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.
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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 23-5
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 23-6
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.
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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.
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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 require d in taxiing, or in any condition in which reverse thrust is to be used.
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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.
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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.
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23.8.2.2.4 The airplane control characteristics are satisfactory with regard to control forces encountered.
23.8.2.2.5 The directional control is adequate using normal piloting skill.
23.8.2.2.6 A determination is made that no dangerous condition is encountered in the event of sudden failure of one engine in any likely operating condition.
23.8.2.2.7 The operating procedures and airplane configuration are such as to provide reasonable safeguards against serious structural damage to parts of the airplane due to the direct effects of the reverse airflow or any resultant buffeting.
23.8.2.2.8 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.
23.8.2.2.9 It is determined that the pilot’s vision is not dangerously obscured by spray due to reverse airflow under normal water operating conditions with seaplanes.
23.8.2.2.10 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:
1. Sufficient power is maintained to keep the engine running at an adequate speed to prevent engine stalling during and after the propeller reversing operation.
2. The propeller does not exceed the approved speed limit of 14 CFR part 35 or the airplane-manufacturer-declared propeller speed limit during and after the propeller reversing operation.
3. This idle setting does not exceed 25 percent of the maximum continuous rating.
23.8.2.2.11 The engine cooling characteristics should be satisfactory in any likely operating condition.
23.9 Turbojet Engine Thrust Reverser System Tests—§ 25.934. [Reserved]
23.10 Turbopropeller-Drag Limiting Systems—§ 25.937.
23.10.1 Explanation: Approval of Automatic Propeller Feathering Systems.
All parts of the feathering device that are integral with the propeller, or attached to it in a manner that may affect propeller airworthiness, should be considered from the standpoint of the applicable provisions of part 35. The determination of the continuing 23-12
eligibility of the propeller under the existing type certificate, when the device is installed or attached, will be made on the following basis:
23.10.1.1 The automatic propeller feathering system should not adversely affect normal propeller operation and should function properly under all temperature, altitude, airspeed, vibration, acceleration, and other conditions to be expected in normal ground and flight operation.
23.10.1.2 The automatic device should be demonstrated to be free from malfunctioning that may cause feathering under any conditions other than those under which it is intended to operate. For example, it should not cause feathering during:
Momentary loss of power or thrust, or Approaches with reduced throttle settings.
23.10.1.3 The automatic propeller feathering system should be capable of operating in its intended manner whenever the throttle control is in the normal position to provide takeoff power or thrust. No special operations at the time of engine failure should be necessary on the part of the crew in order to make the automatic feathering system operative. (See also §§ 25.101,
25.111, 25.121, and 25.1501.)
23.10.1.4 The automatic propeller feathering system installation should be such that not more than one engine will be feathered automatically, even if more than one engine fails simultaneously.
23.10.1.5 The automatic propeller feathering system installation should be such that normal operation may be regained after the propeller has begun to feather automatically.
23.10.1.6 The automatic propeller feathering system installation should incorporate a switch, or equivalent means, by which to make the system inoperative.
23.10.2 Procedures: Propeller Feathering System Operational Tests.
23.10.2.1 Tests should be conducted to determine the time required for the propeller to change from windmilling (with the propeller controls set for takeoff) to the feathered position at the takeoff safety speed, V2.
23.10.2.2 The propeller feathering system should be tested to demonstrate non-rotation or as a minimum, non-hazardous rotation at up to 1.2 times the maximum level flight speed, with one engine inoperative or the speed employed in emergency descents, whichever is higher, with:
The critical engine inoperative, Wing flaps retracted, 23-13
Landing gear retracted, and Cowl flaps (if applicable) closed.
23.10.2.3 A sufficient speed range should be covered to assure that the propeller feathering angle, established on the basis of the high-speed requirement, will not permit hazardous reverse rotation at the lower speeds. In addition, the propeller should not inadvertently unfeather during these tests.
23.10.2.4 In order to demonstrate that the feathering system operates satisfactorily, the propeller should be feathered and unfeathered at the maximum operating altitude established in accordance with § 25.1527. The following data should be recorded:
Time to feather the propeller at the one-engine-inoperative cruising speed, Time to unfeather the propeller to the minimum declared governing speed at maximum operating altitude (note that some driftdown may occur) and one-engine-inoperative cruising speed, Altitude of propeller feathering tests, and Ambient air temperature of propeller feathering tests.
23.10.2.5 In order to demonstrate that the feathering system operates satisfactorily, the propeller should be feathered at the condition within the airplane operating envelope that is critical for the propeller.
23.11 Turbine Engine Operating Characteristics—§ 25.939.
23.11.1 Explanation.
The turbine engines of a transport category airplane must continue to operate safely during normal and emergency operation within the range of operating limitations of the airplane. Generally, compliance with § 25.939(a) can be determined to some extent while ascertaining compliance with other part 25 requirements, such as performance, controllability, maneuverability, and stall speed characteristics. Turbine engines should be stable in their operation and run free of adverse characteristics throughout the normal flight envelope. Certain adverse characteristics are allowed in specific flight regimes if they do not present a hazardous condition.
23.11.2 Reference.
See AC 25.939-1 for comprehensive guidance in the evaluation of turbine engine operating characteristics to show compliance with the requirements of § 25.939(a), only. The referenced AC does not provide guidance for compliance with § 25.939(c), which addresses inlet compatibility.
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23.11.3 Inlet Compatibility—§ 25.939(c).
23.11.3.1 Explanation.
Section 25.939(c) requires substantiation that the engine inlet systems on turbine engines not cause harmful vibration to the engine as a result of airflow distortion. This should be verified in both static and transient power or thrust conditions.
23.11.3.2 Procedures.
Compliance with § 25.939(c) may require special instrumentation of the inlet itself, or the engine’s most critical component (i.e., fan blades). Inlet rakes permit the applicant to verify that the installed airflow distortion patterns are within the limits established by the engine manufacturer. In addition, accelerometer and/or strain gauge data could be acquired in flight tests to verify that vibration and stress level limits are not exceeded during operation in the normal flight envelope.
23.12 Inlet, Engine, and Exhaust Compatibility—§ 25.941. [Reserved]
23.13 Negative Acceleration—§ 25.943.
23.13.1 Explanation.
Section 25.943 requires that no hazardous malfunction of an engine, APU, or any component or system associated with their operation, should result from airplane operation at negative accelerations. A hazardous malfunction in this case is considered to be one that causes loss or sustained malfunction of the engine or APU, or improper operation of the engine accessories or systems. This requirement can be satisfied by flight test demonstrations that take into consideration the critical airplane, engine, and APU configurations. The range of negative accelerations to be tested is prescribed by the flight envelope for the airplane, as defined in § 25.333. The duration of the negative acceleration excursions is intended to represent anticipated non-normal operational events such as atmospheric upsets, collision avoidance maneuvers, etc.
23.13.2 Procedures.
23.13.2.1 In conducting negative acceleration tests, consideration should be given to engine accessory configurations, and critical levels of fuel and oil.
23.13.2.2 Accelerations should be measured as close as practicable to the airplane’s CG position.
23.13.2.3 With the engines operating at maximum continuous power or thrust, and the APU operating with normal load (if flight operable), the airplane should be flown at a critical negative acceleration within the flight envelope. The duration of each test condition should be a minimum of 23-15