The pylon is the structure that hangs the engine off the wing, and the track and support fittings inside it carry every pound of thrust and engine weight into the airframe. They live in a brutal environment. Vibration runs through them on every flight, thermal cycling works the metal, and the highest stress concentrates exactly where you cannot see it: the fillet radii, the lug bores, and the fastener holes. That is why a pylon track support fitting inspection by eddy current shows up on so many heavy check work cards and service bulletins. It finds the cracks that a flashlight and a borescope never will. Because the pylon is where the engine mounts to the airframe, this task sits inside the broader NDT in aircraft engine and MRO scope.
Most of these fittings are machined from high strength steel or titanium, with some aluminum brackets in the surrounding structure. The damage we are chasing is fatigue cracking that starts at a stress riser and grows under load. On steel and titanium fittings the geometry is unforgiving, so the inspection has to be tailored to the part rather than run as a generic sweep.
Where the cracks actually start
On a typical engine strut, the track fittings, support links, and mid spar fittings transfer load through bolted joints and machined lugs. Three areas account for most of the findings. First, the fastener holes and bolt bores, where bearing load and fretting kick off subsurface cracks at the hole wall. Second, the fillet radii at the base of lugs and flanges, where the section changes and stress piles up. Third, the bushing bores, where a worn or migrated bushing can mask a crack that started under it.
The OEM nondestructive test manual drives all of it. The strut and nacelle chapter, Boeing chapter 54 territory and the equivalent sections in the DC-10 and MD-11 manuals, calls out the specific fitting, the access, the probe, the frequency, and the calibration standard. We do not freelance on a primary structure fitting. If the work card references a service bulletin or an airworthiness directive, the method and the acceptance criteria come straight from that document. Reading the AD correctly is half the job, which is one reason we keep our process aligned with FAA airworthiness directive NDT compliance from the first work card to the final record.
Surface eddy current versus bolt hole eddy current
Two techniques cover most of this work. For exposed surfaces, fillet radii, and the faces of a fitting, we run high frequency surface eddy current with a shielded pencil probe, usually in the 100 kHz to 2 MHz range depending on the material and the depth of interest. Conductivity and lift off get sorted out on a reference standard with EDM notches that match the expected crack orientation before the probe ever touches the part.
For the fastener holes and bushing bores, we use bolt hole eddy current with a rotating scanner and a bore probe sized to the hole. The fastener comes out, the hole is cleaned, and the probe sweeps the full bore so a crack at any clock position shows up as a clean signal. When the manual calls for subsurface coverage through a thick flange or a doubler, we drop to low frequency eddy current to push the field deeper. Eddy current is the right tool here because it reads cracks at and just below the surface without removing paint or primer in many cases, which keeps the structure intact and the turnaround fast. If you want the method fundamentals, our guide to eddy current testing walks through probes, frequency, and signal interpretation, and our overview of eddy current crack detection on aircraft covers why it dominates aluminum and steel airframe work.
Calibration, personnel, and records
Every setup is referenced to a calibration standard of the same material and surface condition as the part, with notches at the smallest rejectable crack size from the manual. We verify the standard, set the gain so the reference notch reads against a known amplitude, and check lift off response before and after the scan. Personnel are qualified and certified to NAS 410, and the technique stays inside the OEM acceptance limits, no interpretation drift.
The pylon is a fracture critical area, so the documentation matters as much as the scan. We record the fitting, the technique, the equipment and probe, the calibration standard, the frequency, and the result against the manual reference. That package is what an FAA Part 145 quality system and the operator’s records need to close the task. The pylon does not get inspected in isolation either. The front spar fittings, lower link fittings, and the wing attach hardware are part of the same load path, and we treat them as one job. Our writeup on eddy current inspection of engine pylon front spar fittings covers the upstream end of that structure, and both sit under the larger picture in our ultimate guide to aircraft NDT inspection.
Why operators send this work to Baron
Pylon fitting work rewards experience. The access is tight, the geometry is awkward, and a missed crack on a primary engine mount is not a small problem. Baron NDT is an FAA Part 145 repair station with Boeing Conformity Review approval, and we run these inspections to the OEM manual and the governing AD every time. If you have a pylon track or support fitting task coming up on a check, or a service bulletin you need covered, we can scope the technique and the access before the aircraft is even open. Reach out to Baron NDT at 904-304-2907 and we will line it up. For the aft attachment, see our writeup on eddy current inspection of the pylon lower link fittings.