c. The FAA published engine torque requirements incorrectly in Amendment 23-26. Part 23, § 23.361 was corrected with Amendment 23-45. The incorrectly written rule failed to require the multiplying factor for the torque load. The applied (incorrectly written) rule can result in lower structural loads than previously required from torque loads. These loads aff ect the engine mount, and either the fuselage or nacelle and wing designs.
d. Policy: Apply the mean torque factors in the manner that existed in part 23 before Amendment 23-26 and corrected in Amendment 23-45. Determine airplane design loads for two engine limit torques and for two flight-load-conditions. These airworthiness standards for engine torque loads constitute the minimum level of safety required by the FAA for the engine mount, and either the fuselage or the nacelle and wing designs. For airplane designs that have a part 23 certification basis that encompasses Amendments 23-26 through 23-44, apply the intent of the regulation depicted by the amendments before or after these amendments. Figure 5 presents a view of torque, aerodynamic, and inertial loads airworthiness standards.
FIGURE 5
VIEW OF TORQUE, AERODYNAMIC, AND INERTIAL LOADS 30
23.363 Side load on engine mount (Original)
61. Is there any policy available on this section as of January 1, 2007? No.
23.365 Pressurized Cabin Loads (Original)
62. How do I determine the cabin pressure loads and what do I combine them with? Multiply the maximum differential pressure loads by the 1.5 factor of safety (see part 23, § 23.303). The maximum differential pr essure loads should include the high side relief valve tolerance pressure. Combine them with the ultimate loads of both the normal flight inertia and the local external aerodynamic pressure distribution conditions.
63. Is the aerodynamic pressure a constant? The airloads will vary over the length of the fuselage.
64. Is there any policy available on paragraph (c) of this section as of January 1, 2007? No.
65. What pressure should be used as limit load (omitting other loads)? For the limit (pressure only) load condition, airplane fuselage structure should be designed to withstand:
P =(1.33)Plim max .relief .valve
66. What pressure should be used as ultimate load (omitting other loads)?
a. For the ultimate (pressure only) load condition, combine the 1.5 factor of safety with the 1.33 burst pressure factor to get an ultimate load case for the pressure vessel structural design. Part 23 special conditions have imposed a 1.67 burst pressure factor for airplanes with design altitudes that exceed 45,000 feet. This practice is consistent with part 25. Aerodynamic and landing impact loads may be ignored for this load case.
P = 15 133 P..ult ( ) max.relief .valve For altitudes that exceed 45,000 feet, P =(1.67)Plim max .relief .valve ..( )PP = 15 167ult max.relief valve .
b. Note that the regulation for pressurization tests requires a strength test on a fuselage designed for pressure to the ultimate load condition given by the above equation (see § 23.843).
67. What is meant by “external door”? External door means an opening, a doorway, in the external surface of the airplane. Evaluate door failure effects regardless of whether the door is inward or outward opening. For this rule, an emergency exit is a door. Also see 23.305, Strength and deformation, paragraph 22, for guidance about conducting static tests for pressure vessels.
23.367 Unsymmetrical loads due to engine failure (Amendment 23-7)
68. What is meant by “critical engine”? Traditionally, the term “critical engine” is used to describe the engine whose failure to operate results in the largest yawing moment, compared to that caused by failure of the other engine. However, in a more general sense, and for this regulation, the term “critical engine” means the engine that, when failed, results in the highest structural loads on the airplane.
69. Is there any policy available on paragraph (b) of this section as of January 1, 2007? No.
23.369 Rear lift truss (Amendment 23-48)
70. What is the purpose of this regulation? Civil Air Regulation (CAR) 3.194 is considered a special supplementary reversed airflow condition for design of the rear lift truss, which has no direct relationship to any of the points on the V-n envelope. It has been historically considered as a downwind taxi for a "tail-wheel" type airplane. The lift truss is the brace (frequently a “V,” sometimes “parallel” struts—one to each existing spar) running from the bottom of the fuselage to the lower spar cap(s) of the wing. These lift truss struts usually attach to the wing at midspan. “Wing struts” are usually loaded in tension (for positive load factor conditions)—except during negative “g” maneuvers or gusts, inverted flight conditions (aerobatic maneuvers), landing, and taxi. In these latter cases, the struts can be loaded in compression; therefore, they are subject to Euler column buckling phenomena. Even when on the ground, the airplane rear lift truss (or strut) can experience significant compression loads if the airplane has a tail wheel. This is especially true when the airplane is tied down or is taxiing downwind.
71. Is there any policy available on paragraph (b) of this section as of January 1, 2007? No.
23.371 Gyroscopic and aerodynamic loads (Amendment 23-48)
72. Is there any policy available on this section as of January 1, 2007? No.
23.373 Speed control devices (Amendment 23-7)
73. Is there any policy available on paragraph (a) of this section as of January 1, 2007? No.
74. Is there any policy available on paragraph (b) of this section as of January 1, 2007? No.
CONTROL SURFACE AND SYSTEM LOADS
23.391 Control surface loads (Amendment 23-48)
75. What is the background on this regulation? The control surface load criteria that previously appeared in Appendix B of part 23 were removed from part 23 at Amendment
23-42. The Appendix B criteria were shown to give inappropriate surface loadings for certain airplane configurations, including:
a. High-performance part 23 aircraft;
b. Aircraft that have spar configurations located aft of the 25 percent chord length; and
c. Aircraft that have horizontal stabilizer leading edges that are not attached at the fuselage.
23.393 Loads parallel to hinge line (Amendment 23-48)
76. Is there any policy available on this section as of January 1, 2007? No.
23.395 Control system loads (Amendment 23-7)
77. Can you explain this regulation schematically? Yes. Figure 6 presents an overall relational view of control system loads airworthiness standards. Also see “Control Systems” requirements that are contained in part 23, §§ 23.671 through 23.701.
FIGURE 6
CONTROL SYSTEM LOADS
CONTROL SYSTEM LOADS
CALCULATE
HM.
MAX P.E.
MIN P.E.
MAX P.
REFERENCES H.M.
< MAX P.E.
NO§ 23.395(a)(1) § 23.397(b) YES H.M. = 125% HINGE MOMENT A.P. = AUTO PILOT FORCE P.E. = PILOT EFFORT G.G. = GROUND GUST A.P.
< MAX P.E.
YES
NO DESIGN TO MAX A.P.
DESIGN TO
MAX P.E.
§ 23.395(a)(2) YES § 23.397(b) H.M.
< MIN P.E.
NO DESIGN TO H.M.
ARE MIN P.E.
INAPP YES
NO DESIGN TO MIN P.E.
§ 23.415(a)(2)
DETERMINE G.G. LOADS G.G.
< .6 MIN P.E.
DESIGN TO G.G. LOADS
DESIGN TO .6 MIN P.E.
§ 23.397(b) NO YES 35
78. Is there anything else I can review to get more information? Yes. See 23.423, Maneuvering loads, for more guidance about control system loads.
79. Is there any guidance for unconventional multipath control systems? The control system loads, and limit control forces, and torque airworthiness standards require that pilot loads be opposed at the attachment of the control system to the control surface horn. For an unconventional multipath control system, which involves control of separate surfaces (each with its own horn), it would be appropriate to expect the pilot forces to be restrained at the control surface horns. If this allows a portion of the system to be designed for less than the minimum pilot-effort forces, special attention should be given to the design of this portion of the system to ensure the rugged system.
80. Does this regulation apply to wing flap control systems? No. The § 23.395 requirements are not applicable to wing flap systems.
23.397 Limit control forces and torques (Amendment 23-45)
81. Is there any policy available on paragraph (a) of this section as of January 1, 2007? No.
82. For paragraph (b) of this section, where should the loads be applied?
a. Apply the 100-pound load on a single fore-and-aft control wheel anywhere along the periphery or at the tip of the grip-handle on U-type wheels, and in both foreand-aft directions.
b. Part 23, § 23.397(b) presents both a symmetric and asymmetric 100-pound wheel load for elevator control. You should also see the Figure 6, which presents an overall relational view of control system loads airworthiness standards.
23.399 Dual control system (Amendment 23-48)
83. Is there any policy available on this section as of January 1, 2007? No.
23.405 Secondary control system (Original)
84. Is there any policy available on this section as of January 1, 2007? No.
23.407 Trim tab effects (Original)
85. What is the maximum out-of-trim condition one should consider when determining control surface loads?
a. This rule only applies when maximum pilot-effort forces are imposed on the control system (refer to part 23, § 23.397). Th en, the trim tab deflection is limited to the maximum out-of-trim condition that can exist under prolonged pilot-effort forces (refer to § 23.143).
b. This interpretation only applies to a trim tab that is attached to a movable control surface that is further attached to a fixed main-aerodynamic surface (i.e., it excludes a trim tab that is attached to a stabilator).
c. See 23.423, Maneuvering loads, for more guidance about trim tabs.
23.409 Tabs (Original)
86. Is there any policy available on this section as of January 1, 2007? No.
23.415 Ground gust conditions (Amendment 23-48)
87. How do the ground gust loads in paragraph (a) fit in with the other control system loads in the design requirements? Refer to the accompanying graphic to
23.395, Control System Loads (Figure 6), which presents an overall view of control system loads airworthiness standards.
88. Is there any policy available on paragraph (b) or (c) of this section as of January 1, 2007? No.
HORIZONTAL STABILIZING AND BALANCING SURFACES
23.421 Balancing loads (Amendment 23-42)
89. Is there any guidance for stabilator design?
a. Yes. You may find stabil ator chordwise air-load distributions in the Basic Glider Criteria Handbook for both the balancing conditions and the maneuvering and gust conditions. Also, stabilator s have historically developed flutter problems; therefore, carefully consider the flutter effects of a stabilator design.
b. The Basic Glider Criteria Handbook, 1962 Revision, Federal Aviation Agency, Flight Standards Service, Washington, D.C., is now available by mail from:
Manager, Standards Office (ACE-110), Federal Aviation Administration, Small Airplane Directorate, DOT Building, 901 Locust, Room 301, Kansas City, MO 64106.
c. Also, see 23.321, General (flight loads), for more guidance about balancing tail loads.
90. Is there any policy available on paragraph (b) of this section as of January 1, 2007? No.
23.423 Maneuvering loads (Amendment 23-42)
91. The condition described in paragraph (a) of the regulations does not appear to be realistic. What is the reason for this?
a. Part 23, § 23.423(a) addresses an artificial load condition intended to provide adequate tail strength for abrupt, unchecked pitch maneuvers up to the design maneuver speed, VA . The condition is artificial because the airplane will pitch, and the load factor will no longer be 1g, before the elevator reaches full deflection.
b. A 6g maneuver involves significant airplane pitch velocity, which relieves pilot input and results in a lower stick force per unit elevator deflection. One may correlate tail angle of attack and airplane pitch velocity with measured and calculated hinge moments to gain confidence in the load calculations.
c. A flight in which the pilot pulls full-back-stick at design maneuver speed, VA, and 1g, initially, would probably result in higher elevator angles than would be calculated at 1g because of airplane response characteristics. However, the tail loads resulting from an analysis accounting for airplane response characteristics would not likely exceed those calculated from an analysis that assumes 1g conditions.
d. The analytical approach is acceptable with the following suggestions:
(1) For elevator deflecti on, divide calculated hinge moment by 1.25 (reference § 23.395(b)).
(2) For an elevator tab configuration, deflect the tab to assist the pilot, per § 23.407, Trim tab effects.
e. See the guidance in 23.421, Balancing loads, for stabilator loads distributions.
23.425 Gust loads (Amendment 23-42)
92. What gust loads should be used for stabilator-type aerodynamic surfaces? For stabilator-type aerodynamic surfaces, use the Basic Glider Criteria Handbook as the information source for air-load distributions.
93. Is there any policy available on paragraph (b), (c) or (d) of this section as of January 1, 2007? No.
23.427 Unsymmetrical loads (Amendment 23-42)
94. What unsymmetrical loads should I use for trim systems? For guidance related to non-aerodynamic trim system designs, see 23.677(b), “Trim systems.”
95. What is meant by “conventional” in paragraph (b) of this section?
Conventional means the airplane’s center of gravity (c.g.) is located within the boundaries of the wing mean aerodynamic chord with the empennage aft and the engine(s) forward.
96. Do the conventional requirements for unsymmetrical loads apply to V-Tail configuration designs? No. A V-tail configuration is considered unconventional;
therefore, the conventional requirements for unsymmetrical loads do not apply.
97. For a T-tail design, is a maneuver at maximum elevator deflection required in combination with a maneuver at maximum rudder deflection at the design maneuver speed, VA?
a. Simultaneous maximum deflection of both the elevator and rudder control surfaces is not required by the regulations, although an airplane designer may choose to design for the combined loads for other reasons. The horizontal and vertical stabilizer maneuver loads regulations require structural substantiation for sudden maximum displacement of elevator and rudder, at the design maneuver speed, VA, as separate conditions (see part 23, § 23.423(a) and § 23.441(a)(1)). However, the airworthiness standard for unsymmetrical tail loads, in § 23.427(a), requires that horizontal stabilizer surfaces and their supporting structure withstand unsymmetrical loads arising from maneuver and gust loads, and yaw and slipstream effects.
b. If maximum simultaneous rudder and elevator deflection at the design maneuver speed, VA, is a proposed maneuver, then this should be a design load condition.
98. For T-tail designs, should a symmetrical load on the horizontal stabilizer be combined with a vertical stabilizer load? This combined load condition is not required by part 23. Horizontal stabi lizer design maneuver and gust loads are normally combined with zero load on the vertical stabilizer. However, the yaw maneuver loads and the lateral gust loads are combined with the horizontal stabilizer balancing load for 1g level flight (see § 23.441 and § 23.443).
99. For T-tail designs, should a 50 feet per second (fps) gust load applied on the horizontal stabilizer be combined with a 50 fps gust load applied to the vertical stabilizer?
a. You are not required to apply a 50 fps gust load to both the horizontal and vertical-tail surfaces simultaneously. The horizontal-tail gust loads regulation requires that 50 fps vertical up and down gusts be applied to the horizontal stabilizer at design cruise speed, VC (see § 23.425(a)(1)). Also, the airw orthiness standard for the verticaltail gust loads requires that a 50 fps lateral gust be applied to the vertical stabilizer at design cruise speed, VC, as a separate condition (see § 23.443(a)).
b. For a T-tail, determine the induced unsymmetrical loads on the horizontal stabilizer when the lateral gust and 1g balance tail loads occur. Examine the resulting combined design load condition to determine if it is critical for the empennage and its supporting structure. Use ra tional methods to calculate the unsymmetrical loading condition.
c. Combining the horizontal and vertical gust loads is not a mandated FAA requirement that is contained within the regulations; it is a suggestion for a designer to consider.
d. The maneuvers and safe entry speeds proposed for certification should be carefully considered for higher combined loads on the empennage and aft fuselage than would be determined by applying part 23 requirements. If higher loads are likely to occur, the applicant should perform more investigations.
e. You may not determine the horizontal stabilizer unsymmetrical load from § 23.427(b)(1) and (2) for a T-tail airplane. This formula first appeared in Civil Air Regulations (CAR) 03.2214, effective November 13, 1945, long before the T-tail configuration came into use.
f. AC 23-9, Evaluation of Flight Loads on Small Airplanes with T, V, +, or Y Empennage Configurations, provides guidance about this topic.
NOTE: If the diagrams are to be reproduced , or the AC revised, the “roll axis and moment reference” and the “moment reference” points should be removed.
g. In summary, the part 23 regulations do not require the designer to combine the symmetric pitch maneuver loads with the yaw maneuver loads (see § 23.423 and § 23.441). However, lateral gust and yaw ma neuver loads should be combined with 1g level flight loads.
100. What guidance is available for other tail designs such as V-tail and Y-tail? AC 23-9, paragraph f, provides guidance about control surface and system loads for airplanes with control surfaces that receive simultaneous inputs from more than one control axis.
101. Do acrobatic category airplanes require any special considerations?
a. Yes. For acrobatic category airplanes, which are intended to perform “flick” or “snap” rolls, the unsymmetrical loading on the horizontal stabilizer should be calculated using conservative assumptions.
b. Instrumented flight-test results may be used instead of conservative assumptions.