CHAPTER 9. CONTINUED AIRWORTHINESS AND MAINTENANCE
9.1 Continued Airworthiness and Maintenance.
The following paragraphs provide guidance for preparing instructions for the continued airworthiness of the display system and its components to show compliance with §§ 25.1309 and 25.1529 (including appendix H to part 25), which require preparing Instructions for Continued Airworthiness. The following guidance is not a definitive list, and other maintenance tasks may be developed as a result of the safety assessment, design reviews, manufacturer’s recommendations, and Maintenance Steering Group 3 (MSG-3) analyses that are conducted.
9.2 General.
Information on preparing the Instructions for Continued Airworthiness can be found in appendix H to part 25. In addition to those instructions, maintenance procedures should be considered for the following:
9.2.1 Reversionary switches not used in normal operation. These switches should be checked during routine maintenance because, if a switch failure is not identified until the airplane is in flight, the switching or backup display/sensor may not be available when required. These failures may be addressed by a system safety assessment and should be addressed in the airplane’s maintenance program (for example, MSG-3).
9.2.2 Display cooling fans and filters integral with cooling ducting.
9.3 Design for Maintainability.
The display system should be designed to minimize maintenance error and maximize maintainability.
9.3.1 The display mounting, connectors, and labeling should allow quick, easy, safe, and correct access for identification, removal, and replacement. Means should be provided (for example, using physically coded connectors) to prevent inappropriate connections of system elements.
9.3.2 If the system has the capability of providing information on system faults (for example, diagnostics) to maintenance personnel, it should be displayed in text instead of coded information.
9.3.3 If the flightcrew needs to provide information to the maintenance personnel (for example, overheat warning), problems associated with the display system should be communicated to the maintenance personnel as appropriate, relative to the task and criticality of the information displayed.
9.3.4 The display components should be designed so they can withstand cleaning without internal damage, scratching, and/or crazing (cracking).
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9.4 Maintenance of Display Characteristics.
9.4.1 Maintenance procedures may be used to ensure that the display characteristics remain within the levels presented and accepted at certification.
9.4.2 Experience has shown that display quality may degrade with time and become difficult to use. Examples include lower brightness/contrast, distortion or discoloration of the screen (blooming effects), and areas of the screen that may not display information properly.
9.4.3 Test methods and criteria may be established to determine if the display system remains within acceptable minimum levels. Display system manufacturers may alternatively provide “end of life” specifications for the displays that could be adopted by the airplane manufacturer.
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Appendix A APPENDIX A. PRIMARY FLIGHT INFORMATION A.1 General.
This appendix provides additional guidance for displaying primary flight information.
Displaying primary flight information is required by §§ 25.1303(b) and 25.1333(b). The requirements for arranging primary flight information are specified in § 25.1321(b).
A.2 Attitude.
A.2.1 Pitch attitude display scaling should be such that during normal maneuvers (for example, approach or climb at high thrust-to-weight ratios) the horizon remains visible in the display with at least 5° pitch margin available.
A.2.2 An accurate, easy, quick-glance interpretation of attitude should be possible for all unusual attitude situations and other “non-normal” maneuvers sufficient to permit the pilot to recognize the unusual attitude and initiate an appropriate recovery within one second. Information to perform effective manual recovery from unusual attitudes using chevrons, pointers, and/or permanent ground-sky horizon on all attitude indications is recommended.
A.2.3 Both fixed airplane reference and fixed earth reference bank pointers (“ground and/or sky” pointers) are acceptable as a reference point for primary attitude information. A mix of these types in the same flight deck is not recommended.
A.2.4 There should be a means to determine the margin to stall and to display that information when necessary. For example, a pitch limit indication is acceptable.
A.2.5 There should be a means to identify an excessive bank angle condition prior to stall buffet.
A.2.6 Sideslip should be clearly indicated to the flightcrew (for example, a split trapezoid on the attitude indicator) and an indication of excessive sideslip should be provided.
A.3 Continued Function of Primary Flight Information (Including Standby) in Conditions of Unusual Attitudes or in Rapid Maneuvers.
A.3.1 Primary flight information must continue to be displayed in conditions of unusual attitudes or in rapid maneuvers (§ 25.1301). The pilot must also be able to rely on primary or standby instrument information for recovery in all attitudes and at the highest pitch, roll, and yaw rates that may be encountered (§ 25.1301).
A.3.2 In showing compliance with the requirements of §§ 25.1301(d), 25.1309(a), 25.1309(b),
25.1309(c), and 25.1309(d), the analysis and test program must consider the following conditions that might occur due to pilot action, system failures, or external events— A-1
Appendix A
• Abnormal attitude (including the airplane becoming inverted),
• Excursion of any other flight parameter outside protected flight boundaries, or
• Flight conditions that may result in higher than normal pitch, roll, or yaw rates.
A.3.3 For each of the conditions identified above, primary flight displays and standby indicators must continue to provide useable attitude, altitude, airspeed, and heading information and any other information that the pilot may require to recognize and execute recovery from the unusual attitude and/or arrest the higher than normal pitch, roll, or yaw rates (§ 25.1301).
A.4 Airspeed and Altitude.
A.4.1 Airspeed and altitude displays should be able to convey to the flightcrew a quick-glance sense of the present speed or altitude. Conventional round-dial moving pointer displays inherently give some of this sense that may be difficult to duplicate on moving scales.
Scale length is one attribute related to this quick-glance capability. The minimum visible airspeed scale length found acceptable for moving scales has been 80 knots.
Since this minimum is dependent on other scale attributes and airplane operational speed range, variations from this should be verified for acceptability. A displayed altitude that is geometrically derived should be easily discernible from the primary altitude information, which is barometrically derived altitude. To ensure the pilot can easily discern the two, the label “GSL” should be used to label geometric height above mean sea level. See paragraph F.5.4.4 of appendix F of this AC for airspeed considerations specific to a head-up display (HUD).
A.4.2 Airspeed reference marks (bugs) on conventional airspeed indicators perform a useful function by providing a visual reminder of important airspeed parameters. Including bugs on electronic airspeed displays is encouraged. Computed airspeed/angle-of-attack bugs such as Vstall warning, V1, VR, V2, flap limit speeds, etc., displayed on the airspeed scale should be evaluated for accuracy. The design of an airspeed indicator should include the capability to incorporate a reference mark that will reflect the current target airspeed of the flight guidance system. This has been required in the past for some systems that have complex speed selection algorithms, in order to give the flightcrew adequate information for system monitoring as required by § 25.1309(c).
A.4.3 Scale units marking for air data displays incorporated into primary flight displays are not required (“knots,” “airspeed” for airspeed, “feet,” “altitude” for altimeters) as long as the content of the readout remains clear. For altimeters with the capability to display both English and Metric units, the scale and primary present value readout should remain scaled in English units with no units marking required. The Metric display should consist of a separate present value readout that does include units marking.
A.4.4 Airspeed scale markings such as stall warning, maximum operation speed/maximum operating Mach number, or flap limits, should be displayed to provide the flightcrew a quick-glance sense of speed relative to key targets or limits. The markings should be A-2
Appendix A predominant enough to confer the quick-glance sense information, but not so predominant as to be distracting when operating normally near those speeds (for example, stabilized approach operating between stall warning and flap limit speeds).
A.4.5 If airspeed trend or acceleration cues are associated with the speed scale, vertically oriented moving scale airspeed indications should have higher numbers at the top so that increasing energy or speed results in upward motion of the cue. Speed, altitude, or vertical rate trend indicators should have appropriate hysteresis and damping to be useful and non-distracting; however, damping may result in erroneous airspeed when accelerating. In this case, it may be necessary to use acceleration data in the algorithms to compensate for the error. The evaluation should include turbulence expected in service.
A.4.6 Airspeed scale graduations in 5-knot increments with graduations labeled at 20-knot intervals are acceptable. In addition, a means to rapidly identify a change in airspeed (for example, speed trend vector or acceleration cue) should be provided on moving scale tapes. If trend or acceleration cues are used, or a numeric present value readout is incorporated in the airspeed display, scale markings at 10-knot intervals are acceptable.
A.4.7 Minimum altimeter graduations should be in 100-foot increments with a present value readout, or 50-foot increments with a present value index only. Due to operational requirements, it is expected that airplanes without either 20-foot scale graduations, or a readout of present value, will not be eligible for Category II low visibility operation with barometrically determined decision heights.
A.4.8 Altimeters present special design problems in that: (1) the ratio of total usable range to required resolution is a factor of 10 greater than for airspeed or attitude, and (2) the consequences of losing sense of context of altitude can be detrimental. The combination of altimeter scale length and markings, therefore, should be adequate to allow sufficient resolution for precise manual altitude tracking in level flight, as well as enough scale length and markings to reinforce the flightcrew’s sense of altitude and to allow sufficient look-ahead room to adequately predict and accomplish level-off. When providing low altitude awareness, it may be helpful to include radio altimeter information on the scale so that it is visually related to the ground position.
A.5 Low- and High-Airspeed Awareness Cues.
A.5.1 Section 25.1541(a)(2) states the airplane must contain: “Any additional information, instrument markings, and placards required for the safe operation if there are unusual design, operating, or handling characteristics.” The part 25 regulations related to instrument systems and their markings were not developed with modern day electronic displays in mind. Consequently, these electronic displays are considered an “unusual design characteristic” according to § 25.1541(a)(2), and may require additional marking to warrant safe operation. In particular, it is considered necessary to incorporate additional markings on electronic airspeed displays in the form of low- and high-speed A-3
Appendix A awareness cues to provide pilots the same type of “quick glance” airspeed awareness that was an intrinsic feature of round dial instruments.
A.5.2 Low-speed awareness cues should provide adequate visual cues to the pilot that the airspeed is below the reference operating speed for the airplane configuration (that is, weight, flap setting, landing gear position, etc.). Similarly, high-speed awareness cues should provide adequate visual cues to the pilot that the airspeed is approaching an established upper limit that may result in a hazardous operating condition. Consider the following guidance when developing airspeed awareness cues:
A.5.2.1 Take into account all independent parameters that may affect the speed against which protection is being provided. This is most important in the low speed regime where all transport category airplanes have a wide range of stall speeds due to multiple flap/slat configurations and potentially large variations in gross weight.
A.5.2.2 The cues should be readily distinguishable from other markings such as V-speeds and speed targets (bugs). The cues should indicate not only the boundary value of the speed limit, but must clearly distinguish between the normal speed range and the unsafe speed range beyond those limiting values (§ 25.1545). Since the moving scale display does not provide any inherent visual cue of the relationship of present airspeed to low or high airspeed limits, many electronic displays use an amber and red bar adjacent to the airspeed tape to provide this quick-glance low/high speed awareness. The preferred colors to be used are amber or yellow to indicate that the airspeed has decreased below a reference speed that provides adequate maneuver margin, changing to red at the stall warning speed.
The speeds at which the low-speed awareness bands start should be chosen as appropriate to the airplane configuration and operational flight regime. For example, low-speed awareness cues for approach and landing should be shown starting at VREF with a tolerance of +0 and –5 knots.
Some FAA approved systems use a pilot selectable operating speed “bug” at VREF supplemented by system-computed low speed cues that vary in color as airspeed decreases below certain multiples of the appropriate stall speed (for example, white below 1.3 VS, amber below 1.2 VS, and red below 1.1 VS). Consider the specific operating needs of other flight regimes when developing the criteria for the associated visual cue.
A.5.2.3 Low-speed awareness displays should be sensitive to load factor (g-sensitive) to enable the pilot to maintain adequate maneuver margins above stall warning in all phases of flight. The accuracy of this gsensitivity function should be verified by flight tests.
A.5.2.4 Flight tests should also be conducted in maneuvering flight and expected levels of turbulence to evaluate proper functioning of any damping routines incorporated into the low-speed awareness software. The level of damping should preclude nuisance/erratic movement of the low speed A-4
Appendix A cues during operation in turbulence but not be so high that it inhibits adequate response to accurately reflect changes in margins to stall warning and stall during maneuvering flight.
A.5.2.4.1 High speed awareness should be provided to prevent inadvertent excursions beyond limit speeds. Symbology should be provided to permit easy identification of flap and landing gear speed limits. A visual cue should be incorporated to provide adequate awareness of proximity to VMO. This awareness has been provided by amber bands, similar to the previously discussed low speed cues, and instantaneous airspeed displays that turn amber (or flash amber digits) as the closure rate to VMO increases beyond a value that sill provides adequate time for pilot corrective action to be taken without exceeding the limit speed.
A.5.2.4.2 The display requirements for airspeed awareness cues are in addition to other alerts associated with exceeding high and low speed limits, such as the stick shaker and aural overspeed warning.
A.6 Vertical Speed.
The display range of vertical speed (or rate of climb) indications should be consistent with the climb/descent performance capabilities of the airplane. If the resolution advisory (RA) is integrated with the primary vertical speed indication, the range of vertical speed indication should be sufficient to display the red and green bands for all TCAS RA information.
A.7 Flight Path Vector or Symbol.
A.7.1 The display of flight path vector (FPV or velocity vector) or flight path angle (FPA) cues on the primary flight display is not required, but may be included in many designs.
A.7.2 The FPV symbol can be especially useful on HUD applications. See paragraph F.5.4.5 of appendix F for HUD-specific FPV considerations.
A.7.3 The FPV or FPA indication may also be displayed on the head-down display (HDD). In some HDD and most HUD applications, the FPV or FPA is the primary control and tracking cue for controlling the airplane during most phases of flight. Even though an FPV or FPA indication may be used as a primary flight control parameter, the attitude pitch and roll symbols (that is, waterline or boresight and pitch scale), which are still required primary indications by § 25.1303, must still be prominently displayed. In dynamic situations, such as during recovery from an unusual attitude, constant availability of attitude indications is required.
A.7.4 If the FPV/FPA is used as the primary means to control the airplane in pitch and roll, the FPV/FPA system design should allow pilots to control and maneuver the airplane with a level of safety that is at least equal to traditional designs based on attitude (§ 25.1333(b)).
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Appendix A A.7.5 There may be existing airplane designs where the HUD provides a FPV presentation and the HDD provides a FPA presentation. However, mixture of the two different presentations is not recommended due to possible misinterpretation by the flightcrew.
The designs that were accepted were found to have the following characteristics:
correlation between the HUD FPV display and the primary flight display FPA display, consistent vertical axis presentation of FPV/FPA, and pilots’ ability to interpret and respond to the FPV and FPA similarly.
A.7.6 It should be easy and intuitive for the pilot to switch between FPV/FPA and attitude when necessary. The primary flight display of FPV/FPA symbology must not interfere with the display of attitude and there must always be attitude symbology at the top center of the pilot's primary field of view, as required by § 25.1321.
A.7.7 Airplane designs that display flight path symbology on the HUD and the HDD should use consistent symbol shapes (that is, the HUD FPV symbol looks like the HDD FPV).
A.7.8 In existing cases where an FPV is displayed head up and an FPA head down on an airplane, the symbols for each should not have the same shape. When different types of flight path indications may be displayed as head up and/or head down, the symbols should be easily distinguished to avoid any misinterpretation by the flightcrew. A mixture of the two types of flight path indications is not recommended due to possible misinterpretation by the flightcrew.
A.7.9 The normal FPV, the field-of-view limited FPV, and the caged FPV should each have a distinct appearance, so that the pilot is aware of the restricted motion or non-conformality.
A.7.10 Implementation of air mass-based FPV/FPA presentations should account for inherent limitations of air mass flight path computations.
A.7.11 Flight directors should provide some lateral movement to the lateral flight director guidance cue during bank commands.
A.7.12 To show compliance with §§ 25.1301(a), 25.1303(b)(5), and 25.143(b), the FPV/FPA FD design must— A.7.12.1 Not have any characteristics that may lead to oscillatory control inputs;
A.7.12.2 Provide sufficiently effective and salient cues to support all expected maneuvers in longitudinal, lateral, and directional axes, including recovery from unusual attitudes; and A.7.12.3 Not have any inconsistencies between cues provided on the HUD and HDD displays that may lead to pilot confusion or have adverse effects on pilot performance.
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Appendix A A.7.13 Performance and system safety requirements for flight guidance systems are found in the following ACs:
• AC 25.1329-1B, Change 1, Approval of Flight Guidance Systems.
• AC 120-28D, Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout.
• AC 120-29A, Criteria for Approval of Category I and Category II Weather Minima for Approach.
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Appendix B APPENDIX B. POWERPLANT DISPLAYS B.1 General.
B.1.1 At the time § 25.1305 was adopted, flight deck powerplant displays were primarily a collection of dedicated, independent, full-time analog “round dial” type instruments.
Typically, there was one display for each required indication. Today, flight deck powerplant displays are primarily electronic displays integrated with other flight deck displays on a few relatively large electronic display spaces. Throughout this technological evolution, the FAA has used certification issue papers and other guidance material to assure that this new technology, with its increased potential for common faults and the challenges of effectively sharing display space, did not adversely impact the timely availability and independence of the powerplant information required to meet the intent of § 25.1305. This AC provides some of that guidance material.
B.1.2 To comply with one of the provisions of § 25.1305, a display should provide all the instrument functionality of a full-time, dedicated analog type instrument as intended when the rule was adopted. (See AC 20-88A, Guidelines on the Marking of Aircraft.)
The design flexibility and conditional adaptability of modern displays were not envisioned when § 25.1305 and § 25.1549 were initially adopted. In addition, the capabilities of modern control systems to automate and complement flightcrew functions were not envisioned. In some cases these system capabilities obviate the need for a dedicated full-time analog type instrument.
B.1.3 When making a compliance finding, all uses of the affected displays should be taken into consideration, including— B.1.3.1 Flight deck indications to support the approved operating procedures (§ 25.1585), B.1.3.2 Indications as required by the powerplant system safety assessments (§ 25.1309), and B.1.3.3 Indications required in support of the instructions for continued airworthiness (§ 25.1529).
Note: For example, compliance with § 25.1305(c)(3) for the engine N2 rotor was originally achieved by means of a dedicated, full time analog instrument. This provided the continuous monitoring capability required to—
• Support engine starting (for example, typically used to identify fuel on point);
• Support power setting (for example, sometimes used as primary or back up parameter);
• “Give reasonable assurance that those engine operating limitations that adversely affect turbine rotor structural integrity will not be exceeded in service” as required by § 25.903(d)(2);
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Appendix B
• Provide the indication of normal, precautionary, and limit operating values required by § 25.1549; as well as
• Support detection of unacceptable deterioration in the margin to operating limits and other abnormal engine operating conditions as required to comply with §§ 25.901,
25.1309, etc.
B.1.4 As technology evolved full authority digital engine controls (FADECs) were introduced. The FADECs were designed with the ability to monitor and control engine N2 rotor speed as required to comply with § 25.903(d)(2). Additionally, engine condition monitoring programs were introduced and used to detect unacceptable engine deterioration. Flight deck technology evolved such that indications could be displayed automatically to cover abnormal engine operating conditions. The combination of these developments obviated the need for a full time analog N2 rotor speed indication, in accordance with the guidance found in paragraph 6.3.3 of chapter 6 of this AC.
B.2 Design Guidelines.
B.2.1 Safety-related engine limit exceedances should be indicated in a clear and unambiguous manner. Flightcrew alerting is addressed in § 25.1322.
B.2.2 If an indication of significant thrust loss is provided, it should be presented in a clear and unambiguous manner.
B.2.3 In addition to the failure conditions listed in chapter 3 of this AC, the following design guidelines should be considered:
B.2.3.1 For single failures leading to the non-recoverable loss of any indications on an engine, sufficient indications should remain to allow continued safe operation of the engine. (See §§ 25.901(b)(2), 25.901(c), and
25.903(d)(2)).
B.2.3.2 No single failure could prevent the continued safe operation of more than one engine or require immediate action by any flightcrew member for continued safe operation. (See §§ 25.901(c), 25.903(b), and 25.1309(b)).
B.2.3.3 Engine indications needed during engine re-start should be readily available after an engine out event. (See §§ 25.901(b)(2), 25.901(c)
25.903(d)(2), 25.903(e), 25.1301, 25.1305, 25.1309, and paragraph 6.3.3 of chapter 6 of this AC).
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Appendix C APPENDIX C. DEFINITIONS C.1 Air Mass System:
An air mass-based system that provides a heading/airspeed/vertical velocity derived flight path presentation. It depicts the flight path through an air mass, will not account for air mass disturbances such as wind drift and windshear and, therefore, cannot be relied on to show the flight path relative to the earth’s surface.
C.2 Alert:
A generic term used to describe a flight deck indication meant to attract the attention of, and identify to the flightcrew, a non-normal operational or airplane system condition.
Warnings, cautions, and advisories are considered to be alerts.
C.3 Annunciation:
A visual, auditory, or tactile stimulus used to attract a flightcrew member’s attention.
C.4 Architecture:
The manner in which the components of a display or display system are organized and integrated.
C.5 Basic T:
The arrangement of primary flight information as required by § 25.1321(b), including attitude, airspeed, altitude, and direction information.
C.6 Brightness:
The perceived or subjective luminance. This should not be confused with luminance.
C.7 Bugs:
A symbol used to mark or reference other information such as heading, altitude, etc.
C.8 Catastrophic:
Failure conditions that result in multiple fatalities, usually with the loss of the airplane.
Note: In previous versions of § 25.1309 and the associated advisory material, a “catastrophic failure condition” was defined as a failure condition that would prevent continued safe flight and landing.
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