire zone on a case-by-case basis.
(6) Severe Fire. See definition in paragraph AC 27.859.
c. Procedures.
(1) The fuel, flammable fluid, and vapor system designs should be reviewed early in certification to insure that all flammable fluid or vapor tanks are properly identified and isolated from engines, engine compartments, and other designated fire zones during both normal and emergency operations such as in-flight engine Page E - 125
AC 27-1B, Chg 4 3/31/2014 compartment or other fire zone fires. In some cases fuel or flammable fluid components must be located in an engine compartment or other designated fire zone. In these cases, an equivalent safety finding (which considers the design, construction, materials, fuel lines, fittings, and controls used in the system, or system segment, contained in the engine compartment or other designated fire zone) should be undertaken as a part of the normal certification process. If the level of safety provided is equivalent to that provided by removing the system or system segment out of the engine compartment or designated fire zone, then the design should be accepted. For fuel tanks only, isolation is required by regulation to be achieved by use of either a firewall (reference paragraph AC 27.1191 for Firewall Requirements) or by use of a shroud. A shroud if used should be fireproof (see § 1.1 for definition and the definition of a Severe Fire for further details) and should be drainable (or otherwise inspectable) to insure the fuel tank is not leaking in service. For other flammable fluid or vapor tanks, the regulations allow either the identical treatment previously described for fuel tanks (i.e., firewalls or shrouds) or, alternatively, use of an equivalent safety finding. The equivalent safety finding, if used, can be made as part of the standard certification process. Regulations require that the equivalent safety finding be based on system design, tank materials, tank supports, and flammable fluid system connectors, lines, and controls. In all cases the flammable fluids, fuels, and vapors should be sufficiently isolated from hazardous heat fluxes during both normal and emergency operations to prevent autoignition.
(2) In addition, the regulations require at least one-half inch of clear airspace between each flammable fluid or vapor tank and each firewall or shroud that isolates the system, unless equivalent means (such as fireproof insulation) are used to prevent hazardous heat transfer from each engine compartment or other fire zone to the flammable fluid or vapor mass (or its container surface) at the fluid or vapor’s minimum autoignition temperature. If in-service structural deflections are significant, they must be taken into account when certifying the one-half inch minimum clear airspace requirement. For example, if a one-half inch clearance exists on the ground but in some normal and emergency flight conditions (e.g., autorotation) the one-half inch is reduced to one-fourth inch at a critical time (in-flight engine fire), then the design (static) configuration should have at least a one-half plus one-fourth equals three-fourths inch static clear airspace to insure the regulation’s intent is met. Alternatively, fireproof insulation or additional stiffeners could be used to insure the regulation’s intent is met
(i.e., the thermal equivalent of one-half inch clearance is maintained at all times). Any material used as insulation on or used adjacent to a flammable fluid or vapor tank, should be certified as chemically compatible with the flammable fluid or vapor and to be non-absorbent in case of fuel or vapor leaks. Otherwise, the material should either be treated for compatibility and non-absorbency or not accepted.
AC 27.1187. § 27.1187 VENTILATION AND DRAINAGE.
a. Explanation. To ensure that any component malfunction which results in fuel, flammable fluid or vapor leaks is safely drained or vented overboard and to ensure that Page E - 126
a fire hazard is not created during either normal or emergency service, there should be complete, rapid drainage and ventilation capability present for each part of the rotorcraft powerplant installation and any other designated fire zone which utilizes flammable fluid or vapor carrying components. As a minimum, the routing, drainage, and ventilation system should accomplish the following:
(1) It should be effective under normal and emergency operating conditions.
(2) It should be designed and arranged so that no discharged fluid or vapor will create a fire hazard under normal and emergency operating conditions.
(3) It should prevent accumulation of hazardous fluids and vapors in engine compartments and other designated fire zones.
b. Definitions. Drip Fence. A physical barrier that interrupts the flow of a liquid on the underside of a surface, such as a fuel tank, and allows any leaked liquid to drip from the surface away from hazardous locations to a safe external drain.
c. Procedures. The design of flammable fluid and gas systems running through engine compartments and other designated fire zones should have a thorough hazard analysis performed early during certification that is updated periodically as design changes dictate. The hazard analysis should identify and quantify all normal and emergency service failures that could result in leakage of fuel, flammable fluids and vapors. Once these potential hazards are identified and quantified, appropriate design features, such as drains, drip fences and vents, that minimize or eliminate the hazard should be provided. These means should be analyzed, tested, or a combination as necessary, to ensure that their size, flow capacity, and other design parameters are adequate to rapidly remove hazardous fluids and vapors safely away from the rotorcraft under normal and emergency flight conditions. Typically a venting or draining system should be designed to a 3-to-1 flow capacity margin over the probable worst case leak to which it could be subjected. Adverse effects such as clogging and surface tension flow reduction should be accounted for in design. Testing, including flight testing, using inert fluids or vapors may be necessary for proper design certification. In some instances it may be appropriate to include ventilation and drainage tests when the aircraft is parked.
AC 27.1189. § 27.1189 (Amendment 27-23) SHUTOFF MEANS.
a. Explanation.
(1) This section establishes the requirements for controlling hazardous quantities of flammable fluids which flow into, within, or through designated fire zones.
(2) When any shutoff valve is operated, any equipment, including a remaining engine, which is essential for continued flight, cannot be affected.
Page E - 127
b. Procedures.
(1) Combustible fluid supply lines which pass into, within, or through a firewall into the fire zone must incorporate shutoff valves. This requirement does not apply to lines, fittings, and components which were certified with and are part of the engine.
These requirements do not apply to oil systems for reciprocating engines with less than 500 cubic inches displacement or to any other installation where all components, including the oil tanks, are fireproof or are located in an area that will not be affected by an engine fire.
(2) Eight fluid ounces or less of a combustible fluid is not considered hazardous and no more than this amount should be present after activating the shutoff valve.
(3) Engine isolation is to be maintained when incorporating shutoff valves into engine fuel and lubrication lines. The design should ensure that when one engine is shut down or fails and the fuel and lubrication fluid shutoff valves are activated, the remaining good engine is not affected in any way, and the rotorcraft can continue safe flight to a landing. This should be demonstrated by test.
(4) Each shutoff valve located in a fire zone should be fireproof. If the shutoff valve is located outside of the fire zone, then it should be at least fire resistant or protected so that it will function under a worst case fire condition within a fire zone. This should be demonstrated by test.
(5) For primary propulsion engine installations, the flammable fluid shutoff should be protected from inadvertent operation. Where electrical shutoffs are used, the switches should be guarded or require double actions. If the shutoffs are mechanically activated, the design of the knob and the location of the lever should be such that inadvertent actuation cannot occur. It must be possible to reopen the shutoff valve after it has been closed and this should be demonstrated by test.
AC 27.1191. § 27.1191 (Amendment 27-2) FIREWALLS.
a. Explanation. This section states the certification requirements for the use of fireproof protective devices such as firewalls, shrouds, or equivalent. These devices are necessary to isolate each engine (including combustor, turbine, and tailpipe sections of turbine engines and auxiliary propulsion units (APU); each APU; each combustion heater; each unit of combustion equipment; or each high temperature device (or source) from personnel compartments and critical components (not already protected under § 27.861). The isolation of these fire zones is necessary to prevent the spread of fires, prevent or minimize thermal injuries and fatalities, and prevent damage to critical components that are essential to a controlled landing. Even though § 27.1191(b) implicitly excludes APU’s, combustion heaters, and other combustion equipment that are not used in flight; they should be protected by fireproof enclosures, because of the requirements of the relevant parts of §§ 27.1183 through 27.1203. This is because, even if the device is rendered inoperative in flight, it typically contains residual heat, Page E - 128
fuel, fumes and potential ignition sources (i.e., “potential hazards”). Each fireproof protective device must, by regulation, meet the following criteria:
(1) Its design and location must take into account the probable fire path from each fire zone or source considering factors such as internal airflow, external airflow, and gravity.
(2) It must be constructed so that no hazardous quantity of air, fumes, fluids, or flame can propagate through it to unprotected parts of the rotorcraft.
(3) Its openings (e.g., shaftholes, lineholes, etc.) must be sealed with close fitting fireproof grommets, bushings, bearings, firewall, fittings, or equivalent that prevent burn through and leakage of hazardous fumes or fluids from the fire zone.
(4) It must be fireproof (see definition).
(5) It must be either corrosion resistant or otherwise safely protected from corrosion.
b. Definitions.
(1) Fireproof Protective Device. A fireproof protective device is a device such as a firewall, shroud, enclosure, or equivalent used to isolate a heat or potential fire source (severe fire) from personnel compartments and from critical aircraft components which are essential for a controlled landing.
(2) Fireproof. Fireproof is defined in § 1.1 “General Definitions.”
(3) Controlled Landing. A landing which is survivable (i.e., does not fatally injure all occupants) but may produce an unairworthy, partially salvageable, or unsalvageable rotorcraft.
(4) Severe Fire. See Definition in paragraph AC 27.859.
c. Procedures. Fireproof protective devices are typically certified by analysis, tests, or a combination conducted during the certification process, including flight tests or simulated flight tests, as follows:
(1) Fireproof protective devices should be provided wherever a hazard exists which requires isolation from a severe fire to avoid fires in personnel compartments and to avoid thermal damage to critical components (such as structural elements, controls, rotor mechanisms, and system components) that are necessary for a controlled landing.
A thorough hazard analysis should be conducted during certification to identify, define and quantify in order of severity (i.e., maximum temperature, hot exposed area, etc.) all thermal hazards or zones that require fireproof protection in a given design. Engines (including the combustor, turbine, and tailpipe sections of turbine engines), APU’s, Page E - 129
combustion heaters, and combustion devices are required by regulation to be isolated.
Other high temperature devices may also require isolation because of local hot spots (which occur during normal operations or from failure modes) that can thermally injure occupants or cause spontaneous combustion of surroundings. A hazard analysis should identify these potential problems and provide proper certification solutions.
(2) Fireproof protective devices should be able to withstand at least 2000 ± 150° F for at least 15 minutes (reference AC 20-135). The fireproof protective device should allow protected parts, subsystems or systems to perform their intended function for the duration of a severe fire (see definitions). For firewalls, examples of flat, geometry materials undergoing uniform heat fluxes with material gauges that automatically meet the certification requirements are given in figure AC 27.1191–1. If firewalls are utilized that involve other materials, significant geometric changes, or significantly non-uniform heat fluxes, then automatic compliance may not be assured.
In such cases the fireproof protective device should be analyzed using the severe fire definition and, in some cases, tested in accordance with AC 23–2 to ensure proper certification. For example, a curved protective surface may absorb a uniform incident heat flux unevenly and create a local hot spot that exceeds 2,150° degrees Fahrenheit that burns through in less than 15 minutes; whereas, a flat surface of equal thickness might not exceed 2,150°degrees Fahrenheit and would not burn through in less than 15 minutes. It should be noted that composite materials are not generally used for protective devices because of their inability to withstand high temperatures (i.e., exceedance of the glass transition temperature); however, some specially formulated composites have been previously certified as engine cowlings. Titanium is an acceptable material for fireproof protective devices such as firewalls. However, use of titanium should always be carefully considered and reviewed, because it can lose all structural ability and burn severely (self combust) above 1,050° F, under certain thermodynamic environments, and contribute to the fire instead of providing the intended fire protection. AC 33–4, “Design Considerations Concerning the Use of Titanium in Aircraft Turbine Engines” and MIL-HDBK-5D contain more detailed information on the unique thermal properties of titanium.
Page E - 130
FIGURE AC 27.1191-1
TABLE OF MATERIALS AND GAGES ACCEPTABLE
FOR FIREPROOF PROTECTIVE DEVICES WITH FLAT
SURFACE GEOMETRIES(1) MATERIAL(2) MINIMUM THICKNESS(3) Titanium Sheet .016 in Stainless Steel .015 in Mild Carbon Steel .018 in Terne Plate .018 in Monel Metal .018 in Firewall Fittings (Steel or Copper Base) .018 in(4) NOTES:
(1) Assumes essentially flat vertical or horizontal surfaces undergoing a uniform heat flux. Any significant variation in either geometry or heat flux distribution should be examined in detail for adequate gauge thicknesses on a case-by-case basis.
(2) Must have corrosion protection if not inherent in the material itself.
(3) The minimum thickness is for thermal containment only. Structural integrity considerations may require thickness increases. MIL-HDBK-5D contains material allowable versus temperature data for most common metallic materials.
(4) This is the minimum wall thickness measured at the smallest dimension (e.g., thread root or other location) of the part.
(5) Distortion of thin sheet materials and the subsequent gapping at lap joints or between rivets is difficult to predict; therefore, testing of the simulated installation is necessary to prove the integrity of the design. However, rivet pitches of 2 inches or less on non load-carrying titanium firewalls of .020 inch or steel firewalls of .018 inch are acceptable without further testing.
Page E - 131
(3) The probable path of a fire (as affected by internal and external air flow during normal flight and autorotation, gravity, flame propagation paths, or other considerations) should be taken into account when performing the hazard analysis of item (1). Such a review will ensure that fireproof protective devices are placed in the proper location for intercepting, blocking or containing a severe fire before occupants are injured and a controlled landing is prevented. If the probable path cannot be readily determined by inspection or analysis, testing using simulated air flows, rotorcraft attitudes, and dyed inert fluids or vapors can be used to aid in this determination.
(4) Each opening in a protective device should be sealed with close fitting sealing devices such as fireproof grommets, bushings, firewall fittings, rotating seals or equivalent that are at least as effective as the fireproof protective device itself. This is necessary to ensure that no local breakdowns in protection occur. For materials not listed as acceptable in item (1), analysis and testing should be required in accordance with FAA/AUTHORITY standards and the definition of a severe fire for proper substantiation.
(5) Each protective device should be fireproof in order to withstand a severe fire. Unless designs and materials have been previously FAA/AUTHORITY approved
(e.g., see Item 1), the protective device’s design and material selection should be tested to ensure its fireproof thermal and structural integrity. A full-scale test of a structurally loaded article or a representative sample should be conducted to ensure proper compliance is achieved. Also, the continued sealing ability of the protective device in its deformed state due to a hard controlled landing should be considered during certification (e.g., use of ductile materials). The corrosion environment should be defined and appropriate protection provided. Phased inspections should be specified, if necessary, to ensure continued corrosion integrity. Certification tests for adequacy of corrosion protection should be conducted using sample plates or by other equivalent means, as required.
AC 27.1193. § 27.1193 COWLING AND ENGINE COMPARTMENT COVERING.
a. Explanation.
(1) Section 27.1193(a) requires the cowling and engine compartment coverings to structurally withstand loads experienced in flight.
(2) In order to prevent pooling of flammable fluids, § 27.1193(b) requires rapid and complete drainage from the cowling and engine compartment.
(3) Section 27.1193(c) requires the drain of paragraph (b) to purge the fluid in such a manner not to create a fire hazard.
(4) Section 27.1193(d) requires the cowling and engine compartment covering to be at least fire resistant and paragraph (e) requires them to be fireproof where they may experience high temperatures due to the exhaust system.
Page E - 132