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h. Cable Attachment to Shock-Mounted Units. Route and support electrical wire bundles and mechanical cables in a manner that will allow normal motion of equipmen t without strain or damage to the wire bundles or mechanical cables.
i. 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 surf aces for best contact (resistance of connections should not exceed
0.003 ohm).
(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.
208. ELECTRICAL LOAD ANALYSIS PROCEDURE.
a. 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.
b. One method for the analysis may be found in the ASTM International, Standard Guide for Aircraft Electrical Load and Power Source Capacity Analysis, F 2490-05.
c. Applicable elements of a previously performed electrical load analysis if available may be reused.
209. ELECTROMAGNETIC COMPAT- IBILITY.
a. Electromagnetic Interference (EMI) may disrupt the performance of systems and has varying degrees of consequence. These consequences may be identified as: no safe ty effect, minor, major, hazardous, or catastrophic. The purpose of electromagnetic compatibility (EMC) analysis and testing is to assure 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. (Refer to Table 2-2.)
TABLE 2-2. RELATIONSHIP PROBABILITIES, 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
b. 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.
TABLE 2-3. COMMONLY USED RADIO FREQUENCIES ON AIRCRAFT Range Hz Mode Function 190-1750 kHz Rx ADF Navigation 2-30 MHz Tx/Rx HF Communications 75 MHz Rx Marker Beacon Receiver 108-112 MHz Rx ILS Localizer Receiver 108-118 MHz Rx VHF Omnirange (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 Emergency Locator Transmitter 960-1215 MHz Tx/Rx DME System 1027-1033 MHz Tx/Rx Transponder & TCAS Systems 1087-1093 MHz Tx/Rx Transponder & TCAS Systems 1575.42 MHz Rx GPS Satellite Navigation
c. 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.
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aircraft. Use of ground power is not recommended, as ground power units are not routinely checked for output quality.
(2) All normally closed circuit breakers should be closed and power should be supplied to all normally powered AC and DC distribution busses during testing.
(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.
(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.
TABLE 2-4. AIRCRAFT ELECTRICAL AND
ELECTRONIC SYSTEMS System ADF Air Data Systems Altitude Alert System ATC Transponder Audio Distribution System Autopilot/Flight Guidance System Compass/Directional Gyro Systems DME Electronic Flight Control System Electronic Flight Instrument System (EFIS) Global Positioning System (GPS) Marker Receiver System VHF Communications
VOR/LOC/GS Newly Installed Electrical/Electronic System
210. FUNCTIONAL HAZARD ASSESSMENT (FHA).
NOTE: Refer to AC 43.1309-1 (current edition) for additional information on FHA.
a. 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 assure 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 assure design safeguards is accomplished through a functional hazard assessment.
b. Alterations that involve systems or equipment that perform critical functions or that include complex designs that have a high degree of integration, use of new technology, or novel applications of conventional technology, must be assessed to determine th e 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.
c. Evaluate the system to determine if it is essential or not 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, unless the Administrator has found that the change is significant in an area.
d. Determine if the equipment has any unacceptable, adverse affect when operated.
e. Determine if the operation of the installed equipment has an adverse affect on equipment not essential to safe operation, and if a means exists to inform the pilot of the effect.
f. Determine if a failure or malfunction of the installed equipment could result in unacceptable hazards.
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g. Design requirements and methodology of hazard resolution differing upon application and type of aircraft (i.e., single-engine, multiengine, commuter use).
(1) Operation of equipment that has an adverse effect on other equipment essential to safe operation of the aircraft is unacceptable.
(2) Operation of equipment that has an adverse effect on other equipment that is not essential to safe operation of the aircraft may be acceptable if there is a means to inform the pilot of the effect.
(3) If a probable failure or malfunction will result in a hazard in a multiengine aircraft it is unacceptable.
(4) If a probable failure or malfunction will result in a hazard in a single-engine aircraft its impact must be minimized.
211. THRU 299. RESERVED
CHAPTER 3. ANTENNA INSTALLATION
300. PURPOSE. The purpose of this chapter is to describe antenna installation methods and practices.
An antenna that is instal led on an aircraft must function properly and may not adversely affect other systems or equipment.
301. HAZARDS AND WARNINGS.
a. Follow antenna manufacturer’s instructions and recommendations when they are available and appropriate, and not contrary to the instructions of the aircraft manufacturer.
b. Extension of the landing gear or flaps may impact belly-mounted antenna performance.
302. ADDITIONAL REFERENCES. For further information concerning acceptable methods, techniques, and practices concerning alteration involving specific structures, refer to the appropriate chapter of Advisory Ci rcular (AC) 43.13-1, Acceptable Methods, Techniques, and Practices―Aircraft Inspection and Repair (current edition), and Civil Aviation Regulation 6, Rotorcraft Airworthiness; Normal Category.
303. STRUCTURAL SUPPORT.
a. 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.
b. 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" appear between the base plate and mounting surface, use of a mounting saddle is recommended.
c. 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.
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FIGURE 3-1. ANTENNA MOUNT WITH SADDLE
Upper skin ref Stringer 30L ref Stringer 30R ref UP OUT Looking aft Scale: Full
d. 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.
e. Consider the factors of flutter, vibration characteristics, and drag load. The approximate drag load an antenna develops may be determined by the 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 lbs.
A is the frontal area of the antenna in sq. ft.
V is the VNE of the aircraft in mph.
Example: Antenna manufacturer specification frontal area = 0.135 sq. ft. and VNE of aircraft is 250 mph.
D=0.000327 x .135 x (250)2 =0.000327 x .135 x 62,500 = 2.75 lbs
f. The above formula may be adapted to determine side load forces by substitution of the apparent frontal area value for A, when the aircraft motion and antenna orientation are not the same.
304. PHYSICAL INTERFERENCE.
a. 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 visibility of a navigation position light or beacon.
b. Antennas should be located so that they don’t obstruct or limit airflow to areas of the airframe that require airflow. Care should be exercised that an antenna is not located where it will be damaged by heat from engine exhaust, fumes from battery vents, or fuel/fluid drains.
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c. 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.
d. 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.
305. ANTENNA SELECTION. The selection of an appropriate antenna will include consideration of system requirements and aircraft characteristics. The size and shape of an antenna varies with frequency, power rating, and maximum design speed of the antenna. See Figures 3-7 through 3-19 for pictures of typical antennas.
306. ANTENNA LOCATION.
a. In general, antenna locations on an aircraft which 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.
b. 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.
c. 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 a location for suitability prior to mounting the antenna.
d. 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.
e. 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. Extension of the landing gear or flaps may also impact belly mounted antenna performance.
f. Antennas need to “see” with a direct lineof-site 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.
g. As a rule of thumb, maintain 36 inches as a minimum distance between antennas. Refer to manufacturer’s installation guidelines for specific system limitations and requirements.
h. Antennas should be located such that cable runs between 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.
VOR/LOC/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.
307. ANTENNA BONDING.
a. 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.
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b. 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-C-5541B.
c. 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.
d. 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.
e. 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 apply RTV around the antenna edges to seal the antenna bond;
however, always ensure chemical compatibility before using any sealant.
308. ELECTROMAGNETIC INTERFERENCE (EMI).
a. Since the purpose of an antenna is to either receive or transmit RF energy (or both), it is essential to consider EMI.
b. Antenna mounting positions should be selected that are as far as possible from an EMI source. In special cases, it may be possible to employ filters or select an antenna that has been specifically designed to be resistant to EMI.
c. EMI test procedures are found in chapter 2.
309. MECHANICAL INSTALLATION.
a. Mounting Hardware.
(1) Typical antenna installations employ either #8-32 or #10-32 stainless steel mounting screws.
(2) Some designs require a pan head screw. For others, a counter sunk screw is required.
(3) Mounting screw length will vary based on each particular installation requirement.
b. General Practices.
(1) Refer to installation drawing before drilling holes in aircraft skin to determine proper size and spacing.
NOTE: When replacing antennas, it is important to match the original mounting holes. Previous mounting holes that are not reused with appropriate hardware must be repaired and the mounting location returned to its design strength.
(2) Use of a structural backing plate is highly recommended. Backing plates strengthen the immediate point of attachment but if they are not attached to load carrying structure they do not provide structural load support.
(3) Mounting screws should be secured with stainless steel nuts with flat washers and lock washers, or with flat washers and lock nuts to secure the antenna properly.
(4) Sandwich the aircraft skin between the antenna base plate and the internally mounted backing plate. Before securing the antenna to the aircraft make sure that all the cables are connected to Par 307 Page 26
Par 309 Page 27 the unit and fit through the connector holes in the aircraft.
(5) Gently tighten the mounting hardware so that uniform stress is placed on each side of the antenna. For #8-32 screws do not exceed 20 in•lbs of torque and for #10-32 screws do not exceed 23 in•lbs of torque. Refer to fastener manufacturer’s torque guidelines to confirm that these recommended settings do not exceed the chosen fasteners torque limits.
b. Most antennas require a ground plane size of approximately 24" by 24". While the rule of thumb is to provide a minimum of 1/4 wavelength of the operation frequency, larger is better and ground plane symmetry is critical. Gaps in antenna coverage or performance may occur if a ground plane is not symmetrical.
TABLE 3-1. WAVELENGTH IN FEET = 984 / FREQUENCY IN MEGAHERTZ (MHZ)
(6) Once the antenna is mounted, any minor gaps between the base plate or gasket and aircraft skin should be filled with RTV silicone adhesive sealant.
Frequency Wavelength (ft ) 1/4 wavelength (in) 75 MHz 13.1 40 125 MHz 7.9 24 1000 MHz 1 3
(7) Double check that a reading of
0.003 ohms between the antenna base plate and ground has been achieved.
310. GROUND PLANE REQUIREMENT (WOOD/FABRIC AND COMPOSITE AIRCRAFT).
c. Wire mesh is the best material to use when a solid plate is not practical. Heavy aluminum foil can also be used. In all cases, electrical continuity from ground plane to airframe ground is essential.
d. Be sure the ground plane is well attached to the airframe with cement or epoxy if not otherwise supported. This will prevent noise problems or erratic operation that could occur if the plane moves.
Capacitance will occur as an antenna base is separated from its ground plane. This may distort antenna coverage or operation.
a. When the antenna is not mounted to a conductive surface capable of providing a required ground plane for operation, a ground plane must be fabricated.
FIGURE 3-2. ANTENNA GROUND PLANE FOR NONCONDUCTIVE MOUNT
FIGURE 3-3. ONE MEANS TO PROVIDE ADEQUATE ANTENNA BONDING THROUGH A
COMPOSITE AIRFRAME
(Length to meet installation requirements) Flathead screw stainless steel Seal Aluminum Grounded Foot Plate Composite Aircraft Aluminum Ground Plate Ground to Aircraft (4 places) Doubler Plate Elastic Stop Nut Stainless Steel or equivalent FIGURE 3-4. USE BUSHINGS FOR ALL SCREWS NOTE: Carbon Fiber composite material, while conductive has not been found to be adequate as a ground plane.
e. Refer to AC 43.13-1 (current edition), chapter 3, for acceptable methods, techniques, and practices applicable to fiberglass and plastics.
311. ANTENNA FEED LINE BALUN.
a. Antenna cables (electrical feed lines) may be designed to be electrically balanced or unbalanced. Certain antenna designs incorporate dual elements with a requirement for balanced input.
In these cases, standard cables which are unbalanced are generally employed for the cable run and a balancing transformer is located at the antenna feed connection.
b. A balun is a device that converts an unbalanced feed into a balanced input and may include a transformer that matches feed impedance to provide maximum signal transfer. Follow the manufacturer’s installation procedures when a balun is required. Some balun designs require that the balun be grounded to the airframe.
c. Refer to AC 43.13-1 (current edition), chapter 11, for bonding practices.
Length to be determined per fastener at installation Outer Diameter as required Par 310 Page 28
FIGURE 3-5. TYPICAL DIPOLE ANTENNA
ASSEMBLY
FIGURE 3-6. TYPICAL VOR BALUN
312. ANTENNA REPAIR. Painting an antenna or applying protective coatings or devices that are not approved are not allowed under this AC. Paint is an RF de-tuner. If an antenna is painted in the field, paint type and paint thickness present uncontrolled variables that will affect an antenna’s performance, and may result in the antenna no longer meeting its specifications or Technical Standard Order (TSO).
313 THROUGH 399 RESERVED Par 312 Page 29
FIGURE 3-7. ADF LOOP
FIGURE 3-8. ADF COMBINED SENSE LOOP
FIGURE 3-9. COM WHIP
FIGURE 3-10. COM WHIP BENT
FIGURE 3-11. ELT
FIGURE 3-12. GPS
FIGURE 3-13. GLIDESLOPE
FIGURE 3-14. MARKER
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