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.
VERTICAL SURFACES
23.441 Maneuvering loads (Amendment 23-48)
102. Is there any policy available on paragraph (a), (b) or (c) of this section as of January 1, 2007? No.
103. May I use Appendix B of part 23 to determine the control surface loads?
No. Do not use Appendix B of part 23 to determine control surface loads. See 23.391 Control surface loads, for more guidance about Appendix B of part 23. Also, avoid misusing information that appears in Civil Air Regulations (CAR) 3.219; a tail torsional moment that is satisfactory for a stabilizer with a main spar at the quarter-chord may be inadequate if the main spar is located nearer mid-span. Part 23, Appendix A, as amended by Amendment 23-48, provides helpful guidance.
23.443 Gust loads (Amendment 23-48)
104. Is there any policy available on this section as of January 1, 2007? No.
105. May I use Appendix B of part 23 to determine the control surface loads?
No. Do not use Appendix B of part 23 to determine control surface loads. See 23.391, Control surface loads, for more guidance about Appendix B of part 23.
23.445 Outboard fins or winglets (Amendment 23-42)
106. Is there any policy available on this section as of January 1, 2007? No.
AILERONS AND SPECIAL DEVICES
23.455 Ailerons (Amendment 23-42)
107. What sections of the regulations are applicable to differential-deflection ailerons? These types are not subject to part 23, § 23.459, Special devices. Instead, apply the airworthiness standards for § 23.455, Ailerons, and § 23.683, Operation tests.
Previously generated data may be used.
108. Is there any policy available on paragraph (b) of this section as of January 1, 2007? No.
23.457 Wing flaps (Amendment 23-48) [Removed]
23.459 Special Devices (Original)
109. What types of devices are subject to this regulation? Dive-brakes and spoilers are subject to this airworthiness standard (§ 23.459).
GROUND LOADS
23.471 General (Original)
110. What load distribution is correct for the forward wheels of a four-wheel landing gear on amphibious airplanes with twin seaplane floats? A 50-50 load distribution is correct.
111. Are there other considerations for ground loads as they relate to amphibious airplane floats? Yes. You should assess the safety characteristics of the float configuration regarding the effects of float deflections and the skidding action of a float.
Figure 7 presents an overall view of landing gear airworthiness standards.
FIGURE 7
LANDING GEAR
LANDING GEAR 14 CFR SECTION 23.473
SELECT LIMIT LOAD
FACTORS OTHER ENERGY SECTION 23.723 (a) SECTION 23.725
ABSORPTION VERIFY LOAD FREE DROP
TEST FACTORS TEST OTHER ENERGY SECTION 23.723(E) SECTION 23.726
ABSORTION SHOW RESERVE FREE DROP
TEST ENERGY TEST ANALYSIS OR BY SECTIONS 23.479 SUESTANTIATE 481, 483 GEAR DYNAMIC DROP SECTION 23.726 TEST 45
23.473 Ground load conditions and assumptions (Amendment 23-48)
112. If my airplane ramp weight is above the 12,500-pound limit for certification to part 23, can I still certify to that standard if my maximum takeoff weight is 12,500 pounds or less?
a. Yes. Ramp weights above 12,500 pounds and ramp weights over the airplane maximum takeoff weight can be used for small airplanes and still retain the part 23 certification basis. To use a ramp weight over the maximu m takeoff weight, apply the following guidelines.
(1) The difference between ramp weight and takeoff weight should be limited to the weight of fuel that can reasonably be burned off by the engines from start up to the point of initiating the takeoff roll. Fuel burn-off considerations will include engine start(s), taxi to the runway, engine run-up, and taxi to the initial takeoff point on the runway. Also, for the specific airplane , make these estimates for an average size airport that the airplane will likely operate out of during its operational use.
(2) Since airplane takeoff performance will be based on the maximum takeoff weight, the amount of fuel used for the takeoff run should not be counted as part of the difference between ramp weight and takeoff weight.
(3) Policy: The increment of weight above maximum takeoff weight should be limited to small values consistent with the above-noted reasoning and, generally, will not exceed 1 percent of the takeoff weight.
(4) Provide the pilot an accurate means to determine the airplane gross weight at the takeoff condition just before brake release.
(5) The airplane design should comply with § 23.485 through § 23.511, excluding § 23.511(b) and (c)(1), with ramp weight substituted for maximum weight.
These airworthiness standards address:
(a) Side load conditions;
(b) Braked roll conditions;
(c) Supplementary conditions for tail wheels;
(d) Supplementary conditions for nose wheels;
(e) Supplementary conditions for skiplanes;
(f) Jacking loads;
(g) Towing loads; and
(h) Ground load; unsymmetrical loads on multiple-wheel units.
1. Two limitations are involved:
(aa) Ramp weight is limited by airplane structural integrity under ground loads, and 46
(bb) Maximum takeoff weight is limited by structural integrity or airplane performance.
(6) The Airplane Flight Manual (AFM) should clearly present these limitations to the pilot.
(7) Display maximum ramp weight limitations in at least two places—the Type Certificate Data Sheet and the AFM.
(8) Obtain the correct fuel capacity and fuel weight to show compliance for design landing weight (reference § 23.473(b)(2)) by using the following:
(a) The entire airplane fuel capacity (including unusable and residual fuel), with 1 The maximum appropriate fuel densities shown in the Note found in the Powerplant Guide for the Certification of Part 23 Airplanes (AC 23-16),
23.955 Fuel flow, (c) Pump Feed Systems, paragraph (3).
2 For avgas, use 6.0 pounds per gallon.
113. Why did a prior amendment level refer to part 25 regulations?
Before Amendment 23-7, § 23.473(c) referred to the requirements of § 25.1001 for fuel jettison requirements.
Warning: The design landing weight of a part 23 airplane may be less than that allowed by § 23.473 when a fuel jettisoning system is installed and the one engine inoperative rate-of-climb requirements of § 23.67(a) are met.
The other requirements related to this subject are so different in parts 23 and 25 that the jettisoning requirements of current part 25 are not compatible with part
23. These differences include structures and loads, aircraft performance, and accounting for temperature and altitude effects.
Therefore, the FAA did not intend the jettison requirements of part 25 (Amendment 25-18) to be applied to part 23. The preamble of Amendment 23-7 states that amended § 25.1001 does not now reflect the appropriate requirements for part 23. Amendment 25-18 was intende d to cater to the needs of airline transport airplanes and to preclude the need for additional exemptions from the pre-amendment jettisoning requirement.
114. Is there any policy available on paragraph (d), (e), (f) or (g) of this section as of January 1, 2007? No.
115. Where can I find more information about an increase in maximum weight, maximum landing weight, or maximum zero fuel weight? More information may be found in 23.307, Proof of structure.
116. What weight should be used in showing compliance with §§ 23.485, 23.493, and 23.499 (side load conditions, braked roll conditions, and supplementary conditions for nose wheels)? The design landing weight may be used in showing compliance to these regulations.
Other ground load conditions, which address ground handling or taxiing conditions, should be substantiated to the design maximum weight.
117. Can I establish separate land and water weight limits for amphibian airplanes? Yes.
Separate weight limits are acceptable for amphibians for operation on land and on water because separate certification criteria exist within the regulations.
118. What are the appropriate ground load conditions for an amphibious airplane design?
a. For an amphibious airplane design, the following ground load conditions and assumptions are appropriate (see § 23.473, paragraphs (e), (f), and (g)) when the design landing weight is less than the maximum weight (see § 23.473 (b)):
(1) § 23.473(d) - Determine the landing descent velocity from 14/ W ⎞ ⎠⎟
(2) § 23.473(e) - Assume a wing lift not exceeding two-thirds of the airplane weight exists throughout the landing impact; and
(3) § 23.473(f) - Energy absorption tests.
119. Can I use a landing weight less than the design maximum weight in § 23.473(g)? No.
That is inappropriate since the airplane inertial load factor may not be less than 2.67.
Correspondingly, the ground reaction load factor may not be less than 2.0 at design maximum weight.
120. How much can the gross weight be increased by analysis of prior drop tests without actually conducting a drop test at the increased weight?
a. Any answer should consider all the factors, such as remaining shock-strut travel, metering pin design, energy absorption characteristics (area under the accelerometer time-history trace), etc., and how those factors change at the increased gross weight configuration. The need to conduct or ignore drop tests should be reviewed cautiously.
● Policy: A decision about whether the maximum weight increase is “small” or “substantial” does not affect the application of guidance in this AC.
⎛⎜⎝ V
44.
;
= S
23.477 Landing gear arrangement (Original)
121. Is there any policy available on this section as of January 1, 2007? No.
23.479 Level landing conditions (Amendment 23-45)
122. Is there any policy available on paragraph (a) of this section as of January 1, 2007?
No.
123. What is the background of paragraph (b) of this section?
a. The drag component for landing gear loads may not be less than 25 percent of the design load factor multiplied by the gross weight of the airplane. This equation is observed in Appendix C of part 23 under main wheel loads. This does not mean that a separate drop of the main gear, at maximum gross weight, is required to obtain the load factor used in making this calculation.
b. Two different incline plane angles are given in the Civil Air Regulations (CAR) and Title 14 of the Code of Federal Regulations (14 CFR).
c. Approvals have been granted using CAR 3, before May 3, 1962, which defines the inclined plane angle to be ARC tan K for the level landing case with the nose wheel just clear of the ground. In CAR 3 (Amendment 3-7), dated May 3, 1962, the regulations show the inclined plane angle to equal ARC tan [nk/(n-L)] (see CAR 3.245(b)(2). See Figure 3-12(b), and § 23.479(a)(2)(ii), Appendix C of part 23). Manu facturers of amphibious floats for aircraft certificated under part 23 have used inclined plane angles as ARC tan K. There are no unfavorable service history records of landing gear failures for part 23 airplanes that were substantiated and approved for the main landing gear drop test using the old CAR 3 regulations.
• Policy: CAR 3 landing gear drop test data substantiated to requirements dated before May 3, 1962, are not acceptable for aircraft certificated to CAR 3 or part 23 on or after the 1962 date.
124. Is there any policy available on paragraph (c) of this section as of January 1, 2007?
No.
125. Is there any policy available on paragraph (d) of this section as of January 1, 2007?
No.
23.481 Tail down landing conditions (Original)
126. Is there any policy available on paragraph (a) of this section as of January 1, 2007?
No.
127. What are “spin-up” and “spring-back” loads?
a. When a landing airplane wheel touches the ground, a near instantaneous wheel spinup occurs (since the wheel tangential velocity should quickly match that of the runway). Due to inertial properties, this phenomenon induces a drag load upon the landing gear. The drag force energy is stored in the landing gear as potential energy that causes the gear to spring-back (a negative drag force). The drag forces of spin-up and spring-back may reach maximum values at different times than when the vertical load on the landing gear is achieved. It would be unusual for both load maximums (drag and vertical) to occur simultaneously.
128. Should there be a drag load (spin-up), combined with the vertical load, for paragraph
(b), tail down landing conditions?
a. A primary concern about the last phrase of paragraph (b) is that it can be interpreted at least two ways. The two interpretations are as follows:
(1) Since the wheel is assumed up to speed, the drag load caused by wheel spin-up is zero.
(2) Maximum wheel spin-up and spring-back occur before the landing gear maximum vertical load is attained; therefore, the drag load due to spin-up or spring-back combines with earlier lower-magnitude vertical loads but does not combine with the maximum vertical load.
b. The second interpretation of the regulation is correct. The different interpretations of tail-down landing requirements come from the Basic Landing Conditions table shown in Appendix C of part 23. The ta ble does not include a load, Dr, which accounts for spin-up and spring-back loads. However, Appendix D provides Dr loads for the same landing conditions shown in Appendix C.
129. How are the landing gear loads typically determined and tested?
a. Airplane landing gear designers often estimate these loads analytically and test for these loads by dropping the airplane or the landing gear units. The drag loads are sometimes approximated using inclined planes (wedge blocks) and, at other times, they are created by reverse spinning the wheel and tire before the drop impact.
b. Incline planes are often suitable for small airplanes where the touchdown velocity is relatively small and the wheel and tire diameters are correspondingly small. Incline planes, which are used to induce the estimated drag load, may restrict proper development of the springback (negative drag) load—and should be viewed accordingly. For small airplane landing gear designs, incline planes may be an adequate method to use to show compliance with part 23.
c. Reverse wheel spin-up, to simulate the drag load caused by wheel spin-up, is frequently used by landing gear designers for larger, faster, airplane designs. Higher touchdown speeds and larger diameter tires cause disproportionate increases in the wheel and tire inertial properties; these create larger magnitude spin-up and spring-back loads.
• Policy: Reverse wheel spin-up and in cline planes are both acceptable methods for imposing the drag inputs into a drop test when they are properly applied. Also see § 23.725(c).
23.483 One-wheel landing conditions (Original)
130. What wing lift should I use for this condition? Use wing lift consistent with that used for the other landing impact conditions.
23.485 Side load conditions (Amendment 23-45)
131. Is there any policy available on this section as of January 1, 2007? No.
23.493 Braked roll conditions (Original)
132. Is there any policy available on this section as of January 1, 2007? No.
23.497 Supplementary conditions for tail wheels (Amendment 23-48)
133. Is there any policy available on this section as of January 1, 2007? No.
23.499 Supplementary conditions for nose wheels (Amendment 23-48)
134. Is there any policy available on this section as of January 1, 2007? No.
23.505 Supplementary conditions for skiplanes (Amendment 23-7)
135. What is the typical method for determining the load factor for a ski installation?
a. A ski installation should use a load factor determined by either of the following two methods:
(1) Perform drop tests, with skis installed, on a surface simulating frozen hardpacked snow or ice; or
(2) Use a conservative formula (see SPECIFICATION—Aircraft skis, National Aircraft Standards Committee, NAS 808, paragraph 5.1(a)).
b. Ski installation factors should include consideration for fittings, tubes, axles, nuts, bolts, etc., which attach the skis to the fuselage. Ski-gear loads normally run about 115 percent to 125 percent of wheel-gear loads. Also see 23.737, Skis.