Oversize fastener holes in a fracture-critical fitting
You are the stress engineer of record for a wing-to-fuselage attachment fitting on a new 19-seat commuter aeroplane. During manufacture, four fastener holes in the titanium fitting were drilled 0.40 mm oversize (6.75 mm instead of 6.35 mm); the supplier reamed them and installed the next oversize fastener. The drawing allows +0.05 mm on hole diameter. The fitting is fracture-critical and single-load-path, and the holes were originally cold-expanded, so reaming removed the cold-worked layer. Static analysis still shows positive margins: net-section stress up 0.8%, bearing stress down 6%. The programme manager says the variation is cosmetic, that the fatigue test article was built with nominal holes anyway, and that the compliance document is due to the authority in ten days. He asks you to sign the statement of compliance for the fitting with a note that the hole size is within manufacturing practice. Your signature block makes you personally responsible for the analysis.
1. Quantify the static effect of the oversize holes and explain why the static margins do not close the issue.
Read the worked answer
Numbers (60 mm wide × 4 mm thick strap, one hole in the section):
- Net section: nominal (60 − 6.35) × 4 = 214.6 mm²; as-built (60 − 6.75) × 4 = 213.0 mm² → stress up 0.75%.
- Bearing: P/(d·t); diameter up 6.3% → bearing stress down 5.9%.
- Edge margin: edge distance reduced 0.2 mm per side; check against the 1.5d/2d requirement - small, but not zero.
So static margins barely move. That is exactly why static analysis is not the closing argument. The fitting is fracture-critical and single-load-path: the substantiation lives at the hole, and reaming changed three things the static numbers cannot see.
- Cold expansion. The beneficial residual stress field at the hole was doing a large share of the fatigue job and has been machined away. Fatigue life at an uncold-worked hole can be a fraction of the cold-worked value.
- Fit and load transfer. Hole surface finish and the interference/clearance condition have changed, altering both local stress amplitude and how much load the fastener transfers.
- Test evidence. The full-scale fatigue article was built with nominal, cold-expanded holes, so it no longer covers the shipset - this is the point the manager has inverted.
Closing it requires fatigue data for the as-built condition (coupon or element tests on reamed-oversize holes with cold work removed) plus a damage-tolerance check on the new hole and fastener.
2. What do you do with the manager's request, in what order, and what would make signing defensible?
Read the worked answer
Do not sign. The statement certifies conformity to the type design and that the analysis supports it. Three facts make the signature indefensible: the hole is 0.40 mm oversize against a +0.05 mm tolerance, so 'within manufacturing practice' is a misstatement of the drawing; the fatigue evidence no longer covers the shipset; and schedule pressure is being applied to a technical conclusion.
Sequence:
- Raise a nonconformance formally (NCR / reject tag) and state in writing the same day that you are not signing, and why. Email is sufficient; keep it.
- Route the disposition through independent verification - chief of stress, then head of airworthiness/CVE - not only through the programme manager. He is the party proposing the deviation and cannot also approve it.
- Offer the defensible options: replace the fittings (clean but costly); or obtain a design deviation supported by fatigue and damage-tolerance evidence specific to the as-built holes, approved through the authority route, with any added inspection defined - not by a local note.
- Record the resolution and its evidence in the certification records.
What makes signing defensible: a substantiated as-built fatigue and damage-tolerance assessment, an approved deviation, and inspection requirements that are achievable in service. Not an assurance that nobody will look.
If pressed: say plainly that you will not sign and that proceeding must be done over your written objection through the proper chain. If the fitting risks being installed without disposition, escalate to the head of airworthiness and, failing that, use the confidential reporting channel or the authority's mechanism - with the documentation already in place.
3. Two years later, the same fitting is found on an in-service aircraft, and the records show the supplier signed a certificate of conformity for the nominal hole size. What are your obligations now, and in what order?
Read the worked answer
This is now a continuing-airworthiness and reporting matter, not a design argument.
1. Contain. Establish how many shipsets are affected, using the supplier's manufacturing records and traceability of the oversize fasteners. Determine which aircraft carry them and their accumulated cycles.
2. Report. Notify the competent authority under the applicable occurrence-reporting obligation - in an EASA system Part 21.A.3A, in an FAA system 14 CFR 21.3 - for a defect that could affect airworthiness. This is a design-organisation obligation and is not satisfied by writing to the supplier. The false certificate of conformity is a separate compliance and legal matter for the supplier chain.
3. Protect the fleet. Issue the service information or inspection requirement that follows from the substantiation, defining the inspection method and interval, or the replacement requirement if the as-built condition cannot be shown to be safe.
4. Preserve and escalate. Keep all records, including your original written refusal - it is now the key evidence that the organisation knew and when. Hand the certification-of-conformity issue to compliance/legal, and do not negotiate it privately with the supplier.
5. Correct the system. The failure was not only a drill: it was a control that let a false certificate be accepted, and a design organisation that could not see the deviation. Fix supplier surveillance, incoming inspection, and the nonconformance-escalation route so the next one surfaces before the aircraft flies.