Module 07A B1 - Maintenance Practices

SAFETY PRECAUTIONS - AIRCRAFT AND WORKSHOP

General Workshop Standards
- Maintaining an orderly and clean environment in hangars, shops, and on the flight line is fundamental to safety and efficient maintenance.
- Safety measures include painting safety lanes, pedestrian walkways, and fire lanes around the perimeters inside hangars to prevent accidents and keep traffic out of work areas.
- Signs must be posted to indicate dangerous equipment, hazardous conditions, and the locations of first aid and fire equipment.
- Communication is essential; technicians and supervisors must monitor their own safety and that of others, intervening if personnel act in an unsafe manner.
Safety Around Electricity
- Physiological Hazards: Electricity applied to the human body causes severe entrance and exit burns and can damage or destroy the nervous system.
- Technician Requirements: Technicians must possess a working knowledge of electrical principles and a healthy respect for electrical power.
- Safety Equipment: Physical protection includes rubber gloves, safety glasses, and rubber or grounded safety mats to provide both physical and psychological assurance.
- Psychological Factors: Fear and overconfidence are major causes of accidents. Lack of respect for the power of electricity often stems from a lack of knowledge, while overconfidence leads to dangerous risk-taking.
Fire Safety Around Electricity
- Heat Generation: Heat is a byproduct of current flow ( $I$ ). If current is excessive, heat can melt protective wire coatings, causing short circuits that lead to metal melting, liquid vaporization, and ignition of flammable substances.
- Prevention: Work areas must be clean and free of unnecessary flammable substances. Power cords and wires must be free of kinks and should never be walked on or run over, as internal wire breakage increases current density in remaining strands, causing overheating.
Safety Around Compressed Gases
- Management Rules:
  - Frequently inspect air hoses for breaks or worn spots; replace unsafe hoses immediately.
  - Maintain all connections in a "no-leak condition."
  - Maintain in-line oilers and drain water sumps at regular intervals.
  - Filter air used for paint spraying to remove oil and water moisture.
  - Prohibitions: Never use compressed air to clean hands or clothing (pressure can force debris into flesh causing infection) and never spray toward other personnel.
  - Storage: Hoses should be straightened, coiled, and stored properly.
- Tire Mounting: Use tire dollies for heavy lifting. Always use tire cage guards during inflation. Use pressure regulators on air bottles to prevent over-inflation of high-pressure tires.
Oxygen Safety Considerations
- Oxidizing Properties: Pure oxygen readily combines with other substances, sometimes violently. It must be kept away from petroleum products to prevent explosions.
- Environmental Precautions: Maintenance should be performed outside when possible. Post "NO SMOKING" placards and cordon off the area. Ensure an adequate fire extinguisher is on hand ( $CO_{2}$ or similar).
- Tooling/Cleanliness: Hands, clothes, and tools must be clean and grease-free. Use tools dedicated specifically to oxygen systems. No smoking or open flames within a minimum of $15\,m$ ( $50\,ft$ ) of the area.
- Maintenance Procedures: Close supply valves and release system pressure before work. Unscrew fittings slowly. Oxygen lines must have at least $5\,cm$ clearance from moving parts, electrical wiring, and fluid lines. Conduct pressure/leak checks every time the system is opened.
Hazardous Materials (Oils and Chemicals)
- MSDS/SDS: Materials Safety Data Sheets are required documents from manufacturers containing handling, storage, and disposal info. They must be available to all employees.
- Globally Harmonized System (GHS): Adopted in the EU via EC 1272/2008, it standardizes information into 16 sections (Identification, Hazard ID, Composition, First-aid, Fire-fighting, Accidental Release, Handling/Storage, Exposure Control, Physical/Chemical properties, Stability/Reactivity, Toxicological, Ecological, Disposal, Transport, Regulatory, and Other info).
- Risk Diamond (US System): A four-color segmented diamond:
  - Red: Flammability.
  - Yellow: Reactivity.
  - Blue: Health.
  - White: Special Hazard (e.g., RAD for radiation, ALK for alkali, ACID for acid, CARC for carcinogen, or W with a line through it for water reactivity).
  - Scale: $0$ (little hazard) to $4$ (very hazardous).
Safety Around Machine Tools
- Drill Press: Wear eye protection, secure work with clamps, set proper RPM for material, do not feed spindle beyond limits, and stop machine for adjustments.
- Lathes: Used for cylindrical work. Wear eye protection, use sharp tools, let the chuck stop on its own (no hand pressure), and never set tools on the lathe bed.
- Milling Machines: Shape/dress parts. Clean the bed, secure the work, select proper tools, and do not change feed speed while operating.
- Grinders: Even with a shield, wear eye protection. Inspect wheels for defects. Flanges/washers should be $1/3$ the wheel diameter. Do not stand in the arc of the wheel during operation.
Welding Safety
- Location: Designated areas only; remove parts from aircraft to a welding shop if possible. Shop requires proper tables, ventilation, and fire control.
- Hangar Welding: If necessary, ensure no aircraft are within $10\,m$ . No open fuel tanks, no painting, and no flammable materials nearby. Only qualified welders may operate.
- Equipment: Immediate fire equipment must be rated at least 20B, with 80B backup. Trained fire watches must be present.
- Aircraft Status: Aircraft must be towable with a tug attached, parking brakes released, and hangar doors open.
Fire Safety Fundamentals
- Fire Triangle: Requires Fuel (combines with oxygen), Heat (accelerates oxidation), and Oxygen (the oxidizing element).
- Classification of Fires:
  - Class A: Ordinary combustibles (wood, paper, fabrics). European designations: A1 (normal) and A2 (fire resistant).
  - Class B: Flammable liquids/gases (US); Liquids with flash point $<100^{\circ}C$ (EU/Australia).
  - Class C: Flammable gases (EU/Australia); Electrical fires (US).
  - Class D: Combustible metals (Lithium, Potassium, Magnesium, Titanium, Zirconium). Require dry powder (NaCl or graphite). Never use water or dry chemicals.
  - Class E: Electrical fires (EU/Australia).
  - Class F/K: Cooking oils/fats ( $F$ in EU, $K$ in US).
Fire Extinguishing Agents
- Water: Best for Class A. Cools and deprives oxygen. Unsafe for Class B/D/Electrical.
- Carbon Dioxide ( $CO_{2}$ ): Used for Class A, B, and Electrical. Avoid Class D. Caution: parts become extremely cold; risk of asphyxiation in confined spaces.
- Halogenated Hydrocarbons (Halons):
  - Halon 104 ( $CCl_{4}$ ): Toxicity rating $3$ . Extremely toxic; produces Phosgene gas. Banned.
  - Halon 1001 ( $CH_{3}Br$ ): Toxicity rating $2$ . Corrosive to Al, Mg, and Zn.
  - Halon 1211 ( $CBrClF_{2}$ ): Toxicity rating $5$ . Colorless, non-corrosive, provides mist similar to $CO_{2}$ .
  - Halon 1301 ( $CF_{3}Br$ ): Toxicity rating $6$ (least toxic). Vapor spray (harder to direct).
- Dry Powder: Specifically for Class D. Smothers the fire. Not for general aircraft use due to cleanup difficulty and damage to electronics.
Flight Line Safety
- Hearing Protection: High-frequency noise from aircraft, APUs, and pneumatic tools causes hearing loss. Use external (earmuffs) and internal (plugs) protection.
- Foreign Object Damage (FOD): Control via ramp cleanliness and tool control programs. Turbine engine intakes create low-pressure areas that suck in loose objects; exhaust propelled items can be lethal.
- Surface Operations: Stay visible to pilots. Propellers and intakes/exhaust are primary hazards. Secure all ground support equipment so it cannot roll into aircraft.
- Helicopters: Observe rotor head level for clearance. Approach in view of pilot. Avoid the rear on masterhead helicopters (tail rotor is invisible). Go around the nose, never the tail.

WORKSHOP PRACTICES

Care and Control of Tools
- General Care: Most aircraft tools are made from alloy steels susceptible to corrosion. They must be cleaned (brush/rag/solvents) and re-lubricated with suitable coatings.
- Benches: Metal surfaces are cleaned with kerosene rags. Wooden benches are scraped or brushed. Protect surfaces with hardwood or lead blocks during punching.
- Machine Care: Vices must be wiped with oily rags; move jaws to the limit to lubricate screw bearings. Drilling machines and grinders must be cleared of abrasive dust which damages bearings.
- Precision Instruments: Must be stored in custom cases, never left on workbenches.
Tool Control Procedures
- Misplaced tools can cause catastrophic FOD and loss of life.
- Shadow Boards: Every tool occupies a specific location (holders for wrenches/sockets) to allow quick visual checks for missing items.
- Tool Cribs/Rooms: All special tools are inventoried by identification number or barcode. Log the name of the engineer upon checkout.
- End of Work Check: The aircraft must not be released until every tool is accounted for and checked against personal and tool room inventories.
Workshop Materials Management
- Materials like toxic chemicals, solder, wire wool, and oil spill powder require issuance control for safety and economy.
- Storage: Flammable materials (oils, greases, adhesives, paints) must be stored in specialized cabinets away from direct sunlight.
- Disposal: Fluids must be disposed of in accordance with national/international regulations, never in domestic drains.
- Expiration: Many liquids (epoxies, sealants) have a fixed "life"; materials must be checked for expiry dates before use.
Dimensions, Allowances, and Tolerances
- Nominal Size: Approximate dimension for identification.
- Basic Size: Theoretical size from which limits are derived.
- Actual Size: The measured dimension of the part.
- Tolerance: Total amount a dimension can vary. For example, if basic size is $4.0\,in$ and tolerance is $0.005\,in$ , the part must measure between $3.995\,in$ and $4.005\,in$ .
- Bilateral vs. Unilateral: Bilateral variation occurs in both directions ( $\pm$ ). Unilateral occurs in one direction only (e.g., $-0.005$ ) to prevent seizure or fitment issues.
- Fit and Clearance: A clearance fit allows sliding; an interference fit involves overlapping dimensions requiring force.
Tool Calibration
- Prescribed by NAA/EASA and company certificates. Annual calibration is common for tools that hold calibration well.
- Traceability: Calibration must be traceable to national standards via an unbroken chain of comparisons.
- Documentation: Tools must have a sticker with the next due date. Certified records must show ID, standard used, results, uncertainty, limits of error, and authority.

TOOLS AND MEASURING INSTRUMENTS

Screwdrivers
- Fitment: A common screwdriver must fill at least $75\%$ of the screw slot to avoid burring.
- Phillips vs. Reed & Prince: Reed & Prince forms a perfect cross and requires a pointed screwdriver; Phillips has a larger cross center and requires a blunt screwdriver. They are not interchangeable.
- Offset: Used for limited vertical space, bent $90^{\circ}$ to the shank.
Pliers and Cutting Tools
- Types: Diagonal cutters (dikes), needlenose (tight places), duckbill ( safety wire twisting), and roundnose (metal crimping).
- Rule: Never use pliers to turn nuts as they damage the fastener.
Punches
- Prick Punch: Small point for transferring paper patterns to metal.
- Center Punch: Heavier body, $60^{\circ}$ point for starting twist drills.
- Drive/Pin Punches: Drive (tapered) punch starts a pin; Pin (drift) punch has a straight shank to finish driving the pin out of the hole.
Wrenches
- Box-end: 12-point design allows for use in places with as little as $15^{\circ}$ swing. Inverting the wrench can change the handle arc.
- Torque Wrench Rules: divide inch-pounds by $12$ for foot-pounds. Do not lubricate threads unless specified. Approach the high side of the range if rotating the bolt head instead of the nut.
- Extension Formula: $TA = TW \times \frac{L+E}{L}$ , where $TA$ is actual torque, $TW$ is indicated torque, $L$ is wrench length, and $E$ is extension length.
Cutting Tools
- Snips: Yellow (straight), Green (curve right), Red (curve left). Compound leverage snips can cut up to $0.051\,in$ material.
- Hacksaws: Teeth must point forward (away from handle). Speed should be $40$ to $50$ strokes per minute.
  - Pitches: $14$ (machine steel), $18$ (aluminum/cast iron), $24$ (thick tubing/pipe), $32$ (thin-walled tubing/sheet).
- Files: Grade based on tooth spacing (Rough to Dead Smooth). Techniques include Crossfiling (edges) and Drawfiling (surface smoothing).
- Reamers: Used to enlarge holes to exact size within load tolerances of $0.003$ to $0.007\,in$ . Rotate in the cutting direction only, never back out in reverse.
- Taps and Dies: Taps cut internal threads; Dies cut external. Tool sets contain Taper, Plug, and Bottoming taps.
Power Tools
- Kett Saw: Electric circular saw; can start anywhere on a sheet without a starting hole.
- Nibbler: blanking action at high speed; cuts up to $1/16\,in$ thickness.
- Pneumatic Drill Motors: Preferred around flammables to avoid sparks. Standard rating is $3\,000\,rpm$ .
Bending and Forming
- Cornice Brake: Bends edges; allows material to pass through from front to rear.
- Box and Pan (Finger) Brake: Removable steel fingers of varying widths for forming pans/boxes.
- Slip Roll Former: Forms sheets into cylinders. Upper roll can be released to remove material without distortion.
Precision Measuring
- Rules: Measurements down to $1/64\,in$ or $0.01\,in$ .
- Micrometers: Read to thousandths ( $0.001\,in$ ). Lead screw has $40\,tpi$ , meaning each rotation moves the thimble $0.025\,in$ . Vernier scales read to ten-thousandths ( $0.0001\,in$ ).
- Dial Indicator: Measures surface variations such as runout or bend in a shaft.

AVIONIC GENERAL TEST EQUIPMENT

Built-In Test Equipment (BITE)
- Continuously monitors circuits for faults. Isolation sequences pinpoint problems shown on the Flight Management System (FMS) or Control Display Unit (CDU).
- ACARS: Automatically sends reports to ground stations while en-route.
Reflectometers
- TDR (Time Domain Reflectometer): Checks for standing waves, partial opens (trays drift up), or pinched cables (dips on graticule).
- SSTDR (Spread Spectrum): Greater precision and portability; does not interfere with digital circuits.
Standardization and Connectivity
- ATE (Automatic Test Equipment): Interfaces with LRUs via software-based prompts.
- VME/VXI: European plug-in card architecture for fast, automated testing.
- Data Bus Analyzers: Portable tools to troubleshoot digital systems by evaluating incoming/outgoing signals to LRUs.

ENGINEERING DRAWINGS, DIAGRAMS, AND STANDARDS

Drawing Types
- Detail: Describes a single part.
- Assembly: Shows the relationship between parts.
- Installation: Final position of parts in the aircraft.
- Sectional Views: Full, Half, Revolved, and Removed views show internal/cross-sectional construction.
Lines and Symbols
- Visible Line (Thick): Edges seen directly.
- Hidden Line (Medium Dash): Invisible contours.
- Phantom Line (One long, two short dashes): Alternate positions or missing parts.
- Datum Line: Fixed reference for fuselage station ( $FS$ ) measurements.
ATA Specification 100/iSpec 2200
- Standardizes manual organization. Chapter $21$ is always Air Conditioning; Chapter $24$ is Electrical Power; Chapter $32$ is Landing Gear.
- Page Blocks: $1-100$ (Description), $101-200$ (Troubleshooting), $201-300$ (Maintenance Practices).

FITS AND CLEARANCES

Classification of Fit
- Clearance Fit: Limits ensure a gap exists; suitable for moving parts.
- Interference Fit: Overlapping limits requires mechanical pressure; permanent assembly.
- Transition Fit: Can be either clearance or interference; light press fit for locating parts/dowels.
Systems and Standards
- BS 4500: System based on standard sized holes ( $H$ designation) with varying shaft sizes ( $h$ designation).
- IT Grades: $18$ grades of accuracy. IT6 to IT11 are preferred fits.
- Checking Procedures: Bow is measured with a straight edge and feeler gauges (general limit: $1$ in $600$ ). Twist is checked via witness marks or strain gauges.

ELECTRICAL WIRING INTERCONNECTION SYSTEM (EWIS)

Insulation Properties
- Resistance: Measured with a megohmmeter (insulation tester) to detect leakage.
- Dielectric Strength: Ability to withstand potential difference without electrostatic breakdown.
Bonding and Grounding
- Requirements: Connection resistance should not exceed $0.003\,\Omega$ . Use at least three fasteners in a pattern to secure grounding brackets.
- Prohibitions: Electrical wiring should not be grounded directly to magnesium parts.
Lacing and Supports
- Clamping: intervals not exceeding $60\,cm$ ( $24\,in$ ). Maintain $5\,cm$ ( $2\,in$ ) minimum between wiring and fluid lines.
- Bend Radii: Minimum $10 \times OD$ for bundles, $3 \times OD$ for supported single wires, and $6 \times OD$ for RF/Coaxial cables.

RIVETING

Rivet Parameters
- Head Markings: AD (Dimple, 2117 aluminum), D (Raised dot, 2017), DD (Two bars, 2024), E (Raised ring, 7050).
- Edge Distance: Minimum $2 \times D$ for universal head; $2.5 \times D$ for countersunk.
- Spacing (Pitch): Minimum $3 \times D$ ; Average $4-6 \times D$ .
Tools and Removal
- Clecos: Color-coded temporary fasteners (3/32 is Zinc, 1/8 Copper, 5/32 Black, 3/16 Brass).
- Countersinking: Preferred depth no more than $2/3$ sheet thickness.

TRANSMISSIONS

Gears and Chains
- Gears: Check for backlash ( $0.003$ to $0.004\,in$ is common). Use dye transfer for contact pattern analysis.
- Chains: Maximum elongation limit is typically $2\%$ . Check for "tight joints" by drawing chain over a rod.
Rod Systems
- Push-Pull Rods: Eliminate varying tension; transfer both compression and tension. Rod ends must move freely without binding.

QUESTIONS & DISCUSSION

Question: 1-3 What class of fire extinguisher should be used on burning jet fuel?
Answer: Class B fire extinguisher.
Question: 1-7 If a risk diamond is not present with a questionable material, how could you determine its level of danger?
Answer: Look up the substances Material Safety Data Sheet (SDS or MSDS).
Question: 1-4 What is the major disadvantage of Halon 1001 (Methyl Bromide) fire extinguishant?
Answer: It is highly toxic.
Question: 1-8 Name three primary safety precautions when working around oxygen.
Answer: No smoking. Turn off all nearby electrical equipment. Have a fire extinguisher at hand.
Question: 1-2 What important safety precaution is critical around all types of machine tools?
Answer: Wear eye protection.
Question: 1-6 How would a radioactive substance be identified on a risk diamond?
Answer: The letters RAD in the bottom square.
Question: 1-5 What class(s) of fire could occur from an electrical short circuit?
Answer: European designation Class E. United States designation Class C.
Question: 1-1 What would a blue number "1" indicate on a risk diamond?
Answer: The substance is mildly toxic.
Question: 2-1 How can you tell if a tool is within its calibration limits?
Answer: A sticker on the tool as applied by the company tool crib manager or calibration agency.
Question: 3-1 Metal head hammers are usually sized according to the… of the head without the handle.
Answer: Weight.
Question: 6-6 To check for roundness of a shaft, place the shaft in a V block and measure with a… or a…
Answer: Dial Test Indicator, scribing block.