Aircraft Wheel Inspection: How It Traditionally Works, and Where It Is Vulnerable (Part One)

An aircraft wheel comes off the airplane and into a shop about every 300 landings, and every one of those visits includes some form of non-destructive inspection. The traditional process works, and it works largely because of the people running it. Most of what gets found in a wheel shop gets found because someone who has handled thousands of them looked at this one and knew something was off.

But there are places where even a well-run process has vulnerabilities, and they are worth understanding before anyone starts talking about how to address them. This blog covers how wheels are traditionally inspected today, what that inspection costs the wheel, and the five vulnerabilities the current process is not built to resolve.

What Wears an Aircraft Wheel Out

Aircraft wheels are almost exclusively forged aluminum, and aluminum has a defined fatigue life. After enough accumulated use a wheel is expected to crack, and eventually to fail, which sounds alarming until you realize it is a damage-tolerant design rather than a performance failure of the wheel. Engineers steer that end-of-life crack somewhere predictable, and many wheels are built so the first one shows up in a spoke, where it is relatively easy to find and relatively low risk. For instance, a wheel can have a spoke cracked all the way through and still not be a risk to flight.

The harder question is when a given wheel gets there, because fatigue accumulates with operational conditions rather than with calendar time. Two wheels of the same age can be nowhere near the same point in their lives. Component maintenance manuals carry a rolling-mile life limit for exactly this reason, but rolling miles turn out to be difficult to track in practice. Landings and takeoffs are easy enough to count; taxi distance is not. Two minutes of taxi at Albuquerque and twenty minutes at Atlanta look identical on paper and are very different wear on the wheel. Brake use compounds aging, since the heat delivered over time softens the aluminum slightly and reduces its resistance to fatigue.

Beyond ordinary fatigue, three things retire wheels.

1. Heat damage

The obvious case is a rejected takeoff, a single large heat event that can take a wheel out of service on its own. The less obvious case is prolonged moderate braking, which puts heat into the wheel gradually and changes the aluminum’s material properties without any single event, such as a fuse plug release, to point at afterward.

2. Corrosion

Corrosion is insidious and hard to chase, which is what makes it one of the bigger problems. A new wheel arrives with real protection built into it: stress rolling and shot peening on many surfaces introduce a compressive residual stress that resists cracking,  then anodizing, then paint. Over successive service intervals those protections can wear away and the aluminum underneath corrodes. Most of that corrosion never threatens anything, but two kinds do. Corrosion in a sealing area can cause the wheel to leak, which retires it even when there is no crack. And corrosion in a moderate or high stress area can develop into a pit, the pit becomes a stress concentration, and the stress concentration eventually becomes a crack.

The standard treatment is to blend the corrosion off, check there is no crack underneath, and retreat the area so corrosion does not return. That works well enough, with two caveats. It removes material from the wheel, and physical blending puts heat into it in a way that is not especially well controlled, which is why some structures engineers at the OEM level will tell you that blending repairs often do more harm than good. Manuals still have to take the conservative position so nothing gets missed, even when that position shortens the life of the wheel. It is safety bought with wheel life, because it is difficult to tell which corrosion needs repair and which is better left alone.

3. Foreign object damage

Debris coming off the runway can nick or dent a wheel, and a nick is an initiation point for a crack.

These build over time. Heat softens the aluminum a little at a time, corrosion works through a coating over years, and fatigue advances with every landing. Even a nick does its damage later, by becoming a crack that grows. A wheel’s condition changes continuously while it is in service, but it only gets a full inspection at a major overhaul.

The Traditional Aircraft Wheel Overhaul Process: Minors and Majors

Wheels come into the shop on a predictable cadence, and what happens to them depends on which visit it is.

A minor is a tire change. The wheel gets a visual inspection and an eddy current inspection of the bead seat, which receives that attention because it is the one area on the wheel capable of releasing a large amount of energy. A failure there is dangerous to anyone near the wheel, and can leave an aircraft with a wheel failure and a landing still to complete. If a wheel has a known problem area, the manual may call for more: an eddy current probe of the fuse plug area for corrosion, an inspection of the tie bolt holes, a torque check on the bearing cup. Minors happen roughly every 300 landings, since a tire gives a few hundred before the tread requires removal.

Every three to five minors, the wheel gets a major overhaul. The hardware comes out, the coatings are stripped, penetrant inspection is performed, and the additional inspections are run. Very few wheels actually fail at a major — in commercial wheel shops, roughly 1 to 3 percent turn up a finding. Some of those are retired, and some are repaired. The rest, which is a vast majority of wheels, get reprimed, repainted, and sent back out.

What Stripping the Coatings Costs the Wheel

To run penetrant at a major, the coatings have to come off. The fluorescent penetrant inspection (FPI) inspection cannot happen without it, and the wheel pays for it every single time.

The obvious costs are well understood. There is the labor and the time. There is the hazmat, since removed paint and spent blasting media both qualify. There are the emissions from priming and repainting, and the part handling itself, which carries its own injury exposure.

The less discussed cost is what the blasting does to the wheel. Media blasting has to be aggressive to remove all the coating material, and across successive shop visits that aggression gradually works away the anodizing and degrades the surface treatments applied at manufacture. The stress rolling and shot peening that were put there specifically to resist cracking are not permanent, and the strip cycle is one of the things wearing them off. Nothing in the standard workflow inspects the wheel afterward to establish what fatigue and corrosion resistance was lost in the overhaul process.

Then there is the recoat. The OEM coating is applied under better conditions than an MRO paint shop can generally match, and it is the coating the wheel was designed to carry. There is a long-running joke in the industry that captures the incentive problem exactly: how good a coating do you want to apply, knowing your own people are going to have to strip it back off?

Everyone involved would rather leave the OEM coating in place as long as possible. The obstacle has never been the desire. It is that penetrant inspection of the full wheel requires bare metal.

Five Vulnerabilities in the Current Process

Every method in the standard workflow measures the wheel against something outside it. Visual inspection compares the wheel to what the inspector has seen across thousands of others. Penetrant and eddy current compare an indication to a written acceptance standard. All three work, and between them they catch most of what there is to catch.

Where they do not, the reason is usually structural rather than a matter of diligence, and it comes down to two limits. The first is what the methods can physically detect. Penetrant only finds cracks that break the surface, and eddy current only finds defects at the surface or very close to it. A defect below the surface, such as an internal forging lap, is not detectable by either. Those are uncommon, but when one exists the standard workflow has no way to find it.

The second is that an external reference says nothing about this wheel in particular, because the wheel has no history in the comparison. Its paperwork records what was found on earlier visits, but a finding is not a measurement, so there is nothing to measure this visit against.

Five conditions sit in those blind spots.

1. Marginal overheat

A technician can usually spot an overheated wheel on sight, from discoloration or missing fuse plugs, often before reading the paperwork. The confirming test is a hardness check at several points. The vulnerability is the marginal case: the fuse plug did not release and there is no discoloration, so unless something in the incoming paperwork to indicates a potential overtempterature, nothing triggers the hardness test and the test does not happen.

2. Out-of-round

A wheel that has gone slightly out of round is difficult to identify visually, and no inspection in the standard workflow is looking for it. Unless the incoming paperwork indicates a hard landing or some other anomalous event, the wheel does not receive a dimensional check.

3. Scratches that present as cracks

A normally developed crack tends to be about twice as long at the surface as it is deep, and a shallow scratch can produce a similar penetrant indication while carrying very different structural weight. Penetrant indications are real indications; the constraint is what a shop is permitted to do about them. In a repairable area you can blend down and re-run penetrant to establish which one it was. In a non-repairable area there is no way to find out, and the only available disposition is to retire the wheel.

4. Spoke cracks between overhauls

A spoke crack is where many wheels are designed to reach end of life, and it will be found at a major. The question is what happens in the three to five visits before that one, when the wheel gets a visual check and an eddy current inspection of the bead seat and nothing is looking specifically at the spokes.

5. Post-repair verification

Some wheels develop characteristic problems: corrosion in an inflation port, loosening around the bearing cups, wallowing of the holes where torque bars attach. Most have defined repairs, including pressing in a bushing. A press fit has to be firm enough that it does not loosen later, which means it deliberately raises the stress level in that area, and that leaves an open question about whether cracks are developing at the interface between the wheel material and the bushing, somewhere penetrant cannot reach.

What All Five Conditions Have in Common

Different as they are, these five unresolved areas come back to the same thing. In every case the wheel is being assessed against something external to it, and in every case what would actually settle the question is knowing how this particular wheel compares to the way it was before. Nothing in the process described above produces that information. The paperwork records what was found on earlier visits, but a finding is not a measurement, so there is nothing to measure this visit against.

Every wheel in your shop has a history. None of it is measurable. In a growing number of wheel shops that is no longer true, and part two is about the method that changed it.