What you can destroy, and how to find a fault

Forty-five elements — the largest unit in General — and twenty-seven of them are knowledge. That ratio is the widest in the section and it needs explaining before anything else.

1. Why digital electronics is in a maintenance standard

K21 semiconductors. K22 digital logic, RAM, ROM, NVRAM, logic gates, rectifiers. K23 binary numbers.

You will not repair a circuit board. You will replace a line-replaceable unit. So why is any of this here?

K24 — electrostatic discharge — is the element that answers it.

The digital content exists so you understand what you can destroy and how. A static discharge you cannot feel — well below the threshold at which you notice a shock — will damage a semiconductor junction. And the damage is frequently latent: the unit works when you fit it, works on test, and fails weeks later in service.

That is why handling discipline is unconditional rather than proportionate to how careful you feel. You will never get feedback telling you that you damaged something, because the failure arrives long after you have gone.

The second reason is diagnostic. A mechanic who understands roughly what is inside the box can reason about a failure well enough to isolate it to the right box — rather than replacing three units and calling it fixed. That is K13 in AF-K, and it starts here.

So: learn K21–K23 as what is fragile and why, and as what the box is doing. Not as electronics theory, which you will not use.

2. Ohm's law is a prediction, not a formula

Everyone can rearrange V = IR. That is GEN-H.

The skill this unit is actually building is predicting what the meter will read before you connect it.

If you know a 28 V bus feeds a 14 Ω load, you know the current will be 2 A. When the meter says 0.2 A, you have learned something immediately: there is roughly ten times too much resistance in that circuit, which is a corroded connection, a poor ground or a partially open conductor.

A measurement without a prediction is a number. A measurement against a prediction is a diagnosis. That habit is the difference between a mechanic who tests circuits and one who troubleshoots them.

Series and parallel, K13, K14 and K26, exist to make the prediction possible:

• Series: resistances add, current is common, voltages divide in proportion to resistance. A 28 V supply across 2 Ω, 4 Ω and 8 Ω gives 2 A and drops of 4 V, 8 V and 16 V — and those drops sum to 28 V, which is your check.
• Parallel: voltage is common, currents add, and total resistance is always less than the smallest branch. That last fact is a check: a parallel total larger than any individual branch is arithmetically impossible.

3. Voltage drop is the measurement that finds real faults

Continuity tells you a wire is connected. It does not tell you the connection is any good.

Most real electrical faults in aircraft are not breaks. They are resistance — corroded terminals, loose connections, chafed strands, a poor ground. A connection with 0.4 Ω of resistance passes a continuity test comfortably and cripples a 10 A circuit.

Voltage drop finds these and continuity does not, for one reason: you measure it under load, with current flowing. A bad joint that looks fine at the microamps a continuity tester uses reveals itself immediately at operating current.

The method is in GEN-A Procedures Procedure 5. The idea:

• Measure across the suspect element — connector, ground, length of wire — with the circuit energized and loaded.
• A good connection drops almost nothing. A bad one drops volts.
• The energy has to go somewhere. A 0.4 V drop at 10 A is 4 watts dissipated in that joint, which is why bad connections get hot and eventually burn.

This single technique will find more faults than everything else in the unit combined.

4. S8 is the only simulator element in General, and it is worth watching

Troubleshoot a circuit is the one element in all 358 General elements delivered by simulator (the delivery-mode analysis).

the landing gear station specification doubts that simulated troubleshooting transfers: a simulator tests whether you know where to put the probe, not whether you can put it there — and on real hardware, getting the probe onto the right terminal in a loom you cannot see into is a large part of the task.

If it turns out not to transfer, this element returns to the bench at almost no cost, because the bench and the meter are already there for S1–S4. That makes it a cheap test of an expensive assumption, and AF-L §2 proposes running it early.

5. Batteries are the hazard in this unit

R2 and R4 make battery work a risk, and S14 puts your hands on one.

Two chemistries, two sets of rules, and mixing them up is dangerous:

• Lead-acid — sulfuric acid electrolyte. Neutralize with bicarbonate of soda.
• Nickel-cadmium — alkaline electrolyte. Neutralize with boric acid or vinegar.

The neutralizer for one makes the other worse, so the first question at any spill is which chemistry it is. Keep the two types' servicing equipment separate; contaminating a nickel-cadmium cell with acid, or the reverse, destroys it.

Both produce hydrogen on charge, which is explosive, so ventilation is not optional and neither is avoiding sparks near a charging battery. Both can deliver enormous short-circuit current: a dropped spanner across the terminals will glow, weld, and spray molten metal. Remove rings and watches, use insulated tools, and disconnect the earth lead first and reconnect it last.

Two more types need their own rules. R2 names them: gel cell and lithium-ion.

• Sealed, or valve-regulated, lead-acid (VRLA). The electrolyte is held in the separators and plates, with no free liquid. A gel cell is a battery of this sealed family; the handbooks describe the absorbed-glass-mat form and do not use the word "gel". On charge the oxygen recombines, so there is no water to add and no hydrometer reading — state of charge comes from open-circuit voltage. Each cell has a pressure relief valve that can vent if the battery is overcharged. The chemistry is still acid. FAA-H-8083-31B Ch. 9, Lead-Acid Batteries and Battery Freezing; FAA-H-8083-30B Ch. 12, Sealed Lead Acid (SLA) Batteries.
• Lithium-ion. Unlike the other rechargeable types, it carries a flammable electrolyte and is kept pressurized. Under some conditions it overheats and catches fire. Charge, store and handle it only as its manufacturer specifies (FAA-H-8083-30B Ch. 12, Lithium-Ion Batteries).

The storage rule has one relaxation. Flooded lead-acid and nickel-cadmium batteries need separate storage and servicing areas; batteries that are sealed can be charged and capacity checked in the same area (FAA-H-8083-31B Ch. 9, Storing & Servicing Facilities). And changing an aircraft's battery type may be a major alteration (Ch. 9, Aircraft Batteries), so a sealed or lithium battery is not a like-for-like swap for the one that came out.

R3, high-voltage circuits such as strobe supplies, adds one more: capacitors hold charge after the power is off. The system being switched off is not the system being safe.

6. The largest station load in General needs no apparatus

Seven station elements — continuity, voltage, current, resistance, shorts and opens, voltage drop, battery servicing — and every one is a bench, a multimeter and a wired board.

No rig, no fault-injection frame, no aircraft. The unit with the heaviest station load in General is among the cheapest to equip (our capital estimate), which is worth knowing because the subject sounds like it needs equipment.

Fault injection here is cheap and effective: a chafed conductor, a corroded terminal, a partially broken strand, a wire routed against something it should not touch. All are invisible, all are realistic, and all are found by voltage drop.

7. What consumes this unit

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