(2) Normal Operating Range. Each normal operating range may be marked with a green arc or green line that 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 displayed in yellow. 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 indications. 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 should be avoided wherever possible by specifying a single limiting value and marking it with a single red indication line. 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 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.
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6/23/23 AC 27-1B, Chg 9
(6) Flight Evaluation. In evaluating quantity indicator markings, to meet the requirements of § 27.1543, the final criterion must be: "Are the markings adequate for correct interpretation by the crew?" FAA 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 should 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 that are obscured by parallax, since these features are unacceptable. If the aircraft is to be approved for night operation, each required indicator must also be evaluated during night lighting conditions, per § 27.773. The same visibility requirements apply for night. Except for minor changes, lighting should be evaluated in flight to correctly evaluate vibration effects and various background lighting conditions.
(7) Digital Instruments.
(i) For purposes of this discussion, the two types of digital indicators considered are an indicator that consists of a column of light segments that illuminate sequentially to display changing values, and an indicator that 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. AC 20-88A specifies that instrument markings are intended to provide necessary information at a glance. Trend information for power indicators is vitally important tor sate operation of a rotorcraft, and this information should be obtainable at a glance. For the columnar light segments, the ability to quickly detect 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 evaluator should use a great deal of caution to ensure adequate trend information is available in primary power and rotor indicators of digital design.
(ii) 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, to meet the requirements of §§ 27.1541(b)(1), 27.1543(b), and 27.1545(a), it is Page G - 25.3
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 6.
(iii) 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 that ensures all digital segments operate satisfactorily. This feature should be encouraged.
(8) Additional Markings. To keep markings standardized and uncomplicated, only the FAA-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 RFM (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.
(1) A red cross-hatched indication should be located at power-off VNE if that value is less than power-on VNE.
(Ii) Many rotorcraft have erratic, unreliable, or non-repeatable airspeed indications at low speed that warrant caution when operating in that speed range. In such cases, a yellow indication on the instrument with an appropriate flight manual explanation has been found acceptable.
(iii) Indicated airspeed values should be utilized for all airspeed indicator markings.
(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.
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623/23 AC 27-1B, Chg 9
(10) Reference Material. Additional procedures for marking powerplant instruments are contained in AC 20-88A. Where differences for rotorcraft exist between AC 20-88A and this guidance, the more recently dated guidance should be utilized.
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AC 27.1545. § 27.1545 (Amendment 27-16) AIRSPEED INDICATOR.
See section 27.1543 of this AC.
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AC 27.1545A. § 27.1545 (AMENDMENT 27-51) AIRSPEED INDICATOR.
a. Explanation. Amendment 27-51 changed the requirements for marking the normal operating range of the airspeed indicator. Most modern glass cockpit designs utilize the "dark, quiet cockpit" concept, which means that yellow or red is presented to indicate abnormal conditions; green ranges are not presented. This approach has required a finding of equivalent safety with § 27.1545(b)(4) for applicants using an unmarked normal operating range instead of green.
(1) Section 27.1545(b)(4) continues to allow the normal operating range to be marked as green, but now permits an option for the normal operating range to be unmarked.
(2) The FAA has found it acceptable to only display the red VNE line when the VNE is in the displayed airspeed range. Section 27.1545(b)(2) was removed and § 27.1545(b)(1)(iii) was added to change the presentation of VNE (power-off). In the past, electro-mechanical gauges simultaneously displayed a red line to represent VNE (power-on) and a red cross-hatched radial line ("barber pole") to represent VNE (poweroff). Modern glass cockpits are able to present VNE (power-off) and VNE (power-on) independently, which was infeasible with electro-mechanical gauges.
(3) Another change to this airworthiness standard is that the references to arcs and radials were removed to reflect the use of airspeed tapes. The technical requirements have not changed.
b. Procedures. The policy pertaining to these requirements, which is found in section 27.1543 of this AC, remains in effect with the following changes and additions.
(1) For compliance with §§ 27.771(a) and 27.1545(b)(4), electronic displays that lack green ranges must include features that permit the pilot to easily interpret (by way of glancing at the instrument) whether a parameter is in a precautionary range (yellow) or beyond a limit (red). In addition, consistency in markings should be maintained between the primary instrument and the standby instrument to reduce any confusion in the cockpit when failures require using the standby instrument.
(2) Section 27.1545(b)(1)(iii) requires that the airspeed indicator be marked with a red indication to show both VNE (power-on) and VNE (power-off). If VNE (power-off) is less than VNE (power-on) and both markings are simultaneously displayed, then each depiction needs to be clearly distinguishable from the other. Some electronic displays have represented both VNE (power-on) and VNE (power-off) with a red line. This has been found acceptable by the FAA provided (1) both VNE depictions are not simultaneously displayed and (2) the airspeed indicator automatically depicts the Page G - 25.7
change from VNE (power-on) to the VNE (power-off) upon power failure. Use of a redwhite barber pole to represent VNE (power-off) would still be acceptable when both VNE limits are simultaneously displayed. The FAA has also found it acceptable to represent the VNE limit as a red range only when the actual limit moves out of view, e.g. airspeed increases substantially above VNE.
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AC 27.1547. § 27.1547 (Amendment 27-13) MAGNETIC DIRECTION INDICATOR.
a. Explanation. This section identifies the requirement and location for a calibration placard for the magnetic direction indicator.
b. Procedures. One means of accomplishing the requirements of this regulation is commonly known as swinging the compass. A surveyed compass rose is laid out on an appropriate surface. The compass rose location should be free from the influence of steel structures, underground pipes and cables, reinforced concrete, and other aircraft.
The aircraft should be in an attitude that permits an accurate result. Normally the engines are in operation; however, if the aircraft is equipped with an auxiliary power unit which can supply electrical power for all electrical/electronic equipment or systems, this can be used instead of engine driven generators. Turn the aircraft on successive headings through 360°. . It is recommended that the increments be every 30°; however, the increments should not exceed 45°. Prepare a placard to show the correction to be applied at each of the selected headings. When deviations of more than 10° are introduced by operation of any electrical/electronic equipment or systems, the placard should also be marked at each calibration heading showing the correction to be applied when such equipment or systems are turned on or energized. The placard resulting from this calibration should be installed on or near the magnetic direction indicator and identify which electrical loads, or combination of loads, are the cause of the excessive deviations.
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AC 27-1B, Chg 4 3/31/2014 AC 27.1549. § 27.1549 (Amendment 27-16) POWERPLANT MARKINGS.
See section 27.1543 of this AC.
AC 27.1549A. § 27.1549 (Amendment 27-23) POWERPLANT INSTRUMENTS.
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.
AC 27.1549B. § 27.1549 (Amendment 27-29) POWERPLANT INSTRUMENTS.
a. Explanation. Amendment 27-29 introduces the optional ratings of 30-second/2-minute OEI. Paragraph 27.1549(e) has been revised to show that the limits for the 30-second OEl 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 OEI limits are required by § 27.1143(e) and therefore the 30-second OEI limits are not required to be marked.
b. Procedures. The method of compliance is unchanged except the marking of 30-second OEI limits are unnecessary.
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AC 27.1549C.§ 27.1549 (AMENDMENT 27-51) POWERPLANT INSTRUMENTS.
a. Explanation. Amendment 27-51 changed the requirements for displaying the normal operating range of powerplant instruments. Most modern glass cockpit designs utilize the "dark, quiet cockpit" concept, which means that yellow or red is presented to indicate abnormal conditions; green ranges are not presented. This approach has required a finding of equivalent safety with § 27.1549(b) for applicants using an unmarked normal operating range instead of green. Section 27.1549(b) continues to allow the green operating range to be displayed as green, but now permits an option for the normal operating range to be unmarked.
b. Procedures. The policy pertaining to these requirements, which is found in section 27.1543 of this AC, remains in effect with the following changes and additions.
For compliance with §§ 27.771(a) and 27.1549(b), electronic displays that lack green arcs must include features that permit the pilot to easily interpret (by way of glancing at the instrument) whether a parameter is in a precautionary range (yellow) or beyond a limit (red). In addition, consistency in displaying all required indications should be maintained between the primary instrument and the standby instrument to reduce any confusion in the cockpit when failures require using the standby instrument.
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AC 27.1551. § 27.1551 OIL QUANTITY INDICATORS.
a. Background. This section states that each oil quantity indicator must be marked with enough increments to indicate readily and accurately the quantity of oil.
b. Procedures. There are several different ways in which the oil quantity indicator may be presented. Some of the ones more prevalent in the industry are:
(1) Oil quantity indicator. (Generally used when large amounts of reserve oil are required.)
(2) Oil quantity dip stick. (Most common method of measuring engine oil.)
(3) Oil quantity sight indicator. (Generally used for measuring transmission and gearbox oil quantities.)
c. No matter what method of oil quantity indicator is used, the indicator should be marked so that the oil quantity can be accurately determined. This can range from increments marked in gallons, such as oil quantity indicators for large amounts of oil, to oil quantity indicators marked in quarts with full and add marks, such as engine dip sticks. Sight indicators with full and add marks have been used successfully for gearboxes. Sight indicators normally do not reflect quantities. These are some of the methods currently in use to indicate the oil quantity. In all cases, those methods identified above have proved to be acceptable methods of showing compliance with the § 27.1551.
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AC 27-1B, Chg 4 3/31/2014 AC 27.1553. § 27.1553 FUEL QUANTITY INDICATOR.
a. Explanation. This section describes the markings necessary to identify the portion of unusable fuel that cannot be used in level flight. Unusable fuel may be present in a design due to the relative configuration of the fuel tank to the fuel tank outlet (e.g., sumps, unusual elevations and/or configurations dictated by aircraft contours, etc.). If the unusable fuel supply for any tank is less than or equal to 1 gallon or is less than or equal to 5 percent of the tank capacity, whichever is greater, this section does not apply.
b. Procedures. For each fuel tank which has an unusable fuel capacity exceeding 1 gallon or 5 percent of the tank capacity, whichever is greater, the following should be accomplished.
(1) Calibration computations, measurements, and/or tests should determine the zero (empty) position on the fuel quantity indicator. (reference § 27.1337).
(2) The lowest reading obtainable in level flight must be determined by computation, measurement, and/or testing.
(3) Once the instrument readings defined by paragraphs b(1) and (2) above have been determined, a red arc should be placed between the readings on the fuel quantity indicator.
(4) Appropriate notations should be made in the flight manual to define the intent of the red arc to the flightcrew (reference § 27.1585(e)).
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