Section 6. MISCELLANEOUS EQUIPMENT
208. ELECTRONIC EQUIPMENT - § 25.1431.
a. Explanation. The possibility of interaction between two systems of communications and navigation equipment should be evaluated. Momentary deflection or flicker should be permitted.
(Flicker frequencies above 55 Hz for stroke symbology or non-interlaced raster and 30/60 Hz for interlaced raster are generally satisfactory.) Loss of required function of the usable and assigned frequencies in the national airspace system should be considered unacceptable.
b. Procedures. None.
[209. - 223.] [RESERVED]
CHAPTER 7 - OPERATING LIMITATIONS AND INFORMATION.
Section 1. GENERAL [RESERVED] Section 2. OPERATING LIMITATIONS [RESERVED] Section 3. MARKINGS AND PLACARDS [RESERVED] Section 4. AIRPLANE FLIGHT MANUAL
224. GENERAL - § 25.1581.
a. Explanation. The primary purpose of the FAA approved Airplane Flight Manual (AFM) is to provide an authoritative source of information considered to be necessary for safe operation of the airplane. Since the flightcrew is most directly concerned with operation of the airplane, the language and presentation of the flight manual should be directed principally to the needs and convenience of the flightcrew, but should not ignore the needs of other contributors to safe operation of the airplane in accordance with the applicable operating regulations.
(1) Section 25.1501 requires that the operating limitations specified in §§ 25.1503 through 25.1533, and other information necessary for safe operation, be included in the AFM, be expressed in markings and placards, and also be made available by any other means that will convey the necessary information to the crew members.
(2) Information and data that are mandatory for an acceptable AFM are prescribed in §§ 25.1581 through 25.1587. The material required by Parts 25 and 36 of the FAR must be included in the AFM. At the option of the applicant, the AFM may be expanded to contain additional FAA approved information.
(3) The manufacturer or operator may include other "unapproved" data in a separate and distinctively identified portion of the AFM.
b. Reference. See Advisory Circular 25.1581, "Airplane Flight Manual," dated July 14, 1997, for detailed guidance on the required content and general structure of the AFM.
[225. - 227.] [RESERVED]
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 of the Federal Aviation Regulations (FAR).
b. Explanation. Some airplanes require artificial stall warning systems, such as stick shakers, to compensate for a lack of clearly identifiable natural stall warning to show compliance with the stall warning requirements of § 25.207. Similarly, some airplanes require a stall identification device or system (e.g., stick pusher, automatic inboard slat segment retraction, auto-trim, etc.) 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) 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 high-energy 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.
(ii) 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 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.
(i) 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.
(ii) 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 0.5g), and while the pilot is following windshear recovery Flight Director guidance.
(iii) A means to quickly deactivate the stall identification system should be provided and be made readily available to the 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.
(iv) 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 adequately ascertain the proper operation of the stall warning and stall identification systems prior to takeoff. This method should be described in the operating procedures section of the Airplane Flight Manual (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)(iii), 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 of the FAR, engineering data must be supplied to satisfy the following criteria, determined in accordance with the procedures of Advisory Circular
25.1309-1A, "System Design Analysis," dated June 21, 1988, where appropriate.
(1) Reliability. Probability of artificial stall warning and stall identification systems not operating when required:
(i) If stall warning is not clearly identifiable by natural characteristics, the loss of artificial stall warning must be improbable (not greater than 10 -5 per flight hour). This reliability requirement is normally met by using dual, independent stall warning systems.
(ii) If the natural stall characteristics are unacceptable, the combination of failure of the stall identification system to operate and entry into a stall must 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.
(iii) If the natural stall cha racteristics are demonstrated to be benign, 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.
(i) 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.
(ii) To ensure that inadvertent operation of the stall identificati on system does not jeopardize safe flight, and to maintain crew confidence in the system, it should be shown that:
(A) No single failure will result in inadvertent operation of the stall identification system; and (B) The probability of inadverte nt operation from all causes is improbable (not greater than 10-5 per flight hour).
(iii) Stall identification systems should be designed so that flight in turbulence will not result in inadvertent operation.
NOTE: In making the assessments of subparagraphs (iv), (v) and (vi), 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.
(iv) 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 must not be greater than 10 -7 per flight hour.
(v) 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 must be extremely improbable (not greater than 10 -9 per flight hour).
(vi) 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, 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 of attack far enough below the point for its activation that inadvertent return to the stall angle of attack is unlikely.
(2) The characteristics of stall identification system s, which by design are intended to apply an abrupt nose-down control input (e.g., stick pushers), should be such that it is unlikely that a member of the flightcrew will prevent or delay their operation.
(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.
229. REDUCED AND DERATED THRUST TAKEOFF OPERATIONS .
a. Explanation. The use of derated and reduced 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 thrust takeoff operations. These methods are as follows:
(1) Derated thrust approvals entail the use of completely new takeoff power setting charts and Airplane Flight Manual (AFM) performance information. The new power settings are less than the engine manufacturer's approved takeoff thrust settings, and the AFM performance is based on the thrust developed at these new, lower thrust settings.
(2) A constant reduced thrust increment can be used for a ll operating conditions. In this method the engine parameter by which thrust is set is reduced by a constant, such as an engine pressure ratio increment ( ∆EPR) of .02. A method is supplied to determine the airplane's takeoff performance, at this reduced thrust level, from the AFM data representative of full takeoff thrust.
(3) The assumed temperature method of reduced 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 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 thrust methods are used, the FAA has certain limitations on the use of these reduced thrust levels. The appropriate guidance for derated and reduced thrust approvals, including limitations and procedures, is presented in Advisory Circular 25-13, "Reduced and Derated 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 of the Federal Aviation Regulations (FAR).
b. Explanation. The sections of Part 25 of the FAR, 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 Airplane Flight Manual (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 app roaches 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 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. PERFORMANCE REQUIREMENTS FOR FLIGHT IN ICING CONDITIONS .
a. Applicable Regulations . Sections 25.103, 25.117, 25.119, 25.121(d) and 25.1419 of the Federal Aviation Regulations (FAR).
b. Explanation.
(1) Section 25.117 of the FAR states that the climb requirements of §§ 25.119 and 25.121 must be complied with at each weight, altitude, and ambient temperature within the operational limits established for the airplane, and with the most unfavorable center of gravity for each configuration.
(2) When approval of an airplane for operation in known icing conditions is requested, compliance with the approach climb requirements of § 25.121(d), and landing climb requirements of § 25.119, should be demonstrated with residual ice accretions on unprotected areas of the airframe. For airplanes with de-ice systems, this would also include any ice not shed from protected surfaces with the system in its normal operating mode. The airplane is assumed to descend into, and be required to hold in, atmospheric conditions meeting the continuous maximum atmospheric icing conditions criteria described in Appendix C to Part 25 of the FAR. Approach and landing climb performance information, contained in the Airplane Flight Manual (AFM), should present the effects of residual airframe ice accretions and ice protection system operation.
c. Procedures .
(1) Artificial ice shapes should be constructed and installed for climb performance testing to be conducted in stable, dry air. The artificial ice shapes used for flight testing should be those that have the most adverse effect on handling characteristics. If it is determined that another reasonably expected ice shape could have a greater impact on performance, the artificial ice shape with the most adverse effect on handling characteristics may be used for performance flight testing, provided that any difference in performance can be conservatively taken into account.
(2) Appropriate artificial ice shapes should be determined by one or more of the following methods:
natural atmospheric icing flight tests; flight behind an icing tanker; icing wind tunnel tests; or analytical prediction methods
(3) Stall testing should be conducted in stable, dry air with the artificial ice shapes installed. Effects of the ice shapes on stall speeds and controllability in the stalling maneuver should be determined. If the reference stall speeds (i.e., 1g stall speeds as defined in Appendix 5) are increased by the greater of 3 percent or 3 knots, the approach and landing climb speeds, and landing reference speeds (V REF ) should be recalculated using those higher stall speeds. This will assure the retention of adequate stall margins.
(4) If landing reference speeds are increased to compensate for the effect of ice accretion on stall speeds, the landing distances should also be adjusted to reflect the effect of the increased landing speeds.
(5) Climb performance testing, with the artificial ice shapes installed, should be conducted in stable, dry air using the methods described in paragraph 17b (Ref: § 25.121) of this AC. The thrust setting for landing climb performance should be determined by the method described in paragraph 16b (Ref: § 25.119) of this AC for the critical air bleed configuration associated with operation of the ice protection system.
(6) The AFM approach and la nding climb performance information must include performance decrements that account for the increased drag due to residual ice accretion and thrust (or power) loss due to operation of the ice protection system. For the landing climb of § 25.119, analytical methods may be employed for the derivation of the increase in drag due to ice accretion on the landing gear.
232. FLYING QUALITIES IN NATURAL ICING CONDITIONS .
a. Applicable Regulations . - Sections 25.143 and 25.1419 of the Federal Aviation Regulations (FAR).
b. Explanation.
(1) Since all aircraft will accumulate ice on unprotected surfaces with the ice protection system functioning normally, the effect of those ice accumulations on flying qualities should be determined. The flying qualities evaluation should include an investigation of the effects of any ice that may accumulate on the high lift devices (e.g., in slat or flap gaps, on flaps and associated structure, etc.). For airplanes with de-icing systems, consideration should also be given to the effects of any ice that would remain on the protected surfaces during normal system operation (this should also include Minimum Equipment List (MEL) dispatch considerations for all types of ice protection systems).
(2) To assure that there are n o unusual or hazardous differences in handling characteristics between tests with artificial ice shapes and those associated with the critical natural ice buildup, an evaluation of airplane flying qualities should be performed following a natural icing encounter. The amount of ice on the airplane should be representative of what would be accumulated in a 45 minute hold in icing conditions prior to approach and landing (typically, one to two inches on the unprotected wing leading edge surfaces has been found acceptable for this testing).
(3) Based on the Failure Modes and Effects Analysis (FMEA) for the ice protection system, annunciations provided to the flightcrew, and recommended AFM procedures following a failure in the ice protection system, a handling qualities evaluation should be conducted with the appropriate amount of ice on the airplane in the appropriate locations. This failure condition handling qualities testing may be conducted with artificial ice shapes, rather than natural ice, if the applicant desires.
c. Procedures .
(1) Since extensive natural icing conditions are quite often difficult to find, applicants are encouraged to combine the flying qualities flight testing with other icing-related tests (e.g., ice shedding capability).
(2) The following flight tests were designed to evaluate maneuvers that can be expected while holding in icing conditions, and in the subsequent approach to landing. Additionally, one full stall should be conducted in the landing configuration.
CONFIGURATION C.G. TRIM SPEED MANEUVER
Flaps UP, Gear UP FWD Holding Level, 40o banked turn Bank-to-bank rapid roll, 30 o - 30o Speedbrake extension, retraction.
Flaps DN, Gear UP FWD Holding Level, 40o banked turn Bank-to-bank rapid roll, 30 o - 30o Speedbrake extension, retraction.
Landing FWD 1.3 VS0 Level, 40o banked turn
(i.e., VREF) Bank-to-bank rapid roll, 30 o - 30o Speedbrake extension, retraction.
Landing FWD Same as used Idle power for uncontaminated 1 kt/sec decel to full stall airplane stalls
(3) During any of the specified maneuvers, there should be no unusual control responses, or any uncommanded airplane motions. Additionally, during the level turns and bank-to-bank rolls, there should be no buffeting or stall warning.
(4) It is acceptable for some ice to be shed during the flying qualities testing due to air loads, wing flexure, etc. However, an attempt should be made to accomplish the test maneuvers as soon as possible after exiting the icing cloud to minimize atmospheric influences on ice shedding.
233. CRITERIA FOR APPROVAL OF STEEP APPROACH TO LANDING .
a. Applicable Regulations . Sections 25.119 & 25.125 of the Federal Aviation Regulations.
b. Explanation.
(1) Sign Convention. In referring to approach path deviations/increments, a “plus” (or “+”) indicates an increase in the approach path angle (i.e., steeper than) and a “minus” (or “-”) indicates a decrease in the approach path angle (i.e., less steep).
(2) Airworthiness Approval. The standard approach angle certified as part of the type design of transport category airplanes for instrument landing operations is 3 degrees, which coincides with the nominal Instrument Landing System (ILS) approach angle. Some ILS installations use a 4.5 degree approach path angle, and a very small number exceed that angle under controlled conditions at dedicated Short Takeoff and Landing (STOL) facilities. Airplanes certificated under 14 CFR part, with features that allow them to make steep approaches (greater than 4.5 degrees), and to take off and land in less than normal distances, have not been given credit for this capability by past FAA policies. In response to requests from several operators for authority to conduct steep approaches to short landing fields, the FAA has, however, developed the following basic airworthiness certification criteria for this type of approval. Additions or deletions to these criteria may be required to address specific design features. It should be noted in the Airplane Flight Manual (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.
(3) Operational Approval. Operational approval at a particular airport 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.
c. General Requirements .
(1) Approach path guidance must be used for all steep approaches. The landing screen height is 50 feet, unless stated otherwise (see paragraph 233c(3)(iii), below).
(2) In lieu of the wind tolerance specified in § 25.21(d), the wind for takeoff and landing performance tests may not exceed 5 knots along the runway, measured at a height of 6 feet above the runway.
(3) In lieu of the corresponding landing performance requirements of § 25.125(a), the horizontal distance necessary to land and come to a complete stop from the landing screen height must be determined (for standard temperatures, at each weight, altitude, and wind within the operational limits established by the applicant for the airplane) as follows:
(i) The airplane must be in the appropriate landing configuration.
(ii) A steady approach must be maintained down to the landing screen height at a calibrated airspeed not less than V REF, with an approach path angle not exceeding the maximum for which approval is sought.
(VREF is the steady landing approach speed for a defined landing configuration.)
(iii) When an approved means of approach path visual guidance is used, and the approach path angle is equal to or greater than 4.5 o, the landing screen height may be reduced to 35 feet.
NOTE: An operating limitation in the AFM should specify that when approach path visual guidance is not available, or the approach path angle is less than 4.5 o, landing performance must be based on a screen height of 50 feet.
(4) In addition to the approach climb requirements of § 25.121(d) and the landing climb requirements of § 25.119, it must be possible to successfully complete a go-around in the landing configuration, with an engine failure at the Decision Height (DH) or Minimum Descent Altitude (MDA), as applicable. For propeller driven airplanes, the propeller of the inoperative engine should be in the position it automatically assumes following an engine failure.
(5) If the parametric method is used to determine landing distance, similar to paragraph 19b(3) of this AC, approach angles should be appropriate to the steep approach path angle desired, but the touchdown sink rate for data expansion should be limited to 6 feet per second.
d. Abuse Testing . Under no-wind conditions, it shall be demonstrated that it is possible to complete an approach, touchdown, and stop without displaying any hazardous characteristics in the following conditions:
(1) The recommended approach path angle ±2 o at VREF.
(2) The recommended approach path angle at V REF minus 5 knots.
(3) For both conditions d(1) and d(2):
(i) The rate of descent must be reduced to less than 3 feet per second before touchd own.
(ii) Below a height of 200 ft. no action should be taken to increase power, apart from those small changes necessary to maintain an accurate approach.
(iii) The airplane should be loaded to the critical weight and c.g. combination, and the tests conducted in the all engines operating, or one-engine inoperative, short landing configuration, as appropriate.
(iv) During the +2o approach path abuse demonstration, the engines must remain above idle when stabilized on the approach path.
NOTE: The approach angle abuse requirements are to account for tailwinds or headwinds on the approach and/or to account for necessary corrections back to the desired approach path after inadvertent excursions. The purpose of the
speed abuse test is to account for an unnoticed speed decrease during the approach, hence the requirement in paragraph 233d(3)(ii) for no power increase to account for the slower speed.
e. Engine-Out Operation .
(1) It must be shown that the airplane is safely controllable during appr oach, landing, and during the transition to a go -around, using recommended operational speeds and procedures. This must be demonstrated after failure of the critical engine, while the airplane is stabilized on V REF at the maximum approved approach path angle, and also at the approved angle plus 2 o.
(2) If the one-engine feathered thrust/drag characteristics are such that it is not possible to maintain the approved approach path angle plus 2 degrees without accelerating, then the airplane must either be able to maintain the approved approach path angle minus 2 degrees at V REF, in the landing configuration with one propeller windmilling (in which case it is permissible to continue the approach), or a go-around must be initiated following any engine failure. The results of this test will determine the appropriate engine-out configuration to be approved in the Airplane Flight Manual. If the airplane cannot maintain the approved approach path angle plus 2 degrees with the inoperative engine's propeller feathered without accelerating, or the approved approach path angle minus 2 degrees with the propeller windmilling, then the AFM limitations section must state that steep approaches are not approved with a failed engine and the airplane must divert to an alternate airport.
NOTE: The intent of this test is to determine an airplane configuration that allows adequate vertical flight path control to permit regaining the approved approach path following deviation from that path for any reason. If an engine failure occurs below DH/MDA, it is permissible to continue the approach with the propeller windmilling, without meeting the requirements of paragraph e(2), above, provided the airplane is safely controllable during the subsequent approach and landing.
f. Airplane Flight Manual .
(1) Limitations, operating procedures, and performance information necessary for steep approach operations should be clearly identified as such.
(2) The limitations section should note that the limitations, operating procedures, and performance information for steep approach operations are predicated on the use of an approved approach path guidance system.
(3) The limitations section should specify the airplane configurations (i.e., flaps, gear, number of engines operating, etc.) for which steep approach operations are approved.
(4) The landing weight limitations should include the most restrictive of the landing climb performance conditions, as determined by the approach climb requirements of § 25.121(d), the landing climb requirements of §
25.119, and the additional climb requirements specified in paragraph 233c(4) of this section.
(5) The operating procedures and performance information should clearly identify the airplane configuration, screen height, and approach path angle that the steep approach landing distances are based upon.
(6) The steep approach landing distances should be presented for standard day conditions and should include corrections for weight, altitude, and winds within the limits established for the airplane. Wind velocities should be factored in accordance with § 25.125(e). At the option of the applicant, corrections may also be presented for non-standard temperatures and runway slope.
234. 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 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:
(i) Surface and sub-ba se bearing strength, usually expressed as "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) system employed by the International Civil Aviation Organization (ICAO).
(ii) Thickness, aggregate size, and depth of the surface material.
(iii) Presence of rutting.
(iv) Drainage.
(v) 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 Airplane Flight Manual (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.
(i) 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.
(ii) The test runway should be the actual runway for which approval is requested or be chosen t o 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.
(iii) 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:
(A) Credit may be taken for any performance benefit provided; and (B) Dispatch without it is prohibited, regardless of whether credit for any performance benefit is taken.
(iv) Landing flare and touchdown characteristics should be evaluated during the landing performance tests.