Study Notes Module 7 7.1

Module 7 — Maintenance Practices

7.1 — Safety Precautions — Aircraft and Workshop

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Safety is the overriding priority in all aircraft maintenance activities. The maintenance environment contains numerous hazards — high-voltage electricity, compressed gases, flammable fluids, toxic chemicals, heavy components, and moving machinery. Every maintenance engineer must understand these hazards, know how to prevent accidents, and be able to take immediate remedial action when incidents occur. All EASA Part 66 licence categories require Level 3 (detailed knowledge) of this section.

General Safety Principles

Safe working practices are governed by a hierarchy of controls:

Priority	Control Type	Example
1 (Highest)	Elimination	Remove the hazard entirely (e.g. substitute a toxic solvent with a non-toxic one)
2	Engineering controls	Ventilation systems, machine guards, interlocks
3	Administrative controls	Procedures, training, warning signs, permits to work
4 (Lowest)	Personal Protective Equipment (PPE)	Safety glasses, gloves, hearing protection, steel-toe boots

Electrical Safety

Electricity is one of the most dangerous hazards in aircraft maintenance. Both the aircraft's own electrical systems (typically 28 V DC and 115 V AC at 400 Hz) and workshop mains supplies (230 V AC / 50 Hz in Europe) can cause fatal electric shock.

Effects of Electric Current on the Human Body

Current (mA)	Effect
1 mA	Tingling sensation — threshold of perception
5 mA	Pain, involuntary muscle contraction
10–20 mA	"Let-go" threshold — muscles freeze, victim cannot release grip
50–100 mA	Ventricular fibrillation — potentially fatal
> 200 mA	Severe burns, cardiac arrest

Electrical Safety Precautions

Isolate before work — use lockout/tagout (LOTO) procedures. Apply personal padlocks and warning tags to circuit breakers and switches.
Verify dead — after isolation, test with a proving unit to confirm the circuit is de-energised. Test the proving unit itself before and after use.
Ground/earth — bond yourself to the aircraft/equipment structure using wrist straps or grounding leads.
Residual Current Devices (RCDs) — all portable power tools and workshop outlets should be RCD-protected (typically 30 mA trip).
Capacitor discharge — capacitors in power supplies and ignition units can retain lethal charges long after power is removed. Always discharge and short-circuit before handling.
Low-voltage working — use reduced-voltage tools (110 V centre-tapped to earth, giving 55 V to earth) where practical.
High-voltage areas — radar systems, HF transmitters, and weather radar magnetrons produce hazardous voltages (up to 30 kV). Observe exclusion zones and interlocks.

Critical: Never assume a circuit is dead. Always test. Even 28 V DC can cause serious burns if short-circuited through a low-resistance path (e.g. a ring or watch strap). Remove all jewellery before working on electrical systems.

Gas Safety

Oxygen

Oxygen is widely used in aircraft for crew and passenger breathing systems (typically stored at 1,800–2,200 psi / 124–152 bar). It is not flammable itself but is an extremely powerful oxidiser — it dramatically accelerates combustion.

Keep all oil, grease, and hydrocarbons away from oxygen systems — even a fingerprint of grease on a fitting can ignite in pure O₂.
Use only oxygen-clean tools and components (cleaned with approved solvents, sealed in protective bags).
Never use PTFE tape on oxygen fittings — use only approved thread sealants.
Store oxygen cylinders upright, secured, away from heat sources, and separated from fuel/oil storage by at least 5 metres.
Oxygen-enriched atmospheres (above 23.5%) are extremely fire-hazardous. Ventilate work areas when oxygen systems are open.

Nitrogen

Nitrogen is used for tyre inflation, strut charging, and as an inert gas blanket in fuel tanks (Nitrogen Inerting System — OBIGGS). It is an asphyxiant — it displaces oxygen in confined spaces without warning (it is odourless and colourless).

Never enter a nitrogen-purged space without self-contained breathing apparatus (SCBA).
Monitor oxygen levels in confined spaces — below 19.5% is immediately dangerous to life.
Nitrogen cylinders are pressurised to 200–300 bar. Handle with care, secure during transport.

Compressed Air and Hydraulic Gases

Pneumatic systems operate at up to 3,000 psi (207 bar). Never direct compressed air at the body — it can cause air embolism.
Hydraulic accumulators are charged with nitrogen — depressurise before removing.
Bleed all pressure from lines before disconnecting fittings.

Aviation Context: Gas cylinders are colour-coded. In aviation/aerospace: Green = Oxygen, Grey = Nitrogen (some standards use black body with grey shoulder). Always verify by reading the cylinder label — never rely on colour alone.

Oils, Fuels, and Chemical Safety

Aviation Fuels

Fuel	Type	Flash Point	Colour Code
Jet A-1	Kerosene	38°C min	Black lettering on white
Avgas 100LL	Gasoline	−40°C	Red lettering on white

Avgas vapour is heavier than air and can accumulate in pits, tanks, and low areas — creating explosive atmospheres.
Jet fuel (kerosene) is less volatile but still flammable above its flash point. Fuel-soaked clothing is extremely dangerous.
Prevent static discharge during refuelling — always bond the fuel bowser to the aircraft before connecting hoses.
No smoking, naked flames, or spark-producing equipment within 15 metres (50 feet) of fuelling operations or fuel storage.
Prolonged skin contact with aviation fuels causes dermatitis — use barrier cream and chemical-resistant gloves.

Hydraulic Fluids

Aircraft hydraulic systems commonly use:

MIL-PRF-83282 / MIL-PRF-5606 — mineral-based (red/purple colour). Petroleum-based, flammable.
Skydrol (Type IV/V) — phosphate-ester-based (purple/dark amber). Fire-resistant but highly corrosive — attacks skin, eyes, and many plastics/paints.

Skydrol Warning: Phosphate-ester hydraulic fluid (Skydrol) causes severe eye damage and skin irritation. Wear splash-proof goggles and butyl rubber gloves. If skin contact occurs, wash immediately. If eye contact occurs, irrigate for at least 15 minutes and seek medical attention. Skydrol also attacks many common rubbers, paints, and wire insulation — use only approved seals and materials.

Solvents and Cleaning Agents

MEK (Methyl Ethyl Ketone) — highly flammable, CNS depressant. Use only in ventilated areas.
Acetone — extremely flammable, rapid evaporation, drying to skin.
Isopropyl Alcohol (IPA) — commonly used for cleaning electrical connectors. Flammable.
Chromate compounds — used in some primers and conversion coatings. Hexavalent chromium (Cr⁶⁺) is carcinogenic. Wear respiratory protection when sanding/spraying chromate products.
Two-part sealants and adhesives — catalysts (hardeners) are often sensitisers causing allergic reactions.

Safety Data Sheets (SDS / MSDS)

Every chemical substance used in the workshop must have an SDS available. The SDS contains 16 sections covering identification, hazards, composition, first aid, fire-fighting measures, handling, exposure controls, physical properties, stability, toxicology, disposal, transport, and regulatory information. Always consult the SDS before using an unfamiliar substance.

Fire Safety and Extinguishing Agents

A fire requires three elements (the fire triangle): fuel, oxygen, and heat (ignition source). Removing any one element extinguishes the fire. For some fires, a fourth element — the chemical chain reaction — must also be broken (the fire tetrahedron).

Fire Classes and Extinguishing Agents

Class	Fuel Type	Examples	Suitable Extinguisher
A	Ordinary combustibles	Wood, paper, cloth, rubber	Water, foam, dry powder
B	Flammable liquids	Fuel, oil, grease, solvents	Foam, CO₂, dry powder
C	Flammable gases	Propane, acetylene, hydrogen	Dry powder (shut off gas supply first)
D	Combustible metals	Magnesium, titanium, lithium	Specialist dry powder (e.g. L2, Ternary Eutectic Chloride)
E / Electrical	Energised electrical equipment	Panels, motors, avionics	CO₂, dry powder (never water or foam)

Key Extinguishing Agents:

Water — cools the fuel below ignition temperature. Class A only. Never use on electrical, oil/fat, or metal fires.
Foam (AFFF) — smothers by forming a film that excludes oxygen. Classes A and B.
CO₂ (Carbon Dioxide) — smothers by displacing oxygen. Leaves no residue — ideal for avionics and electrical fires. No cooling effect — fire may re-ignite.
Dry Powder (ABC) — chemical interruption of the chain reaction. Versatile but leaves residue that can damage avionics.
Halon alternatives (HFC-227ea / Novec 1230) — clean agents that interrupt the chemical chain reaction without residue. Used in aircraft engine/APU fire suppression and avionics bays.

Magnesium fires: Burning magnesium (common in aircraft wheels and casings) produces intense white light, extreme heat (3,100°C), and reacts violently with water — producing hydrogen gas and risk of explosion. Use only specialist Class D extinguishing agents. Never apply water, foam, or CO₂.

Accident Remedial Action and First Aid

Electric shock — isolate the supply first (switch off or pull plug). Do not touch the victim while they are in contact with the live source. If breathing has stopped, begin CPR immediately.
Chemical burns — flush affected area with copious clean water for at least 20 minutes. Remove contaminated clothing. Seek medical attention.
Chemical splash in eyes — irrigate with clean water or eyewash for at least 15 minutes, holding eyelids open. Seek immediate medical attention.
Fuel/solvent inhalation — move to fresh air. If unconscious, place in recovery position and call emergency services.
Fire on a person — stop, drop, and roll. Smother with fire blanket if available. Do not use CO₂ extinguisher directly on a person (risk of cold burns and suffocation).

Reporting

All accidents, near-misses, and hazardous occurrences must be reported immediately to the supervisor and documented in the company's safety reporting system. In EASA-regulated environments, serious incidents must also be reported to the competent authority.

Additional Workplace Hazards

Noise

Aircraft engines, APUs, pneumatic tools, and riveting guns produce noise levels well above 85 dB(A) — the threshold for mandatory hearing protection. Prolonged exposure causes permanent noise-induced hearing loss (NIHL). Wear ear defenders or ear plugs in all designated hearing protection zones.

Working at Height

Use approved platforms, docking systems, and scaffolding. Never stand on boxes, drums, or improvised platforms.
Use fall-arrest harnesses when working on top of the fuselage, wings, or tail.
Ensure platforms have guard rails, toe boards, and non-slip surfaces.

Confined Spaces

Fuel tanks, cargo compartments, and wheel wells are confined spaces. Entry requires a permit to work, atmospheric monitoring (oxygen level, flammable gas concentration), a safety attendant outside, and rescue equipment on standby.

Foreign Object Debris (FOD)

FOD is any object or substance in the aircraft environment that could damage the aircraft or injure personnel. Tools, fasteners, safety wire, rags, and personal items must be accounted for at all times. Many organisations use tool shadow boards and FOD walks to maintain clean working areas. Lost tools require an aircraft search and documentation before the aircraft can return to service.

Aviation Context: FOD damage costs the aviation industry an estimated $4 billion per year. A single bolt left in an engine intake can destroy turbine blades worth hundreds of thousands of dollars. Every tool should be accounted for — shadow boards, etched markings, and calibrated tool inventories are standard practice.

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