(5) A failure mode and effects analysis (FMEA) should establish that the failure or malfunction of any engine or the failure of any system that can affect any engine will not-- Prevent the continued safe operation of the remaining engines for the appropriate limited time period.
(iI) Require immediate action by a crewmember for continued safe operation.
(6) As cited earlier, by example, selection of specific engine isolation rules from Part 29 is not effective in assuring that a sudden, complete engine power loss does not occur.
(7) Under the limited time period concept, failure of the second engine must be considered upon expiration of the limited time period. The Rotorcraft Flight Manual must provide the appropriate operating limitations, pilot operating procedures, and performance information limitations to assure continued safe operation following failure of the second engine.
Par 780 1075
TABLE 780-1 Part 29 Engine Isolation Rules § 29.861(a) $ 29.901(c) § 29.903(b) and (c) and (e) $ 29.908(a) § 29.917(b) and (c)(1) § 29.927(c)(1) $ 29.953(a) § 29.1027(a) § 29.1045 § 29.1047(a) § 29.1181(a) § 29.1189(c) § 29.1191(a)(1) § 29.1193(e) § 29.1195(a) and (d) § 29.1197 § 29.1199 § 29.1201 § 29.1305(a)(6) and (b) § 29.1309(b)(2)(i) and (d) § 29.1331(b) 1076 Par 780
781. §§ 27.1541, 27.1543, 27.1545, AND 27.1549 (Amendment 27-16) INSTRUMENT MARKINGS.
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 confusion 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) Advisory Circular 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.
(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 gages. 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.
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АС 27-1A 7/30/97
(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.
(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 Rotorcraft Flight Manual.
(4) OEL Markings. OEI ratings represent a special challenge for retaining simplicity and clarity in powerplant instrument markings. OE 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 2½-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. OLl markings should be consistent between gages. For example, a 30-minute marking on an N, or torque gage should be similar in appearance to the 30-minute marking on the engine temperature gage.
(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 specifying a single limiting value and marking it with a single red line (radial). If readings in excess of that value were indicated, it would then be obvious to the crew that a limit had been exceeded. A red arc may be used to indicate a transient vibration range as indicated in § 27.1549(d); however, if the range is a cautionary range and not a prohibited range, use of a yellow arc is recommended. The fuel quantity indicator configuration described in § 27.1553 is considered a special application of red arcs.
Occasionally a red arc has been utilized when limits vary with other parameters.
Discontinuities in width could conceivably represent limits when other parameters are 1078 Par 781
considered. The use of integrating instruments would alleviate much of the problem and should be encouraged although there may be no regulatory requirement for them.
(6) Flight Evaluation. In evaluating quantity indicator markings, the final criterion must be: "Are the markings adequate for correct interpretation by the crew?"
FAA/AUTHORITY evaluations of quantity indicator markings should begin early in a certification program utilizing a cockpit or aircraft mock-up whenever possible. All required quantity indicators and quantity indicator markings must be readable from each pilot station. Depending on cockpit and window geometry, quantity indicators should be evaluated in direct sunlight unless they are located high on the panel underneath a substantial glare shield. Evaluation in direct sunlight is especially important for any displays using light bars or digital lighting segments, such as digital radar altimeter presentations or vertical scale instruments using light segments. Required quantity indicators must be readable without upper body movement or extensive head movement by the crew. Evaluators should be especially alert to any scale markings or range markings which are obscured by parallax, since these teatures are unacceptable.
If the aircraft is to be approved for night operation, each required indicator must also be evaluated during night lighting conditions. The same visibility requirements apply for night. The evaluator should look particularly for lighting features which may change, mask, or obscure the colored instrument markings. For example, in one case, red indicator markings were totally obscured by red instrument lighting. The use of red lighting is satisfactory if the instrument markings are visible even though the color of the markings may be distorted. Except for minor changes, lighting should be evaluated in flight to correctly evaluate vibration effects and various background lighting conditions.
(7) Digital Instruments.
For purposes of this discussion, the two types of digital indicators considered are an indicator which consists of a column of light segments which illuminate sequentially to display changing values, and an indicator which consists of horizontal and vertical line segments in the configuration of a block "g to display numerical values. Both indicator types work well for parameters where trend information is generally not needed such as engine oil pressure or temperature.
However, for rapidly changing parameters such as engine exhaust gas temperature, torque, or RPM, trend information may not be attainable. Advisory Circular 20-88A specifies that instrument markings are intended to provide necessary information at a glance. Trend information for power indicators is vitally important for safe operation of a rotorcraft, and this intormation must be obtainable at a glance. For the columnar light segments, the ability to detect quickly trend information is largely a function of the resolution provided by single segments (e.g., if there are two segments for each percent RPM, the ability to detect trend information is better than if there is only one segment for each percent RPM). For digital indicators displaying numerical values, trend information may be unattainable because rapidly changing parameters produce a blur, and this design may be unsuitable as a single source of information. The Par 781 1079
АС 27-1A 7/30/97 evaluator should use a great deal of caution to ensure adequate trend information is available in primary power and rotor indicators of digital design.
(її) Another area of concern in digital and moving tape instruments is the ability to determine when limits are being approached. Color code markings are frequently incorporated on the moving face of a tape or digital presentation. In such cases, it is mandatory that limit markings be affixed adjacent to the presentation, or that another means be provided so that the pilot can anticipate approaching a limit. The beginning and end of normal and cautionary ranges should be marked adjacent to the display. The entire range need not be color coded adjacent to the display if the colors are integral on the face of the tape or in the individual digital segments. Marking of limit values solely on the tape or in the colored light segments alone is unsatisfactory.
Marking of digital indicators displaying numerical values is adequately addressed in AC 20-88A, Paragraph 3.
(ш) Appropriate failure modes should be evaluated during the system analysis. This will ordinarily include portions of the digital display. Such failures should be detectable whenever they affect reading accuracy. As a result of this analysis, the system may incorporate a test feature which ensures all digital segments operate satisfactorily. This feature should be encouraged.
(8) Additional Markings. To keep markings standardized and uncomplicated, only the FAA/AUTHORITY-approved ranges and limits should be included. Items such as manufacturer's recommended values or manufacturer's warranty information are inappropriate for instrument markings and should not be included. Such information may be presented elsewhere. Transient limits may be indicated by a small red index such as a dot or triangle. Information defining allowable conditions for each transient index should be in the Rotorcraft Flight Manual (e.g., maximum for starting - 12 seconds).
(9) Airspeed Indicator. While the foregoing information is generally applicable to airspeed indicators, some particular features warrant additional attention.
A red cross-hatched radial line should be located at power-off VNE if that value is less than power-on VNE•
(i) Many rotorcraft have erratic, unreliable, or nonrepeatable airspeed indications at low speed which warrant caution when operating in that speed range. In such cases, a yellow arc on the instrument with an appropriate flight manual explanation has been found acceptable.
(i¡i) Indicated airspeed values should be utilized for all airspeed indicator markings.
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(iV) Airspeed "bugs" may be used to highlight important takeoff, landing, or limit speeds. This concept may generally be encouraged; however, there are a maximum number of "bugs" that can be utilized without confusion for any given indicator. Typically, two "bugs" are acceptable and three or more are questionable.
"Bugs" may also be used on a variety of instruments other than the airspeed indicator.
(10) Reference Material. Additional procedures for marking powerplant instruments are contained in Advisory Circular 20-88A. Where differences for rotorcraft exist between AC 20-88A and this AC, the more recently dated guidance should be utilized.
781A. § 27.1549 (Amendment 27-23) POWERPLANT MARKINGS.
a. Explanation. Amendment 27-23 adds the requirement of marking each OEI limit and operating range. The limits should be clearly differentiated from other limits and ranges marked on the instrument. Refer to AC 20-88A, Instrument Markings.
b. Procedures. The method of compliance is unchanged.
781B. § 27.1549 (Amendment 27-29) POWERPLANT MARKINGS.
a. Explanation. Amendment 27-29 introduces the optional ratings of 30-second/2-minute OEl. Paragraph 27.1549(e) has been revised to show that the limits for the 30-second OE rating are not required to be marked. Use of the 30-second OEI rating is limited to critical phases of operation after a failure or precautionary shutdown of an engine. During this critical stage of operation the crew should not be required to monitor engine instruments to avoid exceedances. Automatic control of the 30-second OEl limits are required by Paragraph 27.1143(e) and therefore the 30-second OE limits are not required to be marked.
b. Procedures. The method of compliance is unchanged except the marking of 30-second OE limits are unnecessary.
782.-783. RESERVED.
Par 781 1081 (thru 1083)
784. FULL AUTHORITY DIGITAL ELECTRONIC CONTROLS (FADEC) FOR INSTALLATIONS WITH CATEGORY A ENGINE ISOLATION.
a. Explanation.
(1) The advent of "microprocessor technology" has resulted in rotorcraft engine controls being implemented by digital process control rather than by conventional means. These digital, processor-based full authority engine controls offer many performance advantages (such as isochronous governing) which were not feasible with conventional technology pneumatic or hydromechanical controls. Because of the incorporation of this advanced technology, some additional considerations must be made of the engine installation to ensure regulatory compliance.
(2) Part 27 does not require engine isolation. The guidance herein does not address FADEC installation certifications which do not desire engine isolation credit.
(3) In showing compliance with certain performance and flight characteristic sections of Part 27, simultaneous malfunction of engines is not considered if Part 29 Category A engine isolation is achieved. Paragraph 780 of this AC describes, in terms of Part 29 Category A requirements, an acceptable approach for determining that engine isolation, adequate for Part 27 performance and flight characteristic credit, has been achieved. That guidance material should be reviewed, but it is not believed that the limited-time-period concept for engine isolation which could be allowed under Part 27 would affect the FADEC installation requirements. Hence, the guidance for a Part 27 aircraft claiming credit for engine isolation is essentially the same as for a Part 29 Category A rotorcraft.
b. Procedures. The following is a discussion of some special attention areas when a FADEC installation is to be shown to comply with the Part 29 Category A engine isolation requirements. Paragraph 621b(4)(i)(D) of this AC contains a general definition of what constitutes a "full authority" control.
(1) Software Qualifications.
Paragraph 621f contains a general discussion of the use of the recommended RTCA document DO-178A which is utilized for the approval of system software. When utilizing that document, one might arrive at the conclusion that the engine control as a required function is essential and thus level 2 software would be appropriate for the control. However, for this level 2 category software, errors are presumed to exist and a software error in a full authority control could result in simultaneous unacceptable malfunctions in all engines. The provisions of § 27.1309(b) for the rotorcraft installations to be designed such that no probable malfunction or failure would result in a hazard to the rotorcraft, and the Part 29 engine isolation rule § 29.903(b) would generally preclude this level 2 classification.
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7/30/97 АС 27-1A (її) System designs which provide redundant distinctive software or an alternate technology control which is automatically selected and meets all of the minimum regulatory requirements would reduce the impact of software errors and may allow the level 2; i.e., essential, software classification. At level 1, it is accepted that the software is sufficiently error free that the software does not require further verification in the installation evaluation.
(2) Lightning Strike Protection. A complete discussion of an acceptable method of demonstrating that the FADEC, as installed, is adequately protected against the catastrophic effects of lightning is contained in Paragraph 621b(4) of this AC.
(3) Electrical Power System Considerations.
Normal Operation. The system should be evaluated with all power sources operating normally. If additional power source capability is being provided that is above the minimum required for certification, a certain portion of the evaluation should be conducted while operating in the minimum configuration.
(її) Malfunction Conditions. Beginning with the minimum configuration that is required for certification, electrical power system malfunctions should be introduced and the impact on continued FADEC operation determined.
(ill) Circuit Protection Location. The circuit protective devices for the FADEC should be located in the cockpit such that they can be readily reset or replaced in flight. The operation of the FADEC system is considered to be essential to safety in flight. Reference § 27.1357(d).
(iv) System Separation. On multiengine applications, each system should be separated from the other system to the maximum extent practical. Wiring should be routed separately. Power should be taken from independent busses and grounds, and system components should be independent of one another.
(v) Periodic Checks. Where periodic checks are appropriate, they should be made at reasonable intervals. This would normally range from preflight checks for certain items of greater concern to a tie-in with normal aircraft maintenance intervals for other items. If a crew check is specified, it should be evaluated to ensure it is a reasonable check. If items to be checked are located in an area that can be covered by interior upholstery, for example, a crew check would not be considered reasonable, and further design considerations may be in order.
(4) Powerplant Installation Considerations.
Paragraph 780 of this AC cites certain Part 29 provisions as being appropriate if engine isolation is claimed for a Part 27 rotorcraft. The guidance which Par 784 1085
follows, in part, references two Part 29 general engine isolation rules, §§ 29.901(c) and
29.903(b)(2), which should be considered.
(її) A demonstration of compliance with § 29.901(c) would generally include a failure mode and effects analysis (FMEA) of the powerplant systems as installed. When a FADEC is utilized, the analysis would consider the control's failure modes, the installed engine reaction, the affect on the aircraft, and the crew response to the situation. Combinations of undetected failures should be considered. Engine failures which may be escalated in severity by the FADEC's response to the initial failure should be analyzed. Potentially hazardous failures should be evaluated during flight testing. The requirements of § 29.903(b)(2) and § 27.1309(b)(2)(i) should be reviewed in determining acceptability of failures.
(iii) Section 29.903(b)(2), Category A engine isolation, is intended to ensure that a failure will not prevent the continued safe operation of the remaining engine (s) or require immediate action of the crew to ensure continued sate operation.
The FADEC's of the individual engines should be independent. Where communication between FADEC's is required (for example, for torque sharing), care should be exercised to ensure that failures which may occur will not result in a power loss to the extent that total power available is less than would be available under OEI conditions.
The no-required immediate-crew-action provision would preclude credit for manually selected or operated backup systems in meeting the § 29.903(b) rule. These unrequired backup systems, which may offer the advantage of get-home multiengine capability rather than forced OEI operation, would be evaluated on a no hazard basis.
(iv) Section 27.939, turbine engine operating characteristics, intends a flight investigation to ensure that no adverse characteristics are present to a hazardous degree during normal and emergency operation in the allowed flight envelope. The evaluation should include assessment of the minimum FADEC system certification configuration; i.e., the minimum proposed by the applicant to meet Part 27 requirements. Reduced capabilities (e.g., restrictions on normal collective movements, limited aircraft maneuvers, etc.) may be acceptable for degraded FADEC modes or backup systems not required to meet Part 27 requirements if those degraded capabilities are reasonable and not hazardous as determined by flight evaluation. The restrictions should be specified in the flight manual.
(v) The rotorcraft with FADEC engines must of course meet all of the Part 27 requirements, but the areas described herein are those which deserve special attention.
785. AGRICULTURAL DISPENSING EQUIPMENT INSTALLATION.
NOTE: This paragraph has been extensively revised and expanded to clarify the restricted category certification of agricultural dispensing equipment installations on rotorcraft.
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a. Explanation. In the early development of the rotorcraft one of its primary usages was agricultural operation. The FAA recognized that the existing requirements, which were designed primarily to establish an appropriate level of safety for passenger-carrying aircraft, imposed an unnecessary economic burden and were unduly restrictive for the manufacture and operation of aircraft intended only for use in rural, sparsely settled areas. Therefore, a special document that established new standards for agricultural dispensing equipment and other special purposes was developed. Restricted Category CAM 8 became effective October 11, 1950.
(1) During the recodification of 1965, CAR 8 ceased to exist as a regulatory basis and selected portions addressing certification were incorporated into FAR 21.
While the specific standards in CAR 8 were not changed substantially when adopted into FAR 21, the less restrictive philosophy of CAM 8 and the policy material that was stated in the preamble to CAM 8 was not clearly conveyed.
(2) Advisory material published in 1965 and revised in 1975, summarized the information contained in the advisory portions of CAM 8. This new advisory material indicated that the CAM advisory material would be applicable to the related FAR's.
Unfortunately, this document specified that CAM 8 could be used in conjunction with certain FAR's for restricted category certification of small agricultural airplanes only.
Rotorcraft were omitted.
(3) A survey of restricted category rotorcraft projects related to agricultural modifications indicates that the CAM 8 philosophy was interpreted to allow the use of AC 43.13-2A structural criteria for most STC's issued through the early 1980's. Since then more restrictive guidance based on CAR 6 and FAR 27 requirements has been applied by some ACO's to several STC applications. Since the more restrictive guidance imposed a significant economic burden on the industry, the HAI requested a meeting with the FAA during the 1990 annual convention in Dallas. As a result of the meeting, an Action Notice to clarify the interpretation of FAR 21.25(a)(1) for restricted category aircraft has been issued.
(4) The following advisory material is a result of a reassessment of past and present policy.
b. Procedures. The certification basis for agricultural dispensing equipment in the restricted category is FAR 21.25(a)(1) as interpreted by Action Notice 8110.22. The accountable Directorate guidance for the substantiation requirements for rotorcraft is as follows:
(1) Substantiation of the agricultural dispensing system hoppers or spray tanks to the load factors provided in Figure 785-1 provides for proof of structure. The load factors of Figure 785-1 address the critical structural load conditions of dispensing equipment mounted in or near the fuselage and provide adequate margins of safety.
Par 785 1087
FIGURE 785-1
ACCEPTABLE ULTIMATE LOAD FACTOR FOR
AGRICULTURAL DISPENSING EQUIPMENT DESIGN
UP DOWN SIDE FORWARD AFT Tanks & Equipment Mounted 1.5g 4.0g 2.0g 4.0g - ...
In Or Near The Fuselage Note 1 Spray Booms 1.5g 2.5g Note 1 2.5g Note 2 Note 1: An ultimate load factor of 2 G's is acceptable for externally side or under fuselage mounted tank and forward mounted spray booms where failure in a minor crash landing will not create a hazard to occupants or prevent occupant exit from the rotorcraft.
Note 2: The aft loads for spray booms may be developed by the applicant based on the 111 percent of VNE for which certification is requested or the load factors of Figure 785-1, whichever is greater.
(2) The applicant may elect to substantiate his product by either static or dynamic testing, by analysis, or any combination thereof.
(3) Lower load factors may be used only when justified by manufacturer's data, rational analysis, or actual rotorcraft flight and ground load demonstrations.
(4) Tank pressure test, while not mandated, is recommended for safety reasons. An acceptable procedure is included in Paragraph c(4).
(5) Dispensing equipment installation attach points. If attach points exist which are an integral part of the rotorcraft and these attach points have been certified to the standard category requirements no further substantiation of the attach point is required if an analysis indicates the dispensing system does not impose loads which exceed those for standard category certification.
(6) Ground clearance for dispensing equipment installation. A 5-inch ground clearance has typically been used for skid gear equipped rotorcraft which incorporate belly mounted supply tanks/hoppers or systems which have dual side mounted supply tanks/hoppers and the design incorporates cross tubes or other system components which are located beneath the bottom of the fuselage when these components are rigidly attached to the airframe structure. The 5-inch dimension is measured vertically from the ground to the lowest point of the installed system, with the rotorcraft in its operational configuration and gross weight (including disposable load) and while resting 1088 Par 785