AC 43.13-1A CHG 1 Page 165 1975 Chapter 10. HYDRAULIC AND PNEUMATIC SYSTEMS Section 1. HYDRAULIC SYSTEMS
392. GENERAL. Maintain, service, and adjust connection fittings: AN-818 nut and AN-819 airplane hydraulic systems in accordance with sleeve.
manufacturers' maintenance manuals and pertinent component maintenance manuals. Certain Minimum bend radil * general principles of maintenance and repair for tightening tube nuts Wrench torque range measured at Inside of bend.
which apply are outlined below. Tube O.D. (Inch pounds) Dimenaton in inches. * (inches)
393. HYDRAULIC LINES AND FITTINGS. Carefully Alum. alloy 1100-H14, Alum. alloy 1100-H114, inspect all lines and fittings at regular intervals 5052-0 Steel 5052-0 Steel to insure airworthiness. Investigate any evidence of fluid loss or leaks. Check metal lines for 30-70 leaks, loose anchorages, scratches, kinks, or other ¾а 40-65 60-80 50-90 70-120 21/32 damage. Inspect fittings and connections for 75-125 00-150 1⅜ leakage, looseness, cracks, burrs, or other damage. 150-250 155-250 15¾8 1¾в Replace or repair defective elements. 200-350 300-400 1¼ 1¾ 300-500 430-575 1⅓ 218
a. Replacement of Metal Lines. When inspec- 500-700 550-750 1¾ 2% tion shows a line to be damaged or defective, 1 600-900 3 replace the entire line or if the damaged section 1¼ 600-900 9% is localized, a repair section may be inserted. 14½ 5¼ In replacing lines, always use tubing of the same 1¾ 2 size and material as the original line. Use the old tubing as a template in bending the new line, FIGURE 10.1.-Tube data.
a template can be made from soft iron wire.
Soft aluminum tubing (1100, 3003, or 5052) under ¼-inch outside diameter may be bent by hand. For all other tubing use an acceptable hand or power tube bending tool. Bend tubing carefully to avoid excessive flattening, kinking, or wrinkling. Minimum bend radii values are shown in figure 10.1. A small amount of flattening in bends is acceptable but do not exceed an amount such that the small diameter of the flattened portion is less than 75 percent of the original outside diameter. When installing the replacement tubing, line it up correctly with the mating part so that it is not forced into line by means of the coupling nuts.
b. Tube Connections. Many tubing connections are made using flured tube ends, and standard In forming fares, cut the tube ends square, file smooth, remove all burrs and sharp edges, and thoroughly clean. The tubing is then flared using the correct 37° aviation flare forming tool for the size of tubing and type of fitting. A double flare is used on soft aluminum tubing ⅜-inch outside diameter and under, and a single flare on all other tubing. In making the connections, use hydraulic fluid as a lubricant and then tighten. Overtightening will damage the tube or fitting, which may cause a failure; undertightening may cause leakage which could result in a system failure.
Mistaken use of 45° automotive flare forming tools will result in improper tubing flare shape and angle causing misfit, stress and strain, and probable system failure.
Chop 10 Par 392
c. Repair of Metal Tube Lines. Minor dents and scratches in tubing may be repaired. Scratches or nicks no deeper than 10 percent of the wall thickness in aluminum alloy tubing, that are not in the heel of a bend, may be repaired by burnishing with hand tools. Replace lines with severe die marks, seams, or splits in the tube.
Any crack or deformity in a flare is also unacceptable and cause for rejection. A dent less than 20 percent of the tube diameter is not objectionable unless it is in the heel of a bend.
Dents may be removed by drawing a bullet of proper size through the tube by means of a length of cable. A severely damaged line should be replaced; however, it may be repaired by cutting out the damaged section and inserting a tube section of the same size and material. Flare both ends of the undamaged and replacement tube sections and make the connection by using standard unions, sleeves, and tube nuts. If the damaged portion is short enough, omit the insert tube and repair by using one union and two sets of connection fittings.
d. Replacement of Flexible Lines. When replacement of a flexible line is necessary, use the same type, size, and length of hose as the. line to be replaced. If the replacement of a hose with swaged-end-type fittings is necessary, obtain a new hose assembly of the correct size and composition. Certain synthetic oils require a specially compounded synthetic rubber hose which is compatible. Refer to the aircraft manufacturer's service information for correct part number for replacement hose. If the fittings on each and are of the collet type or sleeve type in replacement may be fabricated as shown figure 10.2. Typical aircraft hose specifications and their uses are shown in figure 10.3. Install hose assemblies without twisting. (See figure
10.4.) Never stretch a hose tight between two fittings as this will result in overstressing and eventual failure. The length of hose should be sufficient to provide about 5 percent to 8 percent slack. Avoid tight bends in flex lines as they may result in failure. Never exceed the minimum bend radii as indicated in figure 10.5.
Teflon hose is used in many aircraft systems because of its superior qualities for certain applications. Teflon is compounded from tetrafluoroethylene resin which is unaffected by fluids Par 393 AC 43.13-1A CHG 1 1975 normally used in aircraft. It has an operating range of -65° F. to 450° F. For these reasons, Teflon is used in hydraulic and engine lubricating systems where temperatures and pressures preclude the use of rubber hose. Although Teflon hose has excellent performance qualities, it also has peculiar characteristics that require extra care in handling. It tends to assume a permanent set when exposed to high pressure or temperature. Do not attempt to straighten a hose that has been in service. Any excessive bending or twisting will cause kinking or weakening of the tubing wall. Replace any hose that shows signs of leakage, abrasion, or kinking.
Any hose suspected of kinking may be checked with a steel ball of proper size. Figure 10,6 shows hose and ball sizes. The ball will not pass through if the hose is distorted beyond limits.
If the hose fittings are of the reusable type, a replacement hose may be fabricated as described in figure 10.2. When a hose assembly is removed the ends should be tied as shown in figure 10.7, so that its preformed shape will be maintained. Refer to figure 10.9 for minimum bend radii of Teflon hose.
All flexible hose installations should be supported at least every 24 inches. Closer supports are preferred. They should be carefully routed and securely clamped to avoid abrasion, kinking, or excessive flexing. Excessive flexing may cause weakening of the hose or loosening at the fittings.
e. O-Ring Seals. A thorough understanding of O-ring seal applications is necessary to determine when replacement must be made. The simplest application is where the O-ring merely serves as a gasket when it is compressed within a recessed area by applying pressure with a packing nut or screw cap.
Leakage is not normally acceptable in this type of installation. In other installations the O- ring seals depend primarily upon their resiliency to accomplish their sealing uction. When moving parts are involved, minor seepage may be normal and acceptable. A moist surface found on moving parts of hydraulic units is an indication the seal is being properly lubricated.
In pneumatic systems, seal lubrication is provided by the installation of a grease-impreg- Chap 10
AC 43.13-1А Page 195 1972
PRESSURE -
SENSITIVE TAPE XYZ15
SLEEVE MARKER
TIED IN PLACE XYZ 15 FIGURE 11.19.-Identification of wire bundles and harnesses.
467.-477. RESERVED.
Chap 11 Par 464
AC 43.13-1A Chg 3 Page 196 1988 Section 5. CONNECTORS
478. GENERAL PURPOSE CONNECTORS. Connectors (plugs and receptacles) are used to facilitate maintenance when frequent disconnection is required. Since the wires are soldered to the connec tor inserts, the joints should be individually installed and the wire bundle firmly supported to avoid damage by vibration. Connectors have been particularly vulnerable to corrosion in the past, due to condensation within the shell. Special connectors with waterproof features have been developed Class C-A pressurized connector with inserts that are not removable. Looks like a Class A connector but the inside sealing arrangement is sometimes different and is used on walls of bulkheads of pressurized equipment.
Class D-Moisture and vibration resisting connector which has a sealing grommet in the back shell. Wires are threaded through tight fitting holes in the grommet, thereby sealing against moisture.
and may be used to replace nonwaterproof type Class K-A fireproof connector used in areas plugs in areas where moisture causes a connector where it is vital that the electric current is not inproblem. Use a replacement connector of the same terrupted, even though connector may be exposed basic type and design as the connector it replaces. to continuous open flame. Wires are crimped to the Connectors that are susceptible to corrosion dilhi- pin or socket contacts and the shells are made of culties may be treated with a chemically inert wa- steel. This class of connectors is normally longer terproof jelly. When replacing connector assem- than the other classes of connectors.
blies, use the socket-type insert on that half which is live or "hot" after the connector is disconnected * NOTE: Inspect connectors for loose soldered connec to prevent unintentional grounding. tions, proper insulation of metallic parts, and fraying of wires in the plug and receptacle inserts.
479. TYPES OF CONNECTORS. Connectors are identified by AN numbers and are divided into types and classes with manufacturer's variations in each type and class. The manufacturer's variations are differences in appearance and in the method of meeting the specification; however, they do not preclude mating plugs and receptacles of different manufacturers. There are six basic types of AN connectors used in aircraft which are further broken down into five classes as indicated in figures 11.20 and 11.21. Each class of connector has slightly different construction characteristics which are noted as follows: Classes A, B, C, and D are made of aluminum; Class K is made of steel.
Class A-Solid one-piece back shell general purpose connector.
Class B-Connector back shell separates into two parts lengthwise. Use primarily where it is important that the soldered connectors are readily accessible. The back shell is held together by a threaded ring or by screws.
480. ARMY/NAVY (AN) CONNECTOR IDENTIFI- CATION. Code letters and numbers are marked on the coupling ring or shell to identify the connector.
This code provides all the information necessary to obtain the correct replacement for a defective or damaged part. To facilitate ready identification the code found on a typical connector (as shown in figure 11.22) is explained as follows:
a. The letters AN indicate the connector was made to a government standard.
b. Type 3106 indicates this is a straight plug.
c. Letter A indicates this is a general purpose connector with a solid back shell.
d. Number 18 indicates the size of the coupling. This size is determined in 1/16-inch increments. Therefore, the size of the plug in the example is 1 1/8 inch.
Chap 11 Par 478
AC 43.13-1A Chg 3 Page 153 1988 Section 3. MISCELLANEOUS EQUIPMENT
360. PARACHUTES. With reasonable care, parachutes normally last at least 5 years. They should not be carelessly tossed about, left in airplanes so that they may become wet, or left in open places where they may be tampered with. They should not be placed where they may fall on oily floors or be subject to acid fumes from adjacent battery chargers. When repacking is scheduled, a careful inspection of the parachute should be made by a qualified parachute technician (rigger). If repairs or replacements of parts are necessary to maintain the airworthiness of the parachute assembly, such work must be done by the original parachute manufacturer or by a qualified parachute rigger, certificated in accordance with FAR Part 65, or by an appropriately rated parachute loft certificated in accordance with FAR Part 149.
Fraying of belts is an indication of wear, and such belt are likely to be unairworthy because they can no longer hold the minimum required tensile load.* Difference of opinion as to the airworthiness of a belt can be settled by testing a questionable belt to demonstrate that it will support the required load.
Airworthy one-person type-certificated belts should be able to withstand a tensile load of 525 pounds, and TSO belts withstand the rated tensile load indicated on the belt label. Most one-person TSO belts are rated for 1,500 pounds. For twoperson belts, double the loads. Since type-certificated belts will not afford the crash protection provided by a TSO or military belt, such type-certificated belts are not to be repaired nor should their buckles or end fittings be reused on safety belts. If replacement of webbing or hardware of TSO or military belt is attempted, use parts of identical design *361. SAFETY BELTS. The FARS require that and material. Make the stitch pattern identical to when safety belts are to be replaced in aircraft the original and the number of stitches per inch manufactured after July 1, 1951, such belts must equal to the number used in the original belt. There conform to standards established by the FAA.* is no objection to having a greater total length of These standards are contained in Technical Stand- stitching, provided one line of stitches is not placed ard Order TSO-C22. Safety belts eligible for instal- over another line. Space lines of stitching at least lation in aircraft may be identified by the marking 3/16-inch apart. Keep a record, preferably in the TSO-C22 on the belt or by a military designation logbook, stating the extent to which the belt was number since military belts comply with the repaired and the date. Retain the original identifica- * strength requirements of the TSO. Each safety belt tion marking on the belt, conforming either to that must be equipped with an approved metal to metal required by TSO-C22 to a deviation from this latching device. Airworthy type-certificated safety* marking, or to the military designation. Operators belts currently in aircraft may be removed for of a fleet of airplanes should follow the above sugcleaning and reinstalled. However, when a type- gestions, but keeping a record of renovations in a certificated safety belt is found unairworthy, re- logbook is impractical, since the belts are never placem it with a TSO-C22 or a new belt is pre- associated with any one particular aircraft for ferre:: any length of time. Therefore, in addition to * a. The webbing of safety belts, even when retaining the original identification label and mildew-proofed, is subject to deterioration due to attaching it to the renovated belt, use constant use, cleaning, and the effects of aging. some additional simple marking to indicate Chap a Par 360
that the belt has been renovated and show the date of renovation. The use of letter "R" followed by the date would be acceptable. This marking could be in the form of an indelible ink stamping or cloth label stitched to the web-
362. FLARES. Parachute flares are made of materials which are subject to decomposition upon aging. Humidity affects the small igniting charge and also the materials of the candle (illuminant). Hence, the percentage of misfires in old flares is likely to be quite high. To assure unfailing performance of flares, periodically inspect the flare installation. Inspect the entire system, starting at the release mechanism in the cockpit and ending at the flare. Only a qualified person should attempt such inspection, since inadvertent discharge of such pyrotechnics may cause serious damage. Past experience has indicated that it is advisable to return all electrically or pistol-operated flares to the manufacturer for reconditioning within a maximum period of 3 years, and that for mechanically operated flares, a maximum period of 4 years is recommended.
363. OXYGEN SYSTEMS.
a. General. The following instructions are to serve as a guide for the inspection and maintenance of aircraft oxygen systems. The information is applicable to both portable and permanently installed equipment.
(1) Aircraft Gaseous Oxygen Systems. The oxygen in gaseous systems is supplied from one or more high- or low-pressure oxygen cylinders. Since the oxygen is compressed within the cylinder, the amount of pressure indicated on the system gauge bears a direct relationship to the amount of oxygen contained in the cylinder. The pressure-indicating line connection is normally located between the cylinder and a pressure-reducing valve.
NoTE: Some of the gaseous oxygen systems do not use pressure-reducing valves. The high pressure is reduced to a useable pressure by a regulator. This regulator is located between the high- and low-pressure system.
(2) Aircraft Liquid Oxygen Systems. Thus far it has not been a practice to use liquid oxygen in Par 361 AC 43.13-1A 1972 civil aircraft due to its complexity. This however, may change at any time as technological advances are made.
(3) Portable Oxygen Systems. The three basic types of portable oxygen systems are: demand, pressure demand, and continuous flow. The component parts of these systems are identical to those of a permanent installation with the exception that some parts are miniaturized as necessary. This is done in order that they may be contained in a case or strapped around a person's shoulder. It is for this portability reason that it is essential special attention be given to assuring that any storage or security provision for portable oxygen equipment in the aircraft is adequate, in good condition, and accessible to the user.
NOTE: Check portable equipment including its security provisions frequently, as it is more susceptible to personnel abuse than a permanently installed system.
b. Inspection. Hands, clothing, and tools must be free of oil, grease, and dirt when working with oxygen equipment. Traces of these organic materials near compressed oxygen may result in spontaneous combustion, explosions, and/or fire.
(1) Oxygen Tanks and Cylinders. Inspect the entire exterior surface of the cylinder for indication of abuse, dents, bulges, and strap chafing.
(a) Examine the neck of cylinder for cracks, distortion, or damaged threads.
(b) Check the cylinder to determine if the markings are legible.
(c) Check date of last hydrostatic test. If the periodic retest date is past, do not return the cylinder to service until the test has been NOTE: This test period is established by the Department of Transportation in the Code of Federal Regulations, Title 49, Chapter I, Paragraph 173.34.
(d) Inspect the cylinder mounting bracket, bracket hold-down bolts and cylinder holding straps for cracks and deformation, cleanliness, and security of attachment.
(e) In the immediate area where the cylinder is stored or secured, check for evi- Chap 8
SECTION 9. CONTROL SURFACE AND SYSTEM LOADS
152. 1 29.391 (through Amendment 29-19) GENERAL.
a. Explanation. This general rule concerns requirements for design loads of tail rotors, control or stabilizing surfaces, and their control system.
b. Procedures. The design criteria and/or the design loads report must contain the loads dictated by the referenced rules. See paragraphs 153, 154, 155, 156, 157, 158, and 159 of this document.
153. § 29.395 (through Amendment 29-19) CONTROL SYSTEM.
a. Explanation. Control system design loads and the application of these loads are contained in this rule.
(1) Paragraph (a) of the rule specifies the way or means of reacting the design loads specified in §$ 29.397 and 29.399 (for dual control systems).
The design loads must be imposed on any locks and stops and irreversible mechanisms in the control system. Both rotor blade horns and control surface horns must react, without failure, the specified loads while the controls are in critical positions.
(2) Paragraph (b) of the rule specifies application of limit pilot forces or of the maximum loads that can be obtained in normal operation, including any single power boost system failure, whichever is greater. However, minimum limit pilot force 0.60 of the loads specified in §§ 29.397 and 29.399, may be used, as specified, in parts of the primary control system that are not stiff enough to react to the loads specified in the first part of paragraph (b) of the rule. Note the objective for a rugged control system.
(3) Control system design feature and test requirements are found in §§ 29.671 through 29.695. Bearing faotors and fitting factors are specified in §$ 29.623 and 29.625, respectively.
b. Procedures.
(1) The design criteria and/or a design loads report that includes the primary control system design loads should be submitted for FAA approval.
(2) The rotorcraft control system may be tested to ultimate design loads or may be analyzed for the ultimate design loads. See paragraph 124 of this document.
(1) It is advisable that the applicant prepare a proposal control system which reflects compliance with the condition specified. It is describing the procedures and techniques to be used in the static testing of the Pa conour that the teste further advisable that the FAA conour that the tests proposed achieve that objective. Omission of these steps may result in the need for retesting. The test results should be documented.
Chap 2 Par 152 249
AC 29-2, CHG 1 11/19/84 * (11) If tests are not conducted, a structural analysis of the control system is required. Appropriate factors from §§ 29.685 (e), 29.623, and
39.025 must be used as specified. A structural analysis report should be used to jocument compliance with § 29.685(d)(1) and (4), and § 29.685(f).
(3) If a part of the control system is not stiff or rigid enough to react the design loads specified in § 29.397, that part of the system may be substantiated for lower loads as prescribed.
(1) The limit design loads are those loads specified in § 29.397;
(11) The limit design loads are the maximum that can be obtained in normal operation, including any single power boost system failure, except for objectives stated for a rugged system; and
(111) In lieu of a rational analysis, the limit design loads may be
0.60 of the loads specified in $ 29.397.
is a part of the helicopter design, the control system for this part must be stiff enough to react the control surface loads without failure and to provide enough surface deflection to control the helicopter. These limit loads may be 60 pounds fore and aft and 40 pounds laterally on the cyclic control stick in lieu of a rational analysis and may be the maximum loads that can be obtained in normal operation.
If a hydraulic power actuation or boost system is part of the helicopter design, the design limit load for the affected parts of the control system will be the maximum output force of the boost at normal operating pressure added to the limit design loads resulting from the loads specified in § 29.397.
If a single failure in the power portion of the hydraulic system results in actuator forces that exceed the maximum output force at normal operating pressure, the highest output loads must be used as noted in subparagraph
(3)(11). This hydraulic system failure standard is specified in § 29.695(a)(1) as well.
(4) Controls proof and operation test is required by $$ 29.307(b),
29.681, and 29.683. This test is conducted using the design 1imit loads approved, under § 29.395 (b). See paragraphs 282 and 283 of this document.
154. § 29.397 (through Amendment 29-19) LIMIT PILOT FORCES AND TORQUES.
a. Explanation. Design forces are contained in the rule.
(1) Primary controls, pilot and copilot, must be designed for the limit pilot forces specified in paragraph (a) of the rule.
(2) For other operating controls, such as flap, tab, stabilizer, rotor brake, and landing gear, design limit forces are specified in paragraph (b).
Chap 2 250 Par 153
D. Procedures.
(1) Design loads specified in the rule must be used in required structural tests and in any structural strength analysis of the control systems submitted in compliance with other rules.
(2) Operation tests of the control systems noted in other rules require application of these forces also.
155. § 29.399 (through Amendment 29-19) DUAL CONTROL SYSTEM.
Explanation. Design limit loads are specified for dual control systems. Pilot effort forces applied in opposition and in the same direction are required for dual control systems.
b. Procedures. 1
(1) Design loads specified in the rule must be used in required structural tests and in any structural strength analysis submitted for compliance with the other rules.
(2) Operation tests of the control systems, noted in other rules require application of these forces also.
156. § 29.401 (through Amendment 29-19) AUXILIARY ROTOR ASSEMBLIES.
a • Explanation.
(1) For rotorcraft equipped with auxiliary rotors, normally called tail rotors, an endurance test is required by § 29.923 and struotural strength substantiation is required. Section 29.401(b) specifically refers to structural strength substantiation for centrifugal loads resulting from maximum design rotor rpm. Due to the pitch feathering requirements, auxiliary rotors typically have detachable blades.
(2) The rotor blade structure must have sufficient strength to withstand not only aerodynamic loads generated on the blade surface, but also inertial loads arising from centrifugal, coriolis, gyroscopic, and vibratory effects produced by this blade movement. Sufficient stiffness and rigidity must be designed into the blades to prevent excessive deformation and to assure that the blades will maintain the desired aerodynamic characteristics. As a design objective the structural strength requirements should be met with the minimum material. Excess blade weight imposes extra centrifugal loads that may increase the operating stress levels. Blade weight and strength should be optimized.
Even though a structural strength analysis for the blade design loads is required, a flight load survey and fatigue analysis are also required by § 29.571.
(3) Section 29.1509 defines the design rotor speed as that providing a 5 percent margin beyond the rotor operating speed limits.
Chap 2 Par 154 251
AC 29-2, CHG 1 11/19/84
b. Procedure. (1) The endurance, and control parol (1) The endurance tests prescribed by $$ 29.923 and 29.927 require achieving certain speeds, power, and control displacement for the auxiliary (tail) rotor as well as the main rotor. The parts must be serviceable at the conclusion of the tests.
(2) Structural substantiation of the auxiliary (tail) rotor is required to assure integrity for the minimum and maximum design rotor speeds and the maximum design rotor thrust in the positive and negative direction. Thrust capability of the rotor should offset the main rotor torque at maximum power as required by § 29.927 (b).
(1) The maximum and minimum operating rotor speed, power-off, is 95 percent of the maximum design speed and is 105 percent of the minimum design speed, respectively.
stant test auth we on red pros (11) The rotor operating speed limits shown during the official FAA flight tests must include the noted 5 percent margin with respect to the design
(111) The auxiliary rotor generally has a positive and negative pitch limit that assures adequate directional control throughout the operating range of the rotorcraft. The power-off rotor speed limits are generally broader than the power-on rotor speed limits because of the required autorotational rotor speed characteristics. Thus, the auxiliary rotor design conditions concern the maximum and minimum design rotor speeds in conjunction with the maximum positive or negative pitch thrust as appropriate. Thrust capability and precone angle of the rotor, if any, will significantly influence the rotor design loads. The variations in rotor design features and an example of substantiation would be too lengthy to include here. However, ANC-9, "Aircraft Propeller Handbook," contains principles that may be applied to tail rotor designs. Tail rotors may be considered a special propeller design.
(iv) Bearings are generally used in the tail rotor installation to allow flapping and feathering motion of the blades. The bearing manufacturer's ratings of these bearings must not be exceeded. Bearings generally used in main and tail rotors are classified as ABEC Class 3, 5, or 7. Class 7 is the highest quality presently available. Satisfactory completion of the endurance tests of §§ 29.923 and 29.927 18 a means of proving that use of a particular bearing 18 satisfactory.
(v) The analysis must include appropriate special factors, casting factors, bearing factors, and fitting factors prescribed by 99 29.619, 29.621,
29.623, and 29.625, respectively. The fitting factor of 1.15 must be applied in the analysis of the tail rotor installation.
Chap 2 252 (thru 254) Par 156
b. Procedures.
(1) The reliability of the means for preventing ground resonance may be substantiated as stated in the standard. An analysis report or a test proposal and subsequent test report may be used to show compliance. The probable range of variations, in service, of the damping action are an important part of the assessment.
§ 29.241. See paragraph 99.
(1) Analysis and tests may be used.
(11) Reliable service history of identical or closely similar systems may be used. The materials and fluids used, clearance or fits, seals, and physical installation are important items to be evaluated and considered for 'closely similar" systems.
(111) Testing of the complete rotorcraft may be used to prove that malfunction of a single means of the damping system will not cause ground resonance.
One method of demonstrating acceptable compliance is by removing all seals, if practicable, from one damper. Another method is to remove all or most of the fluid, in conjunction with considering the allowable ranges of damping of the other parts of the rotorcraft damping system and operating the rotorcraft throughout the rotor speed range from start to maximum rotor speed. Investigation of elastomeric dampers may require innovative test procedures and preliminary discussions of these prior to preparation of a test proposal. The rotorcraft cyclic control should be displaced as noted in paragraph 99 of this document to assure that the possible rotorcraft resonance frequencies are excited. If vibrations are damped in all tests, the damping system is satisfactory. Each critical rotor damper and landing gear damper, which includes shock struts and tires, should simulate a malfunction to comply with the standard. The testing discussed, however, could become very extensive if one were to attempt to test all combinations of all maintenance adjustments of all components which contribute to the prevention of ground resonance, while at the same time rendering each of the pertinent components ineffective in turn and then repeating all of the maintenance tolerance testing each time. Fortunately, rational analytical methods are available which will permit the evaluation of such combinations so that only the combinations with the least amount of margin used are physically tested.
(2) The pylon damper variation can affect ground resonance. The variations in stiffness and/or damping of pylon mounts should be evaluated except the pylon mounts on contemporary conventional helicopters may have little influence on "classical" ground resonance stability. The dynamics of the rotorcraft on its landing gear is generally established by the airframe properties and the landing gear properties under the influence of the rotor system, with the "pylon" having little effect. For air or flight resonance, the rotor generally couples with the rigid body modes of the fuselage. For a specific design, a relatively simple analysis may be used to show the effect of the pylon mount system stiffness on air and ground resonance stability, and if not important, variations in the system may be omitted from the test program. * Chap 2 415 Par 268
AC 29-2A, CHG 2 9/24/91
(3) The probable ranges of damping shall be established and investigated as prescribed and noted in paragraph (b) of § 29.633. An approved test proposal and test results report should be used for complying with § 29.663(b). For example, if a conventional wheel landing gear is used on the rotorcraft, the probable ranges of tire pressure or the lowest probable tire pressure should be stated in the test proposal and effects of the tire pressure investigated during the test.
addition, the effects of strut pressures should be investigated also. See paragraph 99, § 29.241, concerning tests and instrumentation of the test associated with complying with § 29.241. The instrumentation noted in paragraph 99 also applies to § 29.663(b).
(4) If the wheel landing gear is equipped with wheel brakes, the evaluation should include brakes "on" and "off." The nose or tail wheel should be locked and unlocked if it swivels to evaluate any possible adverse effects of this feature.
269.-275. RESERVED.
416 (thru 432) Chap 2 Par 268