Measuring, torquing, wiring, swaging

Same five-part form as GEN-H Procedures. Guided first attempts at the hand skills are in GEN-E Hands-on; this component is the procedure once you know how to hold the tool.

Procedure 1 — Check a micrometer for calibration (S14)

Do this before any measurement you intend to sign for.

Given — a micrometer and its calibration standard. Sought — confidence that the instrument reads true.

Method

1. Clean the anvil and spindle faces. A fingerprint is a tenth of a thousandth.
2. Close on zero using the ratchet or friction thimble, never the barrel. Read it. A 0–1 in micrometer closes on itself; larger ones close on their standard.
3. Check at more than one point. Zero alone proves the zero, not the screw. Use gauge blocks or the standard at, say, a quarter, a half and three-quarters of range.
4. Check the frame and faces — chips, wear, a frame sprung by dropping.
5. Record the result against the instrument's identification.

Check — an instrument that reads correctly at zero and wrongly at range has a worn or bent screw, and that is the failure a zero check alone will never find. Two points minimum.

Failure — measuring with a dropped micrometer. It reads consistently, so nothing about the measurement looks wrong, and every dimension taken with it is quietly false (GEN-E Concepts §6).

Procedure 2 — Make a precision measurement (S6)

Given — a part and a calibrated instrument.

Method

1. Clean the part where you will measure.
2. Close the instrument on the work using the ratchet or friction device only — consistent feel is the whole basis of repeatability.
3. Read the barrel for the coarse figure, the thimble for thousandths, and the Vernier scale on the sleeve for ten-thousandths where fitted.
4. Measure at least three times, and at more than one position on the feature.
5. Record the reading and the instrument used.

Check — three readings that disagree are telling you something. Either the technique is inconsistent, or the part is not round, not parallel, or tapered. Do not average them until you know which; a mean of a taper is a number describing nothing.

Failure — turning the barrel instead of the ratchet. Feel varies between operators and between attempts, and the resulting spread is bigger than the tolerances being measured.

Procedure 3 — Check the concentricity of a shaft (S7)

Given — a shaft, V-blocks and a dial indicator.

Method

1. Support the shaft in V-blocks on a surface plate, on bearing journals or the specified support points.
2. Set the dial indicator to bear on the surface being checked, perpendicular to it, with preload on the plunger.
3. Zero the indicator, rotate the shaft one full turn, and note the highest and lowest readings.
4. Total indicator reading is the difference between them — that is the runout. Compare against the manual's limit.

Check — rotate a second full turn and confirm the same TIR. A different answer on the second turn means the shaft is moving in the blocks or the indicator is not held rigidly.

Failure — reporting the maximum reading as runout instead of the difference between maximum and minimum. It roughly doubles the figure and condemns serviceable shafts.

Procedure 4 — Determine and properly torque hardware (S2)

Given — a fastener, the aircraft maintenance manual, a calibrated torque wrench.

Method

1. Find the specified value in the maintenance manual, not a generic table. Note whether it is specified dry or lubricated, and note the units — GEN-H Procedures Procedure 8.
2. If the nut is prevailing-torque, run it down and measure the running torque, then add it to the specified value.
3. Prepare the joint: correct washer, correct thread engagement, threads and faces clean and in the condition the specification assumes.
4. Apply torque smoothly in one continuous motion to the value. Do not jerk, and do not tighten past and back off.
5. Follow the specified sequence where one exists — a pattern rather than round the circle, in stages rather than one pass.
6. Apply a witness mark where the manual or local practice requires one.
7. Release the wrench to its lowest setting before putting it away.

Check — a click-type wrench should be re-checked with a second click at the same setting and must not move. If the fastener turns further, the first application did not reach value. And a fastener that reaches specified torque unexpectedly early has something wrong — cross threads, wrong length, bottoming out, missing washer. Stop.

Failure — dry value on a lubricated thread, over-stretching the bolt with no outward sign (GEN-E Concepts §1).

Procedure 5 — Install safety wire (S1)

Given — fasteners to be wired, wire of the specified gauge, safety wire pliers.

Method

1. Decide the direction first. Establish which way each fastener loosens, and plan the run so the wire is pulled tight by any loosening.
2. Select wire gauge and the correct number of fasteners per run as specified — typically no more than three in a group.
3. Thread the wire, take up slack, and twist evenly — a consistent twist rate, tight but not over-twisted. Over-twisting work-hardens the wire and it will fail at the twist.
4. Enter the next fastener so the pull remains in the tightening direction.
5. Finish with a pigtail of three or four twists, cut, and bend it back under so it cannot snag.

Check — push each fastener in the loosening direction. The wire must go taut. If it goes slack, the run is backwards, and the answer is to cut it out and start again — not to adjust it.

Failure — a neat installation running the wrong way. It looks correct, it will not be questioned, and it holds nothing (GEN-E Concepts §4).

Procedure 6 — Fabricate a cable assembly with a swaged fitting (S9)

Given — cable, a swage fitting, a swaging tool, a go/no-go gauge.

Method

1. Cut the cable square by the specified method, without unlaying the strands.
2. Insert the cable fully into the fitting. Confirm the cable end is visible in the inspection hole where the fitting has one.
3. Swage in the specified number of passes, in the specified order, rotating the fitting between passes as required by the tool.
4. Gauge the swaged diameter with the go/no-go gauge at each swaged position.
5. Inspect for cracks in the fitting and for the cable having pulled back.
6. Proof load the assembly where the specification requires it.

Check — the go/no-go gauge is the check, and it is pass or fail. A swage that the "no-go" side accepts is under-swaged and will pull out under load. Visual appearance proves nothing here.

Failure — cable not fully inserted before swaging. The inspection hole exists for exactly this and looking through it takes a second.

Procedure 7 — Select and install aircraft bolts (S5)

Method

1. Read the head marking and confirm material and strength against the requirement (GEN-E Concepts §5).
2. Confirm diameter, length and grip. The grip — the unthreaded portion — should match the material thickness so that threads do not bear in the joint.
3. Fit the correct washer under the turning element.
4. Confirm thread engagement: at least the specified number of threads showing beyond the nut, and never fewer.
5. Torque per Procedure 4.

Check — no threads inside the bearing surfaces, and at least the minimum threads showing. Both are visible after installation, and both are common findings on inspection.

Failure — a bolt of correct diameter and wrong grip length. It fits, it torques, and the joint bears on threads.

The checks, collected

License

CC BY-SA 4.0 (Licensing).