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• CAR 4a .532 thr ough 4a.537 provide the regulatory basis for supplemental variations for installation of instruments in CAR 4, Airplane Airworthiness, non-air carrier (NAC) airplanes.
• T itle 14 CFR § 23.1321 provides the regulatory basis for the arrangement and visibility of instruments in 14 CFR part 23 airplanes.
12.4.1 Structure. Chapter 2, Structural Data, provides guidance for determining structural integrity. Paragraphs 3.3 and 3.4 provide information pertinent to instrument panel installations.
12.4.2 Location. In the absence of specific regulatory requirements, for installation of instruments required for operation under instrument meteorological conditions (IMC), the recommended configuration is in the form of the basic “T” (see Figure 12-1).
Figure 12-1. The Basic “T” Visual Meteorological Conditions (VMC) Instrument Configuration
12.4.2.1 The aircraft attitude indicator is located at the top center of the installation.
12.4.2.2 The airspeed indicator is located adjacent to and left of the attitude indicator.
12.4.2.3 The altimeter is located adjacent to and to the right of the attitude indicator.
12.4.2.4 The directional indicator is located adjacent to and immediately below the attitude indicator.
Note: As instrument panel installations have become more complex, the basic “T” has become more difficult to accomplish. With the relocation of the directional indicator, the relationship of the Air Speed Indicator (ASI)-Attitude Indicator (ATI)-Altitude Indicator (ALT) combination is assumed.
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12.5 Installation. Throughout the development of the complexity of aircraft, the requirements for the mounting of all instruments have remained the same. All instruments must be mounted so they are visible to the crewmember primarily responsible for their use.
12.5.1 Structure. Before initiating changes to instrument panel installations, determine the load-bearing requirements of the component. In some aircraft, the instrument panel is stressed as the primary structure; with the majority, the panel is treated as the secondary structure with load-bearing responsibilities limited to instruments and equipment installed. Regardless of the structural nature, the altering technician is responsible for the installation to its original or properly altered state.
12.5.1.1 In all cases, where available, refer to the manufacturer’s instructions for continued airworthiness (ICA) when considering alterations to any structure.
For aircraft manufactured before the regulatory requirement for the manufacturer to provide such information, refer to AC 43.13-1, Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair, Chapter 4, Metal Structure, Welding, and Brazing, Section 4, Metal Repair Procedures, and/or Chapter 3, Communication, Navigation, and Emergency Locator Transmitter System Installations, of this AC for methods and techniques of retaining structural integrity.
12.5.1.2 If those sources are not available or applicable, we recommend that the altering individual have a Designated Engineering Representative (DER) approve the data before accomplishing the task.
12.5.2 Instrument Plumbing. The majority of CAR 3 and CAR 4 general aviation aircraft use instrumentation identified as “wet” gauges. This term has nothing to do with what is being measured by the gauge but rather the material to be measured is delivered directly to the gauge. Typical “wet” systems are fuel pressure, engine oil pressure, and, when so equipped, hydraulic pressure. While not usually considered “wet” systems, the airspeed and vacuum systems use this same system. The tubing used is generally thin-walled seamless aluminum or stainless steel. However, flexible rubber and/or neoprene lines are prevalent in airspeed and vacuum sensing lines.
12.5.2.1 The most reliable source of information on the material used in a particular aircraft is the manufacturer’s ICA. If that source is not available, AC 43.13-1, Chapter 8, Engines, Fuel, Exhaust, and Propellers, provides inspection and maintenance guidance on fuel lines while Chapter 9, Aircraft Systems and Components, provides the same level of information on hydraulic systems.
12.5.2.2 The installation of remote sensing systems per 14 CFR part 23 has replaced the “wet” systems with system pressures being remotely measured by transducers and electrically transmitted to the flight deck indicators.
12.5.2.3 Aircraft with long histories of alteration may contain a mixture of mechanical and/or electrical analogue instruments and digital presentations.
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12.5.3 Venturi Vacuum Source. A venturi is a relatively low-cost means of producing the vacuum to operate gyroscopic instruments, such as the turn and bank, directional gyro, and artificial horizon. It is mounted on the exterior of the fuselage, parallel with the longitudinal axis of the aircraft. As the aircraft moves through the air, an area of reduced pressure, a partial vacuum, is created in the throat of the venturi. The venturi throat is connected by a tube to the gyro instrument case, resulting in a reduction of pressure within the case. Ambient air, entering the instrument through the inlet port of the case, passes over the gyro rotor, causing it to spin rapidly, and causes the instrument to activate. Venturis are available in sizes providing 2, 4, and 9 inches of mercury (inHg) of vacuum.
12.5.3.1 The vacuum requirement for the operation of each instrument is generally provided on the data plate attached to its case. If it is not, obtain the data from the manufacturer or another acceptable source, such as Army/Navy and Air Force/Navy (AN) and MS standards.
12.5.3.2 If the vacuum available exceeds the requirements of the installation, it will be necessary to install an in-line regulator to adjust the flow within requirements.
12.5.4 Pumped Vacuum Source. Current aviation technology has produced durable light-weight, mechanically-driven pump and control systems for providing instrument operating vacuum. Selection of the correct pump remains dependent on the vacuum requirements of the system being driven. Control is provided by either the system itself or an in-line regulator. Production and aftermarket standby vacuum systems are available to provide operating capabilities in the event of failure of the primary mechanically-driven system.
12.5.5 Calculating Vacuum Loads. When a venturi vacuum source is selected, do not assume the venturi selected will provide sufficient flow and negative pressure to operate the instrument package. Even within the M/M of the venturi selected, tolerance may be insufficient to meet the requirements of the system selected. Therefore, it is essential that the vacuum load requirements be carefully evaluated.
12.5.5.1 Gyroscopic Instruments. Gyroscopic instruments require optimum value of airflow to produce their rated rotor speed. For instance, a specific bank and pitch indicator required approximately 2.30 cubic feet per minute (CFM) flow and a resistance, or pressure drop, of 4 inHg. If the altering technician has selected a 2 inHg Venturi, the resultant vacuum would be insufficient to drive the instrument.
12.5.5.2 Examples. The following vacuum driven items are examples for the purposes of calculation:
12-5 Table 12-1. CFM/Vacuum Values Instrument Flow Vacuum Bank and Pitch Indicator 2.3 CFM 4.0 inHg Directional Gyro Indicator 1.3 CFM 4.0 inHg Turn and Bank Indicator 0.5 CFM 2.0 inHg Total flow required 4.10 CFM
12.5.5.3 Table of Instruments. The above listed instruments are listed in Tables 12-2 and 12-4, Differential Pressure Across the Instrument Inlet and Outlet of the Example Instrument Packages. Optimum values are shown in Table 12-4. The negative pressure air source must deliver not only the optimum vacuum value for the instruments but must also have sufficient volume capacity to accommodate the total flow requirements of the various instruments it serves.
Note: The example components listed above and in Tables 12-2 and 12-4 are accepted as viable, and the operating parameters related to them are accurate. The nomenclature and the related properties have no effect on the resulting computations.
Table 12-2. Instrument Air Consumption for Example Vacuum-Driven Autopilot and Instrument Installations Instrument Air Consumption at Sea Level Differential Drop inHg Suction (Optimum) Cubic Feet per Minute Automatic Pilot System (Type A-2, A-3, and A-3A) Directional gyro control unit across mount assembly 5.00 2.15* Bank and climb gyro control unit across mount assembly 5.00 3.85* Total ― 6.00* Automatic Pilot System (Type A-4) Directional gyro control unit 5.00 3.50* Bank and climb gyro control unit 5.00 5.00* Total ― 8.50* Bank and pitch indicator 4.00 2.30 Directional gyro indicator 4.00 1.30 Turn and bank indicator 2.00 0.50 Note: Asterisk (*) indicates it includes air required for operations of pneumatic relays.
12-6 Table 12-3. Equivalent Straight Tube Line Drops for Each 90-Degree Elbow Installed in Any Given System (By O.D.)
Tubing Size Pressure Drop in a 90˚ Elbow in Terms of Length of Straight Tube Equivalent to a 90˚ Elbow O.D.
Inch Wall Thickness Inch Feet 1/4 X 0.035 0.28 3/8 X 0.035 0.46 1/2 X 0.042 0.62 5/8 X 0.042 0.81 3/4 X 0.049 0.98 1 X 0.049 1.35 Table 12-4. Differential Pressure Across the Instrument Inlet and Outlet of the Example Instrument Packages Instrument Suction in inHg Minimum Optimum Maximum Automatic Pilot System (Type A-2, A-3, and A-3A) Directional gyro control unit across mount assembly 4.75 5.00 5.25 Bank and climb gyro control unit across mount assembly 4.75 5.00 5.25 Gauge reading (differential gauge in B and C control unit) 3.75 4.00 4.25 Automatic Pilot System (Type A-4) Directional gyro control unit 3.75 5.00 5.00 Bank and climb gyro control unit 3.75 5.00 5.00 Bank and pitch indicator 3.75 4.00 5.00 Directional gyro indicator 3.75 4.00 5.00 Turn and bank indicator 1.80 2.00 2.20
12.5.5.4 Calculation of Equivalent Straight-Line Pressure Drop Due to Routing. In addition to the effects of the instrumentation itself, consideration must be given to the parasitic resistance to flow provided by the routing of the tubing providing the source pressure to the instruments. For this problem, assume the example system has four right-angle elbows and 20 feet of 1/2-inch O.D. x
0.042-inch tubing.
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12.5.5.5 Solution to the Stated Problem (Step-by-Step).
12.5.5.5.1 Flow Requirements. Accept the flow requirements of the example instruments listed in paragraph 12.5.5.2 as 4.10 CFM. The pressure drop for one 90-degree 1/2-inch O.D. elbow is equivalent to 0.62 feet of straight tubing.
Therefore, the pressure drop of the four 90-degree elbows is equivalent to
2.48 feet of tubing (see Table 12-3, Equivalent Straight Tube Line Drops for Each 90-Degree Elbow Installed in Any Given System (By O.D.)).
12.5.5.5.2 Pressure Drop. Determine the pressure drop through 22.48 feet (20 feet plus the above 2.48 feet) of 1/2-inch O.D. x 0.42-inch tubing at 4.10 CFM flow (see Figure 12-2, Pressure Drop Data for Smooth Tubing).
Figure 12-2. Pressure Drop Data for Smooth Tubing
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1. The pressure drop for each 10-foot length of that tube is 0.68 inHg.
2. Divide 22.48 feet of tubing to determine the number of 10-foot lengths
(i.e., 22.48 divided by 10 equals 2.248).
3. Multiply the number of sections (2.248) by 0.68 inHg to obtain the pressure drop through the system (0.68 x 2.248 = 1.53 inHg).
4. The pump must, therefore, be capable of producing a minimum pressure differential of 5.63 inHg (i.e., 4.10 inHg) for maximum instrument usage + 1.53 inHg (determined) at a flow of 4.10 CFM.
12.5.5.5.3 Filters. Filters are used to prevent dust, lint, or other foreign matter from entering the instrument and vacuum system. Filters may be located at the instrument intake port or the manifold intake port when instruments are interconnected. Determine if the flow capacity of the filter is equal to or greater than the flow capacity of the vacuum system. If it is not, the restriction will create a pressure drop in the system. AC 43.13-1, Chapter 12, Aircraft Avionics Systems, paragraph 38 provides additional guidance on the installation and maintenance of venturi and engine-driven vacuum and instrument systems.
12.5.5.5.4 Electrical Supply for Instruments. AC 43.13-1, Chapter 11, Aircraft Electrical Systems, provides information on the inspection and installation of electrical systems.
12.5.5.5.5 Instrument Lighting. The regulatory requirements for instrument lighting have changed little as certification specifications have developed.
1. CAR 3.696 and 3.697 provide two paragraphs simply stating that instrument lights (1) should be installed safely, (2) provide sufficient illumination to make instruments and controls easily readable, and (3) shall be installed in such a manner as not to shine in the pilot’s eyes.
2. CAR 4a.577 adds that instrument lighting shall be equipped with a rheostat control for dimming unless it can be shown a non-dimming light is satisfactory.
3. Title 14 CFR § 23.1381 consolidates the language of the earlier regulatory requirements with no appreciable changes.
12.5.5.5.6 Magnetic Direction Indicator. The magnetic compass is commonly referred to as the whiskey compass. The maximum deviation limitations of magnetic compasses have changed little since 1928.
1. CAR 3.666 specifies that the Magnetic Direction Indicator must be installed so that its accuracy is not excessively affected by the airplane’s vibration or magnetic field. After compensating for this in the installation, the deviation level in flight must not exceed 10 degrees on any heading.
12-9 CAR 3.758 provides and requires the installation and maintenance of the compass deviation card.
2. CAR 4a.562 refers to dampening and compensation of the installation and acknowledges the effects of electrical disturbances resulting from the increase in proliferation of aircraft electrical systems. As a result, specific aircraft required to meet the provisions of CAR 4a may be required to display two compass correction cards: one for operation with electrical power off and one for electrical power on operation.
3. Title 14 CFR § 23.1327 provides location and deviation considerations also provided for under CAR 3.666 and CAR 4a.562. This section additionally provides for more than 10 degrees deviation of the non-stabilized magnetic compass due to the operation of adjacent high draw electrical components: if either a magnetic stabilized indicator, not having a deviation of greater than 10 degrees on any heading, or a gyroscopic direction indicator is installed.
12.5.5.5.7 Alternative Installations to Liquid-Filled Magnetic Direction Indicators. With the development of the vertical reading dry compass, it is practical to replace the liquid-filled compass with a viable and approved alternative.
1. Vertical reading dry compasses provide the pilot with a true relationship of the aircraft to azimuths as opposed to the reverse reading requirements of the liquid filled compass.
2. The vertical reading dry compass does not leak and requires no periodic refilling or cleaning to make it stable and readable.
3. The vertical reading compass provides no relief from the requirement for magnetic correction compensation. Depending on the manufacturer and individual installation, the unit may even require supplemental compensation to meet the maximum deviation requirements.
4. Vertical reading compasses are available on the aftermarket in both Technical Standard Order (TSO) and non-TSO versions. The altering technician must be aware of the limitations and certification responsibilities of the unit selected.
12.6 Testing, Marking, and Placarding.
12.6.1 Testing the Venturi Tube-Powered Systems. At normal in-flight cruise speed, or an accurately generated representative airflow, check the venturi tube-powered system to ensure that the required vacuum is being supplied to the system.
12.6.2 Testing the Vacuum Pump-Powered System. When the system is powered by either engine or auxiliary driven vacuum pumps, check the system for proper output at their rated revolutions per minute (rpm). The output should be measured at the point of delivery to the instrument.
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12.6.3 Testing of Altimeters and Static Systems. Before performing an altimeter or static system test, determine that the system is free of contaminating materials such as dirt and water.
With all instruments disconnected from the system, purge the plumbing with low pressure dry filtered air or nitrogen. Title 14 CFR part 43 appendix E provides guidance and practical guidance on altimeter and static systems tests and inspections.
Note: Altimeter tests in accordance with 14 CFR part 43 appendix E must be accomplished by a properly certificated repair agency.
12.6.4 Testing Instrument Electrical Supply. Subsequent to major repairs or alterations that result in addition, replacement, or rearrangement of instrument electrical circuitry, verify continuity and current availability before connecting the instruments. With the individual instruments isolated from the power circuits, determine if the current available is in accordance with the manufacturer’s requirements.
12.6.5 Fuel, Oil, and Hydraulic Fluid Supply (Wet Instruments). Before connecting the fluid transmission lines to the instruments, verify that they are free of residual material, dirt, and water. Purge them using a low-pressure application of dry filtered air or nitrogen.
12.6.5.1 On many CAR 3 and CAR 4a aircraft, common-sized fluid connections can lead to incorrect fluid line connections to the wrong instruments. Be careful to ensure fluid delivery to the correct instrument.
Note: Paragraph 12.6.5.1 speaks to line fittings. Note that these wet instruments require a restricted fitting where the line attaches to the engine pressure outlet port. The restricted fitting is necessary to prevent fuel/oil/etc. from being sprayed into the flight deck during an instrument malfunction. There have been cases where this restricted fitting was removed and a normal fitting was installed, a fatal error for some personnel and airplanes.
12.6.5.2 Verify fluid pressures at both the source and instrument to confirm the absence of restrictions or blockage before connection to the instrument.
12.6.6 Fuel, Oil, and Hydraulic Fluid Supply (Dry Instruments). Use the same care exercised on the verification of security, source, and hygiene of wet instrument systems when checking dry systems.
12.6.6.1 The primary difference is the delivery of the material to be measured to a transducer or pressure transmitter, rather than to the back of an instrument.
Use of the wrong fluid can damage an instrument beyond repair.
12.6.6.2 The additional difference between the two instrument systems is the attention required to provide the correct electrical circuitry and current between the transmitter and the instrument. (See paragraph 12.6.4.)
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12.6.7 Instrument Markings and Placards.
12.6.7.1 When additional instruments are installed, they must be appropriately marked.
Refer to the applicable CAR and 14 CFR for specific instrument marking and placard requirements.
12.6.7.2 Especially when the instrument panel was replaced, the altering individual should refer to the applicable ATC, type certificate (TC), Group 2, Bulletin 7a, or Type Certificate Data Sheet (TCDS) of the aircraft to ensure proper installation and placement of the required placards and limitations.
12.7 Electronic Display Instrument Systems or Electronic Display Indicators. The installation of these systems, regardless of the certification basis of the airplane into which they are being installed, are defined and specified under the provisions of 14 CFR § 23.1311.
12.7.1 Electrostatic Discharges (ESD).
12.7.1.1 With new technologies come the requirements to learn new skills. While a certain amount of care and attention to detail while handling mechanical and electrical instruments has always been required, the hazards have been based on circumstances that could be seen and felt. With the glass flight deck, a different kind of hazard has surfaced as a real threat to airworthiness of instrumentation. That threat is called ESD.
12.7.1.2 ESD is not a new phenomenon. It has been with the industry for a long time.
However, with the advent of the electronic display instrument system and its multiple presentations, the problem can now disrupt entire display systems as a result of one event. Any unit containing electronic components such as diodes, transistors, integrated circuits, programmable read-only memory (PROM), ROM, and memory devices must be protected from ESD. The simple act of improperly carrying an electronic device across a room without adequate protection can render it unairworthy.
CAUTION: To prevent damage due to excessive ESD, use proper gloves, finger cots, or grounding bracelets. Observe the standard procedures for handling equipment containing electrostatic sensitive devices or assemblies in accordance with the recommendations in the manufacturer’s maintenance instructions.
12.7.2 Electronic Display Instrument Systems Provide No Relief From the “Basic T”. Title 14 CFR § 23.1311(a)(5) requires systems using electronic displays to have independent backup systems to provide basic IMC reference in case electrical power is lost.
12.7.2.1 Title 14 CFR § 23.1311 requires an independent magnetic indicator and either an independent secondary mechanical altimeter, airspeed indicator, and attitude instrument; or
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12.7.2.2 Individual electronic display indicators for altitude, airspeed, and attitude that are independent from the airplane’s primary electrical system.
12.7.2.3 These secondary instruments may be installed in panel positions that are displaced from the primary positions as specified by 14 CFR § 23.1321(d), but must be located where they meet the pilot’s visibility requirements of 14 CFR § 23.1321(a).
12.7.3 Regulatory Requirements. AC 23.1311-1, Installation of Electronic Display in Part 23 Airplanes, Paragraph 3.0, Related Regulations and Documents, provides valuable interface references relating to the relationships between the CAR and 14 CFR regulatory requirements installation of this technology to older and current technology in airplanes.
12.8 Environmental Conditions.
12.8.1 General. The equipment environmental limits established by the manufacturer should be compatible with the operating environment of the airplane. When evaluating the equipment installation, consider such factors as the maximum operating altitude of the airplane and whether the equipment is located within a temperature- and pressure-controlled area. Applicable methods for testing the performance characteristics of the equipment for specific environmental conditions are provided in AC 21-16, RTCA Document DO-160 Versions D, E, F, and G, “Environmental Conditions and Test Procedures for Airborne Equipment.” Either test or analysis, or both, ensures the compatibility between the operational environment and the environmental equipment category of the laboratory tests.
12.8.2 Temperature. An electronic system’s reliability is strongly related to the temperature of the solid-state components in the system. Component temperatures depend on the internal thermal design and external cooling. In evaluating the temperature requirements, consider the additional heat generated by the equipment, especially in an area where air flow is restricted. To determine if adequate cooling is provided, the evaluation must make maximum use of previous data from compatible installations. This will assist in limiting the ground and/or flight tests of those installations that cannot be verified by other means.
When the equipment operating environment cannot be verified from previous experience or from evaluation of temperature values in that equipment location, a cooling test must be conducted.
12.8.3 Attitude Information. Attitude information should continue to be presented for a minimum of 30 minutes after the in-flight loss of cooling for the primary instrument when in the normal operating environment (temperature/altitude). If proper performance of the flight instrument functions is adversely affected due to loss of in-flight cooling, such conditions must be annunciated. Consider incorporation of an over-temperature shut-down of the system in case of cooling system failure. If such systems are used, Aircraft Flight Manual (AFM) documentation should be established, requiring subsequent pilot actions. Additionally, applicable placards must be provided for pilot situational awareness of the critical condition. These actions should include procedures to
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12.8.4 Annunciation. Annunciation of in-flight loss of cooling or fan monitors may not be required if shown by a safety analysis or test demonstration that a hazardous or catastrophic condition is not indicated. The safety analysis should consider the reliability of the fans, redundancies of functions, reversionary features (such as the ability to transfer critical functions), the enunciation of over-temperature and its response time, and the availability of other flight instrumentation. In some systems, cooling fans may be installed to improve the operating environment of the components and reduce the possibility of a failure condition or shutdown of the equipment. With supplementary installations, fan monitoring or additional temperature sensors may not be required. If cooling fans are needed to prevent a hazardous or catastrophic failure condition, installation of fan monitoring or other methods of determining the status of the cooling fan must be provided prior to flight.