FAA Aviation Mechanic - Airframe (AMA) common mistakes
FAA Aviation Mechanic - Airframe (AMA) · Understand the traps and how to avoid them.
On a riveted sheet-metal repair layout, measures edge distance from the edge of the rivet hole to the edge of the sheet instead of from the rivet center, and accepts a value below the 2D minimum; also copies the original row spacing without checking pitch limits.
The 2D edge-distance rule is memorized as a number but not as a reference point, and hole-edge-to-part-edge is what the eye measures on a drawing. Pitch minimums (about 3D) get lost because the original structure usually looks 'fine' on the print.
What to do instead
Work layout problems with a scale drawing and mark rivet centerlines first, then measure to the part edge. Drill the pair of rules together: edge distance minimum 2D (center of rivet to edge of material); pitch typically 4D-6D with 3D minimum and 2D transverse row spacing.
Selects a bolt by overall length or by measuring the shank with a ruler, and installs it so the thread runout sits inside the bearing (shear) plane of the joint.
Bolt dash numbers encode grip length, not overall length, and the two differ by roughly the head and thread allowance. Under time pressure the candidate picks the closest-looking bolt instead of computing the material stack thickness.
What to do instead
Compute grip = combined thickness of the parts being joined, then choose the next longer grip so the unthreaded shank (not the threads) bears on the hole; no more than one thread may fall in the shear plane. Practice decoding AN/MS dash numbers so 'AN3-11' is read as a 1-1/8 inch grip, not a length.
Calls a long, feathery, greenish-white inner cone a neutral flame, or deliberately chooses a carburizing flame for 4130 steel 'because excess acetylene protects the metal.'
The three oxyacetylene flame types are memorized as pictures rather than as acetylene-to-oxygen ratios, and the feather is easy to read as 'soft, therefore neutral.' Filler for 4130 is also confused with mild-steel RG-45/ER70S.
What to do instead
Learn the adjustment sequence, not the picture: open acetylene, add oxygen until the inner cone is sharply defined with no feather = neutral; a distinct feather = carburizing (excess acetylene); a short, pointed cone with a hiss = oxidizing. Weld 4130 with a neutral flame and ER80S-D2 (or 4130) filler.
Sets control cable tension straight off the rigging chart regardless of ambient temperature, and safety-wires the turnbuckle so the wire crosses over the barrel rather than passing through the holes and lying along it.
Tension charts are published at a standard temperature and the airframe and cable expand differently, so the chart value is only correct at that temperature. The turnbuckle wire pattern is learned visually and is easy to mirror incorrectly.
What to do instead
Apply the chart's temperature correction, rig with the controls at neutral and rigging pins installed, then recheck. Wire through the turnbuckle holes so tension pulls the wire onto the barrel (single- or double-wrap per AC 43.13-1B) and confirm the terminal thread engagement limits before sign-off.
Tops off a system that requires phosphate-ester fluid with MIL-PRF-5606, or assumes any red hydraulic fluid on the shelf is interchangeable with what is in the reservoir.
The three fluid families look alike and two of them are dyed red; the candidate matches on color and the word 'hydraulic' instead of on specification number and seal compatibility.
What to do instead
Sort by spec: MIL-PRF-5606 is mineral-base (red), MIL-PRF-83282 is synthetic hydrocarbon (red, limited emergency mixing with 5606), and phosphate-ester (Skydrol-type, purple/green) requires EPR/butyl seals and is never mixed with the others. Read the reservoir placard and the maintenance manual before adding fluid.
Adds branch resistances directly when loads are in parallel, or reasons that 'current takes the path of least resistance,' so all current is assigned to the lower-resistance branch.
Series rules are learned first and over-applied, and the 'least resistance' phrase is a misleading simplification that survives from basic training.
What to do instead
Use 1/RT = 1/R1 + 1/R2 + ... with the same voltage across each branch and IT = I1 + I2. After any calculation, sanity-check that total resistance is smaller than the smallest branch resistance and that branch currents sum to the source current.
Installs or troubleshoots a rate-of-rise detector in a cargo compartment and a smoke detector in an engine nacelle, or expects a fixed-temperature unit to give early warning of a smoldering fire.
Detector types are memorized as a list without the physical reason each is chosen: a powerplant fire produces a rapid temperature rise and needs a fast, flame-responsive unit, while a cargo compartment with little airflow smolders and produces smoke first.
What to do instead
Tie each detector to what it senses: rate-of-rise plus fixed-temperature (often combined, and often continuous-loop Kidde-type) in powerplants and APU compartments; photoelectric or ionization smoke detectors in cargo and lavatories. Be able to explain why the other choice fails in that zone.
Writes up a manufacturer Service Bulletin as 'must comply' on an inspection, or assumes every AD for that model applies regardless of serial number, part number, or configuration.
Both documents come from the manufacturer and the FAA and both look official. Under time pressure the applicability paragraph and compliance section of the AD are skipped.
What to do instead
Read the AD's applicability paragraph and compliance time line by line - serial ranges, part numbers, and effective dates decide whether the aircraft is affected. Treat an SB as mandatory only when an AD or the operator's approved inspection program incorporates it, and record AD compliance in the aircraft records.
Chooses dye penetrant to find a subsurface flaw, or picks magnetic particle inspection for an aluminum spar or a composite panel.
The NDT methods are studied as a list of names rather than as two physical gates: is the discontinuity open to the surface, and is the material ferromagnetic?
What to do instead
Ask the two gates before choosing: penetrant and eddy current find surface-connected discontinuities only; magnetic particle needs a ferromagnetic part and finds surface and near-surface flaws; ultrasonic and radiographic methods reach internal flaws. Practice by classifying a described defect ('porosity 3 mm below the surface in a steel fitting') before naming the method.
Cleans aluminum corrosion with a steel wire brush or steel wool, then primes over the remaining pits, or dismisses layered, lifting exfoliation as ordinary surface oxidation and just scuffs it.
Corrosion types are memorized by name without their signature appearance and preferred alloy, and 'scrub it and paint it' satisfies the visual goal of a clean surface without addressing depth or galvanic contamination.
What to do instead
Identify by appearance, location, and alloy: exfoliation lifts the layers of wrought aluminum, intergranular attacks grain boundaries (notably 2024/7075), filiform crawls under organic coatings, and dissimilar-metal contact drives galvanic attack. Remove products with the mildest method (abrasive pad or aluminum wool - never steel wool or a steel brush on aluminum), evaluate remaining depth against the structural repair limits, then treat and prime per AC 43.13-1B.