174 with Cargo and Baggage Compartments, § 25.855; Exhaust Systems, § 25.1121; and Fire Extinguishing Agents, § 25.1197. AC 25-20, “Pressurization, Ventilation, and Oxygen Systems Assessment for Subsonic Flight Including High Altitude Operations,” dated September 10, 1996, also provides guidance for ventilation requirements.
(2) The objective of the inflight smoke evacuation test is to confirm that the flightcrew emergency procedures and the ventilation system are capable of handling heavy smoke, and to show that when using the emergency procedures, the smoke will dissipate at a reasonable rate.
This is a quantitative and qualitative evaluation.
b. Procedures .
(1) Flight testing should be conducted to ensure the amount of ventilation air provided meets the requirements specified and the flightcrew is able to accomplish their duties without undue fatigue and discomfort. Ventilation system controls in the flight deck should be demonstrated to perform their intended function.
(2) The passenger and crew compartment should be monitored for the presence of carbon monoxide. Various flight and equipment configurations should be tested. A carbon monoxide test kit is normally used for this evaluation.
(3) Inflight smoke evacuation testing should be conducted in accordance with AC 25- 9A, “Smoke Detection, Penetration, and Evacuation Tests and Related Flight Manual Emergency Procedures,” dated January 6, 1994.
85. Cabin Ozone Concentration - § 25.832 [Reserved]
86. Combustion Heating Systems - § 25.833 [Reserved]
Section 9. Pressurization
87. Pressurized Cabins - § 25.841.
a. Explanation .
(1) Section 25.841(a) specifies cabin pressure altitude limits, as a function of the external pressure altitude, for cabins and compartments intended to be occupied. AC 25-20 provides additional guidance for pressurized cabins. These cabin pressure altitude limits, to be demonstrated by flight testing, are:
(a) Not more than 8,000 ft., at the maximum operating altitude of the airplane, for normal operation of the pressurization system.
(b) For airplanes to be operated above 25,000 ft., the airplane must be able to maintain a cabin pressure altitude of not more than 15,000 ft. in the event of any reasonably probable failure or malfunction in the pressurization system.
(2) Though not addressed by § 25.841, airplanes may incorporate a “high altitude mode” to permit takeoff and landing at airports above 8,000 ft. For takeoff and landing altitudes above 8,000 ft., the cabin pressure limits to be demonstrated by flight testing are:
(a) Prior to beginning a descent into a high altitude airport, not more than 8,000 ft.
(b) During a descent into or climb out of a high altitude airport, not more than 15,000 ft.
(c) After a takeoff from a high altitude airport and after the cabin pressure altitude decreases to 8,000 ft., not more than 8,000 ft.
(3) Section 25.841(b)(6) requires an aural or visual warning to the flightcrew when the cabin pressure altitude exceeds 10,000 ft. For airplanes incorporating a high altitude mode that resets the cabin pressure altitude warning threshold, the reset warning activation level should not exceed 15,000 ft.
b. Procedures . It is recommended that the test airplane have the maximum allowable leakage rate permitted by the type design specifications. If an airplane does not meet this criteria, it will be necessary to substantiate compliance by additional testing or analysis for the maximum leakage rate allowed by the type design.
(1) Normal Operating Conditions - 8,000 Ft. Cabin Pressure Altitude .
(a) With the pressurization system operating in its normal mode, the airplane should be stabilized in the cruise configuration at the maximum operating altitude it is to be certificated for.
(b) The condition should be held long enough to record any cyclic fluctuations in cabin pressure due to relief valve operation.
(c) The cabin pressure altitude should not exceed 8,000 ft.
(2) Failure Conditions - 15,000 Ft. Cabin Pressure Altitude .
(a) The critical reasonably probable system failure should be identified. The cabin pressure altitude warning system should be set to the high altitude side of its tolerance band or additional testing or analysis may be necessary for compliance. If more than one system failure mode is determined to meet the “reasonably probable” criteria, the flight test should be conducted for each failure mode identified.
(b) The airplane should be stabilized in the cruise configuration at the maximum operating altitude it is to be certificated to, with the pressurization system operating in its normal mode.
(c) After initiating the critical failure to allow the cabin pressure altitude to increase, the flight test crew should immediately don their oxygen masks, but take no further corrective action until 17 seconds after the 10,000 ft. cabin pressure altitude warning activates.
Emergency descent procedures should then be initiated and the descent continued to an altitude below 15,000 ft.
(d) The cabin pressure altitude should not exceed 15,000 ft. at any time during the test.
(3) High Altitude Takeoff Conditions (Greater than 8,000 Ft.) .
(a) The pressurization system should be placed in its high altitude mode. It may be necessary to configure the oxygen system to prevent deployment of the oxygen masks at 10,000 ft., if the system is so designed. AFM procedures may need to be developed. The same applies to high altitude landings above 10,000 ft.
(b) The cabin pressure altitude should not exceed 15,000 ft. during an actual climb-out from a high altitude airport to the maximum operating altitude the airplane is to be certificated for, or during an inflight simulation of such a climb-out. The simulation should commence the climb, from a starting altitude of 8,000 ft. or more, with the airplane unpressurized.
(c) The cabin pressure altitude should eventually decrease to 8,000 ft. and then not exceed 8,000 ft. for the duration of the test.
(4) High Altitude Landing Conditions (Greater than 8,000 Ft) .
(a) The airplane should be stabilized in the cruise configuration prior to beginning a descent.
(b) The pressurization system should be placed in its high altitude mode.
(c) The cabin pressure altitude should not exceed 15,000 ft. during the descent to an actual, or simulated, landing at a high altitude airport.
88. Tests For Pressurized Cabins - § 25.843.
a. Explanation .
(1) Section 25.843(b)(3) requires flight testing to evaluate the performance of the pressurization system and all related sub-systems at maximum altitude and under the dynamic conditions of climbing and descending flight. This testing substantiates the ability of the pressurization system to function correctly in stable and dynamic external pressure conditions.
(2) Section 25.843(b)(4) requires an investigation of the functionality of all doors and emergency exits after the flight test required by § 25.843(b)(3). The concern is the potential for jamming caused by the variable relative positions of doors and fuselage structure during the pressurization/depressurization cycle.
b. Procedures . The following tests may be initiated from an airport at any altitude within the airplane’s proposed takeoff limitations. The pressurization system should be operated in its normal mode. MMEL dispatch may be taken into consideration and additional testing required to verify MMEL configuration.
(1) Steady Climb/Descent . The steady climb/descent pressurization system tests should be performed under conditions (i.e., weight, altitude, temperature, and configuration) that will result in rates of climb/descent corresponding to the maximum attainable within the operating limitations of the airplane.
(a) After takeoff, maintain a stable, continuous climb to the maximum operating altitude the airplane will be certificated to.
(b) Maintain that altitude until the cabin pressure altitude has stabilized.
(c) Initiate a steady, maximum rate of descent within the operating limitations of the airplane down to the airport.
(2) Stepped Climb/Descent . The stepped climb/descent pressurization system tests should be performed under conditions (i.e., weight, altitude, temperature, and configuration) that will result in rates of climb/descent corresponding to the maximum attainable within the operating limitations of the airplane.
(a) After takeoff, initiate a stepped climb to the maximum operating altitude the airplane will be certificated to. Step increments should be 5,000 to 7,500 ft.
(b) Maintain each level-off altitude long enough for the cabin pressure altitude to stabilize.
(c) Initiate a stepped descent from the maximum altitude allowing the cabin pressure altitude to stabilize at each level-off altitude. Step increments should be 7,500 to 10,000 ft.
(3) Positive Pressure Relief. If two valves are provided, one should be deactivated for this test.
(a) After takeoff, climb to the operating altitude that provides maximum cabin differential pressure.
(b) Manually close the outflow valve, allowing the cabin differential pressure to increase.
(c) Verify the cabin pressure differential pressure warning functions properly.
(d) Verify that the relief valve functions and the maximum cabin differential pressure is not exceeded.
(4) Negative Pressure Relief/Emergency Descent. If two valves are provided, deactivate one for this test.
(a) At cruise altitude, perform an emergency descent with the airplane in the critical condition for negative pressure on the fuselage.
(b) Verify that the maximum negative differential pressure for the fuselage is not exceeded.
(5) Manual Cabin Pressure Control. If manual means for pressure control are provided, these means should be evaluated under normal and emergency operations of the airplane and flight envelope.
(6) Testing of Doors and Exits.
(a) Prior to flight, all doors and exits should be checked for proper operation.
(b) After returning from a flight where the airplane is subjected to the maximum certificated altitude and cabin pressure differential pressure, and immediately upon landing, all passenger doors and emergency exits should be opened. There should be no change in the operating characteristics of any door or emergency exit relative to its pre-flight operation.
Section 10. Fire Protection [Reserved]
Section 11. Miscellaneous [Reserved]
Chapter 5 - Powerplant
Section 1. General
89. Installation - § 25.901. [Reserved]
90. Engines - § 25.903.
a. Engine Isolation - § 25.903(b) .
(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:
(a) Prevent the continued safe operation of the remaining engines; or
(b) Require immediate action by any crewmember for continued safe operation.
(2) Procedures .
(a) 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).
(b) 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.
b. Control of Engine Rotation - § 25.903(c) .
(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 shut down turbine engine will not cause:
(a) Powerplant (including engine and accessories) structural damage that will adversely affect other engines or the airplane structure;
(b) Flammable fluids to be pumped into a fire or into an ignition source; or
(c) A vibration mode that will adversely affect the aerodynamic or structural integrity of the airplane.
(2) Procedures . None.
c. Turbine Engine Installations - § 25.903(d) .
(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.
(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:
(a) When one lower thrust engine is installed, the normal AFM performance level is reduced by an increment appropriate to the decrease in thrust resulting from the intermix.
(b) When more than one lower thrust engine is installed, the AFM performance should be based on the thrust of the lower/lowest rated engine.
(c) The minimum control speeds (V MCG, VMCA & VMCL) should be based on the highest thrust engine(s).
(d) 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.
(e) A maximum of two takeoff 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.
(f) 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 part 121 of the CFR.
d. Engine Restart Capability - § 25.903 (e) .
(1) Explanation .
(a) Approval of Engine Restart Capability . An altitude and airspeed envelope must be established by the applicant for in-flight engine restarting, and means provided to restart each engine in flight within this envelope.
(b) Engine Restart Envelope . For turbojet and early turbofan engines with low bypass ratios (approximately one), inflight restart capability was generally provided by “windmilling” of the engine. The windmilling airflow through these engines was capable of providing enough rotational energy to permit inflight restarting throughout most of the approved airspeed-altitude operating envelope. From the experience gained with these first generation engines, acceptable restart capability has been established as 30 seconds from “FUEL ON” to ignition, and 90 seconds from “IGNITION” to stabilized idle. The 90 second ignition to idle time upper limit is based on the ability of the flightcrew to perceive the progression of the engine start by monitoring the time rate-of-change of N2 and exhaust gas temperature. As bypass ratios increased, and rotational energy requirements increased, the “windmilling” restart envelopes became smaller. Some higher bypass ratio engines require an assist from an auxiliary pneumatic source, such as bleed air from an operating engine or an inflight operable auxiliary power unit (APU), to provide restart capability over a large portion of the airplane operating envelope.
Turboprop engines, particularly those with free-turbines, also require some sort of “auxiliary” start assist; this is generally provided by an electric starter or, in some cases, an inflight operable APU. Engine restart envelopes should clearly distinguish between the areas of different restart capabilities in terms of time to restart, windmilling only restart, and auxiliary air assisted restart.
If an APU is required for restart over a significant portion of the operating envelope, a separate inflight start envelope should be substantiated by flight test for the APU in the “cold-soaked” condition associated with medium to high altitude cruise conditions. Engine and APU (if applicable) restart envelopes, and related procedures, should be included in the limitations section of the AFM.
(c) All-Engine Restart Capability . As technological advancements increased specific engine thrust output with a simultaneous decrease in specific fuel consumption, and wing designs produced greater lift, the number of engines required for a given airplane weight has decreased to the point where the majority of modern transport category airplanes are equipped with just two engines. Service history has recorded a number of inflight all-engine thrust loss incidents from various causes: flameouts from inclement weather conditions, volcanic ash ingestion, fuel nozzle coking, fuel contamination and several cases of flightcrew fuel mismanagement. These incidents have occurred during takeoff climbout and in high altitude cruise. Consequently, it should be substantiated by flight test that restart capability exists following the inflight shutdown of all engines. This should be demonstrated for the momentary shutdown from a high power setting, associated with the takeoff and climb flight
regimes, and the extended shutdown at medium to high altitude where the engines may be “windmilling” and/or “cold soaked.”
(2) Procedures .
(a) Tests should be conducted to determine that inflight restarting can be accomplished within the envelope provided. Restarts at the conditions of the critical corners of the envelope and at or near the high altitude extremes of the envelope should be conducted to verify the boundary conditions of the envelope.
(b) Inflight engine restart capability should also be evaluated with regard to suction feed climb conditions at the minimum suction feed relight altitude, and loss of normal electrical power (Ref. § 25.1351(d)).
(c) The engine operating characteristics should be evaluated during each restart.
In particular, any tendencies for the engine to surge, dwell for abnormally long periods below idle, or produce other unusual vibration or audible noises should be noted.
(d) To address the dual engine flameout case during takeoff and initial climb-out, with the engines initially at maximum climb thrust (or higher), it should be demonstrated that the test engine will accelerate to the initial power setting after a brief shutdown (up to 15 seconds), when the fuel source is restored.
(e) To address dual engine flameout at high altitude (greater than 20,000 feet), it should be shown, by a combination of flight test and analysis, that the engines can be restarted, and the airplane’s descent arrested, with a total altitude loss of not more than 5000 feet.
Additionally, engine restart should be accomplished prior to exceeding 300 knots indicated air speed (IAS).
(f) If an onboard APU is required to provide power assist for inflight engine restart, the minimum APU start reliability should be determined, and its operation evaluated by flight tests in critical portions of the flight envelope.
(g) If start cartridges are used to provide restart capability, they should be of sufficient capacity (or number) to provide at least two start attempts for each engine.
(h) The inflight engine restart envelope presented in the AFM should identify a “core windmill relight envelope” that will provide engine ignition within 30 seconds and stabilized idle within 90 seconds of restart initiation. A larger envelope, which includes appropriately labeled longer restart times, may be allowed if it can be shown that indication of a clear progression of engine start is provided to the flightcrew. The AFM should also contain the restart procedures appropriate to each phase of flight (i.e. low altitude “hot start,” high altitude “cold soaked” start, etc.).
91. Automatic Takeoff Thrust Control System (ATTCS) - § 25.904.
a. Explanation .
(1) Beginning in the 1970’s, 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, was subsequently installed on turbopropeller airplanes.
(2) Takeoff performance credit was granted for ATTCS based upon prescribed system functional and reliability requirements, and performance related restrictions (e.g., initial takeoff thrust not less than 90 percent of that set by the ATTCS).
(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 part 25 as § 25.904 and appendix I by amendment 25-62.
b. Procedures . Certification of an airplane with an ATTCS requires flight test demonstration of certain performance and functional aspects of the system, as outlined below:
(1) In order to comply with the part 25 airplane performance requirements, as required by Appendix I, I25.3(c), 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.
(2) Engine operating characteristics should be investigated during operation of the ATTCS, and with the operating engines at the steady-state maximum thrust level achieved following operation of the ATTCS. (Refer to the guidance material provided in paragraph 99 of this AC.)
(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 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 levers, and it must be easily accessible to the pilot’s hand that normally controls the power lever position in accordance with Appendix I, I25.5(b)(2). It should also be demonstrated that the thrust/power level can be manually decreased at any time following ATTCS operation.
(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 automatic takeoff thrust control system, itself, must be shown to be improbable (not greater than 10-5 per flight hour).
(5) Performance credit for an operating ATTCS is not to be taken when operations are conducted using reduced takeoff thrust methods. If the ATTCS is armed during reduced thrust takeoffs, the relevant takeoff speeds should meet the required controllability criteria of part 25 at the 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.
(a) 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 thrust operations with an armed ATTCS.
(b) Takeoff with an armed ATTCS is not restricted when airplane performance is based on an approved “derate” thrust rating that has corresponding airplane and engine limitations approved for use under all weight, altitude, and temperature (WAT) conditions.
92. Propellers - § 25.905.
a. Explanation . When an automatic control system for simultaneous RPM control of all propellers is installed, it should be shown 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.
(2) Cause a loss of thrust 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.
b. Procedures . Compliance with this policy may be shown by analysis, flight demonstration, or a combination of analysis and flight demonstration.
93. Propeller Vibration and Fatigue - 25.907 [Reserved]
94. Propeller Clearance - § 25.925 [Reserved]
95. Propeller Deicing - § 25.929.
a. Explanation . None.
b. 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 min.) 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.
96. Reversing Systems - § 25.933.
a. Turbojet Reversing Systems - § 25.933(a) .
(1) Explanation .
(a) 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 thrust position.
(b) 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.
(2) Procedures .
(a) 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.
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