Aircraft fluid lines—whether they carry hydraulic fluid, fuel, oil, or bleed air—are among the most safety-critical yet easy-to-overlook components on any aircraft. A line that looks serviceable from the outside may be hiding internal fatigue cracks, wall-thinning from corrosion, or a stress concentration from a kink that can rupture under system pressure. The FAA's maintenance handbooks establish clear, measurable criteria for determining when a metal or flexible fluid line must be repaired, rerouted, or replaced. Every Aviation Maintenance Technician (AMT) must internalize these standards—not only to pass the FAA General knowledge test, but to prevent the kind of in-flight fluid loss that can lead to fires, control failures, or engine seizures.
This article walks through each major inspection category—dents, kinks, corrosion, and chafing—defining the acceptance limits, explaining the engineering reason behind each limit, and highlighting the specific situations the FAA knowledge test likes to probe.
Types of Fluid Lines Found on Aircraft
Before diving into defect criteria, it helps to understand the two broad families of lines you will inspect. Rigid (metal) tubing is manufactured from aluminum alloy, corrosion-resistant steel (CRES), or titanium. It is used in fixed runs where vibration is low and where definite support can be provided at regular intervals. Flexible hose (rubber or PTFE-lined, with braided metal or fiber reinforcement) is used where relative motion exists between components—such as between the airframe and an engine mount—or where vibration damping is needed. The defect criteria differ somewhat between the two, but the underlying principle is the same: the line must maintain structural integrity and full flow area under all operating conditions.
Dent Criteria
A dent is a smooth depression in the wall of a tube caused by contact with another object, an improper bending technique, or mishandling during installation. The key word is smooth—a dent without sharp edges does not necessarily create a stress riser the way a crease or kink does. According to FAA inspection standards (FAA-H-8083-31, Chapter 15), a dent in a metal tube is generally acceptable if its depth does not exceed 20 percent of the tube's outside diameter. If a dent exceeds that limit, the section must be replaced. Additionally, dents are not acceptable in any location that falls within a bend—curved sections are already stressed during the bending process, and adding a dent creates an unacceptable stress concentration that could initiate a fatigue crack.
Why the 20 percent rule? When a tube wall is pushed inward, it reduces the cross-sectional flow area and introduces localized stress in the material. At 20 percent of the outside diameter, engineering analysis shows that the remaining wall structure retains adequate strength for normal pressure cycles. Beyond that threshold, fatigue life drops sharply. The inspector should use a small ruler or depth gauge across the mouth of the dent to measure depth accurately—eye estimates are unreliable on small-diameter tubing.
Kink Criteria
A kink is fundamentally different from a dent. Where a dent is a localized smooth depression, a kink is a sharp bend or fold in the tube wall that has caused the metal to buckle. Kinks are not acceptable in aircraft fluid lines and require replacement of the affected section. The reason is straightforward: the buckling process work-hardens the metal unevenly, collapses the inner wall partially, and creates a geometric stress concentration that no amount of external reshaping can remove. Even if you could restore the tube's approximate round cross-section, the internal grain structure is permanently compromised. Always follow the specific aircraft maintenance manual, as it takes precedence over general guidance for any particular installation.
Kinks most often occur when tubing is bent without proper tooling (a tube bender), when a line is forced into place to connect fittings that are slightly misaligned, or when a line is inadvertently bent by technicians stepping on or leaning against it. Prevention is the best policy: always use the correct-size tube bender, never spring a line more than a few degrees to align it with a fitting, and protect open lines during maintenance with caps or plugs to make them visible.
Corrosion Criteria
Corrosion on fluid lines takes several forms depending on material and environment. On aluminum alloy tubing, oxidation appears as a white or gray powdery film; on steel tubing, it appears as reddish-brown rust. Surface corrosion—oxidation confined to the outer surface with no pitting—can sometimes be cleaned and the line returned to service if the cleaned area shows no pitting and the wall thickness is confirmed to be within limits. However, intergranular or pitting corrosion, which penetrates below the surface, is cause for rejection and replacement.
The FAA standard is based on wall-thickness integrity. Any corrosion that has caused pitting or that has visibly thinned the tube wall to the point where remaining wall thickness cannot be confirmed by visual or non-destructive means requires replacement. There is no single FAA-wide numeric percentage for how much wall thinning is allowable—always consult the aircraft manufacturer's maintenance manual or the applicable FAA advisory circular for specific limits, because some lines carry higher internal pressures and have tighter tolerances.
Corrosion under clamps and at fittings is particularly insidious because it is hidden. Best practice is to remove clamps periodically during scheduled maintenance, inspect the tube underneath, and reinstall with a protective sleeve or cushioning clamp of the correct material to prevent galvanic or crevice corrosion from restarting.
Chafing Criteria
Chafing results from repeated rubbing of a line against an adjacent structure, wire bundle, or another line. It is one of the most common fluid-line defects found during inspections because vibration is constant in an operating aircraft and even a small amount of clearance between a line and its neighbors can be consumed over time. Chafing that has worn through the protective coating (paint or anodizing) alone and has not reached bare metal may be dressed, re-coated, and returned to service with a corrected support interval. However, chafing that has cut into the base metal of a rigid tube—or that has worn through the outer braid or reinforcement layer of a flexible hose—requires replacement.
For flexible hose, any chafing that exposes the inner liner or causes the braid wires to be cut is an automatic rejection. The braid is the primary structural reinforcement that contains system pressure; once it is compromised, the hose can balloon and burst under pressure spikes. For rigid tubing, the concern is wall thinning in an elongated area that, unlike a dent, runs along the tube axis where bending stress is highest during vibration.
Correcting the root cause of chafing is equally important. Simply replacing the line without adding a proper clamp, chafe guard, or increasing clamp support intervals will result in the same damage recurring. FAA guidance (AC 43.13-1B, Chapter 7) provides tube support and clamp spacing tables keyed to tube diameter, with larger-diameter lines generally requiring wider spacing than smaller ones—the specific aircraft maintenance manual and applicable spacing tables take precedence over any generalized figure.
Why It Matters: In-Flight Consequences
Fluid line failures are rarely gradual—they often progress from a small leak to a catastrophic rupture faster than a crew can respond. A hydraulic line failure can eliminate flight control authority or braking ability. A fuel line failure introduces flammable fluid into hot areas of the engine compartment. An oil line failure leads to rapid engine oil loss, overheating, and seizure. Recognizing and correcting marginal tubing during scheduled maintenance is the only reliable defense. The FAA's defect criteria exist precisely because in-service experience and materials engineering have shown where the failure probability rises to an unacceptable level.
Key Numbers and Rules
- Dents: Acceptable if depth is 20% or less of the tube's outside diameter, and the dent is not located in a bend.
- Kinks: Not acceptable—replace the kinked section per FAA and manufacturer guidance.
- Corrosion: Surface oxidation without pitting may be cleaned; pitting that penetrates the wall or cannot be confirmed within limits requires replacement. There is no single FAA-wide percentage threshold—consult the applicable maintenance manual or advisory circular.
- Chafing: Acceptable only if damage is limited to protective coating; any base-metal penetration on rigid tubing or braid damage on flexible hose requires replacement.
- Flexible hose: Any crack, bulge, soft spot, or braid exposure is cause for replacement regardless of apparent leak history.
- Clamp intervals: Follow manufacturer's maintenance manual and AC 43.13-1B support spacing tables, which vary by tube diameter.
Common Test Traps
- Dent location matters. A dent within a bend is rejectable even if its depth is less than 20 percent of the tube OD. The test may describe a dent that seems within limits but is located in the curved section—the correct answer is replace, not accept.
- Kinks versus dents. The FAA test may describe a tube with a sharp bend and ask whether it can be straightened and returned to service. The expected answer is no—kinking permanently damages the metal and the line must be replaced.
- Surface versus pitting corrosion. Surface oxidation without pitting may be cleaned and re-inspected. Pitting corrosion is a rejection criterion. Know the difference and do not confuse cleaning with structural repair.
- Flexible hose braid damage. A question may describe a hose with external braid wires that are frayed or cut but no visible leakage. Because the braid is the pressure-containment reinforcement, this hose must be replaced regardless of whether it is currently leaking.
- Chafing and root cause. Simply replacing a chafed line without fixing the source of chafing (adding clamps, guards, or rerouting) is incomplete maintenance. The FAA expects you to address the cause, not just the symptom.