374.-383. RESERVED.
Intentionally Left Blank Par 374 563 (thru 575)
SECTION 23. MISCELLANEOUS (DESIGN AND CONSTRUCTION)
384. § 29.871 LEVELING MARKS.
a. Explanation. Reference marks are required for leveling the rotorcraft on the ground. These marks are necessary for accurate determination of weight and balance effects, particularly after modifications to the basic rotorcraft.
b. Procedures.
(1) Reference marks are sometimes provided in pairs, one high in the cabin and one low. The plumb weight is suspended from the high mark by an appropriate mechanical attachment, and the lower mark is used to level the rotorcraft by centering the plumb weight. The lower reference mark should be a raised or depressed target symbol and shall be applied to a permanent structural component or permanently attached plate in a readily accessible location. Seat tracks, floors, or door sills which are attached with permanent fasteners are typical locations.
(2) Horizontal reference marks for support of bubble levels may also be used, particularly for smaller rotorcraft.
(3) Proper reference should be made to identify the leveling marks or points on the rotorcraft. Design provisions should be made to ensure these locations are not obscured by equipment, fairings, repair, or rework.
385. § 29.873 BALLAST PROVISIONS.
a. Explanation.
(1) This rule requires that ballast provisions prevent inadvertent ballast shifting while in flight or as a result of a landing. Shifting of the ballast may cause a hazardous change in the center of gravity thereby affecting rotorcraft controllability.
(2) Other rules noted here allow removable and fixed ballast and require markings or placards to prevent overloading the ballast installation.
Section 29.29 specifies that the rotorcraft empty weight will include any fixed ballast. Section 29.31 allows the use of removable ballast to comply with the flight requirements. However, ballast may not be adjusted (moved, reduced or increased) in flight.
Section 29.1541 requires conspicuous and durable markings or placards. Section 29.1557 requires placards stating allowable maximum weight, distributed loading, if necessary, and other appropriate limitations for ballast installation.
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(3) Section 29.1583(c) concerns Rotorcraft Flight Manual instructions and information about removable ballast or loading information. The instructions must be included in the operating limitations section of the flight manual to allow ready observance of the limitations.
b. Procedures.
(1) The ballast installation may be substantiated by analysis or by static test.
The design ultimate load may be derived from flight, landing, or minor crash conditions load factor specified in the rules. Substantiation by analysis will require use of the fitting factor prescribed by § 29.625 where appropriate. If static tests are to be conducted, a test plan should be prepared, submitted for evaluation and agreed upon prior to the test.
(2) Ballast installations in the aft part of the fuselage and tail boom may be subject to significant landing condition angular inertia load factors as well as the usual linear load factors.
(3) Substantiation methods and procedures acceptable for the airframe substantiation may be used for the ballast installation as well.
(4) Removable ballast will require attention to assure the ballast is secured easily and properly and will remain secured under the appropriate ballast design load factor requirements. The flight manual instructions should be evaluated for compliance with § 29.1583(c) by flight test and airframe personnel.
(5) The installation must be designed and placarded or marked for the maximum allowable ballast load and for other appropriate loading limits. Normally compliance with § 29.1541 is accomplished with a drawing review by airframe personnel along with an EMDO compliance and conformity inspection. An additional compliance inspection by airframe personnel can be conducted if desired.
386. § 29.877 ICE PROTECTION.
NOTE: § 29.877 was removed and replaced by § 29.1419 in Amendment 29-21. This material is retained since this is one way to show compliance with § 29.877.
a. Background.
(1) In March 1984, the FAA/AUTHORITY for the first time certificated a rotorcraft for flight into known icing conditions. Several other manufacturers are pursuing designs for icing flight capability with certification planned for 1985 or 1986.
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(2) Most rotorcraft icing technology has been developed for military rotorcraft.
The only U.S. military rotorcraft equipped and approved for flight into icing conditions is the UH-60A (Blackhawk). The UH-60A is limited to supercooled cloud conditions where liquid water content (LWC) does not exceed 1.0gm/m and outside air temperature (OAT) is not below -20° C.
(3) Many rotorcraft operators have voiced a high priority on obtaining rotorcraft approved for operation in icing conditions.
(4) The icing characteristics envelope of FAR Part 25, Appendix C, has served as a satisfactory design criteria for fixed-wing operations for two decades. The envelope, as presented, extends to 22,000 feet with possible extents to 30,000 feet but does not present icing severity as a function of altitude. At the time the envelope was derived, it was assumed that all transport category airplanes would operate to at least 22,000 feet. For present state-of-the-art rotorcraft, this assumption is not valid. As such, an altitude limited icing envelope based on the same data used to derive the Part 25, Appendix C, and the Part 29, Appendix C, envelopes is presented as an alternate to the full icing envelope. In addition, a second icing envelope which effectively characterizes supercooled clouds from ground level to 10,000 feet is presented as a second alternative to the Part 29, Appendix C, envelope. The second altitude limited envelope described in reference 386d(2) was derived from recent additional airborne measurements.
b. Explanation.
(1) General.
The discussion in this paragraph pertains generally to certifications to the full icing envelope of Part 29, Appendix C, within the altitude limitations of the rotorcraft or to one of the altitude limited icing envelopes based on a 10,000-foot pressure altitude limit. The actual icing envelope considered may be further restricted based on the actual pressure attitude envelope for which certification is requested. It envisions certification with full ice protection systems (rotor blades, windshields, engine inlets, stabilizer surfaces, etc.). With the exception of pilot controllable variables such as altitude and airspeed, limited certification (either in terms of icing envelope or protection capability) is not envisaged at this time due to the difficulty in forecasting the severity of icing conditions, relating the effects of the forecasted conditions to the type of aircraft, and relating the effects of reported icing among various types of aircraft, particularly between fixed and rotary-wing aircraft. In addition, with a limited protection capability, viable escape options may not be operationally available if limitations are exceeded.
(ii) The discussion in this paragraph, regarding rotor blade ice protection, is oriented primarily toward electrothermal rotor deicing systems, since these have the most widespread acceptance and projected use within the industry. Also, most of the 578 Par 386
testing and research into rotorcraft ice protection to date has been conducted with this type of system. Research is continuing with other types of systems such as anti-icing fluid systems, and information will be added to address certification of these as necessary. It should also be noted that most of the rotorcraft icing experience accumulated to date has been on rotorcraft with symmetrical airfoil sections. The application of this experience to rotorcraft with asymmetrical airfoils should be carefully evaluated. Limited experience has been gained during development and qualification testing of the Army Blackhawk on asymmetrical airfoil icing characteristics. The most prominent difference appears to be a more rapid degradation of airfoil performance.
Rapidity of performance degradation is also dependent upon severity of the icing condition (primarily a function of liquid water content) and ice shape (primarily a function of OAT and median volumetric droplet diameter (MVD)).
(iii) The effects of ice can vary considerably from rotorcraft to rotorcraft.
Experience gained for a rotor system with an identical blade profile could provide valuable information but should be used cautiously when applied to another rotorcraft.
Assumptions cannot necessarily be made based on icing test results from another rotorcraft. Particular care should be exercised when drawing from fixed-wing icing experience as the widely different and varying conditions seen by the rotor blades make many comparisons with fixed-wing results invalid. Likewise, icing effects on rotor blades vary significantly from those on other parts of the rotorcraft. This is due to changing blade velocity as compared with the constant velocity of the remaining parts.
(2) Reference Material. Prior to commencement of efforts to design and certify a rotorcraft, the references listed in Paragraph 386d should be reviewed. FAA Technical Report ADS-4, Engineering Summary of Airframe Icing Technical Data, December 1963, although somewhat dated, is recommended for basic aircraft icing protection system design information.
(3) Objective. The objective of icing certification is to verify that throughout the approved envelope, the rotorcraft can operate safely in icing conditions expected to be encountered in service (i.e., Appendix C of Part 29 or one of the altitude limited icing envelopes presented herein). This will entail determining that no icing limitations exist or defining what the limitations are, as well as establishing the adequacy of the ice warning means (or system) and the ice protection system. A limiting condition may manifest itself in one of several areas such as handling qualities, performance, autorotation, asymmetric shedding from the rotors, visibility through the windshield, etc.
Prior to flight tests in icing conditions, sufficient analyses should have been conducted to determine the design points for the particular item of the rotorcraft being analyzed (windshield, engine inlet, rotor blades, etc.). After the analyses are reviewed and found adequate, tests should be conducted to confirm that the analyses are valid and that the rotorcraft can operate safely in any supercooled cloud icing condition defined by Part 29, Appendix C, or one of the altitude limited icing envelopes. References 386d(1) and (3) may be useful in determining the design points and extrapolation of test data to the desired design points.
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(4) Planning. For best utilization of both the applicant's and the FAA/AUTHORITY's resources, the applicant should submit a certification plan at the start of the design and development effort. The certification plan should describe all efforts intended to lead to certification and should include the following basic information:
Rotorcraft and systems description.
Ice protection systems description.
Certification checklist.
Description of analyses or tests planned to demonstrate compliance.
Projected schedules of design, analyses, testing, and reporting efforts.
Methods of test - artificial vs. natural.
Methods of control of variables.
Data acquisition instrumentation.
Data reduction procedures.
(5) Environment.
Definitions.
(A) Supercooled Clouds. Clouds containing water droplets (below 32° F) that have remained in the liquid state. Supercooled water droplets will freeze upon impact with another object. Water droplets have been observed in the liquid state at ambient temperatures as low as -60° F. The rate of ice accretion on an aircraft component is dependent upon many factors such as droplet size, cloud liquid water content, ambient temperature, and component size, shape, and velocity.
(B) Ice Crystal Clouds. Glaciated clouds existing usually at very cold temperatures where moisture has frozen to the solid or crystal state.
(C) Mixed Conditions. Partially glaciated clouds at ambient temperatures below 32° F containing a mixture of ice crystals and supercooled water droplets.
(D) Freezing Rain and Freezing Drizzle. Precipitation existing within clouds or below clouds at ambient temperatures below 32° F where rain droplets remain in the supercooled liquid state.
(E) Sleet. Precipitation of transparent or translucent pellets of ice which have a diameter of 5mm or less.
(F) Hail. Solid precipitation in the form of balls or pieces of ice (hail stones) with diameters ranging from 5mm to more than 50mm.
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Appendix C of Part 29 defines the supercooled cloud environment necessary for certification of rotorcraft in icing except that the pressure altitude limitation is that of the rotorcraft or that selected by the applicant, provided the remaining altitude envelope is operationally practical. Due to air traffic system compatibility constraints, approval of a maximum altitude less than 10,000 feet pressure altitude should be discouraged. However, there are operations where a lower maximum altitude has no effect on the air traffic system and would still be operationally useful. Figures 3 and 6 of Appendix C, Part 29, relate the variation of average LWC as a function of cloud horizontal extent. These relationships should be used for design assessment of the most critical combinations of conditions as a function of en route distance. This, in combination with a capability to hold in icing conditions for 30 minutes at the destination, is commensurate with policies previously established for fixed-wing aircraft.
Figures 3 and 6 should be used in conjunction with the altitude limited criteria of Figures 368-1 through 386-4 herein. The new criteria of Figure 386-5 includes "duration" (horizontal extent) as the third dimension. It is emphasized that LWC extremes expressed in Part 29, Appendix C, criteria and the alternate envelopes represent the maximum average values to be anticipated within an exceedance probability of 99.9 percent. Transient, instantaneous peak values of much higher LWC have been observed. These instantaneous peak values appear to be of little significance to the design of protected and unprotected surfaces; however, these high values, if encountered, may induce shedding of ice from some unprotected surfaces.
This is due to radical changes in the rate of release of latent heat and resultant changes in the structural properties and adhesion force of ice.
A recent analysis performed at the FAA Technical Center concludes that the aircraft icing environment below 10,000 feet is not as severe in terms of LWC and OAT as that depicted in Part 29, Appendix C, envelope. This AC presents two different altitude limited envelopes that may be employed by those applicants who elect to certify with a 10,000-foot pressure altitude limit. One of these altitude limited envelopes is based upon the same data that were used to derive the design criteria of the Part 29, Appendix C (Figures 386-1 thru 4), while the other is based upon a recently established characterization of supercool clouds below 10,000 feet (Figure 386-5). The applicant may select either of the approaches to altitude limitation. At the present time, applicants have not consistently selected one or the other. If experience shows a unanimous preference for one or the other, the one not used will be deleted in a future revision. The data used to derive these limited envelopes cannot be used to further define icing conditions between 10,000 feet and 22,000 feet; hence, above 10,000 feet, the Part 29, Appendix C, envelopes should be used. It should be noted that the engine inlets should still meet the icing requirements of § 29.1093. The limited icing envelopes may be used on an equivalent safety basis to show compliance with the intent of § 29.1093 if the altitude limit established for the rotorcraft is not greater than 10,000 teet.
(iv) Significant effects can result from various combinations of parameters. For example, most rapid ice accumulations occur at the high values of Par 386 581
АС 29-2B 7130/97 liquid water content, and the greatest impingement area occurs at the high values of droplet size. Most critical ice shapes are a function of each of these parameters in addition to airspeed, surtace temperature, and surface contour. Care should be taken to explore the entire specified ranges of these parameters during the design, development, and certification efforts.
Mixed conditions (i.e., a combination of ice crystals and supercooled water droplets) and freezing rain or freezing drizzle are not addressed in the Part 29 environmental criteria but can present more severe icing conditions than those defined.
Although the probability of encountering freezing rain is relatively low, mixed conditions commonly occur in supercooled cloud formations. Little data have been gathered on the effects of encountering mixed conditions (see Reference 386d(7)). There are no criteria for certification in mixed conditions or freezing rain at present. In addition to the hazards of operating any aircraft in icing, certain aspects of rotorcraft icing (relatively low altitude operation, asymmetric shedding with resulting vibration, and ice damage or ingestion) warrant a caution notice in the RFM advising that the rotorcraft is not certified for operation in freezing rain or freezing drizzle. Avoidance procedures (e.g., climb or descent) may also be usetul.
(6) Eight Test Prerequisites.
The prototype rotorcraft should be capable of IF and IMC flight.
(її) Sufficient analyses should be developed, submitted, and accepted by FAAJAUTHORITY to show that the rotorcraft is capable of safely operating to the selected design points of both the continuous maximum and intermittent maximum conditions of Part 29, Appendix C, or one of the altitude limited icing envelopes. A detailed failure modes and effects analysis (FMEA) should be performed.
(™) Specific attention should be given to (1) assuring that the selected design condition(s) of atmospheric and rotorcraft flight envelopes have been identified;
(2) qualification and design of ice protection systems and components; and
(3) component installation and ice formation effects upon basic rotorcraft structural properties and handling qualities. These assurances can be established from analyses, bench test, and/or dry air flight tests or simulated icing tests, as appropriate prior to flight tests in natural icing.
The applicant should assess rotor blade stability with ice deposits to assure that dynamic instability will not occur in icing conditions. This assessment may be accomplished by analysis including consideration of failure of the most critical segment of the rotor blade ice protection system. It also may be accomplished by experimental means such as attaching dummy ice shapes to the blades and using a whirl stand or wind tunnel.
c. Procedures.
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(1) Compliance
(1) In general, compliance can be established when there is reasonable assurance that while operating in the specified icing environment (1) the engine(s) will not flameout or experience significant power losses or damage; (2) stress levels are not reached with ice accumulations that can endanger the rotorcraft or cause serious reductions in component life; (3) the handling qualities, performance, visibility, and systems operation are defined and are not deteriorated unacceptably; (4) inlet, vent or drain blockage (such as fuel vent, engine, or transmission cooler) is not excessive; and
(5) autorotation characteristics are acceptable with maximum ice accretion between deice cycles. Assessment of performance loss should include not only the drag and weight of the ice itself but electrical or other load demands of the ice protection system and any performance changes resulting from modified rotor blade contours.
it is emphasized that ice formations (shape, weight, etc.) vary significantly under varying conditions of outside air temperature (OAT), liquid water content (LWC), median volume diameter (MVD), airspeed, attitude, and rotor RPM.
The most critical conditions should be defined by means of analyses or test and verified by test. Pertormance changes under these various conditions should be determined and found acceptable.
(III) Laboratory, icing tunnel, ground spray rig, and airborne icing tanker tests are all very useful in developing an ice protection capability, but none of these, either individually or collectively, can satisfy the full requirements for certification. None can presently duplicate the combinations of liquid water content, droplet size, flow field, and random shedding patterns found in natural icing conditions. Airborne tankers hold considerable promise of being able to fulfill certification requirements (in addition to the advantage of being able to produce an icing environment on demand rather than having to wait for it to occur in nature), but tankers have not been able to generate droplet sizes that cover the complete envelope for certification. Many improvements have been made in some tankers in recent years; however, large droplet sizes have typically been a problem. Also, the size of existing tanker clouds is not of sufficient cross section to immerse the entire rotorcraft. There are also solar radiation and relative humidity effects to be considered and correlated with natural icing when using a tanker. The tanker should be able to immerse the entire rotor system as a minimum and should have a means of controlling and changing the cloud characteristics uniformly and repeatably. Until an artificial method has been successfully demonstrated and accepted, icing certification should include flight tests in natural icing conditions.
(iv) Flight testing in natural icing conditions also has limitations.
Reference 386d(16) contains information that may be useful in planning natural icing flight tests. The key limitation of natural icing flight tests is being able to find the combinations of conditions that comprise critical design points. This is especially true of those points falling near the 99.9 percentile of exceedence probability; e.g., high LWC Par 386 583
at low OAT with large MVD. It is emphasized that some more severe design points, however, may exist within the atmospheric icing envelope rather than near the edges or corners of the envelope. This does not mean that natural icing tests must be conducted at all the selected design conditions. Natural icing tests should be conducted in conditions as close to design points as possible and sufficient correlation shown with the analyses to assure that the rotorcraft can operate safely throughout the design envelope.
(v) Certification flight testing should be extensive enough to provide reasonable assurance that either induced or random ice shedding does not present a problem. The most likely indication of a problem if it exists will be ice impact on the airframe or rotor imbalance resulting in vibration. The following should be considered sufficient for rejection:
(A) Vibrations sufficient to make the instruments difficult to read accurately.
(B) Vibrations sufficient to exceed the structural or fatigue limits of any rotorcraft part such as blade, mast, or transmission components.
(C) Ice impact damage to essential parts, such as the tail rotor, that could create a flight hazard. Cosmetic, nonstructure flaws that do not exceed wear and tear characteristics or maintenance criteria are acceptable. Any ice shedding effects that require immediate maintenance action are unacceptable.
(vi) There should be a means identified or provided for determining the formation of ice on critical parts of the rotorcraft which can be met by a reliable and safe natural warning or an ice detection system. A system utilizing OAT must include an accurate OAT measurement since the onset of icing can occur in a very narrow temperature band requiring sensitive and accurate OAT measurement. OAT accuracy should be relative to the true temperature of the air mass. Total system accuracy should be t0.5° C in the -5.0° to +5.0° C range and 11° C throughout the remaining temperature range. The location of the sensor has been shown to be very critical and, in effect, there can be a position error or other errors induced by ice formations or solar radiation. If the system measures liquid water content, consideration should be given to the fact that the actual LWC fluctuates considerably as the rotorcraft passes through an icing environment. A warning system displaying or utilizing a peak or average LWC value (rather than an instantaneous readout) should include sufficient conservatism to provide a margin of safety. The value of an LWC detecting system lies in its utility as a warning that ice is being encountered. The actual magnitude of LWC in combination with OAT and MVD can be used to indicate the icing severity level. The U.S. Army is currently developing an advanced ice detection system for potential application to rotorcraft.
(2) Instrumentation and Data Collection 584 Par 386
Instrumentation proposed for certification tests, including flight strain surveys, should be reviewed as early as possible in the program to establish that it will provide the necessary data. The need for accurate OAT measurement previously noted for operation in icing also applies to the certificated configuration. Mechanical devices such as the rotating multicylinder and rotating disc have been used for measuring ice accretion rate which is relatable by calibration to average LWC and MVD. More recently, hybrid mechanical/electronic LWC measuring devices have been used. Devices that rely on ice accretion as a signal source are subject to the Ludlam limit (the limits whereby latent heat of fusion is not totally absorbed, thus resulting in incomplete freezing of the moisture and some inaccuracy in the indication). The Ludlam limit is a function of various parameters including OAT, airspeed, LWC, and MVD. The Ludlam limit may vary from one device to another. (See References d(8) laser nephelometers, have been used to measure droplet size. Other calibrated and (9)(i) for further information). Gelatin slides, soot and oil slides, and more recently, droplet siz Reference d(16) describes devices intended for measurement of LWC should be used. Reference d(16) describes several of these devices. Photographic coverage of critical areas may be necessary to ascertain that ice protection systems are functioning properly and that there are no runback problems. (The term "runback" refers to liquid water that has not been evaporated by surface deice equipment and flows back to an unheated area subject to freezing.) Reference 386d(19) highlights use of video techniques and equipment for this purpose. Some systems will require acceptable calibration techniques and data.
(i¡) Gelatin, soot, and oil slides provide data that can be used to estimate MVD at discrete intervals while laser nephelometer data can provide time histories of MVD droplet size distributions. Gelatin slide data should be taken frequently during test flights to properly characterize the cloud. Laser nephelometer data have been found to be highly dependent upon knowledge of the equipment and calibration. Proper calibration, maintenance, and data processing techniques should be utilized and demonstrated. Additional information on the subject may be found in Reference 386d(18).
(iii) Structural instrumentation requirements should also be established as early as possible in the program. Flight strain measurements are strongly recommended in assessing the ice imposed stress on the rotorcraft. The flight strain measurements should determine the effect on fatigue life due to ice accumulation for such items as main rotor blades, main rotor hub components, rotating and fixed controls, horizontal stabilizer, tail rotor, etc. The subsequent proper operation of retractable devices such as landing gear should be demonstrated with representative ice accretion. In addition, the static and fatigue strength of the blade with heater mat must be substantiated. Any effect of the heater mat on fatigue strength of the blades must be considered.
(3) Additional Considerations. The following are items to consider in an icing certification program. They are not intended to be all-inclusive, and the possibility of Par 386 585
widely differing characteristics and critical areas among various rotorcraft in icing should be considered.
The rotorcraft should be shown by analysis and confirmed by either simulated or natural icing tests to be capable of holding for 30 minutes in the design conditions of the continuous maximum icing envelope at the most critical wight, CG, and altitude with a fully functional ice protection system. For those applicants who elect to certify their rotorcraft to the new supercooled cloud characterization of Figure 386-5, the rotorcraft should be shown by analysis and confirmed by either simulated or natural icing tests to be capable of holding for 30 minutes in the design conditions of the icing envelopes up to a maximum of 0.8 grams per cubic meter of LWC at the most critical weight, CG, and altitude.
A single ice protection system and power source may be considered acceptable provided that after any single failure of the ice protection system, the rotorcraft can be shown by analysis and/or test to be capable of safe operation (no hazard) for 15 minutes following failure recognition in the continuous icing envelope used as the basis for certification within the same icing limits used for the 30-minute hold criteria. During this 15-minute period the rotorcraft may exhibit degraded characteristics. Pilot controllable operating limitations such as airspeed may be used to satisfy this continued safe flight criteria. For purposes of determining performance and handling qualities degradation, ice protection system failure need not be considered to occur simultaneously with engine failure unless ice protection system operation is dependent upon engine operation.
(¡i) Although current airborne weather radar technology systems may be useful in avoiding potential icing conditions by detecting precipitation, the use of weather radar is not an FAA/AUTHORITY requirement for icing certification.
(iv) If the ice protection is not operating continuously, there must be a means to advise the crew when the rotorcraft is in icing conditions in order that the system may be activated.
(V) No autorotational performance data is required for rotorcraft which have Category A powerplant installations. All rotorcraft certified for flight in icing conditions must be capable of full autorotational landings with the ice protection system operating. Autorotational entry, steady state, and flare entry flying qualities and performance should be evaluated with an ice load. Since the Category A en route performance can vary as the ice protection system operates, a mean value of cyclic torque is acceptable provided at no time does the rate of climb fall below zero. The rotorcraft is assumed to be clear prior to takeoff, and therefore the takeoff performance is not degraded. The landing performance can be based on the in-flight assessment of overall performance degradation. Items such as fuel burns can be used as part of the in-flight performance degradation determination. Regardless of the methods used to determine performance degradation, it must be easily used by the crew. The hover 586 Par 386