30-second and 2-minute power ratings using terminology similar to that developed for the 2½-minute and 30-minute power ratings. Additionally, these paragraphs require the ability to detect any damage which occurs due to the use of either 30-second or 2-minute OE limits and requires that the procedures to inspect for such damage be provided in the instructions for continued airworthiness for either the engine and/or the airframe.
b. Procedures. All of the policy material pertaining to this section remains in effect. Additionally, the following procedures should be considered:
Sections 27.1521(j) and (k) require limitations for 30-second/2-minute OEl operation for multi-turbine engine powered rotorcraft. The same parameters required for the takeoff and continuous ratings should be established as limitations for each approved OEI rating (i.e., maximum rotational speed, time, gas temperature, and torque). These new ratings can only be approved as a rating in conjunction with the other. That is, a rotorcraft with a 30-second OEI rating must also have a 2-minute OEI rating and vice-versa. The 30-second and 2-minute OEI ratings are also limited to use for continued operation of the remaining engine(s) upon failure or precautionary shutdown of an engine. Upon the use of 30-second or 2-minute OEI ratings an inspection for damage to the airframe and/or engine may need to be conducted. The inspection should be accomplished per the procedures furnished by the airframe and engine manufacturers and any damage occurring due to the use of these new ratings should be detected using these inspection procedures. Section 27.923 includes requirements for qualification of the rotor drive system for 30-second and 2-minute OEI powers.
Section 27.1501(a) requires that information necessary for safe operation should be established as limitations. The limitation information provided in this paragraph should be provided in the flight manual. This includes a possible requirement for an inspection prior to further flight after the use of either 30-second or 2-minute OEI rating.
AC 27.1523. § 27.1523 MINIMUM FLIGHTCREW
a. Explanation.
(1) This rule requires that the minimum crew necessary to show compliance with the requirements of Part 27 or for safe operation of the rotorcraft be established as an operating limitation.
(2) The determination of minimum crew requirements is typically based on a subjective pilot assessment of the crew requirements for safe operation of each rotorcraft design. Certain regulations, such as the requirements for instrument flight rules (IFR), have specific quantitative differences between single-pilot and two-pilot requirements. However, most often the minimum crew requirement will be based on more subjective considerations such as location of necessary controls, pilot workload to accomplish required tasks, type of operation, and overall complexity of the rotorcraft design.
Page G - 11
(3) Minimum crew requirements for the same type design may vary with the kind of operation. Many rotorcraft have been approved for a single-pilot crew for visual flight rules (VFR) operations but require a two-pilot crew for IFR operations. Other kinds of operations that may require more than one crewmember to meet type certification requirements are night operations, operations into known icing conditions, operations in falling and blowing snow, extended overwater operations, and external load operations.
(4) It is important to distinguish between the minimum crew requirements for compliance with Part 27 type certification regulations and the minimum crew requirements of the various operating regulations (Parts 61, 91, 121, 133, 135, and
137). A rotorcraft may be type certified for a minimum crew of one and still be required to have a crew of two or more by the operating regulations for certain types of operation or by the workload associated with an operating environment. Therefore, an applicant should carefully consider the possible operational uses of any rotorcraft design and become familiar with the applicable operating regulations as well as the type certification requirements early in the design process.
(5) Although the rotorcraft configuration is typically certified with the pilot-in-command station in the right seat, the left seat may be used for the pilot-in-command if, in addition to the flight controls required to control the rotorcraft, the following are included for the pilot: throttle control including ability to shut down all engines, airspeed indication, altitude indication, rotor and engine RPM, and engine torque and exhaust gas temperature. The authority should evaluate a change to the pilot-in-command station.
(6) The applicant is encouraged to contact the responsible type certification office as early in the design phase as possible to initiate the qualitative assessment process. Cockpit layout drawings, instrument panel mockups, and full-scale cockpit mockups can be used to determine if required controls are accessible and to begin the pilot workload assessment for certain operations.
b. Procedures.
(1) General.
(i) A systematic evaluation and test plan is required for any new or modified rotorcraft. The methods for showing compliance should emphasize the use of acceptable analytical, simulation, and flight test techniques. The crew complement should be studied through a logical process of estimating, measuring, and then demonstrating the workload imposed by a particular cockpit design. When the minimum crew requirements have been determined, they should be included in the limitations section of the Rotorcraft Flight Manual in accordance with § 27.1583(d).
(ii) Appropriate analysis should be conducted by the applicant early in the design process. The specific method(s) of analysis should be selected on the basis of Page G - 12
its predictive validity, sensitivity, reliability, applicability to the particular cockpit configuration, and availability of a suitable reference for comparison.
(2) Analytical Approach.
(i) One analytical approach defines workload as a percentage of the time available to perform tasks (Time Line Analysis). This process may be applied to an appropriate set of flight segments in which operationally important time constraints can be identified. This method is useful for evaluation of cockpit changes relating to overt pilot work such as control movements and data inputs. The generally accepted practice involves careful selection of the limited set of flight scenarios and time segments that represent the range of operational requirements (including the range of normal and non-normal procedures.) Time line analysis yields useful data when tasks must be performed within operationally significant time constraints. The adequacy of this method is very much dependent on an accurate determination of the time available.
Absolute standards are not available for interpretation of obtained time required scores, but such records can be used to identify high or simultaneous workload demands for later testing in a simulator or aircraft, and comparisons can be made with overt workload demands in proven aircraft. However, the impact of cockpit changes on planning and decision-making is difficult to quantify by this method.
The most frequently used basis for deciding that a new design is acceptable is a comparison of a new design with previous designs proven in operational service. By making specific evaluations using the acceptable human factors techniques, and comparing new designs to a known baseline, it is possible to proceed with confidence that the changes incorporated in the new designs accomplish the intended result. When the new cockpit is considered, certain components may be proposed as replacements for conventional items, and some degree of rearrangement may be contemplated. New avionics systems may need to be fitted into existing panels, and newly automated systems may replace current indicators and controls. As a result of this evolutionary characteristic of the cockpit design process, there is frequently a reference cockpit design, which is usually a conventional aircraft that has been through the test of operational usage. If the new design represents an evolution, improvement attempt, or other deviation from this reference cockpit, the potential exists to make direct comparisons. Service experience should be researched to assure that any existing problems are understood and not perpetuated.
(•) If preliminary analyses by the certification team identify potential problem areas, these areas should receive more extensive evaluation and data collection in order to verify compliance with § 27.1523. These concerns should be adequately addressed in the manufacturer's demonstration plan when submitted to the FAA/AUTHORITY.
(iv) If the new design represents a significant change in level of automation or pilot duties, analytic comparison to a reference design may have lessened value. Without a firm data base on the time required to accomplish both Page G - 13
normally required and contingency duties, more complete and realistic simulation and flight testing will be required.
(3) Testing.
In the case of the minimum crew determination, the final decision is reserved until the rotorcraft has been flown by experienced flight test pilots trained and current in the aircraft. More assurance is derived from actual flight tests than from earlier simulator tests or other synthetic or computer model procedures.
(її) The test program should address the workload functions and factors listed below. For example, an evaluation of communications workload should include the basic workload required to properly operate the aircraft in the environment for which approval is sought. The goal is to evaluate workload with the proposed crew complement during realistic operating conditions, including representative air traffic and weather.
(A) Basic workload functions. The following basic workload functions are considered:
(1) Flight path control.
(2) Collision avoidance.
(3) Navigation.
(4) Communications.
(5) Operation and monitoring of aircraft engines and systems.
(6) Command decisions.
(B) Workload factors. The following workload factors are considered significant when analyzing and demonstrating workload for minimum flight crew determination:
(1) The accessibility, ease, and simplicity of operation of all necessary flight, power, and equipment controls, including emergency fuel shutoff valves, electrical controls, electronic controls, and engine controls.
(2) The accessibility and conspicuity of all necessary instruments and failure warning devices such as fire warning, electrical system malfunction, and other failure or caution indicators. The extent to which such instruments or devices direct the proper corrective action is also considered.
Page G - 14
(3) The number, urgency, and complexity of operating procedures with particular consideration given to the specific fuel management schedule imposed by center of gravity, structural or other considerations of an airworthiness nature, and to the ability of each engine to operate at all times from a single tank or source which is automatically replenished if fuel is also stored in other tanks.
(4) The degree and duration of concentrated mental and physical effort involved in normal operation and in diagnosing and coping with malfunctions and emergencies.
(5) The extent of required monitoring of the fuel, hydraulic, electrical, electronic, deicing, and other systems while en route.
(6) The actions requiring a crewmember to be unavailable at his assigned duty station, including: observation of systems, emergency operation of any control, and emergencies in any compartment.
(Z) The degree of automation provided in the aircraft systems to afford (after failures or malfunctions) automatic crossover or isolation of difficulties to minimize the need for any flight crew action to guard against loss of hydraulic or electric power to flight controls or to other essential systems.
(8) The communications and navigation workload.
(9) The possibility of increased workload associated with any emergency that may lead to other emergencies.
AC 27.1525. § 27.1525 (Amendment 27-21) KINDS OF OPERATION.
This rule states that the kinds of operation to which the rotorcraft is limited are established by demonstrated compliance with applicable certification requirements (primarily flight) and the equipment requirements established for that kind of operation.
The basic flight characteristics requirements of Part 27 are suitable for day VFR approval. Additional night considerations appear in § 27.141(c) and in the operating rules. IFR requirements are addressed in § 27.141(c) and Appendix B to Part 27.
Additional IFR equipment requirements are contained in the operating rules. External load requirements for certification may be found in §§ 27.25(c) and 27.865(c) in addition to Part 133. Related § 27.1583(d) further requires that the approved kinds of operation must be listed in the operating limitations section of the Rotorcraft Flight Manual. That equipment necessary to comply with applicable airworthiness requirements of Part 27 should also be listed in the limitations section of the flight manual.
Page G - 15
AC 27-1B, Chg 1 2/12/03 AC 27.1527. § 27.1527 (Amendment 27-14) MAXIMUM OPERATING ALTITUDE.
a. Explanation. This rule requires that the maximum altitude for operation of the rotorcraft must be established as an operating limitation. The rule is intended to establish en route altitude as an operating limit. The requirements for maximum takeoff and landing altitude are contained in other portions of the rule. (See discussion in paragraph AC 27.143.) The en route limit may be established by any of the preceding subparts of the rule involving flight, structure, powerplant, equipment or related functional requirements of those subparts. Maximum operating altitude is ordinarily specified initially by the manufacturer and substantiated throughout the type certification program by each engineering discipline. Maximum operating altitude must be established in terms of pressure altitude unless the pilot is provided with some equally functional means of observing specified altitude limits (e.g., a density altitude indicator if maximum altitude is specified in terms of density altitude). A related requirement in § 27.1583 specifies that maximum operating altitude must be established as an operating limitation in the RFM and further that any limiting factors must be identified and explained.
b. Procedures. Each FAA/AUTHORITY engineering discipline must ensure that data and testing are adequate to properly substantiate and quality all critical components to the maximum operating altitude of the rotorcraft. The design or maximum substantiated altitude should be specified in the Type Inspection Authorization. The flight test program must include at least one test flight to the maximum approved pressure altitude. This flight should include functional testing of all critical aircraft components. Although altitude extrapolation of performance and flying qualities test results may be allowed, an altitude limit higher than the maximum pressure altitude at which functional capability of critical aircraft systems has been demonstrated by flight test should not be approved.
AC 27.1529. § 27.1529 (Amendment 27-18) INSTRUCTIONS FOR CONTINUED AIRWORTHINESS (MAINTENANCE MANUAL).
a. Airworthiness Limitations Section
(1) Explanation. The FAA/AUTHORITY has long recognized the necessity to have a maintenance manual for rotorcraft due to the unique and generally complicated and critical design features.
Amendment 27-3, in 1968, established the requirement for a specific airworthiness limitations section. Amendment 27-18, in 1980, revised the rule and added Appendix A containing requirements for preparation of instructions for continued airworthiness, including the airworthiness limitations section. The operating and maintenance rules require compliance with the airworthiness limitations section. The maintenance rules §§ 43.15 and 43.16 and § 91.163(c) of the operating rules also refer to or require compliance with certain parts of the instructions for continued airworthiness. The limitations were intended to "define the limits of this type certification Page G - 16
approval of the fatigue characteristics of critical flight structure." Refer to FAA Order 8620.2, Applicability and Enforcement of Manufacturer's Data, November 2, 1978, for further information.
(ii) Critical components must be identified by part number (or equivalent) and serial number (or equivalent). Section 27.1529(a)(1) and (2) of Amendment 27-3 and/or § 45.14 list the requirements. The part numbers of parts and/or components requiring inspections and/or replacement as a result of § 27.571 must be listed in the airworthiness limitations section of the manual or another separate, segregated section of the manual appropriate to the rules.
(¡¡i) Rotorcraft type designs are unique in comparison to airplane designs in that transmissions and rotors and some elements of flight control systems have critical components that may be adversely affected by operating conditions and time in service. The FAA/AUTHORITY-approved airworthiness limitations section may include such items as gear sets, bearings, etc., of the rotorcraft type design if a finite life was established during the type certification program and/or if the FAA/AUTHORITY determined that mandatory inspections and/or replacement of the component (part) was necessary to maintain airworthiness of the rotorcraft. For example, a drive spline, gear, or bearing was serviceable after concluding the ground endurance test and/or FAA/AUTHORITY flight test program. However, an FAA/AUTHORITY-mandated inspection or replacement of the component was considered essential for airworthiness of the rotorcraft type design and necessary for type certification. Time between overhaul (TBO) of components is not part of the airworthiness limitations but is a recommendation from the manufacturer (See Part 27, Appendix A, A27.3(b)(1)). If an inspection or replacement of a part in an assembly is required, the inspection interval or replacement time and the part number should be included in the limitations. The inspection interval or replacement time may or may not coincide with the recommended overhaul interval of the assembly. (See the comments for Proposal 8-25, § XX.4 in the preamble of Amendment 27-20 (45 FR 60154), September 11, 1980). Note that parts considered unserviceable at the conclusion of the ground endurance test of § 27.923 are not acceptable for type certification.
(2) Procedures.
General.
(A) The rule states that the manual must contain all information that the applicant considers essential for proper maintenance. Amendment 27-3 added the requirement for an airworthiness limitations section and Amendment 27-18 revised § 27.1529 and added Appendix A that contains the requirements for preparation of the manual. The airworthiness limitations section of the manual, and any revisions thereto, must be FAA/AUTHORITY approved. The manufacturer's recommendations for continued airworthiness are not FAA/AUTHORITY approved.
Page G - 17
AC 27-1B, Chg 1 2/12/03 (B) The airworthiness limitations section contains information derived primarily but not solely from the data approved under § 27.571. In addition to replacement times and inspections, where appropriate, some of the basic usage assumptions made in the fatigue evaluation, which the operator can reliably assess (such as numbers of ground-air-ground cycles) should be identified. These should be noted in the airworthiness limitations so that the operator may take appropriate action (see MG 8 and MG 11). Approval of this section of the manual (draft form is acceptable) must be accomplished before type certification. See Part 27, Appendix A, paragraph A27.4 of Amendment 27-17. (For further information, see comments for Proposal 8-25, § XX.4, in the preamble of Amendment 27-18 (45 FR 60154, September 11, 1980).)
(C) Part 27, Appendix A, paragraphs A27.3(a) and (b) pertain to the content of the instructions for continued airworthiness. For example, scheduling and servicing information is included in this section of the manual.
(її) Identifying and Serializing Fatigue Critical Components.
(A) Part numbers and serial numbers must be applied to fatigue critical components as noted in § 45.14 and § 27.1529(a)(1) and (2). Electric arc marking method should not be used due to possible internal arcing, pitting of surfaces, and changes in physical or chemical characteristics due to the local high temperature at the arcs.
(B) Vibrating pencils, name plates, or permanent inks may be used.
However, serial numbers should be applied on each part such that material is upset or displaced on the part, thereby attaining a more permanent number. When material is upset or displaced, the component's integrity and fatigue tolerance must not be compromised, for example, the least critical or lowest stressed area should be used.
(C) For small parts, the rule (§ 45.14) allows markings that are equivalent to part and serial numbers. Markings or symbols may be used to enable the identification of a part as one for which a replacement time, inspection interval, or related procedure is specified in the airworthiness limitations section. The FAA/AUTHORITY-stated identification of such small parts is clearly essential for safety and may not be relieved. With adoption of Amendment 27-18, the marking requirements are contained in § 45.14, Amendment 45-12.
(iii) A draft copy of the manual should be available to the FAA/AUTHORITY for use during the F&R program if such a program is conducted under § 21.35(b). The content of the manual may be limited to the information necessary to maintain the aircraft during the F&R program. The manual must be completed and furnished with each aircraft receiving an airworthiness certificate, § 21.50(a) and (b).
Page G - 18
(iv) Service experience may dictate additional and subsequent (to type certification) changes to the airworthiness limitations section. ADs may be used to revise the limitations. (The relationship between ADs and the process of changing the limitations is covered in the preamble of Amendment 27-4 (33 FR 14104;
September 18, 1968.) Whenever the revised limitations are made restrictive for aircraft in service, the Administrative Procedures Act requires "notice and public procedure" to persons that may be affected and to satisfy the requirement for notification of the changes and identification of the correct issue of the airworthiness limitations, if appropriate. This procedure is also used for restrictive or reduced operation limitations in the RFM.
b. How to Prepare Instructions for Continued Airworthiness
(1) Explanation. The FAA/AUTHORITY recognized the need for Instructions for Continued Airworthiness to maintain the rotorcraft in an airworthy condition.
(i) Amendment 27-18, 45 FR 60177, September 11,1980, states the applicant must prepare Instructions for Continued Airworthiness, in accordance with Appendix A of this part, that are acceptable to the Administrator.
(ii) Section 21.50(b), Amendment 21-51, 45 FR 60170, September 11, 1980, requires the holder of a design approval to furnish at least one set of complete Instructions for Continued Airworthiness prepared in accordance with § 27.1529 of this chapter. The design approval can include either the type certificate or supplemental type certificate for an aircraft or aircraft engine for which application was made after January 28, 1981. The approved and accepted set of complete Instructions for Continued Airworthiness should be furnished upon delivery or upon issuance of the first standard airworthiness certificate for the affected aircraft, whichever occurs later.
The ICAs or ICA supplements are also required for PMAS, major design change, or alteration approvals that the existing ICAs do not adequately cover.
In such instances, it is the responsibility of the PMA holder, or the person who receives the design change or alteration approval to produce the required ICAs and distribute them in accordance with the requirements of § 21.50(b). The acceptance for these ICAs or supplements should follow the procedure in Section b(2).
(2) Procedure General. When the rule requires Instructions for Continued Airworthiness for a rotorcraft, its rotors, and appliances, the applicant must prepare Instructions for Continued Airworthiness in accordance with Appendix A to FAR/JAR Part 27, that are acceptable to the FAA/AUTHORITY.
(ii) Guidance. The FAA/AUTHORITY has guidance material to assist an applicant in preparing Instructions for Continued Airworthiness for the type design change. AC 27-1B, Appendix A, provides a detailed template describing the Page G - 19
AC 27-1B, Chg 1 2/12/03 requirements, standard industry practices, and guidance for Instructions for Continued Airworthiness.
(II) Preparation. The applicant must prepare Instructions for Continued Airworthiness that is applicable to the type of certification. The holder of a design approval for a type certificate normally will comply with all requirements specified in Appendix A to FAR/JAR Part 27. The holder of a design approval for a supplemental type certificate would only comply with requirements specified in Appendix A to FAR/JAR Part 27 that are applicable to their type design change.
(IV) Submittal. The applicant must submit to the FAA/AUTHORITY the applicant's Instructions for Continued Airworthiness, and any authorized publication referenced in the applicant's Instructions for Continued Airworthiness, i.e., FAA/AUTHORITY-accepted engine and appliance Instructions for Continued Airworthiness.
(v) Evaluation.
(A) The FAA/AUTHORITY will evaluate the applicant's Instructions for Continued Airworthiness to determine that they meet the requirements of FAR/JAR § 27.1529 and Appendix A to FAR/JAR Part 27.
(B) For a type certificate, the determination is made in two parts. The first is an FAA/AUTHORITY review, followed by FAA/AUTHORITY personnel conducting an Aircraft Maintainability Evaluation or equivalent, when requested to determine the acceptability of the Instructions for Continued Airworthiness. For a supplemental type certificate the determination is made by an FAA/AUTHORITY review, unless the supplemental type certificate affects multiple major appliances or systems; then the process for type certificate would be used.
(vi) Acceptance as per Authority procedures. For FAA, acceptance of the Instructions for Continued Airworthiness is indicated by a signed and dated acceptance statement on the List of Effective Pages in the Instructions for Continued Airworthiness.
AC 27.1529A. § 27.1529 (Amendment 27-23) INSTRUCTIONS FOR CONTINUED AIRWORTHINESS (MAINTENANCE MANUAL).
a. Explanation. Amendment 91-21, 54 FR 41211, October 5, 1989, recodified certain paragraphs in FAR Part 91. This revision corrects a reference from FAR § 91.163 to FAR § 91.403.
b. Procedures. Correct the references in paragraph AC 27.1529a(1) from §§ 43.15, 43.16, and 91.163(c) to §§ 43.15, 43.16, and 91.403 of the operating rules.
Page G - 20
SUBPART G - OPERATING LIMITATIONS AND INFORMATION
MARKINGS AND PLACARDS AC 27.1541. § 27.1541 MARKINGS AND PLACARDS - GENERAL.
See paragraph AC 27.1543.
AC 27.1543. §§ 27.1541, 27.1543, 27.1545, AND 27.1549 (Amendment 27-16) INSTRUMENT MARKINGS: GENERAL.
a. Background and Explanation.
(1) Aircraft instruments have historically been marked in a variety of ways and with an interesting assortment of symbols. A limited number of regulatory requirements have been incorporated in part 27, Subpart G, "Markings and Placards," and these efforts have standardized some basic aspects of instrument marking for rotorcraft. As rotorcraft have become increasingly complex with an increased number of engines, OEI ratings, more sophisticated instrumentation, etc., the need for more specific standards has greatly increased.
(2) It is vitally important that instrument markings be standardized among rotorcraft. When markings are not standardized, considerable contusion and additional workload may be introduced into the cockpit environment. If markings are not standard, it is conceivable that a marking in one rotorcraft could mean the opposite of a similar marking in another rotorcraft. The results of such a situation could be troublesome when pilots fly several rotorcraft models.
(3) The following guidance is offered for the purpose of obtaining a general standardization of instrument markings. It is realized that there are a great many variations in instrument presentations for which all guidance may not apply. This is particularly true of new designs, such as cathode ray tube (CRT) displays currently being presented. It is of overriding importance that the philosophies included here be administered, even if specific guidance cannot be applied for particular designs.
Instrument markings are provided to aid interpretation of instruments quickly and accurately. Good instrument markings should indicate operating conditions at a glance.
The best markings are ordinarily the simplest markings.
(4) AC 20-88A, dated 9/30/85, Guidelines on the Marking of Aircraft Powerplant Instruments (Displays), should be used in conjunction with this advisory material for rotorcraft.
b. Procedures.
Page G - 21
AC 27-1B, Chg 4 3/31/2014
(1) Limits. Each maximum allowable limit substantiated for safe operation must be marked with a red line. This marking should be a red radial line for circular gauges.
If there is a minimum allowable limit for safe operation, this value should also be marked with a red radial line. The use of multiple red radial lines should be avoided except where their use is readily apparent to the flight crewmember. Normally, no more than one maximum and one minimum red radial line should be incorporated on any one instrument to minimize confusion and avoid potential flightcrew errors; however, use of multiple red radial lines may be permitted if such markings can be presented in an acceptable manner.
(2) Normal Operating Range. Each normal operating range should be marked with a green arc or green line which does not extend beyond the maximum and minimum values for continuous safe operation. Discontinuities in width have been used when normal ranges vary with other parameters. Integrating instruments in place of these markings should be encouraged although there may be no regulatory requirement for them. An equivalent safety finding is required for electronic displays if the applicant decides not to use green to mark normal operating ranges.
(3) Cautionary Ranges. Time limited ranges, precautionary ranges, or ranges for which special operating procedures are required should be marked with a yellow arc or yellow line. If a yellow range is used to indicate a special operating procedure, information describing the special procedure should be included in the RFM.
(4) OEI Markings. OEI ratings represent a special challenge for retaining simplicity and clarity in powerplant instrument markings. OEI ratings are eligible to be used only during an extremely small portion of total flight time; therefore, they should not dominate the presentation or obscure other markings. They are needed only for reference. Indices for 2 ½-minute and 30-minute power may be marked above the takeoff power redline on engine power instruments. OEI reference markings should be clearly distinct from the normal all-engines-operating markings. One acceptable means of marking OEI limits has been narrow dashed radials with yellow for 30-minute and red for 2 ½-minute limits. OEl markings should be consistent between gauges. For example, a 30-minute marking on an N1 or torque gauge should be similar in appearance to the 30-minute marking on the engine temperature gauge.
(5) Red Arcs or Ranges. Sections 27.1549(d) and 27.1553 allow the use of red arcs. Experience has proven that when red arcs are used to indicate maximum or minimum values, the meaning of a red line loses its significance. Therefore, the use of red ranges or arcs to indicate limit values should be discouraged. Red is conventionally used to represent a limit (maximum or minimum) for which an aircraft or component has been substantiated. A "range" of limits for a given parameter is not consistent with the definition of the terms "limit," "minimum," or "maximum." In addition, a red arc tends to imply that more than one value is limiting, that a scale is provided to show operation within a range of values, and that an absolute limit may not exist until the extreme of a red range is attained. These implications must be avoided wherever possible by Page G - 22