Fluid Line Flaring for the A&P Practical: Single and Double Flares

October 20, 2026 · Primary sources checked October 19, 2026

The Airframe and Powerplant (A&P) oral and practical (O&P) test includes hands-on work. This guide to A&P practical flaring follows the job from a square, deburred tube end to a correctly formed and aligned connection. Practice the workmanship with an instructor and the specified equipment; use the applicable manufacturer’s data for the actual installation.

What does the O&P practical evaluate?

The Federal Aviation Administration (FAA) Airman Certification Standards (ACS) identify the skills below. A Designated Mechanic Examiner (DME) evaluates the assigned project against its stated performance standard, including your selection of appropriate tools and data. Each project has a skill element and two practical questions. The project is pass/fail: the skill demonstration must succeed on the first attempt, and both practical questions must be answered successfully. These study topics do not promise a particular test assignment.

FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, para. 5-5-11-7C3), p. 16

FAA-S-ACS-1, printed 6, PDF 11

Follow the applicable manufacturer’s instructions and approved data for the assigned work. The ACS safety appendix calls for current maintenance publications, safe tool handling, equipment preparation, safety glasses, hearing protection, and any other required personal protective equipment (PPE). Disregard for safety will result in failure. If a project action would jeopardize safety, the evaluator will ask you to simulate that portion.

FAA-S-ACS-1, printed A-3, PDF 67

How do I prepare the tube and bend?

Choose the specified tube material, size, fittings, and tooling before cutting. A square end and careful deburring matter: inside and outside burrs must be removed without thinning or fracturing the tube wall. Remove the filings. Select the bender that matches the tube diameter and make a smooth bend without excessive flattening, kinks, or wrinkles.

1. Cut the tube square with the appropriate cutter or saw; excessive cutter pressure can deform the end.
2. Deburr inside and outside, smooth the end, and remove the filings without damaging the wall.
3. Lay out the bend with the selected tool’s instructions and pull steadily to the required angle. Inspect the bend before flaring.

FAA-H-8083-30B, General, printed 9-2, PDF 355 · FAA-H-8083-30B, General, printed 9-3, PDF 356

How much flattening is too much?

The general bend-inspection criterion is a minor diameter of at least 75% of the original outside diameter. Tubing with flattened, wrinkled, or irregular bends should not be installed; excessive flattening causes fatigue failure. Illustration only: for a 3/8-inch tube, 0.75 × 0.375 = 0.28125 inch, or 9/32 inch. That calculation explains the handbook criterion; it does not replace the assigned aircraft’s limits or authorize a damaged bend.

FAA-H-8083-30B, General, printed 9-3, PDF 356

How do I form a single flare?

Put the coupling nut and sleeve on the tube before forming the flare. Use an aircraft flaring tool; aircraft tube flaring dies are made with 35° to 37° flaring angles, and a 45° automotive tool must not be used. The FAA handbook limits rolling-type flaring tools to soft copper, aluminum, and brass tubing; do not use them with corrosion-resistant steel or titanium. Rolling-type tools differ slightly in procedure, so always read the tool manufacturer’s instructions.

1. For a compatible tube and tool, clamp the prepared end squarely with the projection specified by the tool instructions.
2. Use the tool’s forming sequence. For the self-gauging rolling-type tool described in the FAA handbook, the tube is clamped flush with the top of the die block; that is not a setting for every tool.
3. Remove the tube and inspect the flare and surrounding tubing for cracks, nicks, dents, scratches, and distortion.

FAA-H-8083-30B, General, printed 9-4, PDF 357 · FAA-H-8083-30B, General, printed 9-5, PDF 358

How does double flaring change the operation?

The FAA handbook says a double flare is used on soft aluminum-alloy tubing 3/8 inch outside diameter and under, where it is necessary to prevent cutting off the flare and failure of the tube assembly under operating pressures. A double flare is also more resistant to the shearing effect of torque. The handbook steps below are for that tubing and the double-flaring tool it describes, not an instruction to double-flare every aircraft line. Follow the specified material preparation and the tool instructions; do not improvise heat treatment.

1. For the double-flaring tool described, set the tube projection to the thickness of the adapter shoulder.
2. Use the adapter to form the initial bell, following that tool’s instructions.
3. Remove the adapter and form the folded flare with the cone. Do not overtighten and thin the wall. Inspect the completed flare before assembly.

FAA-H-8083-30B, General, printed 9-5, PDF 358

How do I inspect the flare and tighten the B-nut?

Reject a cracked, split, rough, scratched, or out-of-round flare rather than trying to seal it with extra torque. Clean the connection and align the tube before tightening the coupling nut, often called the B-nut. Do not pull the tube into alignment by tightening the nut. Keep compound off the flare and fitting faces: the seal depends on metal-to-metal contact. Use the prescribed torque for the particular fitting, tube material, and application; a flared-fitting table is not a flareless-fitting procedure.

1. Inspect the flare, sleeve, nut, and mating fitting before assembly.
2. Check alignment without using the nut to draw the line into place.
3. Apply the specified fitting torque with the appropriate wrench arrangement. Inspect the completed connection using the applicable procedure.

FAA-H-8083-30B, General, printed 9-4, PDF 357 · FAA-H-8083-30B, General, printed 9-10, PDF 363 · FAA-H-8083-30B, General, printed 9-12, PDF 365 · FAA-H-8083-30B, General, printed 9-14, PDF 367

Which mistakes should I catch before completion?

An out-of-square cut can produce a crooked flare. A flare that is too small may leak or pull apart; one that is too large can interfere with thread engagement. Excessive fitting torque can cut off the flare or ruin the sleeve and nut, while insufficient tightening can allow leakage or line separation. Diagnose the preparation, shape, alignment, and specified torque instead of treating every leak as a request for more force.

FAA-H-8083-30B, General, printed 9-4, PDF 357 · FAA-H-8083-30B, General, printed 9-12, PDF 365

How should I rehearse before test day?

Our study suggestion: work through the assigned task with an instructor, name the applicable source before selecting tools or values, and inspect the finished work against that source. Keep a note of the step you need to repeat in supervised practice. Rehearse the explanation separately from the physical task.

For examiner fees and planning, read the DME exam cost guide.

Related guides

• Fluid Lines and Fittings Oral Exam Questions and Answers
• Aircraft Hardware and Materials Oral Exam Questions and Answers
• A&P General Oral Questions: The Complete Breakdown

Sources

FAA sources checked October 19, 2026.

• FAA-S-ACS-1
• FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, Administer an Oral and a Practical Test (CHG 929, 9/17/2024), para. 5-5-11-7C3), p. 16
• FAA-H-8083-30B, General

About this guide: drafted with AI from the FAA sources listed on this page and checked claim by claim against them. Not reviewed by an A&P mechanic or DME. How we make our guides