Every fluid system in an aircraft—whether it carries hydraulic fluid, fuel, oil, or compressed air—depends on connections that are perfectly leak-free under vibration, pressure cycling, and temperature extremes. Those connections begin long before the fitting is torqued down. The quality of the cut, the thoroughness of deburring, and the cleanliness of the tube end determine whether a fluid line will seal reliably for thousands of flight hours or fail dangerously on the ramp. Aviation Maintenance Technicians (AMTs) must master these foundational steps, because even a small burr or metal chip introduced at assembly can erode a valve seat, score a cylinder, or ignite a fuel leak.
This article covers the complete process of fluid line end preparation for rigid metal tubing and flexible hose: the correct cutting methods for each material, the deburring techniques that eliminate sharp edges without weakening the tube wall, and the cleaning and inspection procedures required before any fitting is installed. Each step has specific FAA-accepted standards that are directly testable on the AMT General knowledge exam.
Materials and Their Characteristics
Aircraft fluid lines are made from several different materials, and the correct preparation technique depends on the material involved. Aluminum alloy tubing (such as 1100, 3003, 5052, and 6061) is widely used for low- to medium-pressure fuel, oil, and instrument lines. Steel tubing (corrosion-resistant steel, or CRES, and carbon steel) handles high-pressure hydraulic and some fuel applications. Copper tubing was used in older aircraft, but it tends to work-harden and crack under vibration, so it has largely been replaced by other materials in modern aircraft fuel and hydraulic systems. Flexible hose—either rubber-lined or PTFE (polytetrafluoroethylene) with metal braid—appears where rigid lines cannot be used because of vibration or movement.
Each material has a specific wall hardness, ductility, and sensitivity to heat. Cutting methods that are appropriate for soft aluminum may damage harder steel or brittle CRES. Understanding the material you are working with is step one of every tube preparation job.
Cutting Rigid Tubing
The goal of cutting is to produce a square, clean end with no distortion of the tube's circular cross-section. A non-square cut means uneven contact between the tube end and the fitting sleeve or flare, which virtually guarantees a leak.
Tube Cutters
A wheel-type tube cutter is the preferred tool for aluminum, steel, and CRES tubing in most aircraft applications. The cutting wheel is advanced gradually against the tube with light pressure while the tool is rotated around the tube circumference. Forcing the cutting wheel in too quickly is one of the most common errors—it deforms the tube end inward, creating a raised internal ridge that cannot be fully removed and may restrict flow or trap debris. The cut should be completed with multiple light passes, advancing the wheel gradually with light pressure to score deeper with each revolution rather than forcing it.
For larger-diameter or harder tubing, a fine-tooth hacksaw may be used, though a wheel-type tube cutter is generally the preferred tool for rigid aircraft tubing. Hacksaw cuts leave a rougher edge than a wheel cutter and always require more deburring attention. Regardless of the cutting tool, the tube must be held firmly—in a vise with protective jaw covers to avoid scratching or crushing—to prevent it from rotating or flexing during the cut.
Flexible Hose Cutting
Flexible hose requires a different approach. Rubber hose and PTFE hose with wire braid are typically cut with a sharp, fine-tooth saw or a specialized hose-cutting tool. Before cutting rubber hose, wrapping the cut area tightly with masking tape helps keep the braid intact and produces a cleaner, square end. After cutting, the tape is removed. Power saws or abrasive cutoff wheels must not be used on rubber hose because they generate heat that melts or degrades the inner liner and can drive rubber particles deep into the hose assembly.
Deburring
Every cutting method—wheel cutter, hacksaw, or saw—leaves some form of burr or sharp edge at the tube end. On the outside of the tube, burrs can prevent a flare or sleeve from seating correctly. On the inside, burrs are even more dangerous: they can break free during system operation and travel downstream, contaminating filters, valves, and actuators. A single metal chip in a hydraulic system can destroy a precision pump or lock a flight-critical control valve.
Deburring Tools and Technique
The standard deburring tool for tubing is a hand deburring tool with a rotating blade designed to remove material from both the inside and outside of the tube end simultaneously, or separate inside/outside cutters used in sequence. For small-diameter tubing, a tapered reamer, a fine file, or a piece of fine-grit abrasive cloth wrapped around a rod can be used to remove the internal burr.
The critical rule is remove the burr—nothing more. Over-aggressive deburring thins the tube wall at the end, reducing the amount of material available to form a proper flare. If the tube wall is thinned too much, the flare will crack during forming or fail under pressure. The technician should work slowly, checking frequently, until the end is smooth and square but the wall thickness is unchanged from the rest of the tube. After deburring, run a clean finger around the inside and outside of the tube end—any remaining sharpness indicates more work is needed.
Checking for Squareness
After cutting and deburring, verify that the tube end is square to the tube centerline. Place the tube end against a flat surface or use a small machinist's square. A cut that is more than a few degrees out of square must be corrected by re-cutting or careful filing before the tube is flared or fitted with a sleeve.
Cleaning the Tube End and Interior
Even a perfectly cut and deburred tube end is unsafe for assembly if it contains metal chips, cutting fluid, grease, or other contamination. The cleaning step is not optional—it is a required part of fluid line preparation.
After deburring, blow out the tube interior with dry, filtered compressed air, directing the airflow from the prepared end toward the opposite end. This removes loose chips and dust. Next, clean the tube interior with a lint-free cloth or swab moistened with an approved solvent appropriate for the system fluid (for example, isopropyl alcohol or a petroleum-based solvent). The solvent must be compatible with the system—never use a chlorinated solvent in an oxygen line, as residue can create a fire or explosion hazard in the presence of high-pressure oxygen.
For critical or high-pressure hydraulic systems, the cleaned tube may be flushed with the actual system fluid (hydraulic fluid) immediately before installation to remove any residual solvent and pre-wet the interior surface. After cleaning, the tube ends should be capped with clean plastic caps or plugs until the tube is ready to be installed. Leaving prepared tubes uncapped, even for a short time in a shop environment, allows airborne dust, metal filings, and moisture to re-contaminate the interior.
Inspection Before Assembly
Before installing any fitting, perform a final inspection of the tube end. Check for: cracks at or near the cut (a sign of excessive force during cutting or a work-hardened material); ovality (the end should be round, not egg-shaped); wall thinning from over-deburring; and any remaining burrs or sharp edges. Aluminum tubing with visible surface scratches, nicks, or corrosion pits within one tube diameter of the cut end should be re-cut at a fresh location if material length allows, or the tube replaced entirely. A defective tube end that is flared will produce an imperfect flare that is virtually certain to leak.
Key Numbers and Rules
- Cutting pressure: Advance the wheel-type cutter gradually with light pressure—never force the wheel to avoid deforming the tube end.
- Wall integrity: Deburring must not reduce the tube wall thickness below the minimum specified for that tube size and material; when in doubt, re-cut from fresh tubing.
- Squareness tolerance: The cut end should be square within a few degrees; a severely angled cut requires re-cutting before flaring.
- Cap all openings: Install clean plastic end caps immediately after preparation and do not remove them until the fitting is ready to be slipped onto the tube.
- Solvent compatibility: Never use chlorinated solvents in oxygen lines; use only approved solvents for the specific system fluid involved.
- Inspection requirement: All cuts, deburrs, and cleaning steps must be completed before any flaring, beading, or sleeve installation begins.
Common Test Traps
- Forcing the cutter wheel: Many questions describe a technician who advances the wheel too quickly. The correct answer is always that this deforms the tube end inward and must be avoided—even if the cut appears complete.
- Deburring replaces cleaning: Some distractors imply that deburring alone is sufficient preparation. Cleaning is always a separate, required step after deburring.
- Wrong solvent for oxygen lines: The FAA specifically tests the prohibition on chlorinated solvents in oxygen systems. The correct answer is that only approved, compatible solvents may be used.
- Leaving tube ends uncapped: Test questions may ask what happens if prepared tube ends are left uncapped. The answer is contamination that can damage downstream components.
- Over-deburring: Students sometimes think more deburring is always better. The FAA position is that excessive deburring weakens the tube wall and makes a safe flare impossible—remove only the burr, not the base material.