This section addresses various electrical and mechanical installation considerations important to the systems engineer. The purpose of this section is to provide an introduction to installation methods and practices most applicable to the system designer. The intent is not to make the student an installations expert, but rather to provide interfacing and coordination information between the various engineering disciplines. This section is broken down into three sections: electrical equipment, electrical wiring, and mechanical.
Electrical Equipment
The Electrical Installation group is responsible for the installation design for electrical and electronic equipment and wiring on the entire aircraft. This includes the fuselage, wings, tail, and landing gear. Some of the equipment/systems include control boxes, interior/exterior lighting, antennas, attendant’s panels, position sensors, relay and circuit breaker panels, and entertainment systems. To narrow the scope of the subject for this section, equipment installations in three areas of the aircraft where special requirements and conditions exist when systems are in close proximity will be covered. These areas are: (1) Avionics Compartment, (2) Center Accessory Compartment, and (3) Flight Compartment. Many of the issues related to these three compartments are applicable throughout the aircraft. Coordination between system design and electrical design must be initiated early in order to address the important issues of system separation, mounting provisions, and envelope size limitations.
Avionics Compartment
The Avionics Compartment, located just below the Flight Compartment, comprises several different racks. The largest and most important rack housed here is the Main Avionics Rack, or MAR for short. For the most part, equipment is installed in this rack for close proximity to controllers in the Flight Compartment. The Auxiliary Avionics Rack, as the name implies, is an extension of the MAR and supports the equipment that could not be installed in the MAR. The Electrical Rack houses equipment primarily connected with the electrical distribution system, such as the static inverter and transformer rectifiers. This rack also houses other ancillary equipment/boxes. The Equipment Rack houses the forward/aft relay and equipment panels. The relay panels are capable of housing several hundred relays, which for one reason or another were not built into a system’s black box or are needed to connect different black boxes. For some customers, these relay panels have become so full that an auxiliary panel has to be added. This auxiliary panel is capable of containing approximately 75 relays. The Miscellaneous Equipment Panel, as its name implies, is used to house miscellaneous equipment such as transformers, couplers, filters, etc., and is located forward of the MAR.
Center Accessory Compartment
The Center Accessory Compartment (CAC) is located about midship. The main part of the CAC is the Electrical Power Center (EPC). The EPC contains the components and wiring necessary to distribute the electrical power from the generators to the various supply buses, along with the required circuit protection. The location of the EPC is a major decision that must be made during the early stages of space allocations within the airplane. Some of the factors in deciding its location include:
- • number of generators and their locations.
- • vulnerability to faults such as engine failures.
- • locations of major component centers.
- • access for maintenance and during flight.
The EPC should be modular in design so that components can be installed and wired before the module is installed in the airplane. The design may be a separate module for each generator bay, or one assembly with a separate bay for all components associated with any one generator. In either design, there must be adequate isolation between bays so that a failure of any one generating system, e.g., a short circuit, will not cause loss or impairment of any adjacent system. The CAC also houses several shelves for the placement of various LRUs, mostly related to controlling the electrical power systems.
Rack installation design needs
An equipment/avionics rack provides a convenient means of installing a number of LRUs in a particular location in the airplane. It provides a method of interfacing the LRU with aircraft wiring, the equipment cooling system, and other equipment in the aircraft. The preferred method of interfacing the LRU with rack and panel connectors is through the use of mounting trays. A mounting tray assembly that includes front panel hold-downs is a complete system that absorbs all LRU insertion and extraction loads.
LRU sizing
Equipment installed in a rack must be designed to ARINC 404A, ARINC 600, or to a combination of these specifications. ARINC 404A, an older specification used on the DC-10, defines the standard Air Transport Rack (ATR) sizes. ARINC 600 defines the Modular Concept Unit (MCU). The MCU is used to define LRU sizes and is the basic building- block model now used in commercial avionics design. LRU size is specified by the MCU designation. Table 7.5.1 shows approximate correlation between the ATR box sizes of ARINC 404A, and the LRU sizes expressed in multiples of MCU size as defined in ARINC 600.
Table 7.5.1
| LRU size | ATR designation | Width (in.) | Length (in.) | Height (in.) |
|---|---|---|---|---|
| 1 MCU | – | 1.00 | 12.56 | 7.64 |
| 2 MCU | Short 1/4 ATR | 2.25 | 12.56 | 7.64 |
| 3 MCU | Short 3/8 ATR | 3.56 | 12.56 | 7.64 |
| 4 MCU | Short 1/2 ATR | 4.88 | 12.56 | 7.64 |
| 5 MCU | 6.19 | 12.56 | 7.64 | |
| 6 MCU | Short % ATR | 7.50 | 12.56 | 7.64 |
| 7 MCU | 8.79 | 12.56 | 7.64 | |
| 8 MCU | 10.09 | 12.56 | 7.64 | |
| 9 MCU | 11.39 | 12.56 | 7.64 | |
| 10 MCU | 12.69 | 12.56 | 7.64 | |
| 11 MCU | 13.99 | 12.56 | 7.64 | |
| 12 MCU | 15.29 | 12.56 | 7.64 |
Equipment cooling
One of the features of the equipment/avionics rack is that it can provide cooling air to the electrical/electronic boxes. The three cooling systems used today are blow-by, draw-through, or blow-through. All require different approaches to the fundamental design of the rack. The cooling system design will be closely coordinated with the Environmental group and approved by them.
Universal arrangement
A rack arrangement that shows all LRUs installed in their unique locations is called the universal arrangement for that rack. Except for equipment that is mutually exclusive, each LRU is given a unique location in the rack. That is, once a unit is given a location in a rack, it will always be installed in that same position for all customer configurations. This is done to avoid the costly redevelopment of wiring that would be required if the location for an LRU were to change from customer to customer.
Equipment location—Questions to ask
The following questions should be asked when installing equipment in the Avionics Compartment or the shelves of the CAC:
- How will it be wired into the overall system?
- Equipment that must be interconnected should be placed on the same shelf in order to minimize wire lengths. Equipment racks are wired in a horizontal matrix, using a minimum of vertical ties.
- Will more than one system be installed?
- System separation may require physical isolation so that only one system is lost due to an incident. For example, Air Data Computer #2 (ADC-2) is mounted on the left side of the MAR on Shelf 1, and ADC-1 is mounted on the right side of Shelf 1.
- Is there electromagnetic interference?
- Isolate LRUs from “dirty” boxes as much as possible.
- Are there any attitude restrictions?
- Equipment that includes gyros, level sensors, or inertia devices may require alignment to the aircraft axes or relative alignment to adjacent units. For example, Inertial Reference Units (IRUs) are required to be mounted very level.
- What is the weight of the unit?
- Heavy boxes should be located so they will not require excessive maneuvering or lifting for installation.
- Are there special access requirements?
- Some equipment includes tapes, cassettes, communication codes, or other adjustments that should be accessible for replacement or maintenance without removing the box from its installed position.
Flight Compartment
A considerable amount of equipment is installed in the Flight Compartment. For the purposes of this section we will focus on three areas: the pedestal, the circuit breakers installed in the aft overhead panel, and the forward overhead panel. However, of these three areas, only the pedestal and forward overhead panel house equipment. The circuit breakers installed in the aft overhead need to be discussed because of their importance and because of space limitations.
Pedestal
The pedestal is located between the captain and first officer on the centerline of the airplane, and houses control panels for communication, radio, and navigation systems. The equipment is mounted on rails that extend fore and aft. Systems engineers planning to have equipment installed in the pedestal must comply with MS25212, Console Type, and Control Panel. This section covers equipment sizes and methods of mounting. The equipment design must consider wiring connections and equipment cooling as applicable. The systems engineer should also be aware of the possible effects of liquids in the area from condensation or inadvertent spillage of water, tea, coffee, etc. Condensation is especially important for equipment mounted on the forward side of the pedestal where the pedestal is attached to the basic structure. Condensation can freeze during flight, melt during descent, and drip into nearby equipment.
Overhead Circuit Breakers
Circuit breakers installed in the overhead panel are grouped according to voltage buses, e.g., Battery Bus, Battery Direct Bus, Left Emer, DC Bus, etc. They are further grouped by systems and functional categories, such as avionics, flight control, hydraulics, engine, etc.
These overhead circuit breakers are essential to in-flight safety, hence their accessibility to both pilots while remaining seated (reference: FAR Part 25.1357). The primary function of these breakers relates to the safe operation of the aircraft, i.e., power to flight instruments, communication systems, navigation systems, and aircraft performance instruments. Of concern to the systems engineer is the extremely limited amount of space available to install any more circuit breakers on the Battery Bus, and of course, the limited capacity of the battery itself.
Forward Overhead Panel
The overhead switch panel is located on the aircraft ceiling, approximately on centerline and accessible to both pilots. Of the equipment installed on the forward panel, the systems controllers are of primary importance. The remaining control panels are packaged per MS25212, except for the depth. Generally, they have to be considerably shallower than the maximum depth allowed by this specification. Similarly, the system controllers are rail-mounted and are packaged per MS25212, except for the maximum depth and width (they are wider than MS25212 allows). Any equipment to be mounted in the overhead panel needs to be coordinated as early as possible with the electrical Installation design engineers. Also of importance to the systems engineer are the separation requirements for equipment to be installed in the overhead panel.
Equipment Installation and Environmental Factors
Vibration, temperature, and humidity, and various environmental factors can affect the aircraft equipment and systems. The effects of a high-impact bird strike on the system control panels and installation can be very serious.
Leave a Reply