22-1 CHAPTER 22. DESIGN AND CONSTRUCTION: MISCELLANEOUS [RESERVED]
23-1 CHAPTER 23. POWERPLANT: GENERAL
23.1 Installation—§ 25.901. [Reserved]
23.2 Engines—§ 25.903.
23.2.1 Engine Isolation—§ 25.903(b).
23.2.1.1 Explanation: Approval of Engine Isolation Criteria.
The powerplants, and all systems controlling or influencing the performance of the powerplants, must be arranged and isolated from each other to allow operation, in at least one configuration, so that a failure of any engine or any said system will not:
Prevent the continued safe operation of the remaining engines; or Require immediate action by any crewmember for continued safe operation.
23.2.1.2 Procedures.
23.2.1.2.1 Automated control functions incorporated to reduce crew workload, which impact the engine operation and control, may require flight tests to demonstrate the effects of various simulated failures, including engine surging and single engine failures. Failures in systems such as the auto-throttles, digital flight guidance computers, engine synchronizers, and electronic engine controls may need to be simulated to ensure that surging or a failure in one engine system will not hazardously affect control and operation of the remaining engine systems. Such failure simulations may include disruption of electrical power to selected control components, as well as simulated engine failures (throttle chops).
23.2.1.2.2 The applicant should submit a comprehensive test plan detailing the test conditions and failure modes to be simulated. Careful consideration should be given to conducting these tests at safe altitudes and airspeeds, since results of the failure simulations may not be entirely predictable. Where possible, without compromising tests results, on-aircraft (i.e., installed engines) tests of this type should be accomplished on the ground.
23.2.1.2.3 For propeller airplanes, when an automatic control system for simultaneous RPM control of all propellers is installed, it should be shown by analysis, flight demonstration, or a combination of analysis and flight demonstration that no single failure or malfunction in this system or in an engine controlling this system will:
1. Cause the allowable engine overspeed for this condition to be exceeded at any time.
23-2
2. Cause a loss of power or that will cause the airplane to descend below the takeoff path, established in accordance with § 25.111, if such a system is certificated for use during takeoff and climb. This should be shown for all weights and altitudes for which certification is desired. A period of five seconds should be allowed from the time the malfunction occurs to the initial motion of the cockpit control for corrective action taken by the crew.
23.2.2 Control of Engine Rotation—§ 25.903(c).
23.2.2.1 Explanation.
Section 25.903(c) requires that a means be provided to stop the rotation of any individual engine in flight. An exclusion is provided for turbine engines whereby a means to stop rotation need only be provided in cases where continued rotation could jeopardize the safety of the airplane. If means are not provided to completely stop the rotation of turbine engines, it should be shown that continued rotation, either windmilling or controlled, of a shutdown turbine engine will not cause:
23.2.2.1.1 Powerplant (including engine and accessories) structural damage that will adversely affect other engines or the airplane structure;
23.2.2.1.2 Flammable fluids to be pumped into a fire or into an ignition source; or
23.2.2.1.3 A vibration mode that will adversely affect the aerodynamic or structural integrity of the airplane.
23.2.2.2 Procedures.
None.
23.2.3 Turbine Engine Installations—§ 25.903(d).
23.2.3.1 Explanation.
Section 25.903(d) presents specific concerns related to turbine engine installations. The requirements presented in § 25.903(d)(2) are intended to ensure that the installed powerplant control devices, systems, and instrumentation will reasonably protect against exceeding engine operating limitations that adversely affect turbine rotor integrity.
23.2.3.2 Intermixing of Engines.
Engines with different ratings, and/or with different cowls, may be intermixed on airplanes, provided the proper limitations and performance information associated with the engine combination are used. In general, for four-, three-, or two-engine airplanes, the performance combination is as follows:
23-3
23.2.3.2.1 When one lower power or thrust engine is installed, the normal AFM performance level is reduced by an increment appropriate to the decrease in power or thrust resulting from the intermix.
23.2.3.2.2 When more than one lower power or thrust engine is installed, the AFM performance should be based on the power or thrust of the lower/lowest rated engine.
23.2.3.2.3 The minimum control speeds (VMCG, VMCA, and VMCL) should be based on the highest power or thrust engine(s).
23.2.3.2.4 The operating procedures should be provided for all engines installed
(i.e., airstart altitude/airspeed envelopes, crew responses to engine warning systems, etc.). Differences in operating methods should be limited to the equivalent of having a maximum of two different engines on the airplane.
23.2.3.2.5 A maximum of two takeoff power or thrust settings, in terms of engine pressure ratio (EPR) or rotational speed of the low pressure compressor (N1) are permitted for airplanes with intermixed engines; this includes differences related to air conditioning pack and compressor bleed configurations.
23.2.3.2.6 A placard, identifying the location of the non-standard engine type, should be installed. All engine limits and instrument markings should be appropriate to the engine installed at each location. EPR, N1, N2, exhaust gas temperature (EGT), etc. limits for each engine, or the ratings at which they will be operated, must be properly presented to the pilot in accordance with §§ 25.1541 and 25.1543 and 14 CFR part 121.
23.2.4 Engine Restart Capability—§ 25.903(e).
23.2.4.1 Explanation.
23.2.4.1.1 Engine Restart Capability Requirements.
Section 25.903(e), Restart capability, requires:
“(1) Means to restart any engine in flight must be provided.
(2) An altitude and airspeed envelope must be established for in-flight engine restarting, and each engine must have a restart capability within that envelope.
(3) For turbine engine powered airplanes, if the minimum windmilling speed of the engines, following the inflight shutdown of all engines, is insufficient to provide the necessary electrical power for engine ignition, a power source independent of the engine-driven electrical power generating system must be provided to permit in-flight engine ignition for restarting.”
23-4
23.2.4.1.2 Methods for Restart.
There are several available methods used for in-flight engine restart. The primary method used to restart a turbine engine in flight is windmill or unassisted restart. The airplane forward speed provides airflow through the engine creating the rotational energy, or “windmilling,” needed to permit an engine restart. An assisted restart is an alternate restart method that may use a pneumatic, electrical, hydraulic, or some other source to power a starter that will assist in rotating the engine enough to restart it. The air for a pneumatic starter typically comes from another operable engine or an in-flight operable auxiliary power unit (APU). An engine accessory generator, hydraulic generator, APU, batteries, or a combination of these sources can provide electrical power for an electrical starter. A hydraulic starter requires an onboard hydraulic source. Start cartridges may also be used.
23.2.4.1.3 Background.
In general, airplane manufacturers use windmill restart to provide in-flight engine restart capability. Turbojet and early turbofan engines with low bypass ratios, approximately 1:1, had windmill restart capability throughout most of the approved airplane altitude-airspeed operating envelope. Since then, the evolution of technology to improve fuel efficiency has increased the bypass ratio of turbofan engines higher, as much as 10:1 on some engines. The high bypass ratio engines generally require higher airspeeds to provide enough airflow through the engine core to create sufficient windmilling rotational energy for an unassisted engine restart. Additional features incorporated in the later technology engines, such as compressors with higher operating pressures, new technology burners, and optimized operating schedules affording smaller stall margins, have also increased the airspeed required for windmilling restart.
As a result, the altitude-airspeed windmill restart envelope has decreased significantly. These engines typically have assisted restart envelopes to provide larger in-flight engine restart envelopes. Turboprop engines, particularly those with free-turbines, have little to no windmill restart capability and mainly rely on assisted restart methods.
23.2.4.1.4 Engine Restart Envelope.
An engine restart envelope should clearly distinguish between the areas of different restart capabilities in terms of time to restart, windmilling only restart, and starter assisted restart. If an APU provides any essential functions, including in-flight restart over any portion of the operating envelope, the applicant should perform a flight test to substantiate a separate in-flight start envelope for the APU. The applicant should furnish engine and APU (if applicable) restart envelopes, and related procedures, in the AFM.
23-5
23.2.4.1.5 All-Engines-Out Restart Capability.
The applicant should demonstrate all-engines-out restart capability for critical conditions using a method agreed to by the FAA.
23.2.4.2 Procedures.
23.2.4.2.1 The applicant should conduct tests to determine if the flightcrew can restart an engine in flight within the envelope provided using the AFM procedures. The applicant should also conduct restarts at the conditions of the critical corners of the envelope and at or near the high altitude extremes of the envelope to verify the boundary conditions of the envelope. The applicant should establish any other critical test points if specific design features may reduce in-flight engine restart capability. The applicant should use a sample size of at least two different engines for each critical test condition. The applicant should conduct a sufficient number of test points to ensure the reliability and repeatability of an in-flight engine restart within the envelope. When establishing the critical test points, the applicant should seek advice from the engine manufacturer on potential needs to account for the effect of normal engine deterioration that occurs in service on restarting characteristics.
23.2.4.2.2 The applicant should evaluate the engine operating characteristics during each restart. In particular, the applicant should note any tendencies for the engine to surge, dwell for abnormally long periods below idle, or produce other unusual vibration or audible noises. The engine should not require a period of “warm-up” time before the engine is able to accelerate to a power necessary to maintain safe flight.
23.2.4.2.3 An in-flight engine restart sequence (from start initiation to stabilized idle) should occur within a maximum of 90 seconds, including a maximum of 30 seconds for ignition time. A longer time from ignition (light-off) to stabilized idle may be acceptable provided that the flightcrew has a clear indication of the engine start progressing. The applicant may use a dedicated flight deck indication or human factors assessment to provide the clear indication of progression. The human factors assessment should show that normally available flight deck instruments provide adequate and clear indications of engine start progression throughout the start. A clear indication of progression of the engine start is necessary to prevent the flightcrew from terminating a successful start attempt. This is important for engines that have characteristics where the engine temperature is climbing with no increase in engine speed and the start appears to the flightcrew to be “hung.” An applicant may need to include appropriate notes in the AFM to describe an acceptable engine start progression to the flightcrew.
23.2.4.2.4 The applicant should evaluate the engine restarting performance impacts due to the effects of altitude and cold temperatures following engine
23-6 shutdown. The applicant should determine critical conditions considering medium-to-high altitude cruise conditions and engine manufacturer recommendations. The applicant should use the following delay periods between engine shutdown or flameout to restart:
Stabilized windmill conditions, and Until the powerplant installation exhaust gas temperature is stabilized at a total air temperature plus 18 °F (10 °C) or lower.
23.2.4.2.5 The applicant should seek advice from the engine manufacturer on the potential need to evaluate the engine restarting performance impact due to any other operating conditions anticipated to occur in service (e.g., rain, hail, or icing). The applicant is not required to perform engine restart testing in inclement weather. Instead, the applicant may adjust restart procedures to accommodate any anticipated impact from operating conditions. An applicant may need to include additional information in the restart procedure to inform the flightcrew of these effects, such as longer start times.
23.2.4.2.6 If an onboard APU is required to provide essential functions, including assisted in-flight engine restart, the applicant should determine the minimum APU start reliability. The applicant should also perform flight tests to evaluate APU operation within critical portions of the flight envelope. The applicant should conduct APU start reliability testing in flight following cold-soak cruise conditions; a maximum of two relight attempts are allowed.
23.2.4.2.7 If batteries are required to provide power for an assisted in-flight engine restart, the applicant should determine the minimum capacity and reliability. The applicant should also perform flight tests to evaluate battery operation within critical portions of the flight envelope. The applicant should include cold-soak cruise conditions when determining critical points.
23.2.4.2.8 The applicant should evaluate in-flight engine restart capability following loss of normal electrical power. (See § 25.1351(d).) The applicant should demonstrate suction feed capability for airplanes that only supply fuel pressure to engines using normal electrical power (electrical power sources excluding the battery). This condition can result in vapor formation in the fuel system that may inhibit restart. In one airplane design, the heat soak from the engine following flameout caused vapor lock in the engine fuel feed system, resulting in the inability to restart either engine. The applicant should conduct the evaluation at the maximum operating altitude with the critical fuel (type and temperature) from the standpoint of flameout and restart characteristics. An all-engines-out condition occurring at a lower altitude (e.g., just above the maximum altitude where fuel suction feed capability exists) may possibly
23-7 result in a lower recovery altitude than if the all-engines-out condition occurred at a higher altitude. If possible, then the applicant should also demonstrate performance under that condition.
23.2.4.2.9 Flight test procedures to address all-engines-out conditions must be agreed to by the FAA.
23.2.4.2.10 Procedures to evaluate an installed engine’s susceptibility to an engine rotor lock or rotor drag condition are found in FAA policy statement PS-ANM-25-02, Guidance for Screening for Engine Rotor Lock in Transport Category Airplanes during Aircraft Certification.
23.3 Automatic Takeoff Thrust Control System—§ 25.904.
23.3.1 Explanation.
23.3.1.1 Beginning in the 1970s, some manufacturers of turbojet powered airplanes elected to equip their airplanes with engine thrust control systems that automatically increased the thrust on the operating engine(s) when any engine failed. A similar system, referred to as an automatic takeoff thrust control system (ATTCS), was subsequently installed on turbopropeller airplanes.
23.3.1.2 Takeoff performance credit was granted for ATTCS based upon prescribed system functional and reliability requirements, and performance related restrictions (e.g., initial takeoff power or thrust not less than 90 percent of that set by the ATTCS).
23.3.1.3 These systems represented “novel or unusual design features” not adequately addressed by the requirements of part 25 at the time.
Consequently, the airworthiness requirements for the early ATTCS certifications were prescribed in special conditions in accordance with § 21.16. The regulatory and technical content of those special conditions was added to § 25.904 and appendix I to part 25, at amendment 25-62.
23.3.2 Procedures.
Certification of an airplane with an ATTCS requires flight test demonstration of certain performance and functional aspects of the system, as outlined below:
23.3.2.1 In order to comply with the part 25 airplane performance requirements, as required by section I25.3(c) of appendix I to part 25, takeoff speeds, as limited by VMCG and VMCA, must reflect the effect of ATTCS operation following failure of the critical engine. It is permissible to publish two sets of takeoff performance data: one for ATTCS unarmed and one for ATTCS armed. In such cases, the AFM limitations, operating procedures, and performance information should clearly differentiate between the two sets of performance data.
23-8
23.3.2.2 Engine operating characteristics should be investigated during operation of the ATTCS, and with the operating engines at the steady-state maximum power or thrust level achieved following operation of the ATTCS. (See the guidance material provided in paragraph 23.11 of this AC.)
23.3.2.3 Manual override of the ATTCS should be verified in flight test. This capability has been provided by the ability of the pilot to push the throttle levers to a higher power or thrust setting, for airplanes that use less than “full-throttle” for takeoff, and by activating an override switch for “firewall-type” fuel control systems. The override switch must be located on or forward of the power or thrust levers, and it must be easily accessible to the pilot’s hand that normally controls the power lever or thrust position in accordance with section I25.5(b)(2) of appendix I. It should also be demonstrated that the thrust/power level can be manually decreased at any time following ATTCS operation.
23.3.2.4 A critical time period must be determined during which the probability of concurrent engine and ATTCS failure must be shown to be extremely improbable (not greater than 10-9 per flight hour). This critical time period is defined in appendix I to part 25 as being from one second before the airplane attains V1, to a time where the actual takeoff flight path (i.e., no gradient reductions), following a simultaneous engine and ATTCS failure, would intersect the normal (i.e., engine failure at VEF and no ATTCS) one-engine-inoperative actual takeoff flight path at no less than 400 feet above the takeoff surface. The probability of failure of the ATTCS, itself, must be shown to be improbable (not greater than 10-5 per flight hour).
23.3.2.5 Performance credit for an operating ATTCS is not to be taken when operations are conducted using reduced takeoff power or thrust methods.
If the ATTCS is armed during reduced power or thrust takeoffs, the relevant takeoff speeds should meet the required controllability criteria of part 25 at the power or thrust level provided by operation of the ATTCS.
The applicant should demonstrate that the airplane has no adverse handling characteristics and the engine(s) do not exhibit adverse operating characteristics or exceed operating limits when the ATTCS operates.
23.3.2.5.1 In accordance with § 25.1585, the AFM must contain information, instructions, and procedures, as required, regarding the peculiarities of normal and abnormal operations when scheduling reduced power or thrust operations with an armed ATTCS.
23.3.2.5.2 Takeoff with an armed ATTCS is not restricted when airplane performance is based on an approved “derate” power or thrust rating that has corresponding airplane and engine limitations approved for use under all WAT conditions.
23-9
23.4 Propellers—§ 25.905. [Reserved]
23.5 Propeller Vibration and Fatigue—§ 25.907. [Reserved]
23.6 Propeller Clearance—§ 25.925. [Reserved]
23.7 Propeller Deicing—§ 25.929.
23.7.1 Explanation.
None.
23.7.2 Procedures.
If the propellers are equipped with fluid type deicers, the flow test should be conducted starting with a full tank of fluid and operated at maximum flow rate for a time period (15 minutes) found operationally suitable. The operation should be checked at all engine speeds and powers. The tank should be refilled to determine the amount of fluid used after the airplane has landed.
23.8 Reversing Systems—§ 25.933.
23.8.1 Turbojet Reversing Systems—§ 25.933(a).
23.8.1.1 Explanation.
23.8.1.1.1 For reversers intended to be operable on the ground only, it must be shown that the airplane can be safely landed and stopped with a critical engine reverser deployed. In addition, if an undamaged reverser inadvertently becomes deployed in flight, it must be shown that it can be safely restored to a forward power or thrust position.
23.8.1.1.2 For turbojet reversing systems intended for ground and/or inflight use, it must be shown that unwanted deployment of a critical reverser under normal operating conditions will not prevent continued safe flight and landing. Flight tests may be required to obtain aerodynamic data with the critical reverser deployed, to confirm that its deployment in the normal operating envelope will not be catastrophic to the airplane.
23.8.1.2 Procedures.
23.8.1.2.1 Turbine engine thrust reversers may be approved provided the following basic criteria are met:
1. Exceptional piloting skill is not required in taxiing, or in any condition in which reverse thrust is to be used.
23-10
2. Necessary operating procedures, operating limitations, and placards are established.
3. The airplane control characteristics are satisfactory with regard to control forces encountered.
4. The directional control is adequate using normal piloting skill. This is of particular importance for airplanes with aft-mounted engines, which may experience a loss of rudder effectiveness with reverse thrust. This may result in reductions in maximum reverse thrust levels (i.e., lower EPR or N1 settings) and/or increases in reverse thrust minimum operational speeds. The stabilizing effect of the nose gear should also be investigated, particularly for the one-engine-inoperative use of reverse thrust on wet runway surfaces.
5. A determination is made that no dangerous condition is encountered in the event of the sudden failure of one engine in any likely reverse thrust operating condition throughout the airplane’s approved operating envelope.
6. The operating procedures and airplane configuration are such as to provide reasonable safeguards against engine foreign object ingestion and serious structural damage to parts of the airplane due to the reverse airflow. This is normally accomplished by specifying in the limitations section of the AFM a minimum airspeed at which the thrust reversers must be retracted.
7. It is determined that the pilot’s vision is not dangerously obscured under normal operating conditions on dusty or wet runways and where light snow is on the runway.
8. For seaplanes, it is determined that the pilot’s vision is not dangerously obscured by spray due to reverse airflow under normal water operating conditions.
9. The procedure and mechanisms for reversing should provide a reverse idle setting such that, without requiring exceptional piloting skill, at least the following conditions are met:
a. Sufficient power or thrust is maintained to keep the engine running at an adequate speed to prevent engine stalling during and after the reversing operation.
b. The engine does not overspeed or stall during and after the reversing operation.
c. The engine cooling characteristics should be satisfactory in any likely operating condition.
10. For airplanes equipped with thrust reversers intended for inflight use, the effect of non-deployment of a reverser (i.e., asymmetric deployment) on airplane controllability should be investigated.
23-11
23.8.1.2.2 For the failed reverser demonstration tests, the following criteria are provided:
1. The landing with a reverser deployed should be conducted with a flap setting and an airspeed such that a landing can be accomplished safely and consistently. The conditions and operating procedures to use when the landing is made with the deployed reverser must be defined and incorporated in the AFM per § 25.1585(a)(2) or (a)(3), whichever is applicable.
2. The restow test should be conducted at a reasonable and safe altitude and at an airspeed where the airplane can be safely controlled (approximately 200 knots). A procedure should be developed so that the reverser can be restowed (if undamaged) safely, without causing unacceptable airplane controllability problems. The restowing procedure, airspeed, and airplane flight controls configuration should be provided in the AFM.
23.8.1.2.3 Aircraft Backing Using Reverse Thrust.
Limited operational approvals have been granted for the use of thrust reverser systems to back away from terminal gates in lieu of a tug pushback. These approvals are granted by the cognizant FAA Flight Standards office for each operator, the approval consisting of an amendment to the operator’s operations specifications identifying the airplane type, the airport, and the specific gates at that airport at which reverse thrust backing may be used. Though reverse thrust backing is not specifically an airworthiness approval item, there are certain areas of concern that overlap airworthiness and operations. (These items are addressed in paragraph 42.12 of this AC.)
23.8.2 Propeller Reversing Systems—§ 25.933(b).
23.8.2.1 Explanation.
None.
23.8.2.2 Procedures.
Reverse thrust propeller installations may be approved, provided the following is acceptable:
23.8.2.2.1 A reliable means for preventing the in-flight selection of a power setting below flight idle is provided.
23.8.2.2.2 Exceptional piloting skill is not required in taxiing or in any condition in which reverse thrust is to be used.
23.8.2.2.3 Necessary operating procedures, operating limitations, and placards are established.