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
12-13 allow possible recovery of a system that has had an over-temperature shutdown condition.
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.
13-1 CHAPTER 13. CARGO TIEDOWN DEVICE INSTALLATIONS
13.1 Purpose. This chapter provides data for making acceptable cargo tiedown device installations in non-pressurized areas of civil aircraft of 12,500 pounds gross weight or less. Engineering assistance is required for floor/attach fittings load analysis and material burn testing.
13.2 Hazards and Warnings.
13.2.1 Structural failure may occur if aircraft floor loading limits are exceeded.
13.2.2 Fire hazards may exist if materials do not meet flame resistant specifications.
13.2.3 Exceeding manufacturer’s Weight and Balance (W&B) limitations create unsafe flight conditions.
13.3 Installation Considerations.
13.3.1 Ensure that the altered aircraft can be operated within the permissible weight and center of gravity (CG) ranges.
13.3.2 Determine that there will be unobstructed access to all equipment and controls essential to the proper operation of the aircraft, required emergency exits, and emergency equipment.
13.3.3 Use only materials that are at least flame resistant for covering floors and webbing material. Refer to the applicable airworthiness standards for the aircraft involved to determine the required flame-resistant qualities. For aircraft in air taxi or other commercial operations, refer to the applicable operating rule for special requirements regarding fire protection, cargo bins, location of cargo with respect to passengers, cargo compartment, or aisle width.
13.4 Fabrication and Installation.
13.4.1 Cargo Tiedown Devices.
13.4.1.1 Cargo tiedown devices may be assembled from webbing, nets, rope, cables, fittings, or other material that conforms to a FAA-Parts Manufacturer Approval (PMA), Technical Standard Order (TSO), National Aerospace Standard (NAS), Army/Navy and Air Force/Navy (AN), or Military Specification (MIL-SPEC) standards. Use snaps, hooks, clamps, buckles, or other acceptable fasteners rather than relying upon knots for securing cargo.
Install tensioning devices or other acceptable means to provide a method of tightening and adjusting the restraint system to fit the cargoes to be carried.
13.4.1.2 Provide covers or guards where necessary to prevent damage to or jamming of the aircraft’s equipment, structure, or control cables.
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13.4.1.3 Straps and nets manufactured with MIL-SPEC webbing and thread must be evaluated to determine that the working load requirements are met. All tiedown assemblies are only as strong as the weakest component in the system, including the point of attachment.
Note: The owner/operator is responsible for ensuring that a basis of approval or acceptance is obtained for the tiedown device before use on aircraft. This is accomplished by providing substantiating engineering data of the devices being used, including aircraft floor load limits.
13.4.2 Structural Attachment. Commercially available seat tracks, rails, or other types of anchor plates may be used for structural attachment, provided they conform to a NAS, AN, or MIL-SPEC standard. This type of hardware permits a ready means of mounting a wide variety of quick-disconnect fittings for cargo tiedown. Typical examples of such fittings and their attachments are shown in Figures 13-1, Extruded Track, Anchor Plates, and Associated Fittings, through 13-5, Typical Cargo Tiedown Straps and Cargo Nets.
13.4.2.1 When installing these fittings, reinforce the existing floorboards or other adjacent structures, or both, to obtain the necessary load carrying capacity.
Seat tracks installed longitudinally across lateral floor beams generally require full-length support for adequate strength and rigidity between beam attach points (see Figure 13-4, Installation of Underfloor Support for Extruded Track).
13.4.2.2 Consider the inherent flexibility of the aircraft structure and install any reinforcement in a manner that will avoid localized stress concentrations in the structural members/areas. Give specific attention to the size, shape, and thickness of the reinforcement, fastener size and pattern, and the effects of any adhesives used.
13.4.2.3 Fittings used for cargo tiedown attachment need not be substantiated by static tests if it can be shown that the fitting’s rated minimum breaking strength would not be exceeded by the applicable static test loads. Existing racks, rails, or other points used for attachment may be verified by static tests, analysis, or a written statement by the aircraft manufacturer attesting to its adequacy to withstand the necessary loads.
Figure 13-1. Extruded Track, Anchor Plates, and Associated Fittings
EXTRUDED TRACK ANCHOR PLATE Light duty Medium duty Heavy duty Extruded track and anchor plates are available in several different styles and load capacities and will accommodate a wide variety of quick attachment fittings.
SINGLE PIN TYPE SINGLE PIN TYPE
HOLD DOWN FITTINGS CARGO TIE DOWN FITTINGS Adjustable These types of fittings are suitable for litter or berth attachment to the extruded track and anchor plate styles shown above.
These types of fittings are suitable for cargo tie down attachment to the extruded track and anchor plate styles shown above.
DUAL PIN TYPE CARGO TIE FITTINGS 360° Rotation Low profile These types of cargo tie down fittings are of greater capacity than the single pin types and are suitable for use with the extruded track style shown above.
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Figure 13-2. Miscellaneous Litter, Berth, and Cargo Tiedown Fittings
PAN FITTINGS SINGLE STUD FITTINGS Stud & Cargo ring Cargo ring only Stud only Round head Hex head
SINGLE STUD CARGO TIE DOWN FITTING
SINGLE STUD HOLD DOWN FITTINGS Lift to release These types of fittings are suitable for cargo tie down attachment to the single stud fittings or stud equipped pan fittings shown above.
STUD/RING These types of fittings are suitable for litter or berth attachment to the single stud fittings or stud equipped pan fittings shown above.
EYE BOLT These types of fittings are suitable for litter, berth, and/or cargo tie down attachment directly to the aircraft structure.
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