3-6 Figure 3-4. Typical Shock-Mounted Base (Wood or Composition Flooring)
3.5.1.2 M etal Flooring. Secure the shock-mounted base assembly directly to the floor using machine screws, washers, and self-locking nuts. Floor area under and around the radio mounting bases may require installation of doublers or other reinforcement to prevent flexing and cracking. Installation of plate nuts on the floor or doubler will facilitate removal and installation of the shock mounts. Install a ground strap between the shock mount foot and the radio rack.
Figure 3-5. Typical Shock-Mounted Base (Metal Flooring)
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3.5.2 Rigid-Mounted Unit Base. Secure radio mounting base plate(s) to the floor (wood, composite, or metal) using machine screws as shown in Figure 3-6. Use a reinforcing plate or large area washers or equivalent under wood or composite flooring. When the mounting base is secured to wood or composite material, install a ground strap between the base and aircraft metal structure.
Figure 3-6. Typical Rigid Base Plate Mount
3.6 Fabrication of Supporting Brackets for Attachment to a Structure Other Than Flooring.
3.6.1 Typical supporting brackets usually consist of a shelf or platform upon which the radio unit mounting base assembly can be installed in the same manner as described in paragraph 3.4.
3.6.2 Fabricate the bracket according to good aircraft design, layout, assembly practices, and workmanship to obtain results compatible with the airframe structure. Generally, the thickness of the bracket material will depend on the size or area of the platform and the load it must sustain according to the provisions set forth in Chapter 2.
3.6.3 Use a rivet size and pattern compatible with the aircraft structure to provide the strength needed to ensure support of the loads imposed under all flight and landing conditions.
3-8 Figure 3-7. Typical Underseat Installation No te: To increase the strength of floor attachment points, metal reinforcement may be installed as needed.
3.7 Reinforcement of the Supporting Structure.
3.7.1 Attachments. Attach equipment to the aircraft’s supporting structure so that its supported load will be transmitted to aircraft structural members. If direct attachment to the existing structure (bulkheads, horizontal stringers, etc.) is not feasible, add the necessary stringers, doublers, bulkhead flange reinforcements, etc., to provide adequate support and ensure load transfer to the primary structure. When attaching to the existing structure, ensure the attachment does not weaken the structure. Alteration to the primary structure may require approved engineering data.
3.7.2 Placard. Fasten a permanent placard onto the shelf or bracket (as the example below) stating the design load that the installed structure is determined capable of supporting.
“Shelf load not to exceed __________lbs.”
3-9 Figure 3-8. Typical Remote Unit Mounting Base-Vertical or Horizontal Other Than Structure to Floor
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.