Flush rivet repair layout on a 1.6 mm fuselage skin
You are laying out a flush-riveted repair on a pressurised fuselage skin of 2024-T3, 1.6 mm thick, with a 2024-T3 doubler 1.2 mm thick - a 2.8 mm stack. The rivet chosen is MS20426AD4, diameter 4.0 mm. The draft layout shows a rivet pitch of 28 mm and an edge distance of 5.0 mm, and a 7.9 mm rivet has been drawn from stock. The approved repair scheme for this area states: pitch 4D to 6D; edge distance not less than 2D; rivet diameter between 2t and 3t of the skin; and, per the shop practice sheet, rivet length = stack thickness + 1.5 × diameter, taken from stock lengths in 0.4 mm increments. Driven-head acceptance is 1.5D diameter and 0.5D height, tolerance ±0.4 mm and ±0.3 mm respectively.
1. Evaluate the pitch and edge distance against the repair scheme and propose compliant values.
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Pitch: 28 mm ÷ 4.0 mm = 7.0D. The scheme allows 4D–6D, i.e. 16–24 mm. 28 mm is 4 mm beyond the maximum: too few rivets per unit length, so the sheet between rivets carries too much load and will pillowing/buckle under pressurisation cycling, and the joint is under-strength.
Edge distance: 5.0 mm ÷ 4.0 mm = 1.25D. The minimum is 2D = 8.0 mm. Too close to the edge - the rivet will tear out and may split the sheet on driving.
Compliant layout: pitch of about 5D = 20 mm (any value 16–24 mm), edge distance 8–10 mm (2D–2.5D), with edge distance measured from the rivet centre to the edge of the sheet, and edges of adjacent panels no closer than about 2.5D to the rivet centre.
Also check the diameter rule: 4.0 mm ÷ 1.6 mm skin = 2.5t, inside the permitted 2t–3t band, so the rivet itself is acceptable. The layout is not.
2. Determine the correct rivet length for the stack, and say what a 7.9 mm rivet would have produced.
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Required length = stack + 1.5D = 2.8 + (1.5 × 4.0) = 2.8 + 6.0 = 8.8 mm
Stock comes in 0.4 mm increments, so select the nearest standard length not less than 8.8 mm - 8.9 mm (or 9.2 mm if that is the next length listed). Never select shorter to 'draw it up tight'.
What 7.9 mm gives: shank protruding above the stack = 7.9 − 2.8 = 5.1 mm = 1.28D. That is about 15 % short of the 1.5D needed, so the driven head will be under-filled and undersize - an under-driven rivet with insufficient head volume, low clamp-up and a loose joint. It is a reject.
Practical points: drill 4.1 mm (or the hole size on the drawing) normal to the surface, deburr both sides, apply sealant as the scheme requires, and check the countersink depth so the flush head sits level with the skin before driving.
3. After driving, a sample rivet is measured with a head diameter of 5.2 mm and a height of 2.2 mm. Accept or reject? What is the likely cause?
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Requirements for D = 4.0 mm:
- Head diameter 1.5D = 6.0 mm, tolerance ±0.4 mm → acceptance band 5.6–6.4 mm.
- Head height 0.5D = 2.0 mm, tolerance ±0.3 mm → acceptance band 1.7–2.3 mm.
Measured 5.2 mm diameter: 0.8 mm below nominal, and 0.4 mm below the lower limit → reject. Height 2.2 mm is acceptable.
A small head with normal height means the material volume is there but it has not spread - typically under-driven (too few or too light hammer blows with the bucking bar held too long), or a rivet that was too long was 'coaxed' in, or the hole is oversize so the shank filled the hole rather than forming the head.
Action: remove the rivet (drill from the head side with a drill no larger than the shank diameter, using a pilot, so the hole is not damaged), inspect the hole, and re-set with correct technique. Inspect the whole run - if the gun pressure or operator technique was wrong, one bad rivet means the batch is suspect. Record the defect.