(vii) Conduct cooling tests with the rotorcraft hovering at both the minimum and maximum hover altitudes.
(vili) 3 for hot day conditions. Use the standard lapse rate of 3.6° F/1,000 feet for consideration of altitude. See Paragraph 621 of this advisory circular for details on temperature correction.
(ix) If at any time during the testing it appears the manufacturer's limits are to be exceeded, the amperage load on the test generator/alternator should be reduced to prevent this from happening.
779. Sections 27.1301, 27.1309, and 27.1322 ANNUNCIATOR PANELS.
a. Explanation.
(1) The annunciator panel design should be reviewed for the presence of failure modes that can cause illumination of multiple panel segments.
(2) Many test circuits that are diode isolated are vulnerable to this condition. A typical sequence begins with the shorting of a test circuit diode. This failure is undetectable and goes unnoticed until an actual failure condition occurs which causes the associated panel segment to illuminate. At this time all panel segments connected to the test circuit will illuminate.
(3) This configuration becomes a special problem when one or more of the panel segments are red. A red light calls for immediate action by the crew, and the crew does not have adequate information for immediate action when many false panel segments are illuminated.
(4) If the design review indicates a problem, a redesign of the panel to eliminate the condition is considered to be the best solution and is highly encouraged.
b. Procedures
(1) An alternative to panel redesign might be the following:
(• Review the annunciator panel design and note which segments are red.
Determine if cross reference information is available in the cockpit to allow elimination from consideration of any of the red segments (e.g., red low fuel pressure light and low fuel pressure gauge). Normal operation of the gauge would be a reason to assume the light did not cause the problem.
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(III) Where a cross reference is available, further design review of that function is not necessary; however, it may be appropriate to include procedural information in the emergency procedures section of the rotorcraft flight manual.
(iv) If cross references are not available for red segments, additional isolation should be incorporated into the annunciator design for those functions.
(2) If cross referencing is not practical, the following approach is encouraged.
(i) Review the annunciator panel design and note which segments are red.
(ii) Determine if isolation diodes are checked during the application of battery or external power before starting the engines (e.g., red low oil pressure light). If isolation diode is shorted, all panel segments will light as soon as battery or external power is applied.
(iii) When the isolation diode can be checked before starting engines, further design review is not necessary.
(iv) If diodes are not automatically checked before starting, then additional isolation, should be considered.
c. Annunciator Panel Arrangement. The annunciator panels should be arranged in a logical manner to reduce the crew's time required to locate faults and to increase their efficiency in following aircraft flight manual procedures. For example, engine annunciators on multiengine rotorcraft should be physically located on the panel to coincide with engine location (left or right) so that properly operating engines are not inadvertently shut down due to crew confusion over which engine has malfunctioned.
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780. DEFINITION OF ENGINE ISOLATION FEATURES AS APPLIED TO §§ 27.79(b)(2), 27.141(b)(1), and 27.143(d)(1) (Amendment 27-19).
a. Explanation.
(1) Each of the cited performance and flight characteristic sections of Part 27 mention multiengine rotorcraft meeting transport Category A engine isolation requirements or refer to engine isolation features which ensure continued operation of the remaining engine. Unlike normal category fixed-wing (Part 23, § 23.903(c)) and the transport category fixed-wind and rotorcraft regulations, Part 27 does not provide a general engine isolation rule to make this determination.
(2) While it is clear that Part 27 does not require complete engine isolation, if credit for this feature is claimed (i.e., sudden complete engine power failure is not considered in showing compliance with the cited section), criteria must be established to allow a satisfactory isolation assessment.
(3) An approach which the FAA/AUTHORITY would find acceptable in making a Part 27 engine isolation determination is given. The FAA/AUTHORITY logic for establishing this criteria is also presented.
b. Criteria.
(1) The engine isolation provided may be for an appropriate limited time period.
(2) The failure or malfunction of any engine or the failure of any system that can affect any engine will not--
(i) Prevent the continued safe operation of the remaining engines for the appropriate limited time period; or
(ii) Require immediate action by a crewmember for continued safe operation.
(3) Each engine must be isolated by a firewall, shroud, or equivalent means from the remaining engines.
c. Criteria Rationale.
(1) Category A Minimum Time for Isolation. The acceptance of a limited time period for engine isolation is consistent with the acceptance of a reduced level of safety for a Part 27 rotorcraft. The criteria is also consistent with the Part 27 philosophy of allowing for a controlled landing following engine failure versus the Part 29 Category A principle of continued safe flight and a controlled landing.
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(2) Installation Analysis.
(i> The degree of engine isolation can be established by an installation assessment against the § 29.903(b) general isolation requirement, as modified for Part 27 by a limited time period concept.
Table 780-1 is a listing of the Part 29 sections that may be involved in Category A engine isolation considerations. Sections 29.901(c) and 29.903(b) are the general isolation regulations under which the other more specific rules naturally fall.
The point that the selection of specific rules from Table 780-1 does not achieve the desired degree of transport Category A engine isolation, and that the general isolation rules (89 29.901(c) and 29.903(b)) must be used, is illustrated by the following examples.
(A) Example #1. No specific requirement from Part 29 (or Table 780-1) can be cited which precludes a common engine mount. The design of the mount could be such that its failure results in sudden, complete power loss from all engines.
(B) Example #2. No specific Part 29 requirement prohibits a common engine induction system. F.O.D., fire in the induction system, or the adverse affect of engine surge on the remaining engine could result in sudden, complete power loss from all engines.
(C) Example #3. Crosstalk between engine fuel controls (possibly used for power matching) or the use of a common input parameter signal to the fuel controls is not prohibited by any specific isolation rule. Signals could be received which command the simultaneous shutdown of all engines.
(iii) These examples clearly illustrate that specific Part 29 isolation rules cannot be selected to establish appropriate Part 27 engine isolation, and that the installation must be evaluated by the general isolation policy set forth. This can be readily accomplished by a failure mode and effects analysis (FMEA).
(3) Firewalls.
CAR 6.483, prior to Amendment 6-4, effective May 15, 1953, requires "All engines, auxiliary power units, fuel burning heaters, and other combustion equipment which are intended for operation in flight shall be isolated from the remainder of the rotorcraft by means of firewalls, shrouds, or other equivalent means."
This rule would clearly require a firewall between engines of multiengine rotorcraft.
(ii) Amendment 6-4 revised § 6.483 to read "Engines shall be isolated from personnel compartments by means of firewalls, shrouds, or other equivalent means. They shall be similarly isolated from the structure, controls, rotor mechanism, Par 780 1073
АС 27-1А 7/30/97 and other parts essential to a controlled landing of the rotorcraft..." (remainder essentially identical to current § 27.1191).
(ії) The preamble explanation of Amendment 6-4 states that these changes are "intended to afford greater protection to the crew and passengers in the event of fire during flighti This revision did not intend to authorize less firewall isolation between the engines than was required by the earlier version. Also, the subsequent paragraphs clearly require firewalls between other combustion equipment and the rest of the rotorcraft (§ 27.1191(b)). To accept anything less for the engine is clearly inappropriate. Further, § 23.1191 requires firewalls or equivalent means between each engine and the rest of the airplane, and current safety requirements pertaining to in-flight fires should be no less stringent for normal category rotorcraft.
(iv) The lack of a firewall between engines or any other design arrangements which, in the event of one engine failure creates definable jeopardy for the remaining engines, will result in a significantly lower level of safety than is being assumed by the operators.
(v) A regulation change to clarity this § 27.1191 rule is planned.
d. Guidance.
(1) The minimum appropriate limited time period of engine isolation which would allow establishment of a one-engine-inoperative HV diagram, § 27.79(b)(2), would be defined by the time increment to recognize the engine failure and to make a landing from the most critical point on the desired HV diagram.
(2) The minimum appropriate limited time period of engine isolation to show compliance with §§ 27.141(b)(1) and 27.143(d)(1) considering the sudden power failure of one engine (rather than sudden complete power failure) would be the time increment to recognize the engine failure and to transition to a flight condition where failure of the remaining engine can be tolerated.
(3) Some existing provisions of Part 27 require isolation of certain systems (oil, fuel, and engine controls) without regard to a limited time period. These existing Part 27 engine isolation provisions must be observed regardless of the policy discussed herein.
(4) The limited time period concept must not be utilized to eliminate protection otherwise required by specific rules of Part 27 or to reduce accepted test conditions.
For example, lines which carry flammable fluids in areas subject to engine fire conditions must be fire resistant (§ 27.1183(a)). Fire resistant hose standards require testing for at least 5 minutes at 2000° F.
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(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.
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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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(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.
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