20-1 CHAPTER 20. DESIGN AND CONSTRUCTION: PRESSURIZATION
20.1 Pressurized Cabins—§ 25.841.
20.1.1 Explanation.
20.1.1.1 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:
20.1.1.1.1 Not more than 8,000 feet, at the maximum operating altitude of the airplane, for normal operation of the pressurization system.
20.1.1.1.2 For airplanes to be operated above 25,000 feet, the airplane must be able to maintain a cabin pressure altitude of not more than 15,000 feet in the event of any reasonably probable failure or malfunction in the pressurization system.
20.1.1.2 The 8,000-foot limit on cabin pressure altitude applies during normal operations at all altitudes, up to the maximum operating altitude of the airplane. For airplanes operating at airports with altitudes higher than 8,000 feet (see paragraph 20.1.1.3), an equivalent level of safety finding must be made in accordance with § 21.21(b)(1) to allow the higher than 8,000-foot cabin pressure altitude needed for these operations.
20.1.1.3 Though not addressed by § 25.841, airplanes may incorporate a “high altitude mode” to permit takeoff and landing at airports above 8,000 feet.
For takeoff and landing altitudes above 8,000 feet, the cabin pressure limits to be demonstrated by flight testing are:
20.1.1.3.1 Prior to beginning a descent into a high altitude airport, not more than 8,000 feet.
20.1.1.3.2 During a descent into or climb out of a high altitude airport, not more than 15,000 feet.
20.1.1.3.3 After a takeoff from a high altitude airport and after the cabin pressure altitude decreases to 8,000 feet, not more than 8,000 feet.
20.1.1.4 Section 25.841(b)(6) requires an aural or visual warning to the flightcrew when the cabin pressure altitude exceeds 10,000 feet. The intent of this requirement is to warn the crew when a safe or preset cabin pressure altitude limit is exceeded. However, complying with this regulation when operating at airports with altitudes higher than 10,000 feet (see paragraph 20.1.1.3) would result in nuisance cabin pressure warnings. For
20-2 systems that set the cabin pressure altitude warning limit above 10,000 feet for operations at high altitude airports, an equivalent level of safety finding must be made in accordance with § 21.21(b)(1).
20.1.2 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 criterion, it will be necessary to substantiate compliance by additional testing or analysis for the maximum leakage rate allowed by the type design.
20.1.2.1 Normal Operating Conditions—8,000-Foot Cabin Pressure Altitude.
20.1.2.1.1 With the pressurization system operating in its normal mode, verify that the cabin pressure altitude does not exceed 8,000 feet during climb, cruise, or descent at any altitude up to and including the maximum operating altitude for which the airplane is to be certificated.
20.1.2.1.2 A stable condition should be held long enough to record any cyclic fluctuations in cabin pressure due to relief valve operation.
20.1.2.2 Failure Conditions—15,000-Foot Cabin Pressure Altitude.
20.1.2.2.1 The critical probable system failure condition 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 “probable failure condition” criterion, the flight test should be conducted for each failure mode identified.
20.1.2.2.2 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.
20.1.2.2.3 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-foot cabin pressure altitude warning activates. Emergency descent procedures should then be initiated and the descent continued to an altitude below 15,000 feet.
20.1.2.2.4 The cabin pressure altitude should not exceed 15,000 feet at any time during the test.
20.1.2.3 High Altitude Takeoff Conditions (Greater than 8,000 Feet).
20.1.2.3.1 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 feet, if the system is so designed. AFM
20-3 procedures may need to be developed. The same applies to high altitude landings above 10,000 feet.
20.1.2.3.2 The cabin pressure altitude should not exceed the maximum allowable takeoff and landing altitude (within an acceptable tolerance) 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 feet or more, with the airplane unpressurized.
20.1.2.3.3 The cabin pressure altitude should eventually decrease to 8,000 feet and then not exceed 8,000 feet for the duration of the test.
20.1.2.4 High Altitude Landing Conditions (Greater than 8,000 Feet).
20.1.2.4.1 The airplane should be stabilized in the cruise configuration prior to beginning a descent.
20.1.2.4.2 The pressurization system should be placed in its high altitude mode.
20.1.2.4.3 The cabin pressure altitude should not exceed the maximum allowable takeoff and landing altitude (within an acceptable tolerance) during the descent to an actual, or simulated, landing at a high altitude airport.
20.2 Tests for Pressurized Cabins—§ 25.843.
20.2.1 Explanation.
20.2.1.1 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.
20.2.1.2 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.
20.2.2 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.
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20.2.2.1 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.
20.2.2.1.1 After takeoff, maintain a stable, continuous climb to the maximum operating altitude for which the airplane will be certificated.
20.2.2.1.2 Maintain that altitude until the cabin pressure altitude has stabilized.
20.2.2.1.3 Initiate a steady, maximum rate of descent within the operating limitations of the airplane down to the airport.
20.2.2.2 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.
20.2.2.2.1 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 feet.
20.2.2.2.2 Maintain each level-off altitude long enough for the cabin pressure altitude to stabilize.
20.2.2.2.3 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 feet.
20.2.2.3 Positive Pressure Relief.
If two valves are provided, one should be deactivated for this test.
20.2.2.3.1 After takeoff, climb to the operating altitude that provides maximum cabin differential pressure.
20.2.2.3.2 Manually close the outflow valve, allowing the cabin differential pressure to increase.
20.2.2.3.3 Verify the cabin pressure differential pressure warning functions properly.
20.2.2.3.4 Verify that the relief valve functions and the maximum cabin differential pressure is not exceeded.
20.2.2.4 Negative Pressure Relief/Emergency Descent.
If two valves are provided, deactivate one for this test.
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20.2.2.4.1 At cruise altitude, perform an emergency descent with the airplane in the critical condition for negative pressure on the fuselage.
20.2.2.4.2 Verify that the maximum negative differential pressure for the fuselage is not exceeded.
20.2.2.4.3 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.
20.2.2.5 Testing of Doors and Exits.
20.2.2.5.1 Prior to flight, all doors and exits should be checked for proper operation.
20.2.2.5.2 After returning from a flight where the airplane is subjected to the maximum certificated altitude and cabin 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.
21-1 CHAPTER 21. DESIGN AND CONSTRUCTION: FIRE PROTECTION [RESERVED]
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.
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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:
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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.”
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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