Are You Flying Too High? Rethinking Legacy High-Frequency Eddy Current Procedures in Aviation

In the aviation industry, tradition can be both a strength and a blind spot.

Eddy current inspection procedures passed down from earlier decades—especially in airframe maintenance and aerospace component testing—sometimes specify fixed test frequencies like 2 MHz or even 6 MHz. These values often trace back to the limitations of older instruments, where high-frequency options were hard-wired into the system: You had 500 kHz, 2 MHz, and 6 MHz—take your pick.

But here’s the question: Are those legacy frequencies still the best choice today?

Historical Context Isn’t Always Technical Justification

Back then, the equipment defined the procedure. Today, we have instruments with fine frequency resolution, phase angle tuning, and multi-frequency capability. That means we should be picking test frequencies based on:

• Flaw size and orientation

• Material conductivity and permeability

• Wall thickness and geometry

• Lift-off conditions and surface finish

• Noise floor vs. signal clarity

Not based on what the tester could do in 1985.

The Physics Problem with Very High Frequencies

At frequencies like 6 MHz, you run into practical limitations:

• Shallow penetration: You’re inspecting only the top skin of the material. Anything subsurface may be missed entirely.

• Increased noise: Surface roughness, paint thickness, even slight probe wobble can become dominant signal components.

• Poor flaw separation: Phase resolution between flaw signals and liftoff effects often decreases.

So, while high frequencies can be useful for detecting very shallow cracks or conductive coatings, they also come with a steep trade-off in signal stability and interpretation.

Quote to Consider: “The maximum signal from the defect does not always give the maximum signal-to-noise ratio.”

— Dodd, Deeds & Spoeri, 1971

This is the heart of the matter. Bigger signal ≠ better result. If you're seeing more noise than usable signal, your inspection reliability may be compromised—even if you’re following the procedure to the letter.

Time for a Tuning Mindset

Modern eddy current analysts should feel empowered to evaluate legacy procedures, not just follow them blindly. Some may still be valid. Others may be ripe for optimization using:

• Multifrequency inspections

• Lift-off compensation techniques

• Lower, more stable frequencies (e.g., 100–500 kHz)

• Improved phase-sensitive instrumentation.

A Note to the Aviation World

This isn’t an attack—it’s an invitation. If you’re using a legacy 2 or 6 MHz frequency, ask:

• Was it chosen based on flaw physics or equipment limitation?

• Does it offer the best signal-to-noise today?

• Have you tried tuning down to see if flaw visibility or phase separation improves?

Let's Talk About It

This post comes in the spirit of continuous improvement. As NDT professionals, we carry the responsibility to challenge what no longer serves—especially when better tools and understanding exist.

Have you had success or struggles with high-frequency EC in aerospace? Drop your experience in the comments. Let’s keep the signal strong and the noise low—both in our inspections and our discussions.

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