The upper fillet radius on a DC-10 or MD-11 pylon is one of those details that looks like nothing and carries everything. It is the curved transition where the upper spar cap meets the side web on the engine mount structure, and it sees the full load path from the engine through the pylon and into the wing. Thrust, inertia, gust loading, and thermal cycling all funnel through that radius. When fatigue cracks start anywhere on a pylon, this is a spot you check first.
Baron NDT runs eddy current on these fillet radii as a recurring task, and it is rarely a one-off. A single pylon teardown or a heavy check on a freighter conversion can stack up dozens of radius readings across the forward and aft fittings. The geometry is tight, the access is awkward, and the acceptance threshold is small. That combination is exactly why operators send it out to a Part 145 shop instead of running it green-card in the field.
Why the upper fillet radius cracks
Pylon structure on the DC-10 and its MD-11 successor is high-strength aluminum and steel fittings carrying a wing-mounted or tail-mounted engine. The upper fillet radius is a stress concentration by design. Any radius is. Add decades of flight cycles, a freighter’s higher gross weights, and the occasional fastener-hole fretting, and you get the classic fatigue picture: a crack initiates at the surface of the radius, often at or near the bore of an attach hole, and grows along the load path before anything is visible to the eye.
By the time a crack is big enough to see under a flashlight, it is well past the point where the OEM wants it found. That is the whole argument for surface eddy current here. It finds tight, surface-breaking cracks in conductive material long before visual or a dye penetrant bleed-out would call them, and it does it without stripping the part down to bare metal in every case.
The eddy current approach Baron uses
This is a surface and near-surface job, so it lives in the higher frequency band, typically in the range the OEM NDT manual calls out for aluminum airframe details, often around 100 kHz to 500 kHz depending on the alloy and the probe. We follow the applicable Douglas / Boeing NDT manual procedure for the specific fitting and effectivity, since the radius dimension and the reference standard change between the DC-10 series and the MD-11.
A few things matter more than the rest:
- Probe selection. The radius is small, so we use a shielded pencil or radius probe sized to ride the curve without lifting off. A probe that bridges the radius reads garbage. Conformance to the actual fillet dimension is the first thing we confirm.
- Reference standard. Calibration is set on a reference block with EDM notches that match the manual, run on the same radius geometry. We set the response on the notch, not on a flat coupon, because lift-off behavior in a radius is not the same as on a plane surface.
- Lift-off control. The signal we care about is small. We rotate the impedance plane so lift-off goes horizontal and the crack response goes vertical, then we scan slowly enough that a tight crack does not get lost in the noise.
- Coverage. The fillet gets scanned along its full length on both the forward and aft fittings where the manual requires it, with the probe indexed to keep the full radius under the coil.
Personnel are qualified to NAS 410 and certified for the method, and the work is documented to the procedure and the customer’s repair station requirements. The point of all of it is repeatability. Two inspectors on the same fitting should call the same thing.
Where it fits in a pylon inspection
The upper fillet radius is one stop on a larger pylon map. The same airframe down event usually pulls a full set of eddy current readings across the mount structure, and those are the related inspections we run in the same visit. The radius work pairs naturally with the front spar fitting inspection at FS625.3, the pylon track and support fittings, and the lower link fittings at the wing. Together they cover the load path top to bottom.
If you want the bigger picture on why eddy current is the right tool for fatigue cracks in aluminum airframe structure, our guide to eddy current testing walks through the physics and the probe choices, and the crack detection overview covers why surface ET beats visual for tight cracks. For the full airframe-zone view of how these task inspections fit a heavy check, see our guide to aircraft NDT inspection.
What we hand back
Every fillet radius inspection comes back with a written report tied to the procedure, the fitting and effectivity, the probe and frequency used, the calibration standard, and a clear accept or reject call against the OEM criteria. If we find a crack-like indication, we document location and extent so engineering can make the disposition. No guesswork, no verbal handoffs.
This is bread-and-butter aviation NDT for us, and the DC-10 and MD-11 fleet running freight today is exactly the population where these fatigue details deserve attention. If you have pylons coming due, talk to Baron NDT about scheduling the eddy current scope.