Source: https://www.federalregister.gov/documents/2011/07/07/2011-16295/special-conditions-boeing-model-787-8-airplane-interaction-of-systems-and-structures-electronic
Timestamp: 2018-09-26 11:12:10
Document Index: 239445842

Matched Legal Cases: ['art 25', '§\u200921', '§\u200921', 'art 25', '§\u200925', '§\u200925', 'art 25', '§\u200925', 'art 25', '§\u200925', '§\u200925', 'art 25', 'art 25', '§\u200925', '§\u200925', '§\u200925']

A Rule by the Federal Aviation Administration on 07/07/2011
39763-39769 (7 pages)
Docket No. NM362
Special Conditions No. 25-354A-SC
https://www.federalregister.gov/d/2011-16295 https://www.federalregister.gov/d/2011-16295
Start Preamble Start Printed Page 39763
Amended special conditions
These amended special conditions are issued to the Boeing Model 787-8 airplane. This airplane will have novel or unusual design features when compared to the state of technology envisioned in the airworthiness standards for transport category airplanes. These design features include limit engine torque loads for sudden engine stoppage. Special Conditions No. 25-354-SC was issued on July 18, 2007, addressing, in part, this condition. We have determined that more clarification is needed on the limit engine torque loads for sudden engine stoppage special conditions, and have therefore added a new requirement. This additional requirement has been applied, via special conditions, to other programs. Since applicable airworthiness regulations, including those contained in Special Conditions No. 25-354-SC, do not contain adequate or appropriate safety standards for this particular design feature, these amended special conditions contain the additional safety standards which the Administrator finds necessary to establish a level of safety equivalent to that established by the existing standards.
Todd Martin, FAA, Airframe and Cabin Safety Branch, ANM-115, Transport Airplane Directorate, Aircraft Certification Service, 1601 Lind Avenue, SW., Renton, Washington 98057-3356, telephone (425-227-1178; facsimile (425-227-1320).
On March 28, 2003, Boeing applied for an FAA type certificate for its new Boeing Model 787-8 passenger airplane. The Boeing Model 787-8 airplane will be an all-new, two-engine jet transport airplane with a two-aisle cabin. The maximum takeoff weight will be 476,000 pounds, with a maximum passenger count of 381 passengers. Special Conditions No. 25-354-SC was issued on July 17, 2007, to address interaction of systems and structures, electronic flight control system control surface awareness, HIRF protection, limit engine torque loads for sudden engine stoppage, and design roll maneuver requirements. Since then, it was determined more clarification was needed on the limit engine torque loads for sudden engine stoppage special conditions.
The limit engine torque loads for sudden engine stoppage special conditions, issued as part of Special Conditions No. 25-354-SC, distinguishes between the more common, less severe engine failure events, and those rare events resulting from structural failures. Paragraph (a) defines limit load conditions for the less severe events, and paragraph (c) defines the ultimate load conditions for the more severe structural failure events.
Compliance with paragraph (a) includes, by definition, assessment of deformation at limit load, as well as assessment of structural integrity at ultimate load. However, since paragraph (c) is defined as an ultimate load condition, it only requires assessment of structural integrity at ultimate load, and does not require assessment of deformation.
New paragraph (e), therefore, is added to the special condition to require assessment of deformation for the structural failures defined in paragraph (c).
If the Administrator finds that the applicable airworthiness regulations (i.e., 14 CFR part 25) do not contain adequate or appropriate safety standards for the Boeing Model 787-8 because of a novel or unusual design feature, special conditions are prescribed under the provisions of § 21.16.
Special conditions are initially applicable to the model for which they are issued. Should the type certificate for that model be amended later to include any other model that incorporates the same or similar novel or unusual design feature, the special conditions would also apply to the other model under § 21.101.Start Printed Page 39764
The 787 is equipped with systems that affect the airplane's structural performance, either directly or as a result of failure or malfunction. That is, the airplane's systems affect how it responds in maneuver and gust conditions, and thereby affect its structural capability. These systems may also affect the aeroelastic stability of the airplane. Such systems represent a novel and unusual feature when compared to the technology envisioned in the current airworthiness standards. A special condition is needed to require consideration of the effects of systems on the structural capability and aeroelastic stability of the airplane, both in the normal and in the failed state.
This special condition requires that the airplane meet the structural requirements of subparts C and D of 14 CFR part 25 when the airplane systems are fully operative. The special condition also requires that the airplane meet these requirements considering failure conditions. In some cases, reduced margins are allowed for failure conditions based on system reliability.
With a response-command type of flight control system and no direct coupling from cockpit controller to control surface, such as on the 787, the pilot is not aware of the actual surface deflection position during flight maneuvers. This feature of this design is novel and unusual when compared to the state of technology envisioned in the airworthiness standards for transport category airplanes. These special conditions are meant to contain the additional safety standards that the Administrator considers necessary to establish a level of safety equivalent to that established by the existing airworthiness standards. Some unusual flight conditions, arising from atmospheric conditions or airplane or engine failures or both, may result in full or nearly full surface deflection. Unless the flightcrew is made aware of excessive deflection or impending control surface deflection limiting, piloted or auto-flight system control of the airplane might be inadvertently continued in a way that would cause loss of control or other unsafe handling or performance situations.
The 787 will use electrical and electronic systems which perform critical functions. These systems may be vulnerable to high-intensity radiated fields (HIRF) external to the airplane. There is no specific regulation that addresses requirements for protection of electrical and electronic systems from HIRF. Increased power levels from radio frequency transmitters and use of sensitive avionics/electronics and electrical systems to command and control the airplane have made it necessary to provide adequate protection.
The 787 will have high-bypass engines with a chord-swept fan 112 Start Printed Page 39765inches in diameter. Engines of this size were not envisioned when § 25.361, pertaining to loads imposed by engine seizure, was adopted in 1965. Worst case engine seizure events become increasingly more severe with increasing engine size because of the higher inertia of the rotating components.
Section 25.361(b)(1) requires that for turbine engine installations, the engine mounts and supporting structures must be designed to withstand a “limit engine torque load imposed by sudden engine stoppage due to malfunction or structural failure.” Limit loads are expected to occur about once in the lifetime of any airplane. Section 25.305 requires that supporting structures be able to support limit loads without detrimental permanent deformation, meaning that supporting structures should remain serviceable after a limit load event.
Given this situation, ARAC has proposed a design standard for today's large engines. For the commonly-occurring deceleration events, the proposed standard requires engine mounts and structures to support maximum torques without detrimental permanent deformation. For the rare-but-severe engine seizure events such as loss of any fan, compressor, or turbine blade, the proposed standard requires engine mounts and structures to support maximum torques without failure, but allows for some deformation in the structure.
The FAA concludes that modern large engines, including those on the 787, are novel and unusual compared to those envisioned when § 25.361(b)(1) was adopted and thus warrant a special condition. This special condition contains design criteria recommended by ARAC.
The 787 is equipped with an electronic flight control system that provides control of the aircraft through pilot inputs to the flight computer. Current part 25 airworthiness regulations account for “control laws,” for which aileron deflection is proportional to control stick deflection. They do not address any nonlinearities [2] or other effects on aileron actuation that may be caused by electronic flight controls. Therefore, the FAA considers the flight control system to be a novel and unusual feature compared to those envisioned when current regulations were adopted. Since this type of system may affect flight loads, and therefore the structural capability of the airplane, special conditions are needed to address these effects.
This special condition differs from current requirements in that it requires that the roll maneuver result from defined movements of the cockpit roll control as opposed to defined aileron deflections. Also, this special condition requires an additional load condition at design maneuvering speed (VA), in which the cockpit roll control is returned to neutral following the initial roll input.
This special condition differs from similar special conditions applied to previous designs. This special condition is limited to the roll axis only, whereas previous special conditions also included pitch and yaw axes. A special condition is no longer needed for the yaw axis because § 25.351 was revised at Amendment 25-91 to take into account effects of an electronic flight control system. No special condition is needed for the pitch axis because the applicant's proposed method for the pitch maneuver takes into account effects of an electronic flight control system.
Accordingly, pursuance to the authority delegated to me by the Administrator, the following amended special conditions (which adds paragraph (e) to Special Condition No. 4) are issued as part of the type certification basis for the Boeing Model 787-8 airplane regarding limit engine torque loads for sudden engine stoppage.
The Boeing Model 787-8 airplane is equipped with systems which affect the airplane's structural performance either directly or as a result of failure or malfunction. The influence of these systems and their failure conditions must be taken into account when showing compliance with requirements of subparts C and D of part 25 of Title 14 of the Code of Federal Regulations. The following criteria must be used for showing compliance with this special condition for airplanes equipped with flight control systems, autopilots, stability augmentation systems, load alleviation systems, flutter control systems, fuel management systems, and other systems that either directly or as a result of failure or malfunction affect structural performance. If this special condition is used for other systems, it may be necessary to adapt the criteria to the specific system.
(a) The criteria defined here address only direct structural consequences of system responses and performances. They cannot be considered in isolation but should be included in the overall safety evaluation of the airplane. They may in some instances duplicate standards already established for this evaluation. These criteria are only applicable to structure whose failure could prevent continued safe flight and landing. Specific criteria defining acceptable limits on handling characteristics or stability requirements when operating in the system degraded or inoperative mode are not provided in this special condition.
(b) Depending on the specific characteristics of the airplane, additional studies may be required that go beyond the criteria provided in this special condition in order to demonstrate capability of the airplane to meet other realistic conditions such as Start Printed Page 39766alternative gust conditions or maneuvers for an airplane equipped with a load alleviation system.
(4) Probabilistic terms: Terms (probable, improbable, extremely improbable) used in this special condition which are the same as those probabilistic terms used in § 25.1309.
(5) Failure condition: Term that is the same as that used in § 25.1309. The term failure condition in this special condition, however, applies only to system failure conditions that affect structural performance of the airplane. Examples are system failure conditions that induce loads, change the response of the airplane to inputs such as gusts or pilot actions, or lower flutter margins.
Although failure annunciation system reliability must be included in probability calculations for paragraph (f) of this special condition, there is no specific reliability requirement for the annunciation system required in paragraph (g) of the special condition.
(i) Loads derived from the following conditions (or used in lieu of the following conditions) at speeds up to VC/MC, or the speed limitation Start Printed Page 39767prescribed for the remainder of the flight, must be determined:
If Pj is greater than 10−3 per flight hour then a 1.5 factor of safety must be applied to all limit load conditions specified in subpart C-Structure, of 14 CFR part 25.
(iii) For residual strength substantiation, the airplane must be able to withstand two thirds of the ultimate loads defined in paragraph (f)(2)(ii) of this special condition. For pressurized cabins, these loads must be combined with the normal operating differential pressure.
If Pj is greater than 10−3 per flight hour, then the flutter clearance speed must not be less than V′.
(1) The system must be checked for failure conditions, not extremely improbable, that degrade the structural capability of the airplane below the level required by part 25 or significantly reduce the reliability of the remaining system. As far as reasonably practicable, the flightcrew must be made aware of these failures before flight. Certain elements of the control system, such as mechanical and hydraulic components, may use special periodic inspections, and electronic components may use daily checks, instead of detection and indication systems to achieve the objective of this requirement. Such certification maintenance inspections or daily checks must be limited to components on which faults are not readily detectable by normal detection and indication systems and where service history shows that inspections will provide an adequate level of safety.Start Printed Page 39768
(h) Dispatch with known failure conditions. If the airplane is to be dispatched in a known system failure condition that affects structural performance, or affects the reliability of the remaining system to maintain structural performance, then the provisions of this special condition must be met, including the provisions of paragraph (e) for the dispatched condition, and paragraph (f) for subsequent failures. Expected operational limitations may be taken into account in establishing Pj as the probability of failure occurrence for determining the safety margin in Figure 1. Flight limitations and expected operational limitations may be taken into account in establishing Qj as the combined probability of being in the dispatched failure condition and the subsequent failure condition for the safety margins in Figures 2 and 3. These limitations must be such that the probability of being in this combined failure state and then subsequently encountering limit load conditions is extremely improbable. No reduction in these safety margins is allowed if the subsequent system failure rate is greater than 10−3 per hour.
In addition to compliance with §§ 25.143, 25.671, and 25.672, the following special condition applies.
In lieu of § 25.361(b) the Boeing Model 787-8 must comply with the following special condition.
(e) Any permanent deformation that results from the conditions specified in paragraph (c) must not prevent continued safe flight and landing.
In lieu of compliance to § 25.349(a), the Boeing Model 787-8 must comply with the following special condition.
Issued in Renton, Washington, on June 23, 2011.
2. A nonlinearity is a situation where output does not change in the same proportion as input.
[FR Doc. 2011-16295 Filed 7-6-11; 8:45 am]