FAA Aviation Mechanic - General (AMG) common mistakes
FAA Aviation Mechanic - General (AMG) · Understand the traps and how to avoid them.
Computes total resistance in a parallel circuit by adding the branch resistances (R1 + R2 + R3), then applies E = IR to that sum, and also mixes up which quantity is constant - adding voltages across parallel branches or adding currents in a series loop.
Series rules are taught first and get over-learned; the reciprocal formula for parallel branches collapses under time pressure, and the 'constant quantity' rule (current is the same everywhere in a series loop, voltage is the same across every parallel branch) is recalled as a slogan rather than applied.
What to do instead
Reduce the circuit in stages and write each stage down: parallel branches combine as R_T = 1 / (1/R1 + 1/R2 + ...), or product-over-sum for exactly two resistors; series sections add directly. Use the sanity check that a parallel combination must be smaller than the smallest branch, and label every unknown with its unit before solving.
Measures a length off the printed drawing with a scale or ruler instead of reading the dimension callout, and works from the print without ever looking at the title block, revision block, or the zone/station/waterline/buttock-line grid.
The drawing looks geometrically to scale, the dimension text is small, and the title and revision blocks sit in a corner that reads like boilerplate - so candidates skip what is actually the controlling information.
What to do instead
Treat every drawing as not-to-scale (reproduction and resizing destroy scale) and take all sizes from the stated dimension and its tolerance. Before using any print, read the title block for the part number and the revision block to confirm you have the current issue, and locate features by station, waterline, and buttock line numbers rather than by eyeballed position.
Assigns the wrong sign to arms and moments - treating everything forward of the datum as positive - or, when equipment is added or removed, finds the new empty-weight CG by averaging the old and new CG values instead of dividing total moments by total weight.
Datum location and direction of positive arm differ between aircraft, and averaging two CG figures 'feels' like the right way to blend them because the numbers are both CGs.
What to do instead
Sketch a number line with the datum at zero and mark each item's arm with its direction before doing any arithmetic. Compute moment = weight × arm for every item including negative arms, sum weights and moments in separate columns, and get CG only by dividing total moment by total weight. Never average CG values; re-average only after recomputing all moments from the new weight list.
Fits a 45-degree flared fitting to 37-degree flared aircraft tubing (or the reverse), or installs a flareless (bite-type) fitting on a tube without verifying that the tube material and wall thickness are acceptable for that fitting.
The two flare angles look nearly identical to the eye, automotive plumbing experience is almost always 45-degree SAE, and the wall-thickness requirement for flareless fittings is buried in the tubing specification rather than on the fitting package.
What to do instead
Aircraft AN and MS flared fittings use a 37-degree flare; 45-degree flares belong to non-aircraft SAE plumbing. Confirm with a flare gauge before assembly. For flareless fittings, check tube outside diameter, wall thickness, and temper against the fitting manufacturer's data and the accepted practices for tubing, and remember that flareless fittings do not tolerate thin-wall or soft tubing.
Treats all aluminum alloys as heat-treatable - for example assuming a 6061-T6 part can be re-heat-treated and quenched in the shop like 2024 or 7075, or assuming a 5052 or 3003 sheet can be hardened by heat treatment at all.
The four-digit designations look arbitrary unless the alloying-element scheme is known, and the temper codes (T3, T4, T6, H32) are memorized as strings rather than as descriptions of a process.
What to do instead
Separate the families: 2024 and 7075 are heat-treatable (solution heat treatment plus precipitation hardening); 5052 and 3003 are strain-hardenable only and never respond to heat treatment; 6061-T6 is heat treated at the mill. Learn the temper meaning (T3 = solution treated, cold worked, naturally aged; T4 = solution treated, naturally aged, no cold work) and never remove more of the pure-aluminum Alclad layer than the allowed limit, since that cladding is the corrosion protection.
Refuels an aircraft without bonding it to the fueling source, leaves the aircraft unattended with fuel flowing, or fills a tank to the very top with no room left for expansion.
The bonding cable is treated as an optional accessory, the filler is only a few feet away so 'unattended' seems harmless, and a 'full' tank reads as good service rather than as a hazard.
What to do instead
Establish and maintain electrical bonding between the aircraft and the fueling equipment before any filler cap is opened, and keep it connected until all caps are closed - the bond exists so aircraft and fuel source sit at the same potential and no spark can jump. The aircraft must be attended throughout fueling, engines and electrical equipment off, and tanks filled only to the specified level to leave expansion space.
Removes aluminum corrosion with steel wool or a steel wire brush, or sands aggressively until the dull gray-white Alclad surface is gone.
Steel wool and wire brushes are the fastest thing at hand and they visibly clean the metal, and the cladding is not obviously different from the core alloy underneath.
What to do instead
Use aluminum wool, nylon abrasive pads, or aluminum-oxide abrasive of the grit specified for the surface; never steel wool or a steel brush, because embedded steel particles become initiation sites for dissimilar-metal corrosion. Remove only the corrosion, not sound metal - the Alclad layer is corrosion protection and its removal is limited. Finish by washing with clean water to remove residue, drying completely, applying a conversion coating where specified, and priming.
Works a computation in mixed units without converting first - multiplying pounds per square inch by dimensions given partly in inches and partly in feet, or mixing gallons and cubic inches in the same volume - and rounds intermediate answers before reaching the final result.
Attention goes to selecting the right formula, unit labels are treated as decoration, and each rounding step seems harmless on its own.
What to do instead
Write the unit beside every number before substituting, convert everything into one system first, and let the units cancel to confirm the formula (for example, psi × in² leaves pounds). Carry full precision through the calculation and round only the final answer, then sanity-check its magnitude against an estimate.
Writes a maintenance entry that only describes the work performed, omitting the certificate number, the kind of certificate held, and the approval-for-return-to-service statement - or signs off work the held certificate does not authorize, such as approving an annual inspection with an A&P certificate alone or completing a major alteration with no FAA Form 337.
A plain-language description reads like a complete record, the required entry elements exist only as a regulation list rather than as a habit, and the boundary between A&P authority and Inspection Authorization authority is easy to blur.
What to do instead
Build every entry against the checklist: description of the work performed or reference to acceptable data, date of completion, and the name, certificate number, and kind of certificate of the person approving. An entry approving for return to service also carries the signature, date, certificate number, and the approval statement. A 100-hour inspection may be approved by an A&P; an annual inspection requires an Inspection Authorization. Major repairs and major alterations require approved data and FAA Form 337.
Treats AC 43.13-1B/2B as approved data and performs a major repair or major alteration from it without any further approval, and uses a manufacturer's maintenance manual without checking that the revision is current and applies to the specific make, model, and serial number.
AC 43.13 is genuinely authoritative-looking, is legitimately used for many minor repairs, and the acceptable-versus-approved distinction is a single word that is easy to read past; manual revisions go stale silently in a shop binder.
What to do instead
Memorize the split: AC 43.13 is acceptable data - methods, techniques, and practices for minor repairs and minor alterations. Major repairs and major alterations require approved data such as the type certificate holder's instructions, an STC, DER-approved data, or an FAA field approval. Before using any manual, verify the revision and effective date, confirm serial-number effectivity, and check that all applicable airworthiness directives are current for that aircraft.