c. When static pressure influences structural loads: The structure experiences a load increase due to the external air load when the external static pressure is different from the internal static pressure. This often happens on secondary structures like engine cowls, windshields, and windows.
d. Also see the question regarding the use of wind tunnel data under section
23.301.
35. What proof-of-strength factors should be considered for airplane design changes that may affect structure? (Also see 23.321, General (flight loads).)
a. When loads increase, the strength of structure is affected by shear, bending, and torsion—not just one of these conditions.
b. In some aircraft certifications, the structure was proven by tests only. Any modifications to these aircraft that alter the loads⎯or the load paths⎯must be assessed to determine if the change is significant enough to require retests. It is inappropriate to assume that additional strength resides in the structures beyond the values proven by tests.
c. For stressed-skin wings, if analyses are used to justify the strength between limit and ultimate load conditions, strain-gauge data coupled with panel-buckling stress data may be used to show that the strength extrapolation is reasonable and correct.
d. Consider the effects of stress concentration factors.
e. A small increase in design maximum weight may cause a severe reduction of fatigue life (see 23.572, Metallic wing, empennage, and associated structures).
f. Any load increase on the wing, tail, or landing gear structures, or passenger, cargo or equipment areas affects the fuselage.
g. Identify specific materials, dimensions, and processes used in the design (see part 21, § 21.31, and part 23, § 23.603).
h. A previously certificated airplane requires structural proof-of-strength substantiation to the Certification Basis regulations when:
(1) Someone puts a turbine engine on a previously certificated reciprocating-engine airplane; or when
(2) A turbine engine substitutes as a single-power-source for two or more reciprocating engines.
36. I want to get an STC for a gross weight increase, but I am not the Original Equipment Manufacturer (OEM), so I do not have access to the original Type Certificate data. Do I have to assume that the margin of safety of all structural components is zero? Yes. Assume that the margin is zero unless you can confirm two things. First, you corroborate that the proof of strength was demonstrated by a combination of analysis and tests. Second, you confirm that these tests showed a margin of safety greater than zero in an "as delivered" condition.
37. Is there any guidance available specifically for composite relative to proof of structure?
Yes. See Policy Statement PS-ACE100-2001- 006, “Static Strength Substantiation of Composite Airplane Structure”.
FLIGHT LOADS
23.321 General (Amendment 23-45)
38. Is there any policy available on paragraph (a) of this section as of January 1, 2007? No.
39. Is there any policy available on paragraph (b) of this section as of January 1, 2007? No. However, you can see 23.307 for guidan ce about airplane weight increases.
See 23.471 for guidance about ramp weight and take-off weight.
40. Is there any policy available on paragraph (c) of this section as of January 1, 2007? No. However, for information a bout canard and tandem wing airplane configurations, see 23.421, balancing loads for horizontal stabilizing and balancing surfaces. Also, laminar flow aerodynamics information may be found in 23.21, proof of compliance.
41. Can you show a graphical relational image of the airworthiness standards that concern airplane loads? Yes. See Figure 2 and Fi gure 3 on the next two pages.
LO/OE/₺ UNSYMMETRICAL §23.347 Vertical surfaces Horizontal stabilizing and balancing surfaces Vertical surfaces Ailerons Rolling Conditions Engine torque Side load on engine mount Gyroscopic and aerodynamic loads combined Unsymmetrical loads due to engine failure ACE-100, E. Gabriel, May 1996 FLIGHT LOADS- Part 23[ xxx| Amendment 45 or winglets Pressurized cabin loads Canard or tandem wing configurations Outboard fins Speed control devices Wing flaps Wing flaps High lift devices High lift devices SYMMETRICAL §23.331 Clean Airplane Horizontal stabilizing and balancing surfaces Balancing horizontal tail load Limit load factor Pitching Wing Flaps Slipstream Effects Rear lift truss
GUST MANEUVER ENGINE OTHER
FIGURE 3
FLIGHT LOADS Flight Loads - Part 23 Amendment 45 UNSYMMETRICAL § 23.347 Vertical Surfaces Lateral gust: + 50 fps @ Vc [§ 23.443(a)] Commuter category [§ 23.443(b)] Gusts normal to plane of symmetry Vc, Vo clean airplane high lift devices Horizontal Stabilizing and Balancing Surfaces [§ 23.445] [§ 23.427] Loads from gusts combined with yawing and slipstream effects, clean airplane and with high lift devices Vertical Surfaces [§ 23.441] - @ V, Yaw, sideslip. and [§ 23.445(d)] rudder deflection Outboard Fins or Ailerons [§ 23.445] Abrupt maximum control movement Winglets @VA. Control deflection requirements @Vc and Vp Rolling Conditions [§ 23.349] - Wing and wing bracing Airload Distribution Condition (See § 23.333) 100%/70% to 75% Normal, Utility, Commuter 100%/60% A and F Wing loads due to aileron deflections § 23.445 Engine Torque [§ 23.361] - Combined with symmetrical limit loads @ VA Side Load on Engine Mount [§ 23.363] Gyroscopic and Aerodynamic Loads [§ 23.371] - Pitching and yawing, applies only to turbine installations Unsymmetrical Loads Due to Engine Failure [§ 23.367] - Turboprops only Pressurized Cabin Loads, combined with flight loads [§ 23.365] Canard or Tandem Wing Configurations [$ 23.302] Category Acrobatic [§ 23.373] Speed Control Devices SYMMETRICAL § 23.331 High Lift Devices Clean Airplane [§ 23.345] Discrete Vertical Gusts [§ 23.333(c), § 23.341] + 25 ft/sec vertical 25 ft/sec head-on + 50 fps @ Vc and + 25 fps @ Vo Wing Flaps + 66 fps @ VB [§ 23.457(a)] (commuter category only) Horizontal Stabilizing and Balancing Surfaces [§ 23.425] Clean airplane and with high lift devices Balancing Horizontal Tail Load [§$ 23.331, 23.421] Limit Load Factor [§ 23.337] Normal or Commuter Category n = 3.8* Utility Category n = 4.4 Acrobatic Category n = 6.0 *May reduce for W > 4,118 Ibs. High Lift Devices [§ 23.345] Pitching: Checked and Unchecked n = 2.0 g Applies to horizontal stabilizing and Wing Flaps balancing surfaces [§ 23.423] [§ 23.457(a)] Abrupt maximum control input @ VA Wing Flaps Slipstream Effects, n = 1.0 [$ 23.457(b)] Rear Lift Truss, reverse air flow [§ 23.369] ANTONE
23.331 Symmetrical flight controls (Amendment 23-42)
42. Is there any policy available on this section as of January 1, 2007? No.
23.333 Flight envelope (Amendment 23-34)
43. Do I have to revise the velocity-load factor (V-n) diagram for Supplemental Type Certificates (STC) involving engine changes or modifications? It is not an automatic requirement. This question usually arises because some foreign certifying agencies require the development of a new V-n diagram if the original minimum design cruising speed (VC) was defined by the wing loading (W/S) [reference: part 23, § 23.335(a)(1) and (2)], instead of the maximum level flight speed (VH) [reference: part 23, § 23.335(a)(3)]. However, we do not require a new V-n diagram unless the applicant wishes to increase VC.
44. Is there any policy available on paragraph (b) or (c) of this section as of January 1, 2007? No.
23.335 Design airspeeds (Amendment 23-48)
45. When applying design airspeed criteria, how may the designer establish a minimum design cruising speed?
a. The designer may establish the minimum design cruising speed, VC min, according to the following:
W33 ≤ V ≤ 0 9 . V , for normal, utility and commuter category C min H S and W ≤ V ≤ 0 9 . V , for acrobatic category36 C min H S 26
b. Both minimum and maximum design cruise speed values, VC min and VC max, may be chosen with the following understanding:
VC ≤ 0.9VH
46. Why would I want to define VC as equal to 0.9 VH? Use this definition if you are designing an airplane that is capable of a sustained speed (VC) higher than that obtained by using the wing loading (W/S) formula.
47. Is there any policy available on paragraph (b) of this section as of January 1, 2007? No.
48. What is the design maneuvering speed VA?
a. The design maneuvering speed is a value chosen by the applicant. It may not be less than Vs√ n and need not be greater than Vc, but it could be greater if the applicant chose the higher value. The loads resulting from full control surface deflections at VA are used to design the empennage and ailerons in part 23, §§ 23.423, 23.441, and 23.455.
b. VA should not be interpreted as a speed that would permit the pilot unrestricted flight-control movement without exceeding airplane structural limits, nor should it be interpreted as a gust penetration speed. Only if VA = Vs √n will the airplane stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor.
c. Amendment 23-45 added the operating maneuvering speed, VO, in § 23.1507.
VO is established not greater than VS√n, and it is a speed where the airplane will stall in a nose-up pitching maneuver before exceeding the airplane structural limits.
49. Is there any policy available on paragraph (d) of this section as of January 1, 2007? No.
23.337 Limit maneuvering load factors (Amendment 23-48)
50. Is there any policy available on this section as of January 1, 2007? No.
51. Are there any related sections? Yes. See Section 23.423, Maneuvering loads.
23.341 Gust load factors (Amendment 23-48)
52. Is there any policy available on this section as of January 1, 2007? No.
23.343 Design fuel loads (Amendment 23-48)
53. Is there any policy available on this section as of January 1, 2007? No.
23.345 High lift devices (Amendment 23-48)
54. Is there any policy available on this section as of January 1, 2007? No.
23.347 Unsymmetrical flight conditions (Amendment 23-48)
55. Is there any policy available on this section as of January 1, 2007? No.
23.349 Rolling conditions (Amendment 23-48)
56. Is there any policy available on this section as of January 1, 2007? No.
23.351 Yawing conditions (Amendment 23-42)
57. Is there any policy available on this section as of January 1, 2007? No.
23.361 Engine torque (Amendment 23-45)
58. Is there any policy available on paragraph (a) of this section as of January 1, 2007? No.
59. How do I determine the limit engine torque load imposed by sudden engine stoppage due to malfunction or structural failure (such as compressor jamming) for turbine engines to satisfy the requirements of paragraph (b)(1)? Title 14 Code of Federal Regulations (CFR) part 33, § 33.23 requires the specification of the maximum allowable limit and ultimate loads for engine mounting attachments and related engine structure. Therefore, the maximum engine st ructure loading information, as specified in the engine Manufacturers Installation Manual, satisfies the limit engine torque load imposed by sudden engine stoppage due to malfunction or structural failure (such as compressor jamming).
60. Is there any policy available on paragraph (c) of this section as of January 1, 2007?
a. This rule requires the design of engine mounts and supporting structure to sustain limit torque at takeoff power and at maximum continuous power, with corresponding propeller revolutions per minute (r.p.m.), for two specified flight 28
conditions. The rule defines limit torque equa l to mean torque multiplied by a factor that is a function of the number of cylinders of a reciprocating engine (see Figure 4).
FIGURE 4
LIMIT ENGINE TORQUE DEFINITION Tlim = Fcyl Tmean Engine type Turbopropeller Five or more cylinders Four cylinders Three cylinders Two cylinders Fcyl 1.25 1.33 2 3 4
b. NOTE: The limit engine torque, Tlim, is not an engine limit in the sense of part 33, § 33.7. For structural installation lo ads purposes, treat it as simply a limit load arrived at by the equation and figure shown. Engine mean torque is available from the engine manufacturer. The engine type multiplying factor, Fcyl, is a constant for a given engine without regard to the speed or power at which the engine is operating.
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.