3-10 Figure 3-9. Typical Shelf Installation N ote 1: Use standard aircraft practices and procedures for fabrication and attachment of the shelf. Reinforce fore and aft corners with gussets or a bulb angle.
Note 2: Fabricate a platform using 2017T4(17ST) or equivalent. Apply standard aircraft practices for fabrication and installation.
Note 3: The equipment manufacturer’s mounting bases that meet load requirements and can be utilized are acceptable.
3-11 Figure 3-10. Typical Attachment of Support Structure to Tubular Frame Other Than Structure to Floor
3.8 Electrical Requirements.
3.8.1 Installing an electrical system or component into an aircraft requires consideration of the electrical load, the appropriate power distribution circuit, and available power capacity.
The specific requirements for a system depend on its electrical characteristics and the criticality of its application for use in the aircraft’s operation.
3.8.2 The total energy available to power electrical systems is referred to as the aircraft’s capacity. It includes available stored power and generated power. This will vary depending on the phase of flight or type of operation.
3.8.3 The critical distribution circuit of an aircraft is designed to transfer power from the source to a system determined to be critical to the operation or function of the aircraft. This circuit is required to include additional capacity and circuit protection as required by applicable regulations. Distribution systems should be designed to facilitate loadshedding procedures. Power distribution system design includes the following concerns.
Refer to AC 43.13-1, Chapter 11, Aircraft Electrical Systems, for:
• Circuit protection.
• Wire selection.
• Connectors, switches, and termination devices.
• Wire routing.
• Wiring/cable support.
• Identification.
3.8.4 Installation of Wiring. Select and use wiring of a type and design that is appropriate for the intended purpose of the application. Install in a manner suitably protected from fuel, oil, water, other detrimental substances, oxygen systems, and abrasion damages.
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3.8.5 Power Sources.
3.8.5.1 Connect radio electrical systems to the aircraft electrical system at the power source protective device, a terminal strip, or use a plug and receptacle connection.
3.8.5.2 Radio electrical systems must function properly whether connected in combination or independent.
3.8.6 Protective Devices.
3.8.6.1 Incorporate a “trip free” (one that will trip open if the ON-OFF mechanism is held in the ON position) resettable type circuit breaker or a fuse in the power supply from the bus. Mount in a manner accessible to a crewmember during flight for circuit breaker resetting or fuse replacement and label.
3.8.6.2 Select circuit breakers or fuses that will provide adequate protection against overloading of the radio system wiring.
3.8.6.3 Connect all leads in such a manner that the aircraft’s master switch will interrupt the circuit when the master switch is opened, unless the equipment is intended to be powered when the master switch is open.
3.8.6.4 Radio system controls are to provide independent operation of each system installed and be clearly labeled to identify their function relative to the unit of equipment they operate.
3.8.7 Wire Bundle Separation From Flammable Fluid Lines.
3.8.7.1 Physically separate radio electric wire bundles from lines or equipment containing oil, fuel, hydraulic fluid, alcohol, or oxygen.
3.8.7.2 Mount radio electrical wire bundles above or on level with flammable fluid lines and whenever possible, maintain a minimum separation of 6 inches.
Where separation is 1/2 to 2 inches, install clamps. Refer to AC 43.13-1, Chapter 11, Paragraph 11-127, Flammable Fluids and Gases.
3.8.8 Cable Attachment to Shock-Mounted Units. Route and support electrical wire bundles and mechanical cables in a manner that will allow normal motion of equipment without strain or damage to the wire bundles or mechanical cables.
3.8.9 Radio Bonding. It is advisable to electrically bond radio equipment to the aircraft in order to provide a low impedance ground and to minimize radio interference from static electrical charges. When electrical bonding is used, observe the following:
1. Keep bonding jumpers as short as possible.
2. Prepare bonded surfaces for best contact (resistance of connections should not exceed
0.003 ohm).
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3. Avoid use of solder to attach bonding jumpers. Clamps and screws are preferred.
4. For bonding aluminum alloy, use aluminum alloy or tinned, or cadmium-plated copper jumpers. Use brass or bronze jumpers on steel parts.
5. When contact between dissimilar metals cannot be avoided, put a protective coating over the finished connection to minimize corrosion.
3.9 Electrical Load Analysis Procedure.
3.9.1 Available Power Supply. To preclude overloading the electric power system of the aircraft when additional equipment is added, perform an electrical load analysis to determine whether the available power is adequate. Radio equipment must operate satisfactorily throughout the voltage range of the aircraft electrical system under taxi, takeoff, slow cruise, normal cruise, and landing operating conditions. Compute the electrical load analysis for the most adverse operating conditions; typically, this is for night and/or instrument flight.
3.9.2 ASTM Method. One method for the analysis may be found in the ASTM International F2490-20, Standard Guide for Aircraft Electrical Load and Power Source Capacity Analysis.
3.9.3 Previous Electrical Load Analysis. Applicable elements of a previously performed electrical load analysis if available may be reused.
3.10 Electromagnetic Compatibility (EMC).
3.10.1 EMI may disrupt the performance of systems and has varying degrees of consequence.
These consequences may be identified as no safety effect, minor, major, hazardous, or catastrophic. The purpose of EMC analysis and testing is to ensure that equipment does not cause interference with any existing aircraft system function and that existing systems do not cause any interference with the new equipment (see Table 3-2, Relationship Probabilities and Severity of Failure Conditions).
3-14 Table 3-2. Relationship Probabilities and Severity of Failure Conditions Classification of Failure Conditions No Safety Effect Minor Major Hazardous Catastrophic Effect on Airplane No effect on operational capabilities or safety Slight reduction in functional capabilities or safety margins Significant reduction in functional capabilities or safety margins Large reduction in functional capabilities or safety margins Normally with hull loss Effect on Occupants Inconvenience for passengers Physical discomfort for passengers Physical distress to passengers, possibly including injuries Serious or fatal injury to an occupant Multiple fatalities Effect on Flightcrew No effect on flightcrew Slight increase in workload or use of emergency procedures Physical discomfort or a significant increase in workload Physical distress or excessive workload impairs ability to perform tasks Fatal injury or incapacitation
3.10.2 When EMC characteristics are known, the need for and extent of EMC testing can be determined by a review of those characteristics. Knowing specific target frequencies, EMC testing can focus on the aircraft systems, and even those susceptible frequencies in the case of tunable systems, likely to be affected by interference. Where sensitive systems or potentially strong sources of EMI are involved, more intensive evaluation will be required.
3-15 Table 3-3. Commonly Used Radio Frequencies on Aircraft Range Hz Mode Function 190–1750 kHz Rx Automatic Direction Finder (ADF) Navigation 2–30 MHz Tx/Rx High Frequency (HF) Communications 75 MHz Rx Marker Beacon Receiver 108–112 MHz Rx Instrument Landing System (ILS) Localizer (LOC) Receiver 108–118 MHz Rx Very High Frequency (VHF) Omnidirectional Range (VOR) Receiver 118–137 MHz Tx/Rx VHF Communications 243 MHz Tx Emergency Locator Xmtr (Satellite)
328.6–335.4 MHz Rx ILS Glide Slope Receiver
406.3 MHz Tx ELT 960–1215 MHz Tx/Rx Distance Measuring Equipment (DME) System 1027–1033 MHz Tx/Rx Transponder and Traffic Alert and Collision Avoidance System (TCAS) Systems 1087–1093 MHz Tx/Rx Transponder and TCAS Systems 1575.42 MHz Rx Global Positioning System (GPS) Satellite Navigation
3.10.3 On-aircraft EMC tests for systems or equipment should be conducted. If lab test data is available, it should be used to guide the planning of these tests. If lab test analysis is not available, a more comprehensive EMC testing on the aircraft systems and equipment typically needs to be performed.
3.10.3.1 The aircraft should not be close to large reflecting surfaces, such as buildings or other aircraft. The use of ground power is not recommended, unless the ground power unit that is used is routinely checked for output quality.
3.10.3.2 All normally closed-circuit breakers should be closed, and power should be supplied to all normally powered alternating current (AC) and direct current (DC) distribution busses during testing.
3.10.3.3 The aircraft should be in flight configuration. Doors and hatches that might be in any interfering signal’s path should be in the position they would normally be in during flight.
3.10.3.4 Aircraft systems being tested should be operated and monitored for indication of interference. It is essential that systems determined critical for the operation of the aircraft are tested. The following systems, if installed, should be included in the aircraft EMC test plan.
3-16 Table 3-4. Aircraft Electrical and Electronic Systems System ADF Air Data Systems (ADS) Altitude Alert System Air Traffic Control (ATC) Transponder Audio Distribution System Autopilot/Flight Guidance System (FGS) Compass/Directional Gyro Systems DME Electronic Flight Control System Electronic Flight Instrument System (EFIS) GPS Marker Receiver System VHF Communications VOR/Localizer (LOC)/Glideslope (GS) Newly Installed Electrical/Electronic System
3.11 Functional Hazard Assessment (FHA).
Note: Refer to AC 23.1309-1 for additional information on FHA.
3.11.1 Many older aircraft designs did not provide for all weather operations or potential increases in the pilot’s reliance on installed systems and equipment. Requirements to ensure design safeguards against hazards have developed as technologies have become available and pilots have increased their reliance on installed systems and equipment. A fundamental analysis to ensure design safeguards is accomplished through an FHA.
3.11.2 Alterations that involve systems or equipment that perform critical functions or that include complex designs with a high degree of integration, use of new technology, or novel applications of conventional technology must be assessed to determine the severity of failure conditions. Complexity in itself does not drive the need to perform a system safety analysis, but the effect of a failure does. Comparison with similar previously approved systems is sometimes helpful.
3.11.3 Evaluate the system to determine whether it is essential to safe operation. For aircraft of 6,000 pounds or less maximum weight, refer to the regulations incorporated by reference in the type certificate (TC), unless the Administrator has found that the change is significant in an area.
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3.11.4 Determine whether the equipment has any unacceptable adverse effects when operated.
3.11.5 Determine whether the operation of the installed equipment has an adverse effect on equipment that is not essential to safe operation, and whether a means exists to inform the pilot of the effect.
3.11.6 Determine whether a failure or malfunction of the installed equipment could result in unacceptable hazards.
3.11.7 Design requirements and the methodology of hazard resolution differ depending on the application and type of aircraft (i.e., single-engine, multiengine, and commuter use).
3.11.7.1 It is unacceptable to operate equipment that has an adverse effect on other equipment essential to the safe operation of the aircraft.
3.11.7.2 Operation of equipment that has an adverse effect on other equipment that is not essential to the safe operation of the aircraft may be acceptable if there is a means to inform the pilot of the effect.
3.11.7.3 It is unacceptable if a probable failure or malfunction will result in a hazard in a multiengine aircraft.
3.11.7.4 If a probable failure or malfunction will result in a hazard in a single-engine aircraft, its impact must be minimized.
4-1 CHAPTER 4. ANTENNA INSTALLATION
4.1 Purpose. The purpose of this chapter is to describe antenna installation methods and practices. An antenna that is installed on an aircraft must function properly and may not adversely affect other systems or equipment.
4.2 Hazards and Warnings.
4.2.1 Follow antenna manufacturer’s instructions and recommendations when they are available and appropriate and not contrary to the instructions of the aircraft manufacturer.
4.2.2 Extension of the landing gear or flaps may impact belly-mounted antenna performance.
4.3 Additional References. For further information concerning acceptable methods, techniques, and practices concerning alteration involving specific structures, refer to the appropriate chapter of AC 43.13-1, Acceptable Methods, Techniques, and Practices―Aircraft Inspection and Repair, and Civil Air Regulation (CAR) 6, Rotorcraft Airworthiness; Normal Category.
4.4 Structural Support.
4.4.1 The antenna’s structural load, plus any required allowances, may not exceed the design capacity of the structure intended to support it. It is important to understand the operational characteristics of the aircraft and consider forces that occur during flight (dynamic loading) as well as those that occur when the aircraft is not in motion (static loading). For example, an aircraft designed without flaps may employ a side slip procedure to lose altitude, during which the direction of airflow across the fuselage is not in line with the aircraft longitudinal axis. Antenna mountings on these aircraft need to be designed and evaluated for the direction of airflow that occurs during such an operation.
4.4.2 Whenever possible, an antenna should be mounted to a flat surface. Minor aircraft skin curvature can be accommodated with the use of an appropriate gasket, but if gaps over
0.020 inches appear between the base plate and mounting surface, use of a mounting saddle is recommended.
4.4.3 Since antenna systems typically require a ground plane (this may be a conductive surface that the antenna mounts to), any separation of an antenna from its ground plane may impact performance. Contact the manufacturer for recommendations if a gasket or mounting saddle is needed.
4-2 Figure 4-1. Antenna Mount With Saddle
4.4.4 Mounting screws must never be over torqued in an attempt to distort aircraft structure to reduce gaps between the antenna base plate and aircraft-mounting surface.
4.4.5 Consider the factors of flutter, vibration characteristics, and drag load. The approximate drag load an antenna develops may be determined by the following formula:
D = 0.000327 AV2 (The formula includes a 90 percent reduction factor for the streamline shape of the antenna.)
D is the drag load on the antenna in pounds.
A is the frontal area of the antenna in square feet.
V is the never-exceed speed (VNE) of the aircraft in miles per hour (mph).
Example: Antenna manufacturer specification frontal area = 0.135 square feet and the VNE of the aircraft is 250 mph.
D = 0.000327 x .135 x (250)2 = 0.000327 x .135 x 62,500 = 2.75 pounds
4.4.6 The above formula may be adapted to determine side load forces by substituting the apparent frontal area value for A when the aircraft motion and antenna orientation are not the same.
4.5 Physical Interference.
4.5.1 Antennas should be located where they will not interfere with the operation of the aircraft or other aircraft systems. One such example of interference could be the obstruction of the visibility of a navigation position light or beacon.
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4.5.2 Antennas should be located so that they do not obstruct or limit airflow to areas of the airframe that require airflow. Care should be exercised so that an antenna is not located where it will be damaged by heat from engine exhaust, fumes from battery vents, or fuel/fluid drains.
4.5.3 Antennas may accumulate ice that can then depart and damage areas behind them.
Special attention should be paid to areas of the airframe near pitot-static ports and sensors, and near flight controls since the antenna may alter airflow characteristics.
4.5.4 Antennas should be located in such a manner that they are not susceptible to damage from misuse, such as near a door where they might be mistaken for or used as a handhold.
4.6 Antenna Selection. The selection of an appropriate antenna will include consideration of system requirements and aircraft characteristics. The size and shape of an antenna vary with frequency, power rating, and maximum design speed of the antenna. See Figures 4-7, ADF Loop, through 4-19, DME/Transponder Blade, for pictures of typical antennas.
4.7 Antenna Location.
4.7.1 In general, antenna locations on an aircraft that provide unobstructed line-of-sight views of the transmitted or received signals are preferred. Objects located in the path of a signal may cause a blanking of antenna coverage and impact the performance of the system.
Note: Global Positioning System (GPS) antennas will not receive a signal if a line-of-sight view of a satellite is not available. Do not mount a GPS antenna on the underside of an aircraft.
4.7.2 The acceptable and unacceptable spacing between an antenna and an obstacle or the permissible interval between antennas is dependent upon operating frequency and system characteristics. When in doubt, contact the antenna manufacturer or system designer for further information.
4.7.3 Antennas should be separated as far as possible from interference sources (other radiating antennas, ignition noise sources, etc.). When known interference sources are present, it may be advisable to temporarily position an antenna and check the location for suitability prior to mounting the antenna.
4.7.4 Very high frequency (VHF) Com 1 should be mounted on the top of the aircraft since this will provide the best unobstructed location. VHF Com 2 can also be mounted on the top, provided there is at least 1/2 wavelength (of the antenna operating frequency) distance available between antennas.
4.7.5 If Com 2 is mounted under the aircraft, a bent whip may be required to provide ground clearance. Bent whips may not provide the best performance because of proximity to the aircraft skin. Signal reflection and obstruction is more of a problem with such locations.
4-4 Extension of the landing gear or flaps may also impact belly-mounted antenna performance.
4.7.6 Antennas need to “see” with a direct line-of-sight to the source. Antenna patterns can be disrupted by landing gear or vertical stabilizers, for example. When mounting antennas, try to locate them in areas where line-of-sight view is not obstructed.
4.7.7 As a rule of thumb, maintain 36 inches as a minimum distance between antennas. Refer to the manufacturer’s installation guidelines for specific system limitations and requirements.
4.7.8 Antennas should be located such that the cables that run between the antenna and equipment are as short as practical. Signal loss of a cable is dependent upon cable design, length, and frequency. In some cases, cables must be specific lengths to provide a required capacitance, attenuation, or signal transmission time. VHF omnidirectional range (VOR)/Localizer (LOC)/glideslope (GS) blade or towel bar type antennas require the cables to their coupling assembly to be the exact same length to maintain a phase relationship.
4.8 Antenna Bonding.
4.8.1 The electrical bonding of the antenna to the aircraft surface is extremely important. The conductive skin of an aircraft is an electrical part of the antenna system. If an antenna is not properly bonded to the aircraft, its pattern may be distorted, and nulls in coverage may appear.
4.8.2 The electrical bonding of the antennas to the aircraft skin of a metal aircraft is best accomplished by direct metal-to-metal contact of the antenna base to the skin. A resistance of no more than 0.003 ohms between the antenna base plate and skin should be achieved.
Note: To achieve this electrical bonding, the aircraft paint in the mounting area will need to be removed and the surface covered with an oxide film
(i.e., aluminum conversion coat) to protect aluminum against corrosion in accordance with MIL-DTL-5541.
4.8.3 An alternate method for providing electrical bonding to metal aircraft skin is through the antenna mounting screws, which attach to a backing plate inside the aircraft, making electrical contact with the backside of the skin. To ensure good contact, remove any interior paint in the area where the backing plate is placed and coat this area in accordance with MIL-C-5541B to minimize corrosion.
4.8.4 Composite or fabric-covered aircraft that do not provide a conductive mounting surface generally require fabrication of a conductive surface (ground plane) and bonding through the mounting screws.
4.8.5 Antenna performance can be severely degraded from corrosion caused by moisture accumulation where the antenna electrically bonds to its ground plane. It is advisable to