ice shapes to the inlet. These shapes should closely match the actual ice shapes defined by test or analysis.
c. Turbine Engines - Snow Protection.
(1) Explanation.
Section 29.1093(b)(1) (il) provides that the turbine engine and its air inlet system operate satisfactorily within the limitations established for the rotorcraft, in both falling and blowing snow. The section does not provide the definition of falling and blowing snow.
Since the regulation provides for certification "within the limitations established for the rotorcraft," the FAA/AUTHORITY can accept a restriction against snow operations in the limitations section of the RFM in lieu of demonstration of compliance. If no restriction on snow operations appears in the RFM, it is presumed that the aircraft may operate in snow at the pilot's discretion.
(2) Guidance.
(i> The FAA/AUTHORITY has accepted that engine induction system operation in falling and blowing snow can be approved without restriction if normal operations under the following conditions are demonstrated:
Visibility: ¼ mile or less as limited by snow.
Temperature: 25° F to 34° F (28° F to 34° F desired), unless other temperatures are deemed critical.
Operations: Ground operations - 20 minutes IGE hover - 5 minutes Level flight - 1 hour Descent and landing
(ii) Rotorcraft Flight Manual visibility restrictions for falling and blowing snow operations are not appropriate.
Time limitations, other than possibly for ground and hover operations, are not appropriate.
(iv) Artificially produced snow should not be used as the sole means of showing compliance.
(3) Guidance Rationale.
Par 532 837
The test conditions specified--visibility, temperature, and operations--are based on previous certification programs, previous FAA/AUTHORITY guidance, and on research by the FAA technical center and others.
(A) Visibility. The test visibility defined, /4-mile visibility or less as limited by snow, represents a heavy snowstorm and is the maximum likely to be encountered in service. Rotorcraft which have been certified to the ¼-mile visibility test criteria have not shown engine inlet snow-related service difficulties. It is important to note that the visibility specified is a test parameter rather than an operational limitation to be imposed on the rotorcraft after the tests are completed.
(B) Temperature.
(1) The ambient temperature specified is conducive to wet snow conditions. Wet snow tends to accumulate on unheated surfaces subject to impingement.
(2) Colder ambients, more conducive to dry snow conditions, may be critical for some induction systems. Colder exterior surfaces may be bypassed, and the snow crystals may stick to partially heated interior surfaces where partial melting and refreezing may occur.
(3) Company development testing or experience with very similar type induction systems may be adequate to determine the critical ambient conditions for certification testing.
(C) Operations.
(1) Ground running, taxiing, and IGE hover operations are generally the most critical since the rotorcraft may be operating in recirculating snow. Twenty-five minutes under these extreme conditions would seem a reasonable maximum, both from the view of pilot stress and the maximum expected taxi time prior to takeoff in bad weather.
(2) One hour of level flight operation under ¼-mile visibility snow conditions should provide ample opportunity for hazardous accumulations to begin to build.
(3) The descent and landing will provide an engine power change, an induction system airflow change, and a variation in the external airflow pattern near the induction system entrance. The initiation of the descent and final flare for landing may also produce additional airframe vibration transmitted to the induction system.
These power, airlow, and vibration changes may provide an opportunity for any level flight accumulations to be ingested into the engine. Hazardous accumulations are not acceptable during or after any test phase.
838 Par 532
Visibility may fluctuate rapidly in snowstorms. It is affected by the presence of fog or ice crystals, is not crew measured or controlled, and is difficult to estimate. A visibility operational limitation based on snow, therefore, is not appropriate.
(ili) Since during cruise in snow conditions the aircraft is likely to be in and out of heavy snowfall, it is not practical for the crew to account for the time spent in snow in level flight conditions. Thus, it is not appropriate to include time limitations in the RFM for level flight snow operations.
(iv) A practical ground and IGE hover time limitation of less than 25 minutes in recirculating snow may be considered. The expected action at the expiration of this specified time period would be shut down and inspection of the inlet system or transition to a safe flight condition where demonstration has shown that moisture accumulations will not intensify or shed and cause engine operational problems.
(v) Artificially produced snow is an excellent development tool and has been successfully used to indicate potential problem areas in induction systems. These devices are usually restricted to use for hover and ground evaluations, and the snow pellets produced by these machines are not sufficiently similar to natural snowflakes to justify the use of artificial snow as the sole basis of certification.
(4) Procedures.
Satisfactory demonstration of the test conditions requires that the engine, induction system, and proximate cowling surfaces remain free of excessive snow, ice, or water accumulation. Excessive accumulation is defined as accumulation that may cause engine instability, damage, or significant loss of engine power. If a questionable amount of snow or moisture accumulates in the inlet, the applicant may elect to demonstrate that this amount in the form of snow or water and ice, as appropriate, can be ingested by the engine without incurring surge, flameout, or damage.
(ii) The conditions specified assume actual flight demonstration in natural snow. The ground operations and IGE hover test conditions assume operation in recirculating snow. Blowing snow, resulting from rotor airflow recirculation, can be expected to be more severe than natural blowing snow if the rotorcraft continues to move slowly over freshly fallen snow. Thus, the blowing snow operational capability is usually demonstrated by the taxi and hover operations in recirculating snow.
(ili) For VFR rotorcraft, the airspeeds for the level flight test condition should include the maximum consistent with the visibility conditions. For IFR operations, the airspeed should be the maximum cruise speed or the maximum speed specified for snow operations in the flight manual limitations, unless other airspeeds are Par 532 839
АС 29-2B 7/30/97 deemed more critical. It is recognized that many rotorcraft initially certified VFR are later IF certified with a resulting possible increase in airspeed in snow conditions.
This factor should be considered if IFR certification is anticipated.
(iv) The visibility specified assumes that visual measurements are made in falling snow in the absence of fog or recirculating snow by an observer at the test site outside the tests rotorcraft's area of influence. An accepted equation for relating this measured visibility to snow concentration is V = 374.9/C°.7734 where C is the snow concentration (grams/meter) and V is the visibility (meters).
(A) This equation can be reasonably applied to all snowflake type classifications and is credited to J.R. Stallabrass, National Research Council of Canada.
(B) Other equations may be applied if they are shown to be accurate for the particular snowflake types for the test program.
(v) The snow concentration corresponding to the visibility prescribed, ¼ mile or less, will be extremely difficult to locate in nature. Data from Ottawa, Canada, research indicate that fewer than 4 percent of the snowstorms encountered there meet the 0.91 grams/m concentration associated with the ¼-mile visibility. Furthermore, the likelihood that the desired concentration will exist for the duration of the testing is even more remote. Because of these testing realities, it is very likely that exact target test conditions will not be achieved. Those involved in certification must exercise good judgment in accepting alternate approaches.
(vi) For some engine induction systems, it may become apparent by inspecting for moisture accumulations that ground and IGE hover operations in recirculating snow are much more severe than the level flight test. In this instance, it is reasonable to accept prolonged IGE operations in recirculating snow and to accept durations of less than 1-hour level flight in ¼-mile or less visibility. Best efforts should be made to assure that at least some level flight time is accomplished at ¼-mile or less visibility to assure that the spectrum is covered.
(vii) It should be determined that the visibility established at the test sight is limited by snow and not by fog or poor lighting (twilight) conditions.
(vili) The concentration of snow approaching the inlet in severe recirculation will far exceed the quantity encountered in the natural snowfall.
Recirculation is necessarily a qualitative judgment by the test pilot. The snow concentration at the inlets during recirculation would vary for different rotorcraft types and would be dependent on rotor characteristics, power setting, and inlet location. For test purposes, recirculation should be the highest snow concentration attainable in the maneuver, or that corresponding to the lowest visibility at which (in the pilot's judgment) control of the rotorcraft is possible in the IGE condition. The visibility specification of 840 Par 532
7/30/97/ АС 29-2B ¼ mile or less outside of the recirculation influence becomes inconsequential provided that fresh, loose snow is continually experienced during the ground operation and IGE hover testing phase. However, since it is intended that the test phases be accomplished sequentially to assure that transition to takeoff and other transients are considered, the conditions at takeoff, level flight, and descent and landing should approximate the ¼-mile visibility criteria.
d. Turbine Engines - Ground Icing.
(1) Explanation. This requirement addresses the situation where extended ground operation in icing exposes the rotorcraft and its engine inlet to icing (ground fog) conditions which may have different droplet impingement patterns and involve different and/or less effective means of ice protection. Note that the requirement is effective at Amendment 10 and is applicable regardless of any desire to prohibit dispatch into known icing conditions.
(2) Procedure. Since this condition assumes zero airspeed, wind tunnel testing may be inappropriate unless conservative extrapolation of low speed tunnel data can be determined to be valid. For protection schemes which are dependent primarily on airspeed for proper functions (external screens have shown this tendency), it may be necessary to verify adequate ground operation protection capability by very low speed tunnels or by the use of outside facilities such as the Canadian National Research Council's spray rig at Ottawa, Canada. For heated systems or for internal bypass schemes, tunnel speed may not be important, and adequate demonstration may be accomplished at higher tunnel speeds provided that internal inlet airflows and heat available are properly considered. Testing should proximate the regulatory test conditions and be continued for 30 minutes using engine power and control manipulation as normally accepted during taxiway operations, followed by an acceleration to takeoff power. The test time may be shortened if deice/anti-ice protection is adequate or if stabilization of ice build-up is affirmed. The induction system should be in condition for safe flight at the conclusion of the test.
532A. § 29.1093 (Amendment 29-26) INDUCTION SYSTEM ICING PROTECTION.
a. Explanation. Amendment 29-26 clarifies that the phrase, "within the limitations established for the rotorcraft" applies only to the requirement in § 29.1093(b)(1)(ii) for demonstrating flight in falling and blowing snow.
b. Procedures. All of the policy material for this section remains in effect with the update that turbine engines and turbine engine inlets should perform satisfactorily in atmospheric icing conditions defined in Appendix C of FAR 29 instead of FAR 25. In addition Paragraph 532, the following procedures should be followed:
(1) A "serious loss of power" in this section has been interpreted to be any power loss that requires immediate pilot action. In addition, the term "adverse effect on Par 532 841
engine operation" in § 29.1093(b)(1)(ii) has been interpreted to be an effect that would prevent the engine from achieving rated aircraft flight manual performance (takeoff/climb/etc.). This term also includes effects on the engine induction system characteristics to an acceptable level established by the engine manufacturer (inlet distortion, etc.).
(2) It should be shown that rotorcraft that are prohibited from flight into falling and blowing snow can exit inadvertent entrance into those conditions without adverse effect upon the operating characteristics of the engine or the rotorcraft.
(3) For full flight capability into snow, both falling and blowing, it should be shown that each engine, and its inlet system, will operate satisfactorily throughout the flight power range of the engine and the operating limitations of the rotorcraft. It should be shown that any build-up or accumulation of snow will not reduce or block the flow of inlet air to the engine. Any accumulations that become dislodged should not affect engine operation.
533. § 29.1101 CARBURETOR AIR PREHEATER DESIGN.
a. Explanation. Each carburetor air preheater must be designed and constructed to:
(1) Ensure ventilation of the preheater when the engine is operated in cold air.
(2) Allow inspection of the exhaust manifold that it surrounds.
(3) Allow inspection of critical parts of the preheater itself.
b. Procedures. Although carburetors of some design and fuel injections are free from icing difficulties, the most common remedy is to preheat the air supply entering the carburetor. In this way, sufficient heat is added to replace the heat lost due to vaporization of fuel, and the mixing chamber temperature cannot drop to the freezing point of water. The air preheater is essentially a tube or jacket through which the exhaust of one or more cylinders is passed with the air flowing over the heated surface raised to the required temperature before entering the carburetor. A control for adjusting the preheater valve is installed in the cockpit so that heat may be applied only when actually required to prevent ice formation.
534. § 29,1103 (Amendment 29-17) INDUCTION SYSTEM DUCTS AND AIR DUCT SYSTEMS.
a. § 29.1103(a):
(1) Explanation. This paragraph is intended to require the design of induction system ducts for engines and auxiliary power units to include fuel and water drains 842 Par 532A
which are effective in the ground attitude and do not discharge into any location where the fuel drainage could be ignited to cause a fire hazard.
(2) Procedures. Determine that each induction duct is provided with at least one drain of sufficient size to minimize clogging and located at the low point of the duct with the rotorcraft in the ground attitude. Discharge from the drain should not create a hazard to the rotorcraft.
b. § 29,1130(b):
(1) Explanation. This paragraph applies to reciprocating engines and is intended to require the induction system to withstand the stresses of explosive backtire which must be expected in these engines.
(2) Procedures. The magnitude of the backfire to be considered is somewhat subjective; however, the rule can generally be satisfied by testing which involves inducing actual backfires in the engine. This can usually be accomplished by crossing ignition leads between cylinders to cause ignition when the intake valve is open. Tests should include both engine cranking and power-on regimes.
c. § 29.1103(c):
(1) Explanation. Induction ducts, particularly on reciprocating engines, involve connections with other ducts and with structure. Flexibility is required to prevent relative motion (expansion, structural deflections, etc.) from prestressing the duct.
(2) Procedures. Review the design for long runs of ducting between the engine and structural supports and between other connections or supports in the duct system. Short segments of the duct constructed of bellows will usually provide the necessary flexibility.
d. § 29,1103(a):
(1) Explanation. The effectiveness of fire extinguisher systems is based, in part, on testing for agent concentration in the fire zone with the airflows to be expected.
Any duct failure (burnout) during an engine compartment fire may be expected to introduce air to dilute the agent concentration, or if the duct passes through a firewall, duct burnout could result in an opening in the firewall. Fireproof ducts, as specified by this rule, are needed to ensure the integrity of the firewalls and the effectiveness of the fire extinguisher system. Fire resistant ducts may be used if located totally within the fire zone.
(2) Procedures. Ducts within a fire zone are usually engine air induction ducts, air bypass ducts, or cooling air ducts. For ducts which penetrate the firewall or other fireproof construction such as fireproof cowling, verify that the duct is of fireproof Par 534 843
construction. Other ducts may be only fire resistant. A duct constructed of material which has been accepted as firewall material would be considered as fireproof without further testing (unless the duct is subject to significant structural loads, in which case, fire testing may be necessary with the loads applied to the duct). The tests for "fireproof" and "fire resistant" qualification differ only in the time exposure; i.e., 15 minutes for "fireproof" and 5 minutes for "fire resistant." If nonmetallics are used in duct construction intended for "fireproof" applications and the integrity of the test specimen is deteriorating towards the end of the 15-minute fire test period, assessment of the situation with respect to possible hazards if the engine fire were to exist beyond 15 minutes is appropriate. Duct burnout should not result in the possibility that fire could escape the fire zone and create hazardous conditions.
e. § 29.1103(e):
(1) Explanation. This rule requires additional fireproofing of the inlet duct of auxiliary power units (APU's) to ensure safe disposal or containment of hot gas reverse flow from the APU from entering any other compartment of the rotorcraft in which a hazard would be created. This rule could, in some designs, require fireproof construction of the inlet duct for the APU to extend upstream beyond the confines of the firewall provided in compliance with § 29.1191(b). The extent of the fireproofing is subjective and may require malfunction testing if no applicable information can be provided by the manufacturer of the APU. For ducting upstream of the fireproof section, materials selected need not be qualified for fire impingement; however, they must be shown to be suitable for the maximum normal heat conditions to be expected.
(2) Procedures. Normally, fireproof ducting upstream of the APU to the contour of the rotorcraft is acceptable for compliance. However, if this distance is less than 36 inches, the possibility of impingement of hot gases on the contour skin of the rotorcraft is required. Fireproofing of contour skin or duct relocation should be considered if the impingement area is a nonmetallic structure or is part of or close to fuel tanks. Other system air inlets in the impingement area should also be evaluated for possible hazards due to ingestion of hot gases in event of reverse flow from the APU.
f. § 29.1103(f):
(1) Explanation. APU inlet ducts subject to reverse flow of hot gases should be constructed of materials that will not absorb fuel or other flammable liquids to avoid induction duct inlet fires which may ignite by backfires or reverse flow from an APU.
(2) Procedures. Any nonmetallic duct material should be shown by test or by previous qualification to be sealed or otherwise free of tendencies to absorb flammable liquids. Tests, if necessary, should follow the guidelines for absorption qualification set forth in TSO's or military specifications for fuel and oil tanks.
844 Par 534
535. § 29.1105 INDUCTION SYSTEM SCREENS.
a. Explanation. This paragraph concerns reciprocating engine installations. If induction system screens are used, the following considerations apply.
(1) Each screen must be upstream of the carburetor.
(2) No screen may be in any part of the induction system that is the only passage through which air can reach the engine unless it can be deiced by heated air.
(3) No screen may be deiced by alcohol alone, and it must be impossible for fuel to strike any screen.
b. Procedures. Inlet screens in the engine induction system are generally provided to prevent the entrance of foreign objects. The induction design may incorporate features which address the concerns identified above. Also, some designs incorporate an alternate air door which, with appropriate consideration, accounts for the requirements of this paragraph. The alternate air source should provide the required air to maintain flight and landing to a suitable landing site at appropriate airspeeds and gross weights.
536. § 29.1107 INTERCOOLERS AND AFTER-COOLERS.
a. Explanation. Each intercooler and after-cooler must be able to withstand the vibration, inertia, and air pressure loads to which it would be subjected in operation.
b. Procedures. In complying with this regulation, the various vibrations, inertia, and air pressure loads should be identified. The installation may be verified by either analysis or test appropriate for the design.
537. § 29.1109 CARBURETOR AIR COOLING.
a. Explanation. It must be shown under § 29.1043 that each installation using two-stage superchargers has means to maintain the air temperature at the carburetor inlet, at or below the maximum established value.
b. Procedures. When the powerplant installation design utilizes a supercharger installation, it should be shown by testing that the air temperature at the carburetor inlet does not exceed established values.
538.-547. RESERVED.
Par 535 845 (thru 855)
SECTION 31. EXHAUST SYSTEM
548. § 29.1121 (Amendment 29-13) GENERAL.
a. Explanation.
(1) This section addresses the arrangement of exhaust components and the protection against hazardous conditions which exist with hot exhaust gases for powerplant and auxiliary power unit installations.
(2) The objective is to ensure safe disposal of exhaust gases without fire hazard or physical impairment to any occupant.
b. Procedures.
(1) During the certification process, carbon monoxide levels should be monitored in the personnel compartments to verify that the gas levels are well within the acceptable range. The conditions under which the measurements are taken should be representative of the normal operating limitations of the rotorcraft. This paragraph is not applicable to gas turbine-engine-powered rotorcraft.
(2) Exhaust system surfaces hot enough to ignite flammable fluids or vapors must meet the isolation or shielding requirements of this section in addition to the requirements of §§ 29.1183 and 29.1185. Good design practice suggests that the isolation and shielding features incorporated would continue to be effective under the emergency landing conditions specified in § 29.561.
(3) Compliance with the § 29.1121(c) fireproof requirements can be accomplished by demonstrating that the material or component will withstand a 2000° F + 50° F flame for 15 minutes while still fulfilling its design purpose. This testing should accurately simulate, as near as practicable, the operating environment of the material or component in service. In addition to the fireproof requirements, the requirements of § 29.1191 must be met.
(4) Compliance with § 29.1121(d) can be accomplished by locating the vents and drains where fumes and fluids cannot interact with the hot exhaust gases. Drains should discharge positively and be a minimum of 0.25 inches in diameter. No drain may discharge where it will cause a fire hazard. This can be demonstrated by discharging a colored liquid through the drain system in flight and on the ground. The dye should not impinge on any ignition source.
(5) It should be demonstrated that exhaust gases are discharged in such a manner that they do not cause distortion or glare seriously affecting the pilot's visibility 856 Par 548
at night. One method of compliance would be a night flight evaluation at critical azimuth and variable wind conditions to verify that no degradation exists.
(6) Hot spots that can occur on exhaust system components should be eliminated by providing deflectors and/or adequate ventilation. Exhaust shrouds can either be ventilated or insulated to keep the temperatures low enough so that ignition of flammable vapors or fluids cannot occur under normal operation or under the emergency landing conditions specified in § 29.561.
(7) Compliance with § 29.1121(h) can be accomplished by ensuring that the drain will not discharge where it might cause a fire hazard. This can be demonstrated by discharging a colored liquid through the drain system in flight and on the ground.
The dye should not impinge on any ignition source.
549. § 29.1123 EXHAUST PIPING.
a. Explanation. This section contains the following requirements that must be met for proper certification of exhaust piping on engines, auxiliary propulsion units (APU), and other similar devices.
(1) § 29.1123(a) requires that the piping be heat and corrosion resistant so that it performs its intended function during its operational life (either the life of the rotorcraft or a specified limited life) without significant metal corrosion, metal erosion, or creation of hazardous hot spots. The piping system should be designed, have an installation design, or a combination that allows performance of its function without thermal expansion (thermal strain) induced structural failures, such as ruptures caused by operating temperature excursions and by overpressurization during its operational
(2) § 29.1123(b) requires that the piping must be supported to withstand the vibration and loading environment (including inertia loads) to which it will be subjected in service.
(3) § 29.1123(c) requires that piping that connects to components between which relative motion exists in service must have the necessary flexibility and structural integrity to withstand the relative motion without exceeding limit load (at the maximum operating temperature) of the piping, or creating unintended loads (or load paths) on the components to which the piping connects.
b. Procedures. Exhaust piping is typically certified by analysis and installation tests conducted during the basic certification process, including flight tests, as follows:
(1) For compliance with § 29.1123(a), because of its durability in the hot exhaust environment, exhaust piping is typically made from stainless steel or alloy steel of the appropriate structurally and thermally derived wall thickness. Hot aircraft exhaust Par 548 857
gases are very corrosive; thus, proper material selection and corrosion protective design should be performed and validated during certification. Advisory Circular (AC) 43-4, "Corrosion Control For Aircraft" contains a detailed discussion of exhaust gas corrosion problems. Analysis and/or verification tests of the exhaust system should be conducted. This work is necessary to ensure thermal and structural integrity; to ensure that thermal expansion does not cause a structural overload or failure; and, to ensure that exhaust piping does not contact (or come close to) ambient temperature materials (such as structure or system components). Hot exhaust piping in contact with (or close to) ambient temperature materials can either create a fire hazard or cause an unintended strength reduction. To ensure that thermal expansion analyses and tests are properly conducted, the maximum in-service temperature excursion should be properly defined. The maximum temperature excursion should be based on the maximum temperature of the piping and exhaust gases, as affected by the insulatory characteristics of the piping's enclosure, and as affected by a worst case hot day. The worst case temperature environment used for analysis can be verified by a temperature survey. If run on cooler days, the survey can be adjusted for the worst case hot day environment using methods identical to those used for engine cooling tests (reference Paragraph 517, Cooling Tests). The piping should be designed to expand freely so that thermal expansion (thermal strain) induced loads on the piping and its restraint system are minimized. If thermal expansion induced loads (in conjunction with deflection induced loads and exhaust flow loads, discussed in b(4)) are significant relative to limit load of any item in the load path, then a fatigue check on the critical design points) should be performed. The fatigue check should establish a safe life or an approved limited life for the critical component(s) in the system. An accurate analytical fatigue check on exhaust piping may be difficult to perform because of erosion, corrosion, etc., in service; therefore, phased inspections should be considered to ensure the exhaust piping's continued airworthiness.
(2) For compliance with § 29.1123(b), exhaust piping should be properly supported so that the maximum loads anticipated in-service are properly distributed and reacted, and, as previously discussed, so that thermal expansion induced loading is minimized. Typically the worst case static design load conditions are either the inertia loads from an emergency impact (reference § 29.561) or the combined loading from thermal expansion, in-flight deflections and internal exhaust gas flow (See Paragraph b(4)). It should be noted that several combinations of these loads should be examined to determine the critical combination. The piping should be supported and restrained such that critical frequencies are avoided and the induced vibration environment's effect is minimized. Flight test vibration surveys may be necessary, in some cases, to properly define or validate the critical modes and environment and their effect on the exhaust piping design. Operating modes such as ground idle, flight idle, 40 percent and 80 percent of maximum continuous power, maximum continuous power, OEl power settings and other power settings should be investigated to determine their vibratory effect on the exhaust gas piping system. The strength reduction of the piping materials at operating temperature (and at worst case temperature) should be properly 858 Par 549