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.
Par 303 Page 24
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
Par 312 Page 30
Par 312 Page 31 (and 32) FIGURE 3-18. TRANSPONDER/DME PROBE FIGURE 3-15. COMBINED COM/VOR
FIGURE 3-16. VOR RABBIT EAR FIGURE 3-19. DME/TRANSPONDER BLADE
FIGURE 3-17. VOR BLADES
CHAPTER 4. ANTICOLLISION AND SUPPLEMENTARY LIGHT
INSTALLATION
400. PURPOSE. This chapter gives procedures and standards to be used when replacing older rotating beacon assemblies and wing lights with strobe or other anticollision systems. This chapter assumes that the newer units have FAA approval in the form of a Parts Manufacturer Approval (PMA) or Technical Standard Order (TSO), and not an experimental or aviation unit.
401. HAZARDS AND WARNINGS. When installing anticollison lights take care to ensure the unit is properly grounded, the airframe structure can support the new unit, and the aircraft wiring is of the correct size. Mechanics should take special precautions to protect their eyes when testing the new unit. Strobe lights are especially hazardous in dark or darken hangars when activated.
402. REGULATIONS AND OTHER REFERENCES. The requirements for anticollision lights are included in Title 14 of the Code of Federal Regulations (14 CFR) part 23, § 23.1401 and part 27, § 27.1401 for non-transport category aircraft. For part 23 aircraft certificated after March 11, 1996, § 91.205 are required to have an anticollision light. The night VFR requirements for part 23 certificated on or before August 11, 1971, must have an approved white or red anticollison light. Aircraft for which an application for type certificate was made before April 1, 1957, may conform either to the above regulations or to the following standards: Additional information can be found in AC 20-74.
a. Civil Aviation Regulation (CAR) 6, Rotorcraft Airworthiness; Normal Category.
b. Anticollision lights (when installed) should be installed on top of the fuselage or tail in such a location that the light w ill not impair the flight crewmembers’ vision and will not detract from the conspicuity of the position lights. If there is no acceptable location on top of the fuselage or tail, a bottom fuselage or wing tip installation may be used.
c. The color of the anticollision light must be either aviation red or aviation white in accordance with the specifications of § 23.1397 or § 27.1397, as applicable.
d. The arrangement of the anticollision light system, (i.e., number of light sources, beam width, speed of rotation and other characteristics, etc.) must give an effective flash frequency of not less than 40, nor more than 100, cycles per minute. The effective flash frequency is the frequency at which the aircraft’s complete anticollision light system is observed from a distance, and applies to each sector of light including any overlaps that exist when the system consists of more than one light source. In overlaps, flash frequencies may exceed 100 but not more than 180 cycles per minute.
e. The system must consist of enough lights to illuminate the vital areas around the aircraft, considering the physical configuration and flight characteristics of the aircraft. The field of coverage must extend in each di rection within at least 75 degrees above and 75 degrees below the horizontal plane of the aircraft. The minimum light intensity and minimum ef fective intensities are given in §§ 23.1401 and 27.1401 respectively.
f. Supplementary lights may be installed in addition to position and anticollision lights required by applicable regulations; provided that, the required position and anticollision lights are continuously visible and unmistakably recognizable and their conspicuity is not degraded by such supplementary lights.
403. OPERATIONAL CONSIDERATIONS:
CREW VISION. Partial masking of the anticollision light may be necessary to prevent direct Par 400 Page 33
or reflected light rays from any anticollision or supplementary light from interfering with crew vision. Determine if the field of coverage requirements are met. An acceptable method of preventing light reflection from propeller disc, nacelle, or wing surface is an application of nonreflective paint on surfaces which present a reflection problem. Perform a night flight-check to assure that any objectionable light reflection, sometimes known as flicker vertigo, has been eliminated. Enter a notation to that effect in the aircraft records.
404. INSTALLATION CONSIDERATIONS.
a. Communication and Navigation. Assure that the installation and operation of any anticollision/supplementary light does not interfere with the performance of installed communication or navigation equipment. Capacitor discharge light (strobe) systems may generate radio frequency interference (RFI). This radiated interference can be induced into the audio circuits of communication or navigation systems and is noticeable by audible clicks in the speaker or headphones. The magnitude of the RFI disturbance does not usually disrupt the intelligence of audio reception.
b. Precautions. RFI can be reduced or eliminated by observing the following precautions during installation of capacitor discharge light systems:
(1) Locate the power supply at least 3 feet from any antenna, especially antennas for radio systems that operate in the lower frequency bands.
(2) Assure that the lamp unit (flash tube) wires are separated from other aircraft wiring placing particular emphasis on coaxial cables and radio equipment input power wires.
(3) Make sure that the power supply case is adequately bonded to the airframe.
(4) Ground the shield around the interconnecting wires between the lamp unit and power supply at the power supply end only.
405. MARKINGS AND PLACARDS. Identify each switch for an anticollision/supplementary light and indicate its operation. The aircraft should be flight tested under haze, overcast, and visible moisture conditions to ascertain that no interference to pilot vision is produced by operation of these lights. If found unsatisfa ctory by test or in the absence of such testing, a placard should be provided to the pilot stating that the appropriate lights be turned off while operating in these conditions.
406. ELECTRICAL INSTALLATION. Install an individual switch for the anticollision light or supplementary light system that is independent of the position light system switch. Data for the installation of wiring, protection device, and generator/alternator limitations is contained in Advisory Circular (AC) 43.13-1B Acceptable Methods, Techniques, and Practices-Aircraft Inspection and Repair, (as amended) chapter 11.
Assure that the terminal voltage at each light is within the limits as prescribed by the manufacturer.
407. ALTERATION OF STRUCTURE.
a. The simplest light installation is to secure the light to a reinforced fuselage skin panel. The reinforcement doubler shall be of equivalent thickness, material, and strength as the existing skin.
(Install as shown in Figure 4-1.)
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FIGURE 4-1. TYPICAL ANTICOLLISION OR SUPPLEMENTARY LIGHT INSTALLATION
IN A SKIN PANEL (UNPRESSURIZED)
Anticollision Light or Supplementary Light Fuselage Skin Existing Stringer Existing Stringer View A-A Reinforcing Doubler Approximately One Inch Spacing of 1/8” Min.Dia.Rivet Alclad 2024-T3 1 1/2” Edge Distance Min.
b. When a formed angle stringer is cut and partially removed, position the reinforcement doubler between the skin and the frame. The doubler is to be equivalent to the stringer in thickness and extend lengthwise beyond the adjacent fuselage frames. The distance between the light and the edge of the doubler is to be twice the height of the doubler flange. (See Figure 4-2 for typical installation.)
c. Engineering evaluation is required for installations involving the cutting of complex formed or extruded stiffeners, fuselage frames, or pressurized skin of pressurized aircraft.
d. Vertical stabilizer installations may be made on aircraft if the stabilizer is large enough in cross section to accommodate the light installation, and if aircraft flutter and vibration characteristics are not adversely affected. Locate such an installation near a spar, and add formers as required to stiffen the structure near the light. (A typical installation is shown in Figure 4-3.)
e. Rudder installations are not recommended because of the possible structural difficulties.
However, if such installations are considered, a FAA engineering evaluation to determine whether the added mass of the light installation will adversely affect the flutter and vibration characteristics of the tail surfaces must be made.
Par 407 Page 35
FIGURE 4-2. TYPICAL ANTICOLLISION OR SUPPLEMENTARY LIGHT INSTALLATION
INVOLVING A CUT STRINGER (UNPRESSURIZED)
Anticollision Light or Supplementary Light Cut Stringer Reinforcing Doubler Alclad 2024-T3 Stringer Stringer Frame Frame Double Flanges at Least the Height of Cut Stringer Flange Stringer and Doubler Existing Rivets Rivets Added as Shown on Frames
Par 407 Page 36