291 Chapter 8 - Airworthiness: Miscellaneous Items
228. Design and Function Of Artificial Stall Warning and Identification Systems.
a. Applicable Regulations. Sections 25.103, 25.201, 25.203, and 25.207.
b. Explanation. Some airplanes require artificial stall warning systems to compensate for a lack of clearly identifiable natural aerodynamic stall warning to show compliance with the stall warning requirements of § 25.207. A stick shaker is a recommended method of providing such a warning, regardless of whether or not the natural aerodynamic stall warning is clearly identifiable. Similarly, some airplanes require a stall identification device or system (e.g., stick pusher,) to compensate for an inability to meet the stalling definitions of § 25.201 or the stall characteristics requirements of § 25.203. In addition to compliance with the flight test requirements prescribed in paragraph 29 of this AC, certain system design and function criteria should also be addressed during the certification process of these airplanes. Included are system arming and disarming, preflight checks, failure indications and warnings, and system reliability and safety. The reliability of these systems can be evaluated in terms of the probability of the system not operating when required, and the safety aspects in terms of the probability of the system operating inadvertently. The required reliability and safety of stall warning and identification systems should be defined as a function of how critical their respective functioning is to safety of flight.
c. Arming and Disarming.
(1) Stall warning systems should be armed any time the airplane is in flight (i.e. from main gear liftoff to touchdown). However, up to the end of the takeoff rotation (i.e., until the takeoff pitch attitude is attained), any phase advance feature of the stall warning system (i.e., the portion of the algorithm for activating the stall warning system that responds to the rate of change in angle-of-attack) can be inhibited.
(a) Arming of stall warning systems has typically been accomplished by a ground/air logic circuit, which requires the nose and/or main gear squat switches to sense air mode before the system is armed. A pitch angle threshold during rotation has also been used to arm the stall warning system. These types of system arming schemes provide stall warning protection during liftoff and initial climb, where a stall would most probably have catastrophic consequences. They also provide protection against nuisance warnings during the takeoff roll, where the angle-of-attack (AOA) sensor vanes may be misaligned. Service history, however, has shown that systems armed around the liftoff point have caused pilots to abort takeoffs due to false alerts resulting from stall warning system faults or failures. In some cases, these highenergy rejected takeoffs have resulted in overruns. Therefore, system faults and failures that would lead to a false stall warning near liftoff should be made evident as early in the takeoff as practicable.
(b) In accordance with the requirements of § 25.207(b), if a stall warning system is required for any normal combination of flap and landing gear position, it must be used for all
292 combinations of flap and landing gear positions. The purpose of this requirement is to provide a standard, consistent warning to the flightcrew of an operational flight envelope limit.
(2) Stall identification systems should be armed any time the airplane is in flight.
(a) The arming should take place automatically and may be provided by the same ground/air sensing system used for arming the stall warning system. The stall identification system may be inhibited during the takeoff rotation, but should become functional immediately after main gear liftoff. For airplanes with both stall warning and stall identification systems, it is permissible to have the stall identification system armed by operation of the stall warning system, provided the resulting probability of the stall identification not to operate when required is not greater than that specified in paragraph 228e, below.
(b) Stall identification systems may incorporate automatic disarming in flight regimes where the risk of stalling is extremely remote or where their unwanted operation would pose a threat to continued safe flight; examples of such inhibits would be high airspeed, and “g” cutouts (typically 0.5 g), and while the pilot is following windshear recovery flight director guidance.
(c) A means to quickly deactivate the stall identification system should be provided and be available to both pilots. It should be effective at all times, and should be capable of preventing the system from making any input to the longitudinal control system. It should also be capable of canceling any input that has already been applied, from either normal operation or from a failure condition.
(d) If a stall identification system is required to show compliance with the stall requirements of part 25 in one (or some) airplane configuration(s), it does not have to be used for stall identification in configurations where compliance can be demonstrated without it. Unlike stall warning, the stall point, be it aerodynamic or artificially induced, represents an end-point outside the in-service operational envelope of the airplane, and, therefore, does not need to be provided by the same means for all flap and landing gear configurations. Additionally, the added system complexity, and increased exposure to malfunctions and failures, would not warrant the use of a stall identification system for configurations where it is not required.
d. Indicating and Warning Devices.
(1) A method should be provided to allow the pilot to determine that the stall warning and stall identification systems are operating properly prior to takeoff. This method should be described in the operating procedures section of the AFM.
(2) Warning that the associated systems for operating the stall warning or stall identification devices has failed should be provided. As far as is practicable, this warning should cover all system failure modes.
(3) A clear and distinctive cockpit indication should be given when the stall identification system has been deactivated by the flightcrew (see paragraph 228c(2)(c), above).
This indication should be present as long as the system is deactivated.
(4) Any related limitations, and normal and emergency operating procedures, together with any information found necessary for safety during operation of the stall warning and identification systems, should be included in the AFM and supplemented by such markings and placards as deemed necessary.
e. System Reliability and Safety. When stall warning and/or stall identification systems are installed to show compliance with the stalling requirements of §§ 25.201, 25.203, and
25.207, engineering data should be supplied to satisfy the following criteria, determined in accordance with § 25.1309.
(1) Reliability. Probability of artificial stall warning and stall identification systems not operating when required:
(a) If stall warning is not clearly identifiable by natural characteristics, the loss of artificial stall warning should be improbable (not greater than 10-5 per flight hour). This reliability requirement is normally met by using dual, independent stall warning systems.
(b) If the natural stall characteristics are unacceptable, the combination of failure of the stall identification system to operate and entry into a stall should be extremely improbable (not greater than 10-9 per flight hour). A stall identification system with a failure rate not greater than 10-4 per flight hour will satisfy this requirement.
(c) If the stall identification system is installed solely for the purposes of identifying the stall, and the stall characteristics would otherwise meet the requirements of Subpart B with the stall identification system disabled, a maximum failure rate of 10-3 per flight hour will be acceptable.
(2) Safety. Probability of artificial stall warning and stall identification systems operating inadvertently.
(a) The probability of inadvertent operation of artificial stall warning systems, during critical phases of flight, should not be greater than 10-5 per flight hour.
(b) To ensure that inadvertent operation of the stall identification system does not jeopardize safe flight, and to maintain crew confidence in the system, it should be shown that:
1 No single failure will result in inadvertent operation of the stall identification system; and
2 The probability of inadvertent operation from all causes is improbable (not greater than 10-5 per flight hour).
(c) Stall identification systems should be designed so that flight in turbulence will not result in inadvertent operation.
NOTE: In making the assessments of subparagraphs (d), (e) and (f), below, it should be assumed that in the climb, cruise, and descent flight regimes, corrective pilot action will not be initiated until three seconds after unwanted operation has been recognized. During takeoff and final approach, this time delay may be reduced to one second.
(d) If inadvertent operation of the stall identification system would result in limit loads being exceeded in any part of the airplane structure, the probability of inadvertent operation should not be greater than 10-7 per flight hour.
(e) If inadvertent operation of the stall identification system would result in ultimate loads being exceeded in any part of the airplane structure, the probability of inadvertent operation should be extremely improbable (not greater than 10-9 per flight hour).
(f) Inadvertent operation of the stall identification system should not cause catastrophic ground contact. This should be achieved by limiting the effect of the stall identification system to that necessary for stall identification purposes, without undue flight path deviation (e.g., by limiting the stroke of a stick pusher). Alternatively, if inadvertent operation could result in catastrophic ground contact, per § 25.1309(b)(1), the probability of inadvertent operation must be extremely improbable. Inhibition of the system close to the ground (e.g., for a fixed time after liftoff or below a radar altitude) would not normally be an acceptable means of compliance with this requirement.
f. System Functional Requirements.
(1) Operation of the stall identification system should reduce the airplane’s angle-ofattack far enough below the point for its activation that inadvertent return to the stall angle-ofattack is unlikely.
(2) The characteristics of stall identification systems, which by design are intended to apply an abrupt nose-down control input (e.g., a stick pusher), should make it unlikely that a flightcrew member will prevent or delay its operation. The required stick force, rate of application, and stick travel will depend on the airplane's stall and stick force characteristics, but a force of 50 to 80 pounds applied virtually instantaneously has previously been accepted as providing this characteristic.
(3) Normal operation of the stall identification system should not result in the total normal acceleration of the airplane becoming negative.
(4) The longitudinal maneuvering capability of an airplane equipped with stall identification systems, at all speeds likely to be encountered in normal operations, should be substantially the same as would be expected for an airplane with acceptable aerodynamic stall characteristics.
g. System Tolerances. See paragraph 29i for additional considerations regarding compliance with stall-related regulatory requirements, including how to address tolerances for stall speed (§ 25.103) and stall characteristics (§ 25.203) testing. See paragraph 29f(2)(f) for how to address tolerances of the stall warning and stall identification systems during testing to show compliance with the stall warning requirements (§ 25.207).
229. Reduced and Derated Power or Thrust Takeoff Operations.
a. Explanation. The use of derated and reduced power or thrust for takeoff operations can produce substantial reductions in operating costs due to lower fuel consumption and increased operating margins. With the appropriate limitations and operating procedures applied, these operations can also offer safety benefits. Three methods have been approved by the FAA for derated and reduced power or thrust takeoff operations. These methods are as follows:
(1) Derated power or thrust approvals entail the use of completely new takeoff power or thrust setting charts and AFM performance information. The new power or thrust settings are less than the engine manufacturer’s approved takeoff power or thrust settings, and the AFM performance is based on the power or thrust developed at these new, lower power or thrust settings.
(2) A constant reduced power or thrust increment can be used for all operating conditions. In this method the engine parameter by which power or thrust is set is reduced by a constant, such as an engine pressure ratio increment (EPR) of 0.02. A method is supplied to determine the airplane’s takeoff performance, at this reduced power or thrust level, from the AFM data representative of full takeoff power or thrust.
(3) The assumed temperature method of reduced power or thrust takeoff operations entails using takeoff performance information determined for an “assumed” temperature above ambient, but not above the temperature at which the takeoff weight would be limited by the takeoff field length available for a particular airport runway or by FAA climb requirements.
Engine power or thrust settings, takeoff speed schedules, takeoff field lengths, and climb performance are determined at the assumed temperature. This assumed temperature method is the most flexible in its application and the most widely used by transport category airplane manufacturers and operators.
b. Procedures. Because there is a reduction in the takeoff performance level when any of the derated or reduced power or thrust methods are used, the FAA has certain limitations on the use of these reduced power or thrust levels. The appropriate guidance for derated and reduced power or thrust approvals, including limitations and procedures, is presented in AC 25-13, “Reduced and Derated Takeoff Thrust (Power) Procedures,” dated May 4, 1988.
230. Runway Gradients Greater Than ± 2 Percent.
a. Applicable Regulations. Sections 25.105, 25.115, 25.119, 25.121, 25.125, 25.1533, and
25.1587.
b. Explanation. The sections of part 25, referenced above, require accounting for the effects of runway gradient. Typically, performance limitations and information are determined for runway gradients up to ±2 percent in the AFM expansion of test data. Though these gradient extremes are adequate for addressing the majority of runways, there are a number of airports frequented by transport category airplanes that have runway slopes greater than ±2 percent.
Consequently, approvals have been granted for operations on runways with slopes exceeding ±2 percent with specific testing and analysis validation for the effects of the higher slopes.
Additional concerns, beyond runway slope effect on acceleration and braking and proper accounting of elevations during obstacle clearance analysis, include takeoff flare from liftoff to 35 feet, minimum takeoff climb gradients, minimum approach and landing climb gradients, landing flare distances, and unique operating procedures.
c. Procedures.
(1) Takeoff Flare from Liftoff to 35 Feet. The AFM expansion of the takeoff data should account for the effect of the runway slope on the portion of the takeoff distance after liftoff. At climb performance-limiting thrust-to-weight ratios, the average gradient of climb will be on the order of 2.0 to 3.0 percent. On a downhill runway of sufficient magnitude, the airplane could attain a height of 35 feet above the runway and have a positive gradient of climb relative to it, but its flight path may continue to descend beyond that point. The transition from liftoff to climbing flight, in the sense of an ascending flight path, should be adequately addressed with respect to obstacle clearance analysis data.
(2) Minimum Takeoff Climb Gradients. At limiting thrust-to-weight ratios, the transition to free air (i.e., out of ground effect) takeoff climb could result in steep uphill runways rising faster than the airplane’s ability to climb. The minimum second segment takeoff climb gradient should maintain the same margin, relative to the increased maximum uphill runway slope, that exists between the minimum gradient specified in § 25.121 and a two percent uphill runway.
(3) Minimum Approach and Landing Climb Gradients. Balked landing go-arounds, at climb limited landing weights, could also result in an uphill runway rising faster than the airplane’s ability to climb. The minimum approach and landing climb gradients should maintain the same margins, relative to the increased maximum uphill runway slope, that exist between the minimum gradients specified in §§ 25.119 and 25.121 and a two percent uphill runway.
(4) Landing Technique and Distance. Final approaches to steep uphill runways will require early flare initiation, to avoid hard landings, and landing flare air distances will be increased for approaches to steep downhill runways using normal approach descent angles. The AFM operating procedures should describe any special piloting technique required for landing on steep runways. The AFM expansion of landing distances should account for the effect of
297 runway gradient, including any expected increase in flare distances, from 50 feet to touchdown, for steep downhill runways.
(5) Operating Procedures. Operating procedures should be provided in the AFM for operations on runways with gradients greater than ± 2 percent. Guidance should be provided on takeoff rotation and landing flare techniques.
(6) Operational Considerations. For runway slopes greater than ± 3 percent, the specific airport(s) should be investigated relative to runway lengths and surrounding terrain and obstacles. Airport-specific operating limitations may be necessary, such as: direction of takeoff and landing, takeoff flap restrictions, prohibition of overspeed takeoffs on downhill runways, requirement for the antiskid system to be operative and on, and restrictions on engine bleed air and power extraction.
(7) Flight Test Requirements. For approval of certification data for runway slopes exceeding ± 3 percent, operational flight tests should be conducted to verify the proposed procedures and performance information.
231. Criteria For Approval Of Steep Approach To Landing.
a. Applicable Regulations. Sections 25.119, 25.121, 25.125, and 25.143.
b. Explanation.
(1) Airworthiness Approval. The standard approach angle assumed as part of the type certification of transport category airplanes is 3 degrees, which coincides with the nominal ILS approach angle. Those evaluations are considered adequate to address approach angles of less than 4.5 degrees. The criteria listed below represent FAA policy for airworthiness approval of steep approach landing capability using an approach angle of 4.5 degrees or more. Additions or deletions to these criteria may be needed to address specific design features. It should be noted in the AFM that the presentation of the steep approach limitations, procedures, and performance information reflects the capability of the airplane to perform steep approaches, but does not constitute operational approval.
(2) Operational Approval. Operational approval to conduct steep approaches in the United States is the exclusive responsibility of FAA Flight Standards Service, and cannot be delegated to FAA Aircraft Certification Service employees, designees, or to foreign civil aviation authorities. FAA Flight Standards Service has assigned this responsibility to the Flight Standardization Board (FSB) with oversight for the airplane type in question. Operational approval will, in part, be based on the results of the airworthiness testing described in this section. Additional testing, for operational concerns, may be combined with the airworthiness testing. Ideally, the testing for operational approval would be conducted by the Flight Standardization Board during the test program for airworthiness certification of steep approach capability.
c. General Criteria.
(1) If approval is sought to conduct steep approaches in icing conditions, compliance with the part 25 requirements applicable to steep approach operations identified below should also be shown for icing conditions.
(2) The following criteria apply when showing compliance with § 25.125 for steep approaches:
(a) The airplane should be in the landing configuration used for steep approaches.
(b) Compliance with the requirement that a stable approach be conducted to a height of 50 feet with a speed not less than VREF (§ 25.125(b)(2)) should be shown with an approach path angle not exceeding the maximum for which approval is sought. The VREF used for steep approaches may be different than the VREF used for normal approaches.
(c) If the parametric method of determining the landing distance is used (see paragraph 19b(3) of this AC), approach angles should be appropriate to the steep approach path angle desired, and the touchdown sink rate for data expansion should be limited to 6 feet per second.
(3) The landing distance established under § 25.125(a) begins at a point 50 feet above the landing surface. If an applicant proposes to use a different height for the beginning of the steep approach landing distance, this must be done through an equivalent level of safety finding, in accordance with § 21.21(b)(1), or an exemption, in accordance with part 11. This has been done in some steep approach certifications to take advantage of precision approach guidance at an airport that guides the airplane to a height over the runway threshold of less than 50 feet.
(4) Compliance with §§ 25.119 and 25.121(d) should be shown using the configurations and speeds established for steep approach operations.
d. Test Conditions For Reasonably Expected Variations In Approach Speed and Path Angle.
(1) The following additional criteria should be applied to show that the airplane is safely controllable and maneuverable during landing (§ 25.143(a)(5)).
(a) Under calm air conditions, demonstrate that it is possible to complete an approach, touchdown, and stop without displaying any hazardous characteristics in the following conditions:
1 An approach path angle 2 degrees steeper than the steepest approach path angle for which approval is sought at the VREF established for a steep approach; and
2 The steepest approach path angle for which approval is sought at a speed 5 knots lower than the VREF established for a steep approach.
(b) For both conditions above:
1 The airplane should be loaded to the most critical weight and c.g.
combination;
2 The airplane should be in the steep approach configuration;
3 The rate of descent should be reduced to no more than 3 feet per second at touchdown;
4 Below a height of 200 feet, no action should be taken by the pilot to increase power or thrust, apart from those small changes needed to maintain an accurate approach;
5 After initiating the flare, the longitudinal control should not be used to depress the nose apart from those small changes necessary to maintain a continuous and consistent flare flight path;
6 The flare, touchdown, and landing should not require exceptional piloting skill, alertness, or strength; and
7 To ensure adequate capability for a go-around or down path adjustment, the engines should remain above flight idle power or thrust when stabilized on the approach path.
NOTE: The 2 degrees steeper approach path angle demonstration is to account for tailwinds on the approach and to take into account necessary corrections back to the desired approach path after inadvertent excursions. The purpose of the test at VREF minus 5 knots is to account for an unnoticed speed decrease during the approach, hence the requirement in paragraph 231d(1)(b)4 for no power or thrust increase to account for the slower speed.
(c) For flight test safety reasons, when conducting the 2 degrees steeper approach path angle test condition of paragraph 231d(1)(a)1, the pilot may begin to flare the airplane (or reduce the approach angle) at a reasonable height somewhat higher than the normal steep approach flare height. If this is done, it should be shown by analysis that there is sufficient pitch control to arrest the descent rate if the flare were to be initiated at the normal steep approach flare height, keeping in mind the criteria in paragraphs 231d(1)(b)3 and 6.
(2) Compliance with § 25.143(b)(1) should be assessed as follows: Demonstrate that the airplane can both safely land and safely transition to a go-around following a failure of the critical engine at any point in the approach under the following conditions:
(a) The steepest approach angle for which approval is sought;
(b) The VREF established for a steep approach; and
(c) The most critical combination of weight and c.g.; and
(d) For propeller powered airplanes, the propeller of the inoperative engine should be in the position it would normally assume without any action taken by the pilot following an engine failure.
(3) The height loss experienced during the maneuver described in paragraph 231d(2) should be determined.
e. One-Engine-Inoperative Steep Approach.
(1) If approval is sought for one-engine inoperative steep approach capability, the following criteria should be met at the most critical weight and c.g. position, using the configuration and speed established for a one-engine-inoperative steep approach:
(a) The demonstrations identified in paragraph 231d(1) above; and
(b) Demonstrate that the airplane can safely transition to a go-around during a one-engine inoperative steep approach.
f. Airplane Flight Manual.
(1) In accordance with §§ 25.1581, 25.1583, 25.1585, and 25.1587, the following information must be provided in the AFM:
(a) Limitations, operating procedures, and performance information necessary for steep approach operations, including the configuration(s), speeds and flight path angle(s) approved for conducting a steep approach; and
(b) Operating limitations prohibiting initiation of a steep approach:
1 With one engine inoperative, unless the airplane is approved for oneengine inoperative steep approaches; and
2 In forecast or known icing conditions unless the airplane is approved for conducting steep approaches in icing conditions.
(c) A statement in the limitations section that the steep approach limitations, procedures, and performance information reflect the capability of the airplane to perform a steep approach, but do not constitute operational approval to conduct steep approach operations.
(d) The height loss determined in accordance with paragraph 231d(3).
232. Takeoff and Landing On Unpaved Runways.
a. Explanation. There are no specific regulatory requirements or established guidance material pertaining to transport category airplane airworthiness certification for operations on
301 runway surfaces other than smooth and hard. However, several transport category airplanes have been certificated by the FAA for operation on various kinds of unpaved runways, including sod, dirt, and gravel. The following general guidance for airplane certification, for operation on unpaved surfaces, reflects the experience and policy developed during those certification programs.
b. Procedures. The considerations described in paragraphs (1) through (6), below, should be addressed in obtaining approval for operation of transport category airplanes on unpaved runways.
(1) Surface Definition. Each type of surface should be defined so that it can be recognized, controlled, and maintained in service. The definition should include specification characteristics of the surface necessary for safe operation, such as:
(a) Surface and sub-base bearing strength, usually expressed in terms of California bearing ratio (CBR). Measurements wet and dry every 500 feet along the runway centerline and 15 to 30 feet either side of the centerline have been used. Other means of defining the suitability of a runway surface to the operation of a particular airplane exist that classify runways based on their load bearing capability; one example is the Aircraft Classification Number (ACN) employed by the International Civil Aviation Organization (ICAO).
(b) Thickness, aggregate size, and depth of the surface material.
(c) Presence of rutting.
(d) Drainage.
(e) Presence of surface vegetation.
(2) Airplane Performance. If special equipment (e.g., low pressure tires, shields, deflectors) or special procedures are required, the effect of such equipment and/or procedures on airplane performance should be determined and presented in the AFM; for example, landing gear retraction time may increase if deflectors are installed on the landing gear, necessitating changes to AFM first segment climb data.
(a) Takeoff, accelerate-stop, and landing performance should be demonstrated and scheduled in accordance with the appropriate airworthiness requirements based on each type of unpaved runway surface for which approval is requested. The flight test demonstrations should be conducted on both wet and dry surfaces. An abbreviated series of test conditions, relative to the test requirements of a conventional smooth, hard-surface runway test program, may be acceptable if reliable adjustments for all flap settings can be established to derive these data from the smooth, hard surface performance data. However, a minimum of four conditions each for takeoff, accelerate-stop, and landing should be conducted, and the heaviest weight demonstrated for takeoff and landing will establish the weight limitations for those modes of operation.
(b) The test runway should be the actual runway for which approval is requested or be chosen to represent the worst characteristics (i.e., high rolling friction, low braking friction, etc.) of each type of unpaved runway for which approval is sought. In this regard, it may not be sufficient to conduct these tests from a runway with a low CBR. Previous tests have shown that rolling friction is primarily a function of CBR, but braking friction is primarily a function of runway surface characteristics and largely independent of CBR and, in some cases, whether the surface is wet or dry. The effects of other variables such as airplane weight and tolerances on recommended tire pressure should also be determined.
(c) A VMCG demonstration should be conducted. Rudder pedal nose wheel steering may be used, provided the runway surface for the test represents the worst case anticipated for operation. The aerodynamic moment applied to the airplane by the rudder, combined with the use of rudder pedal nose wheel steering, may result in the nose wheel plowing the unpaved runway surface. This can result in the runway surface elements impacting critical airframe and powerplant surfaces. The test should be closely monitored to ensure this damage source does not exist. If the test is conducted with rudder pedal nose wheel steering:
1 Credit may be taken for any performance benefit provided; and
2 Dispatch without it is prohibited, regardless of whether credit for any performance benefit is taken.
(d) Landing flare and touchdown characteristics should be evaluated during the landing performance tests.
(e) Climb performance should account for any additional drag or power/thrust loss due to special equipment installations.
(3) Airplane Handling. Airplane handling characteristics must meet the appropriate airworthiness requirements in each configuration specified for operation. Any special procedures or techniques associated with unpaved runway operation, such as use of thrust reversers, brakes, nose wheel steering, etc., should be identified.