The Airbus A220 (originally the Bombardier CSeries) has matured into one of the most heavily utilized narrowbody aircraft in regional and mainline fleets, and with high utilization comes a predictable set of structurally significant inspection points. Among the most important is the wing bottom cover buttstrap runout region, addressed by scheduled task 57-21-57. This inspection targets a load path where fatigue cracking can initiate quietly and grow toward a size that matters, and phased array ultrasonic testing (PAUT) has become the preferred method for finding those cracks before they become a problem.
Where the Buttstrap Runout Sits and Why It Matters
The wing lower cover on the A220 carries tension loading through every flight cycle. Where large integrally stiffened panels or splice regions transition, manufacturers use a buttstrap: a reinforcing strap that bridges a joint and redistributes load across a row of fasteners. A buttstrap has to end somewhere, and that termination is called the runout. At the runout, the strap tapers out of the load path and stiffness changes abruptly over a short distance. That geometric discontinuity, combined with the fastener holes clustered in the region, concentrates stress exactly where you least want it.
Fatigue damage in metallic wing structure almost always initiates at these stress risers. The bottom cover is the tension surface of the wing during normal flight, so the buttstrap runout fastener rows see cyclic tensile stress on nearly every cycle. Cracks tend to nucleate at the bore of a fastener hole or at the faying surface between the strap and the skin, then propagate outward along the net section between holes. Because the crack often starts on a buried or faying surface, it can be well established before any indication reaches an accessible surface. That hidden-initiation behavior is precisely why a volumetric method is required rather than a surface-only technique.
Why Phased Array UT Is the Right Tool Here
Conventional single-element ultrasonics can interrogate this structure, but the buttstrap runout geometry makes it slow and operator dependent. Phased array changes the economics and the reliability of the inspection. A PAUT probe contains many small elements that are pulsed with programmed time delays, allowing the beam to be electronically steered and focused across a range of angles from a single probe position. That capability lets the technician sweep a sector of angles through the fastener row and the runout taper without physically re-aiming a fixed-angle wedge for every reflector orientation.
For this task the practical advantages are concrete. Electronic focusing improves signal to noise at the depth of the faying surface and the far side of the strap. Sector scanning covers cracks that grow at varying angles out of a hole bore. Encoded linear scanning produces a permanent, position-referenced record so a later inspection can be compared against the baseline. These are the same PAUT principles Baron applies across airframe work, and readers who want the method fundamentals can review our ultimate guide to ultrasonic testing and our applied examples such as phased array UT crack detection per the 737 NDT manual.
The Scan Plan for 57-21-57
A defensible PAUT inspection of the bottom cover buttstrap runout starts with a written scan plan derived from the applicable NDT task in the aircraft maintenance data. The plan defines probe frequency and aperture, the wedge, the angular sweep range, the index and scan axes relative to the fastener rows, the coupling method, and the calibration reference. Typical setups for this thickness range use an appropriate array frequency with a wedge selected to place the useful angular range through the expected crack plane at the hole bore and net section.
Calibration is performed on a reference standard that represents the part thickness and the relevant reflectors, commonly side-drilled holes and notches that bracket the rejectable flaw size. The technician sets a time-corrected gain so that response is uniform across the swept angles and depths, then establishes the recording threshold from the reference reflectors. Scanning is normally encoded along the fastener row so the collected S-scan and C-scan data are tied to physical position. Access dictates a lot: the bottom cover is inspected from the accessible skin surface, which means surface preparation, sealant and finish considerations, and sometimes removal of adjacent systems to reach the runout cleanly.
The inspector interrogates each fastener location and the intervening net sections, watching for corner and bore reflectors that break the recording threshold and behave like a crack under angle and position changes. Because the buttstrap runout is a known initiation site, the scan is biased to fully cover the taper and the first fastener rows inboard and outboard of the runout, where the stress gradient is steepest.
Interpretation and Acceptance
Acceptance is dictated by the aircraft maintenance manual and the associated NDT task, not by an inspector’s judgment call. A relevant indication is one that exceeds the recording threshold and exhibits crack-like behavior in its amplitude, position, and response to beam angle. When such an indication is found, the technician characterizes its position along the fastener row, its depth, and its estimated length, then reports against the disposition criteria in the task. Findings route to engineering for evaluation, and confirmed cracks drive repair or replacement per the structural repair manual. Corroborating methods are common in this region: radiography is frequently used to confirm and map subsurface indications, as described in our article on radiographic inspection of outer wing buttstrap runouts and the related work on wing lower skin under doublers and brace fittings.
Thorough documentation matters as much as detection. A complete report includes the scan plan reference, calibration records, the encoded data files, indication maps, and a clear statement of accept or reject against the task revision in effect. That record is what allows the operator to demonstrate compliance and to trend the structure over successive inspection intervals.
Why This Belongs in an FAA Part 145 Shop with NAS-410 Technicians
Task 57-21-57 is primary structure work on a pressurized, high-cycle aircraft, and the consequences of a missed crack are significant. Operators route this inspection to an FAA Part 145 repair station because the certificate carries the quality system, procedure control, and record retention that regulators expect for structural NDT. The technicians performing and interpreting the data should be qualified and certified to NAS-410, the aerospace personnel qualification standard, at the level appropriate to setting up the technique and dispositioning results. That combination of a controlled procedure, calibrated equipment, and a properly certified inspector is what makes the result trustworthy.
Baron NDT performs this A220 buttstrap runout PAUT work as recurring aviation structural inspection out of our Jacksonville aviation operation, alongside related airframe phased array such as phased array UT of 737 lap seams and structural PAUT on regional types like the CRJ family. Operators looking for the broader picture of how these methods fit together across an airframe can start with our ultimate guide to aircraft NDT inspection.
Bottom Line
The A220 bottom cover buttstrap runout is a textbook fatigue location: a stiffness discontinuity stacked on top of a fastener row on the tension surface of the wing. Task 57-21-57 exists to catch cracks there while they are still small, and phased array ultrasonics gives the speed, angular coverage, focusing, and encoded records to do it reliably. Performed under a written scan plan by NAS-410 certified technicians inside an FAA Part 145 quality system, PAUT turns a structurally critical inspection into a repeatable, well documented event that keeps A220s flying safely between intervals.