Appearance
An aircraft manufacturer routing wiring through a 12 m wing spar (7075-T6) drilled 24 conduit pass-through holes of 12 mm x 400 mm deep on a 5-axis CNC gantry (long-reach carbide 12 mm x 600 mm drill, Vc = 100 m/min, f = 0.08 mm/rev, emulsified oil at 30 bar). Each hole was cold-worked by pulling a 12.3 mm oversize mandrel through, expanding the hole by 0.10-0.15 mm and creating -100 to -200 MPa compressive residual stress (XRD verified). Cold-working increased fatigue life by 3-5x.
Wing Spar and Fuselage Frame Conduit Drilling
Wiring routing holes in aircraft structures require a combination of precise drilling and post-drilling cold-working to maintain structural integrity under cyclic loading. The cold-working process (split-sleeve cold expansion or mandrel expansion) involves pulling a slightly oversized mandrel through the drilled hole, which plastically expands the material around the hole radially. When the mandrel passes through, the elastic recovery of the surrounding material creates a compressive residual stress zone (typically 100-300 MPa compressive) that extends 2-5 mm from the hole edge, retarding fatigue crack initiation by 3-5x compared to a non-cold-worked hole.
| Parameter | Wing Spar (Al 7075-T6) | Wing Rib (Al 2024-T3) | Fuselage Frame (Al 2024) | Fuel Tank Wall (Al 7075) |
|---|---|---|---|---|
| Hole diameter | 8-16 mm | 6-12 mm | 6-10 mm | 6-12 mm |
| Hole depth (material thickness) | 10-50 mm | 3-8 mm | 3-6 mm | 8-20 mm |
| Drill type | Long-reach carbide twist | Carbide twist drill | Carbide twist drill | Carbide gun drill |
| Cutting speed (Vc) | 60-120 m/min | 80-140 m/min | 80-140 m/min | 60-100 m/min |
| Feed rate (f) | 0.04-0.08 mm/rev | 0.05-0.10 mm/rev | 0.05-0.10 mm/rev | 0.03-0.06 mm/rev |
| Coolant | Emulsified oil (30 bar) | MQL or flood | MQL or flood | Emulsified oil (40 bar) |
| Cold-working method | Split-sleeve mandrel | Mandrel or none | Mandrel or none | Split-sleeve mandrel |
| Mandrel interference | 0.2-0.4 mm | 0.1-0.2 mm | 0.1-0.2 mm | 0.2-0.3 mm |
| Fatigue life improvement | 3-5x | 2-3x | 2-3x | 3-5x |
| Hole finish after cold-work (Ra) | < 1.6 microns | < 1.6 microns | < 1.6 microns | < 1.6 microns |
The initial hole must be within +/- 0.02 mm of the specified diameter and have a surface finish of Ra less than 1.6 microns to ensure consistent mandrel interference and prevent galling. Drilling is performed on a CNC gantry machine or using a hand-drill with a drill guide template. The drill point angle is 130-140 degrees with a split-point chisel edge.
Fuel Tank Wiring Passages and Sealing
Fuel tank wiring penetration is the most critical wiring drilling application in aircraft. The hole through the fuel tank wall must be sealed with a fuel-tight connector. US Patent 20040145122 describes an expandable annular bush that is inserted into the drilled hole and crimped to seal against the tank wall. The seal is tested by pressurising the fuel tank with air at 3.5 kPa (0.5 psi) and applying a soap solution to the wiring penetration.
| Parameter | Fuel Tank Conduit | Avionics Bay Pass-Through | Static Wick Mount | Wing Tip Light Conduit |
|---|---|---|---|---|
| Hole diameter | 6-12 mm | 10-25 mm | 4-8 mm | 8-15 mm |
| Hole depth | 8-20 mm (tank wall) | 3-10 mm (bulkhead) | 10-30 mm (tip) | 5-15 mm |
| Drilling method | CNC gun drill | CNC or hand drill | Hand drill with template | Long-reach CNC |
| Seal type | Expandable bush (US 20040145122) | Feed-through connector | Weather seal | Grommet + sealant |
| Leak test pressure | 3.5 kPa (0.5 psi) | N/A (not fuel zone) | N/A (exterior) | N/A |
| Separation from fluid lines | 12.7 mm min (MIL-W-5088L) | 12.7 mm min | N/A | 12.7 mm min |
| Separation from fuel lines | 150 mm min | 150 mm min | N/A | 150 mm min |
The separation of wiring conduits from fluid lines is specified by MIL-W-5088L: wire bundle diameter must not exceed 80% of the conduit inside diameter; wire bundles must maintain a minimum separation of 12.7 mm from fluid lines and 150 mm from combustible fluid lines (such as fuel lines in the wing tank area).
Quality Control for Aerospace Wiring Holes
| Inspection Parameter | Method | Acceptance Criteria | Frequency |
|---|---|---|---|
| Hole diameter (pre-cold-work) | Air gauge or plug gauge | +/- 0.02 mm of spec | 100% of holes |
| Hole surface finish (pre-cold-work) | Profilometer (replica) | Ra < 1.6 microns | Sample (10%) |
| Mandrel pull force | Force transducer on puller | Within +/- 10% of calculated | 100% of holes |
| Hole diameter (post-cold-work) | Air gauge or plug gauge | Specified + 0.10-0.15 mm | 100% of holes |
| Residual stress (compressive) | X-ray diffraction (XRD) | -100 to -300 MPa | First article + sample |
| Fuel seal integrity | Soap solution + 3.5 kPa air | No bubbles for 30 seconds | 100% of fuel penetrations |
| Conduit position | CMM or template | +/- 1 mm from spec | 100% of holes |
| Edge distance to structure | CMM or scale | Min. 2x hole diameter | 100% of holes |
Frequently Asked Questions
Why is cold-working necessary for wiring holes in aircraft wing spars?
Cold-working is necessary because the wing spar is a primary structural component that experiences cyclic tensile and compressive loads during flight. A drilled hole creates a stress concentration at its edge, which is the most common starting point for fatigue cracks in aircraft structures. The cold-working process introduces compressive residual stress around the hole, which counteracts the tensile service loads and prevents crack initiation. Tests show that cold-worked holes have 3-5 times longer fatigue life than non-cold-worked holes, which is critical for aircraft designed for 30,000-60,000 flight cycles.
What is the minimum separation required between wiring conduits and fluid lines?
MIL-W-5088L specifies that wire bundles must maintain a minimum separation of 12.7 mm (0.5 inch) from fluid lines and 150 mm (6 inches) from combustible fluid lines. This separation prevents wiring chafing against fluid lines and reduces the risk of fluid leaks contacting electrical wiring. In wing spar drilling, the conduit positions must be selected to maintain these clearances, which often requires additional coordination between the structural engineering and electrical routing teams.
How are fuel tank wiring penetrations sealed after drilling?
Fuel tank wiring penetrations are sealed using an expandable annular bush as described in US Patent 20040145122. After the hole is drilled and cold-worked, the bush is inserted into the hole and crimped using a special tool that expands the bush against the tank wall. The bush provides a fuel-tight seal around the wiring bundle while maintaining electrical bonding between the wiring shield and the aircraft structure. The sealed penetration is tested by pressurising the fuel tank with air at 3.5 kPa (0.5 psi) and applying a soap solution.
What drill type is used for long-reach wing spar drilling?
Long-reach wing spar drilling uses a carbide twist drill with a length-to-diameter ratio of up to 50:1 (e.g., 12 mm diameter x 600 mm length). The drill has a split-point or helical point geometry with a point angle of 130-140 degrees for aluminium alloys. The long-reach drill is supported by drill bushings at the entry and exit surfaces to maintain straightness. The drilling parameters for 7075-T6 aluminium are Vc = 60-120 m/min, feed f = 0.04-0.08 mm/rev, with emulsified oil coolant at 25-40 bar through the drill's internal coolant passages.
Can wiring holes be drilled in composite aircraft structures?
Yes, but composite structures require different drilling techniques than metal structures. Carbon fibre composite spars and fuselage panels are drilled using diamond-coated or PCD-tipped drills at Vc = 100-200 m/min, feed f = 0.02-0.05 mm/rev, with compressed air cooling (liquid coolant can penetrate the composite matrix and cause delamination). Cold-working is not applicable to composites (which do not plastically deform), so the fatigue performance of holes in composites is improved by careful edge finishing and by using bushed inserts that distribute the bearing load.
The information provided in this article is for general informational purposes only. Data and recommendations are based on published research and industry experience as of 2026. Always consult your aircraft manufacturer or regulatory authority for application-specific requirements.