Section 6. PERSONNEL AND CARGO ACCOMMODATIONS
60. - 61. [RESERVED]
62. PILOT COMPARTMENT VIEW - § 25.773 .
a. Explanation. (Reserved.)
b. Procedures . For detailed guidance on complying with pilot compartment view requirements, refer to Advisory Circular 25.773-1, "Pilot Compartment View for Transport Category Airplanes," dated January 8, 1993.
63. - 72. [RESERVED]
Section 7. EMERGENCY PROVISIONS
73. [RESERVED]
74. EMERGENCY EVACUATION - § 25.810(c) .
a. Explanation.
(1) Installation of slip-resistant escape route. (See § 25.810(c), formerly § 25.803(e) See Advisory Circular 25-17, “Transport Airplane Cabin Interiors Crashworthiness Handbook,” § 25.803(e), for guidance regarding slip resistant material.
(2) The effect of the slip-resistant surfaces on airplane performance, flight characteristics, and buffet should be evaluated and should, if necessary, account for significant drag increments.
b. Procedures . None.
75. - 83. [RESERVED]
Section 8. VENTILATION AND HEATING
84. VENTILATION - § 25.831 .
a. Explanation.
(1) This requirement deals with minimum ventilation requirements for each occupant of the airplane, control of the ventilating air, accumulation and evacuation of smoke and harmful or hazardous concentrations of gases or vapors, and failure conditions of the ventilation system. Specific quantities of fresh air along with carbon monoxide and carbon dioxide concentration limits are specified in the rule. Reference should be made to the paragraphs in this AC dealing 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,” 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 ensu re 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 Advisory Circular 25-9A, "Smoke Detection, Penetration, and Evacuation Tests and Related Flight Manual Emergency Procedures," dated January 6, 1994.
85. - 86. [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, “Pressurization, Ventilation, and Oxygen Systems Assessment for Subsonic Flight Including High Altitude Operation,” dated 9/10/96, provides additional guidance for pressurized cabins.
These cabin pressure altitude limits, to be demonstrated by flight testing, are:
(i) Not more than 8,000 ft., at the maximum operating altitude of the airplane, for normal operation of the pressurization system.
(ii) 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:
(i) Prior to beginning a descent into a high altitude airport, not more than 8,000 ft.
(ii) During a descent into or climb out of a high altitude airport, not more than 15,000 ft.
(iii) 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.
(i) 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.
(ii) The condition should be held long enough to record any cyclic fluctuations in cabin pressure due to relief valve operation.
(iii) The cabin pressure altitude should not exceed 8,000 ft.
(2) Failure Conditions - 15,000 Ft. Cabin Pressure Altitude.
(i) 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.
(ii) 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.
(iii) 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.
(iv) 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.).
(i) 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. Airplane Flight Manual (AFM) procedures may need to be developed. The same applies to high altitude landings above 10,000 ft.
(ii) 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.
(iii) 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).
(i) The airplane should be stabilized in the cruise configuration prior to beginning a descent.
(ii) The pressurization system should be placed in its high altitude mode.
(iii) 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. The Master Minimum Equipment List (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.
(i) After takeoff, maintain a stable, continuous climb to the maximum operating altitude the airplane will be certificated to.
(ii) Maintain that altitude until the cabin pressure altitude has stabilized.
(iii) 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.
(i) 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.
(ii) Maintain each level-off altitude long enough for the cabin pressure altitude to stabilize.
(iii) 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.
(i) After takeoff, climb to the operating altitude that provides maximum cabin differential pressure.
(ii) Manually close the outflow valve, allowing the cabin differential pressure to increase.
(iii) Verify the cabin pressure differential pressure warning functions properly.
(iv) 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.
(i) At cruise altitude, perform an emergency descent with the airplane in the critical condition for negative pressure on the fuselage.
(ii) 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.
(i) Prior to flight, all doors and exits should be checked for proper operation.
(ii) 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:
(i) Prevent the continued safe operati on of the remaining engines; or
(ii) Require immediate action by any crewmember for continued safe operation.
(2) Procedures .
(i) 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).
(ii) 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:
(i) Powerplant (including engine and accessories) structural damage that will adversely affect other engines or the airplane structure;
(ii) Flammable fluids to be pumped into a fi re or into an ignition source; or
(iii) 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:
(i) When one lower thrust engine is installed, the normal Airplane Flight Manual (AFM) performance level is reduced by an increment appropriate to the decrease in thrust resulting from the intermix.
(ii) When more than one lower thrust engine is installed, the AFM performance should be based on the thrust of the lower/lowest rated engine.
(iii) The minimum control speeds (V MCG, VMCA & VMCL) must be based on the highest thrust engine(s).
(iv) The operating procedures must be provided for all engines installed (i.e., airstart altitude/airspeed envelopes, crew responses to engine warning systems, etc.). Differences in operating methods shall be limited to the equivalent of having a maximum of two different engines on the airplane.
(v) A maximum of two takeoff thrust settings (EPR or N 1) are permitted for airplanes with intermixed engines; this includes differences related to air conditioning pack and compressor bleed configurations.
(vi) A placard, identifying the location of the non-standard engine type, must be installed. All engine limits and instrument markings should be appropriate to the engine installed at each location. EPR, N 1, N2, 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 FAR.
d. Engine Restart Capability - § 25.903 (e) .
(1) Explanation.
(i) 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.
(ii) 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-ofchange 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 Airplane Flight Manual.
(iii) 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 .
(i) 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.
(ii) 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 AC power (Ref. § 25.1351(d)).
(iii) The engine operating characteristics should be evaluated during each restart. In par ticular, 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.
(iv) To address the dual engine flameout case during takeoff and initial climb-out, wit h 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.
(v) To address dual en gine 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 IAS.
(vi) If an onboard auxiliary power unit (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.
(vii) 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.
(viii) The inflight engine restart envelope presented in the Airplane Flight Manual (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 SYSTEMS (ATTCS) - § 25.904 .
a. Explanation.
(1) Beginning in the 1970's, some manufacturers of tu rbojet 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 Power 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 must not be less than 90 percent of that set by the ATTCS).