AC 27-1A

7/30/97/ AC 27-1A

639. RESERVED.

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640. § 27.1335 (Amendment 27-13) FLIGHT DIRECTOR SYSTEMS.

a. Explanation. This section prescribes the accepted display criteria for a rotorcraft three-cue flight director providing command guidance for pitch, roll, and power. Three-cue flight directors for rotorcraft use the usual pitch and roll command cues with the third cue displayed on the left side of the attitude director indicator (ADI).

These instruments can be used in either the two-axes or three-axes modes. In either mode, the lateral command cue controls the roll attitude, and the vertical command cue controls the pitch attitude. The rotorcraft attitude, controlled by the cyclic control, is changed to satisfy the flight director commands. The third cue, when displayed, commands collective pitch position and is used when an airspeed or pitch attitude mode and a vertical mode (altitude hold, glide slope, etc.) are selected.

(1) The general convention for flight director design is that each command bar is a "fly to" command. The motion of the flight director indicator is such to command a corresponding sense of control system motion. This is true of flight director pitch and roll commands and should hold true for additional commands such as collective pitch.

(2) Some consideration should be given to the collective, or third cue, display.

For example, if the collective symbol is selected as the fixed index, the command cue and collective pitch control should move in opposite directions when collective pitch changes are made. This configuration would constitute a conventional "fly to" indicator.

If the collective symbol is selected for the movable index, the direction of motion of the collective symbol will coincide with the direction of collective pitch changes. In this case the moving collective symbol does not comply with the "fly to" convention; however, this configuration has been approved by the FAA with special symbology, special background effects, and special color coding and has performed satisfactorily in service.

b. Procedures. The recommended display for a three-cue flight director incorporates the standard pitch and roll command symbols, either pitch and roll bars or the "V" bar display. The third cue, or collective symbol, should be located on the left side of the ADI. The shape of the moving cue and the background display should be unique to avoid being confused with a glide slope display or angle of attack display.

One display uses a third cue, shaped like a small handle, to aid in identifying it as the collective pitch symbol.

(1) The color of the pitch and roll command indicators, the aircraft symbol, the background marking of the third cue, and third cue itself, should be consistent. The optimum color scheme uses the same color for the aircraft symbol and the collective symbol. This is usually fire orange. The command cues including the collective cue also should use the same color, usually yellow. The rationale for the different colors is that the aircraft symbol and the collective symbol (the same color) are moved toward their respective command cues. If the pitch command cue is above the center, the aircraft symbol is raised (nose pulled up) and, if the collective command cue is above 852 Par 640

7/30/97 АС 27-1A the collective symbol, the collective pitch is raised, moving the collective symbol towards the command cue.

(2) If the attitude director indicator (ADI) provides a monochromatic display, the collective pitch cue and its background markings must be distinctive to reduce the chance of being confused with the glide slope indicator. This can be accomplished through the use of different shaped cues and background marks. A round cue with a chevron-shaped background marking has been satisfactory.

641. § 27.1337 (Amendment 27-12) POWERPLANT INSTRUMENTS.

PARAGRAPH (b) - FUEL QUANTITY INDICATOR.

a. Explanation. Section 27.1337(b) requires, in part, a means to indicate to the flightcrew the quantity of useable fuel in each tank. Since the flight attitude of a rotorcraft may vary significantly with CG (center of gravity) and airspeed, several attitudes for calibration of the fuel gage may be appropriate. Accordingly, the manufacturer should review the operational envelope (with respect to attitudes) and select at least three attitudes for gage calibration with the provision that these attitudes should be useful to the intended operation of the rotorcraft and demonstrate gage accuracy (within limits suggested herein). Selection of ground attitude for one of these conditions may, in some cases, be appropriate.

b. Procedures.

(1) Determine the rotorcraft pitch attitudes for most forward and most aft CG at a median gross weight and at an airspeed of 0.9 VNE or 0.9 VH, whichever is less. The mean attitude of the extremes defined above, further adjusted for lateral CG effects, if necessary, define the rotorcraft attitude for fuel gage calibration.

(2) After establishing the calibration attitude, the requirements of § 27.1337(b) can be accomplished. The aircraft should be placed in the calibration attitude. Add fuel to the filler neck spillover level. Defuel the aircraft in increments corresponding to fuel gage increment markings or at least 10 increments until gage zero is obtained.

Precautions should be taken during this step to be sure that the fuel transmitter is sensing fuel level and not simply reflecting a physical "STOP" or end point in the system range. The fuel remaining in the tank below the "ZERO" mark must not be less than that amount determined by flight testing under § 27. 959. (Otherwise, the zero point must be adjusted upward.) The gaging system accuracy is acceptable when it meets a tolerance of t2 percent of the total useable fuel plus 14 percent of the remaining usable fuel at any gage reading, provided that the gage indicates zero fuel with unusable fuel in accordance with § 27.959 in the tank. (For a 100-gallon tank this formula would allow a t6-gallon error at the full level, ‡4-gallon error at 50-gallon level, converging to a t2-gallon error at low fuel with the further provision that the zero mark accurately reflects unusable tuel.)

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(3) Certain other aspects of a fuel gaging system need attention in order to minimize fuel exhaustion incidents:

(i) Gage reading with the aircraft at ground attitude is frequently used by the crew in calculating range, weight and balance, and actual gross weight. Significant gage errors in either direction during this reading can introduce hazards to the operation of the aircraft. If a calibration at this attitude indicates an unconservative error in excess of 6 percent of the gage reading, corrective information should be applied adjacent to the fuel quantity gage or be made available to the crew in other handbook data.

Flight during hover with maximum rearward wind may introduce significantly different fuel gage readings. A check should be made to ensure that the gage is either accurate or at least does not read high (unconservative) in this attitude.

If external loads are approved, the attitude for this type of operation should be considered.

(4) Fuel gaging system transmitters which are strictly volumetric measuring devices (float-actuated variable rheostats) introduce a gage readout error of about 5 percent if calibrated with a fuel temperature of 0° C and subjected to -55° C fuel or +55° C fuel. This error may be minimized by calibrating the gage with fuel temperature in the middle of the useful range; i.e., 15° C. These types of gaging systems should be recalibrated during periodic major maintenance intervals. The manufacturer should be urged to include procedures and requirements for this in the "Instructions for Continued Airworthinessi for the model rotorcraft.

(5) Capacitance transmitters have become the standard for most modern fuel systems. These transmitters ordinarily need no temperature compensation since the fuel volume and the fuel dielectric constant vary inversely as temperature changes.

The basic capacitance transmitter does not compensate for the different dielectric values to be expected with different type fuels. An add-on capacitance located where it will be submerged in fuel at all times can be devised to automatically compensate for other fuels.

641A. § 27.1337 (Amendment 27-23) POWERPLANT INSTRUMENTS.

a. Explanation. Amendment 27-23 adds § 27.1337(e) that requires certain rotor drive system transmissions and gearboxes to be equipped with chip detector systems.

These detectors will sense and signal the presence of ferromagnetic particles to the flight crew. This amendment will improve the level of safety available with the installation of chip detector systems.

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b. Procedures. All of the policy material pertaining to this section remains in effect. Additionally, the following information is added about chip detectors. The chip detectors should:

(1) Indicate the presence of ferromagnetic particles in the transmission or gearbox;

(2) Be easily removable for inspection of the magnetic poles for metallic chips;

and,

(3) Prevent the loss of lubricant in the event of failure of the retention device for the removable portion of the chip detector (debris monitor).

642.-651. RESERVED.

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SECTION 36. ELECTRICAL SYSTEMS AND EQUIPMENT

652. § 27.1351 (Amendment 27-13) GENERAL.

a. Explanation. With the advent of more sophisticated rotorcraft and operations under more critical conditions, such as IFR and icing, it is essential that the electrical system be very carefully analyzed and evaluated to assure proper operation under any foreseeable operating condition and that hazards do not result from any malfunctions or tailures.

b. Procedures.

(1) Electrical System Capacity. Rotorcraft electrical systems have grown in capacity, complexity, and impact on safety. This paragraph requires adequate electrical system capacity for safe operation of load circuits essential for safe operation at continuous rated power. If this capacity can be shown by electrical measurements, an electrical load analysis is not required.

Load circuits (systems) that are essential for safe operation are those systems necessary to maintain controlled flight and land safely and are generally those systems required to show compliance with the certification regulations. This includes most electrical utilization systems.

An electrical utilization system is a system of electrical equipment, devices, and connected wiring using electric energy to perform a specific aircraft function.

The specific utilization systems, which are necessary to maintain controlled flight and land safely, will vary with the type of rotorcraft and with the nature of operations. Examples of systems which may be essential are basic flight instruments, minimum navigation equipment, minimum radio communications, and flight control systems.

(2) Function.

(1) Electrical equipment, controls, and wiring must be installed so that failure of a source will not interrupt supply of power by other sources to any systems that are essential for safe operation. Generating systems should be analyzed, inspected, or tested to ensure that no probable malfunction in the generating system or in the generator drive system may cause permanent loss of service to systems essential for safe operation. A probable malfunction is any single electrical or mechanical malfunction or failure which is considered probable on the basis of past service experience with similar components in rotorcraft applications. The experience with similar components in other aircraft may be used, if applicable, when there is 866 Par 652

insufficient rotorcraft experience. This definition should be extended to multiple malfunctions when:

(A) The first malfunction would not be detected during normal operation of the system, including periodic checks established at intervals which are consistent with the degree of hazard involved; or (B) The first malfunction would inevitably lead to other malfunctions.

(I)) The generator drive system includes the prime movers (propulsion engines or other) and coupling devices such as gearboxes or constant speed drives.

(ill) Where crew corrective action is necessary:

(A) Adequate warning should be provided for any malfunction or failure requiring such corrective action;

(B) Controls should be located to permit such corrective action during any probable flight situation;

(C) If corrective action must be taken within a specified time to continue safe operation of the generating system, it should be demonstrated that such corrective action can be accomplished within the specified time during any probable flight situation; and (D) The procedure to be followed by the crew should be detailed in the Rotorcraft Flight Manual.

(iv) Chapter 11 of Advisory Circular 43.13-1A, "Acceptable Methods, Techniques, and Practices; Aircraft Inspection and Repair," includes guidance on installation of electrical systems (routing, separation, tying, clamping, j-box installations, etc.). Special emphasis should be placed on wire routing during the rotorcraft compliance inspection. Control wires to the rotorcraft's generators should be routed separately from generator output wiring. This should begin at the generator and continue to the voltage regulator.

(3) Generating System. When electrical power is needed for essential equipment, this paragraph requires at least one generator with adequate capacity for safe operation. Complete electrical failures have been caused by loss of voltage control in the voltage regulator. Overvoltage conditions can destroy electronic equipment. An acceptable method of overvoltage protection is the use of a separate overvoltage sensing relay to trip the generator off the line when overvoltage is detected.

Another acceptable method is use of a voltage regulator with built-in overvoltage protection.

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(4) Instruments. Voltage and current supplied by each generator are parameters which define system operation. Some systems are provided with voltmeters and ammeters to display these parameters to the crew. These instruments may be multifunctional with switches to select the functions displayed. Some designs have annunciated sate operation of each generator with lights and have no voltmeter and ammeter. If additional limitations, such as maximum loading of portions of the systems, are necessary to account for fault condition, that information should be made avallable to appropriate personnel (crew, owner, modifier, etc.) to ensure the limits are not exceeded.

(5) An external power source supplying reverse polarity or reverse phase sequence to the rotorcraft electrical system could seriously damage the system. This paragraph requires a means to prevent such an occurrence. This can be accomplished by use of a standard polarized receptacle and protective relays.

653. § 27.1353 (Amendment 27-14) STORAGE BATTERY DESIGN AND INSTALLATION.

a. Explanation. Batteries must not be designed and installed to create a hazard to the rotorcraft under any operating conditions.

b. Procedures.

(1) As part of the electrical system evaluation, the battery installation should be reviewed to ensure the battery is vented and drained. If there is some doubt regarding the ability of the drain to satisfactorily dispose of corrosive fluids, TIA tests should be conducted to resolve the issue. Normally this is done by expelling a dye solution through the drain system during different phases of flight to ensure that fluids are drained clear of the rotorcraft. Some aircraft rely on the installation of a sump jar to dispose of corrosive fluids.

(2) If nickel cadmium batteries are used for engine starts, compliance with § 27.1353(g) may be achieved through:

A battery charge control system may be used that automatically controls the battery charge to prevent battery overheating. Unless otherwise specified by the battery manufacturer, temperatures above 140° F are considered overheat for NI-CAD batteries. The system is acceptable if the charge rate is automatically adjusted by controlling the charging current as a function of battery temperature, and in an over-temperature condition, the charge current is automatically reduced to a safe value.

Zero to 10 amperes has been considered safe for batteries rated at less than 34 amp-hours, and zero to 15 amperes has been considered safe for batteries rated at 34 amp-hours or more. The actual number chosen should be substantiated. Means and/or procedures should be provided for the crew to monitor the charger performance or the battery condition. If there is an automatic disconnect of the charger from the 868 Par 652

batteries and associated bus on an over-temperature condition, provisions should be provided in the cockpit to warn of a disconnect.

(il) If a temperature monitoring system is used, the temperature sensor should be located in a position that will most accurately reflect the internal battery temperature without causing adverse effects to the sensor. The location normally used is near the center of the battery. If the sensor is placed between two cells, the indication should be very close to the actual temperature within the cell. If the sensor is placed in a cell strap, there will normally be a period of time just after a heavy current drain (e.g. engine start) when the sensor shows a temperature that is hotter than the actual cell temperature.

(ill) Battery failure sensing and warning systems have also been used to show compliance with this rule.

(3) Other aspects of the battery installation can be resolved by reviewing AC 43.13-1A, "Acceptable Methods, Techniques, and Practices: Aircraft Inspection and Repairs" and AC 43.13-2A, "Acceptable Methods, Techniques, and Practices: Aircraft Alterations."

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654. RESERVED.

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655. § 27.1357 (Amendment 27-13) CIRCUIT PROTECTIVE DEVICES.

a. Explanation. Circuit protective devices are normally installed to limit the hazardous consequences of overloaded or faulted electrical circuits. These devices are resettable (circuit breakers) or replaceable (fuses) to permit the crew to restore service when nuisance trips occur or when the abnormal circuit condition can be corrected in flight. Chapter 11 of Advisory Circular 43.13-1A, "Acceptable Methods, Techniques, and Practices: Aircraft Inspection and Repair," includes guidance on selection of circuit protective devices.

b. Procedures.

(1) The circuit protective devices for systems essential to flight safety should not be tripped by faults in other circuits.

Systems that are "essential to flight safety" are generally those systems that are required to show compliance with the regulations. These essential systems include the basic electrical system, the distribution system, and many electrical utilization systems.

An electrical utilization system is a system of electrical equipment, devices, and connected wiring using electrical energy to perform a specific aircraft function.

(ill) The specific utilization systems, which are necessary to maintain controlled flight and land safely, will vary with the type of rotorcraft and with the nature of operations. Examples of systems which may be essential are basic flight instruments, minimum navigation equipment, minimum radio communications, and flight control systems.

(2) Automatic reset circuit breakers, which automatically reset themselves, should not be used as circuit protective devices. If an abnormal circuit condition cannot be corrected in flight, the decision to restore power to the circuit should result from a careful analysis by the flightcrew and cannot be performed by automatic reset circuit breakers.

To ensure crew supervision over the reset operation, circuit protective devices should be designed to require a manual operation to restore service after tripping. Circuit breakers must be designed such that the tripping mechanism cannot be overridden by the operating control. These are known as the "trip free" type.

(3) This paragraph requires protective devices for circuits essential to safety in flight to be accessible to the crew in the cockpit. Again, this generally applies to systems required for compliance as discussed above. If continued safe flight to the destination is sufficiently assured, certain required circuits have been excepted from Par 655 871

АС 27-1A 7/30/97 this accessibility. Voltmeter and ammeter circuit protective devices are examples of ones that have been excepted. Some utilization systems, although not specifically required by Part 27, may be required for the particular design to be certified. Circuit protective devices for these systems should be accessible. The following are considered to be acceptable compliance with the "readily reset" provision of this paragraph:

(i) For operation by a single pilot with seat belt and shoulder harness normally adjusted, the pilot should be able to identify and reset or replace the opened circuit protector while flying the rotorcraft. Circuit protection should not be located aft of a vertical plane passing left to right (laterally) through the pilot's body.

For a crew of two, it is satisfactory for one crewmember to move his seat and loosen his shoulder harness to identify and reset or replace the circuit protective device. It is not satisfactory for one of the crewmembers to leave his seat to reset or replace the circuit protective device.

(4) The spare fuse requirement applies only to fuses protecting systems required to show compliance with the regulations. Spare provisions are encouraged but not required for nonrequired convenience type installations. The spare fuses should be stored in a location readily accessible to the crew. For spare fuses not directly visible to the crew, location information should be provided. One acceptable location is on the fuse panel in a holder without wire terminations. The spare fuse should be identified "spare" with the fuse rating.

(5) Passive circuit protection has been utilized to a limited degree in some designs. To accommodate special installation problems, unprotected wire runs of up to 2 feet have been accepted in a few instances when associated with detailed specific installation data and regular periodic inspections. Specific installation data would normally include information such as routing requirements, clamp locations, requirement for conduit, etc. Electrical master junction boxes usually rely to some degree on passive circuit protection for protection against short circuits on distribution bars. This reliance is normally supported by considerations such as careful layout to minimize the possibility of shorts from loose objects, extensive use of nonconductive materials, terminal covers for relays, etc. Periodic inspections are also normally required. It is desirable to install junction boxes so loose objects will tend to fall away from internal circuitry. Also, careful consideration should be given to flammability characteristics when selecting a nonconductive material.

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