Approval of New Avionics Equipment Installations
Most of the avionics equipment discussed in the Communication and Navigation series is only repairable by the manufacturer or FAA-certified repair stations that are licensed to perform specific work.
The airframe technician, however, must competently remove, install, inspect, maintain, and troubleshoot these increasingly complicated electronic devices and systems. It is imperative to follow all equipment and airframe manufacturer’s instructions when dealing with an aircraft’s avionics.
The revolution to GPS navigation and the pace of modern electronic development result in many aircraft owner-operators upgrading flight decks with new avionics. The aircraft technician must only perform airworthy installations
The avionics equipment to be installed must be a TSO-authorized device that is approved for installation in the aircraft in question. The addition of a new piece of avionics equipment and/or its antenna is considered a minor alteration if previously approved by the airframe manufacturer.
A certificated airframe technician is qualified to perform the installation and return the aircraft to service. The addition of new avionics not on the aircraft’s approved equipment list is considered a major alteration and requires an FAA Form 337 to be completed.
A technician with an inspection authorization is required to complete the approval for return to service following a major alteration and to sign the corresponding approval for return to service block on Form 337.
Most new avionics installations are approved and performed under an STC. The equipment manufacturer supplies a list of aircraft on which the equipment has been approved for installation. The STC includes thorough installation and maintenance instructions the technician must follow.
Regardless, if the equipment is not on the aircraft’s original equipment list, the STC installation is considered a major alteration and an FAA Form 337 must be filed. The STC is referenced as the required approved data.
Occasionally, an owner-operator or technician wishes to install an electronic device in an aircraft that has no STC for the model aircraft in question. A field approval and FAA Form 337 must be filed showing that the installation will be performed in accordance with approved data.
Considerations
There are many factors which the technician must consider prior to altering an aircraft by the addition of avionics equipment. These factors include the space available, the size and weight of the equipment, and previously accomplished alterations.
The power consumption of the added equipment must be considered to calculate and determine the maximum continuous electrical load on the aircraft’s electrical system. Each installation should also be planned to allow easy access for inspection, maintenance, and exchange of units.
The installation of avionics equipment is partially mechanical, involving sheet-metal work to mount units, racks, antennas, and controls. Routing of the interconnecting wires, cables, antenna leads, etc. is also an important part of the installation process.
When selecting a location for the equipment, use the areas designated by the airframe manufacturer or the STC. If such information is not available, select a location for installation that will carry the loads imposed by the weight of the equipment, and which is capable of withstanding the additional inertia forces.
If an avionics device is to be mounted in the instrument panel and no provisions have been made for such an installation, ensure that the panel is not a primary structure prior to making any cutouts. To minimize the load on a fixed instrument panel, a support bracket may be installed between the rear of the electronics case or rack and a nearby structural member of the aircraft. [Figure 1]
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| Figure 1. An avionics installation in a stationary instrument panel may include a support for the avionics case |
Avionics radio equipment must be securely mounted to the aircraft. All mounting bolts must be secured by locking devices to prevent loosening from vibration. Adequate clearance between all units and adjacent structure must be provided to prevent mechanical damage to electrical wiring or to the avionics equipment from vibration, chafing, or landing shock.
Do not locate avionics equipment and wiring near units containing combustible fluids. When separation is impractical, install baffles or shrouds to prevent contact of the combustible fluids with any electronic equipment in the event of plumbing failure.
Cooling and Moisture
The performance and service life of most avionics equipment is seriously limited by excessive ambient temperatures. High performance aircraft with avionics equipment racks typically route air-conditioned air over the avionics to keep them cool. It is also common for non-air-conditioned aircraft to use a blower or scooped ram air to cool avionics installations.
When adding a unit to an aircraft, the installation should be planned so that it can dissipate heat readily. In some installations, it may be necessary to produce airflow over the new equipment either with a blower or through the use of routed ram air.
Be sure that proper baffling is used to prevent water from reaching any electronics when ducting outside air. The presence of water in avionics equipment areas promotes rapid deterioration of the exposed components and could lead to failure.
Vibration Isolation
Vibration is a continuous motion caused by oscillating forces. The amplitude and frequency of vibration of the aircraft structure will vary considerably with the type of aircraft. Avionics equipment is sensitive to mechanical shock and vibration and is normally shock mounted to provide some protection against in-flight vibration and landing shock.
Special shock mounted racks are often used to isolate avionics equipment from vibrating structure. [Figure 2]
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| Figure 2. A shock mounted equipment rack is often used to install avionics |
Such mounts should provide adequate isolation over the entire range of expected vibration frequencies. When installing shock mounts, ensure that the equipment weight does not exceed the weight-carrying capabilities of the mounts. Radio equipment installed on shock mounts must have sufficient clearance from surrounding equipment and structure to allow for normal swaying of the equipment.
Radios installed in instrument panels do not ordinarily require vibration protection since the panel itself is usually shock mounted. However, make certain that the added weight of any added equipment can be safely carried by the existing mounts. In some cases, it may be necessary to install larger capacity mounts or to increase the number of mounting points.
Periodic inspection of the shock mounts is required, and defective mounts should be replaced with the proper type. The following factors to observe during the inspection are:
- Deterioration of the shock-absorbing material
- Stiffness and resiliency of the material
- Overall rigidity of the mount
If the mount is too stiff, it may not provide adequate protection against the shock of landing. If the shock mount is not stiff enough, it may allow prolonged vibration following an initial shock.
Shock-absorbing materials commonly used in shock mounts are usually electrical insulators. For this reason, each electronic unit mounted with shock mounts must be electrically bonded to a structural member of the aircraft to provide an electrical ground path.
This is accomplished by secure attachment of a tinned copper wire braid from the component, across the mount, to the aircraft structure as shown in Figure 3. In some installations, bonding is accomplished using solid aluminum or copper material where a short flexible strap is not practical.
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| Figure 3. A bonding jumper is used to ground an equipment rack and avionics chassis around the non-conductive shock mount material |
When is an FAA Form 337 required for an avionics installation?
What is the difference between a minor and major avionics alteration?
Why must shock-mounted avionics be electrically bonded to the airframe?
What physical factors must a technician calculate before adding new equipment?
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