Reading and producing drawings

Six procedures. Same form as GEN-H Procedures, and the check matters most here, because a misread drawing produces work that is competent and wrong.

Procedure 1 — Establish that a drawing applies (R3, R4, before anything else)

This runs before every other procedure in this unit. It is not one of the ACS skills; it is what makes the skills safe.

Method

1. Drawing number — does it match the one the maintenance manual or SRM sent you to?
2. Revision letter and date — is this the current revision? Check against the revision index or the manual's list of effective pages.
3. Effectivity — does the serial-number or model range include the aircraft in front of you?
4. Approval status — is this approved data, or acceptable data, or neither? (GEN-I §1.)

Check — state out loud which aircraft this drawing applies to. If you cannot name the model and serial range from the document, you have not established applicability.

Failure — every failure in this unit begins here. A part that fits is not a part that applies.

Procedure 2 — Interpret lines and symbols (S2)

Method — identify each line type before reading any geometry: object lines, hidden lines, center lines, section lines, phantom lines, and the leaders and extension lines that carry dimensions.

Check — hidden lines describe something you cannot see from this view. If a feature you believe is visible is drawn hidden, you are looking at the wrong view or the wrong side. Confirm your reading against a second view before acting on it.

Failure — reading a hidden line as an edge, which invents a feature that is not there.

Procedure 3 — Interpret dimensions (S3, with R1)

Method

1. Find the title block and read the general tolerance note — it governs every dimension not individually toleranced.
2. Read the dimension itself, and note whether it is to a surface, a center line or a datum.
3. Apply the tolerance: a stated 2.500 ± 0.005 accepts 2.495 to 2.505 inclusive.
4. Check whether tolerances stack. If the feature you care about is the sum of two dimensioned steps, the worst case is the sum of both tolerances.

Check — the dimensions on a view should close. Individual dimensions plus the overall dimension must agree. If they do not, one of them is being read from the wrong datum.

Failure — treating a measurement at the limit as acceptable by rounding it inward. A value at or beyond a limit is a rejection (GEN-B Concepts §2).

Procedure 4 — Identify changes on a drawing (S4)

Method — find the revision block, read the description of each revision, and locate the changed areas on the drawing itself. Changed features are conventionally flagged, often with a revision triangle carrying the revision letter.

Check — the revision block and the drawing must agree. If the block records a change you cannot find on the drawing, you may be holding a superseded sheet of a multi-sheet drawing.

Failure — working to a superseded sheet inside a current drawing set. The set is current; the sheet is not.

Procedure 5 — Determine material requirements (S5)

Method — read the bill of material or material note: specification, alloy, temper, form and thickness. Cross-check against the general notes, which frequently carry substitution restrictions.

Check — the material call-out must name a specification, not just a metal. "Aluminum" is not a call-out; a specification with an alloy and temper is (GEN-E). If all you have is a common name, you have not found the call-out yet.

Failure — substituting on the basis of appearance or availability. Temper is invisible, and AF-A R1 is about exactly this.

Procedure 6 — Interpret a graph or chart (S6)

Method

1. Name both axes and their units before reading anything.
2. Note whether either scale is non-linear.
3. Enter on the known value, follow to the correct curve, read across.
4. If your entry falls between curves, interpolate — and record that you did.

Check — read a second point you can predict. Most charts have an obvious anchor: a standard-day value, a zero, a known maximum. If the chart returns the expected answer there, your axes and units are right.

Failure — reading to the nearest curve instead of interpolating. It produces a plausible number and the error is invisible.

Procedure 7 — Sketch a repair or alteration (S1)

Given — a repair you have designed from approved data. Sought — a sketch that supports a Form 337 and lets the next person understand what was done.

Method

1. Orient the view and label it — which way is forward, which surface is shown.
2. Draw the damaged area and the repair, in reasonable proportion. Proportion, not scale; a sketch is not a drawing and should not pretend to be.
3. Dimension the repair: patch size, edge margins, fastener spacing, thickness.
4. Call out material specification and fastener type and size.
5. Reference the approved data the repair was designed from.
6. Add anything a reader could not infer — sealant, doubler stack-up, treatment.

Check — hand it to someone who was not there and ask them to describe the repair. If they can, it is sufficient. If they ask what material, which fastener, or which way up, it is not.

Failure — a beautiful sketch that omits the approved data reference. The sketch records what was done; the reference is what made it permissible, and GEN-I needs both.

The checks, collected

License

CC BY-SA 4.0 (Licensing).