What is the difference between a flared and a flareless fitting?

A flared fitting requires you to form a cone on the end of the tube; that cone is clamped between the fitting's seat and a sleeve, and the flared face itself is the seal. A flareless fitting leaves the tube end square. A separate sleeve or ferrule bites into the tube wall as the nut is tightened, sealing against both the tube and the fitting body.

Why does bend radius matter when fabricating a rigid fluid line?

A bend that is too tight flattens or kinks the tube. That reduces the cross-sectional area and restricts flow, and it leaves a stress concentration where fatigue cracks start under vibration and pressure cycling. Keeping the radius as large as practical also gives the line the flexibility it needs to absorb vibration and thermal expansion instead of transmitting the load into the fittings.

How is aircraft rigid tubing sized?

By outside diameter in sixteenths of an inch, paired with a wall thickness. A dash eight line is eight sixteenths, or one half inch, on the outside, and the wall thickness is called out separately because two tubes of the same outside diameter can have very different pressure ratings. Fitting sizes use the same sixteenths convention, which is why a fitting dash number matches the tube it serves.

Can I substitute a different tubing material or size for a replacement line?

Not on your own judgement. A replacement line has to match the material, diameter and wall thickness called out in the manufacturer's maintenance data, because alloy and wall thickness set the pressure rating of the installation. Any deviation has to be supported by approved data, and the work and the parts used are recorded in the maintenance record required by 14 CFR Part 43.

Fluid Lines and Fittings — FAA A&P Test Questions (ACS AM.I.D)

Fluid Lines and Fittings is a compact General subject area built almost entirely around shop practice. You are asked which flare angle belongs on an AN fitting, where a flared connection actually seals, what a flareless sleeve does, why bend radius matters when you fabricate a replacement line, what a lay line on a flexible hose is for, and what the color bands and symbols on an installed line are telling you. Almost every question maps to a step you would perform at the bench, so the fastest way through the area is to walk the fabrication sequence in your head.

What ACS AM.I.D covers

This area follows the fluid lines and fittings chapter of FAA-H-8083-30 and divides into rigid tubing, flexible hose, fittings, and identification. Rigid tubing questions cover material selection and the markings that identify a tube's alloy, sizing by outside diameter in sixteenths of an inch together with wall thickness, and the fabrication sequence: measure, cut square with a tube cutter, deburr the ends inside and out, bend to as large a radius as practical, then flare. Expect stems that pick one step out of that sequence and ask what comes next, or ask what a too-tight bend does to the tube. Fitting questions center on the two families you will meet on an aircraft. The AN flared fitting seals where the flared face of the tube is clamped against the cone seat of the fitting, with the sleeve and nut supplying the force, and uses a 37-degree flare. The flareless or MS fitting has no flare at all; a separate sleeve bites into the tube as the nut is drawn up and seals against the fitting. Flexible hose questions cover the lay line printed along the hose, installation slack, minimum bend radius, support and clamping, and the fire sleeve. Identification closes the area: the color-coded bands, geometric symbols and wording that tell you which system a line belongs to and what it carries, which matters because a line that returns to service must still match approved data under 14 CFR Part 43.

Where this sits on the test

ACS AM.I.D is tested on the FAA General written test, one of 1,558 ACS-tagged questions in the General bank. Every question tagged to this area carries a worked rationale and its FAA handbook reference, so you can drill the code itself rather than the whole test.

FAA handbook references

FAA-H-8083-30

Three traps candidates fall into

AN flared fittings use a 37-degree flare, while the 45-degree flare is automotive. The two look alike and will start on the same threads, so a 45-degree flare in an AN fitting bears on a line contact and then leaks or cracks in service. When a stem gives you a flare angle, that number is usually the entire question.
Candidates say the nut makes the seal. It does not. On a flared connection the seal forms where the flared face of the tube meets the cone seat of the fitting, and the sleeve and nut only provide clamping force. On a flareless connection the sleeve bites into the tube and seals against the fitting body.
Flexible hose is installed with slack rather than pulled taut, because hose changes length under pressure. Candidates also install hose with the printed lay line spiralling around it, which is the visible sign that the hose has been twisted during installation; the stripe must run straight along the hose once it is in place.

7 free sample questions from ACS AM.I.D

AM.I.DTap an answer
What color marking on aircraft tubing identifies it as fabricated from aluminum alloy that should be used for that application?
AM.I.DTap an answer
When fabricating a replacement rigid fluid line, the bend radius should be kept as large as practical primarily to prevent which condition?
AM.I.DTap an answer
A flared tube fitting forms its fluid-tight seal at which location?
AM.I.DTap an answer
Which flare angle is standard for AN (Army-Navy) flared tube fittings used in aircraft fluid lines?
AM.I.DTap an answer
After cutting a length of metal tubing with a tube cutter, the next step before flaring should be to
AM.I.DTap an answer
A flareless (MS) tube fitting depends on which component to bite into the tube and form the seal?
AM.I.DTap an answer
A flexible hose marked with a lay line (a stripe running along its length) is checked at installation to confirm the hose is not

What color marking on aircraft tubing identifies it as fabricated from aluminum alloy that should be used for that application?

ACS code: AM.I.D

Correct answer: A stamped code and color band identifying the alloy

Rationale: Aircraft metal tubing is identified by markings stamped or coded on the tube indicating the material and, for aluminum alloy tubing, color bands or codes that identify the specific alloy (such as 1100, 3003, 5052, or 6061). A mechanic must verify the correct material before installation because each alloy has different strength and corrosion characteristics. There is no convention of a single full-length red stripe or a generic yellow end band serving as the material identifier.

When fabricating a replacement rigid fluid line, the bend radius should be kept as large as practical primarily to prevent which condition?

ACS code: AM.I.D

Correct answer: Flattening or wrinkling of the tube wall

Rationale: Bends in metal tubing must be made with as large a radius as practical, and tube benders are used so the tube is not flattened, wrinkled, or kinked. A flattened or wrinkled bend restricts flow and creates a stress concentration that can lead to cracking. Bend radius does not control fitting-nut tension, and the weight difference between a tight and generous bend is negligible.

A flared tube fitting forms its fluid-tight seal at which location?

ACS code: AM.I.D

Correct answer: Between the flare cone and the fitting seat

Rationale: In an AN/MS flared fitting, the tube end is flared to a cone, and the seal is made where that flared cone is compressed against the mating cone-shaped seat of the fitting by the sleeve and coupling nut. The coupling-nut threads only provide the clamping force; they do not seal. The sleeve backs up the flare but does not itself form the fluid-tight joint.

Which flare angle is standard for AN (Army-Navy) flared tube fittings used in aircraft fluid lines?

ACS code: AM.I.D

Correct answer: 37 degrees

Rationale: AN aircraft flared fittings use a 37-degree flare, whereas automotive and some plumbing fittings use a 45-degree flare. Mixing a 37-degree tube flare with a 45-degree fitting (or vice versa) produces a poor seal and can fail under pressure, so the mechanic must match the flare angle to the fitting. The 30-degree value is not a standard aircraft flare angle.

After cutting a length of metal tubing with a tube cutter, the next step before flaring should be to

ACS code: AM.I.D

Correct answer: remove burrs from the inside and outside of the tube

Rationale: After the tube is cut, the ends must be deburred (reamed) inside and out so the metal is smooth before flaring. Burrs left on the tube can cause a defective flare, restrict flow, or break loose and contaminate the system. Annealing every cut end is not required, and applying anti-seize before flaring would contaminate the seating surface.

A flareless (MS) tube fitting depends on which component to bite into the tube and form the seal?

ACS code: AM.I.D

Correct answer: A ferrule or sleeve that cuts into the tube

Rationale: Flareless fittings use a ferrule (sleeve) that, as the nut is tightened, bites into the outer surface of the tube to grip it and form the seal—no flare is made on the tube. This is why flareless tubing must not be over-tightened, which can cut the tube. By definition a flareless joint has no flared cone, and the seal is metal-to-metal through the ferrule rather than an O-ring in the nut.

A flexible hose marked with a lay line (a stripe running along its length) is checked at installation to confirm the hose is not

ACS code: AM.I.D

Correct answer: twisted

Rationale: The lay line is a stripe printed along the length of a flexible hose; if it spirals after installation, the hose has been twisted, which weakens it and can cause early failure. The mechanic installs the hose so the lay line stays straight. The lay line indicates twist, not length or service life—hose age and cure date are read from separate stamped markings.

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Frequently asked questions

What is the difference between a flared and a flareless fitting?: A flared fitting requires you to form a cone on the end of the tube; that cone is clamped between the fitting's seat and a sleeve, and the flared face itself is the seal. A flareless fitting leaves the tube end square. A separate sleeve or ferrule bites into the tube wall as the nut is tightened, sealing against both the tube and the fitting body.
Why does bend radius matter when fabricating a rigid fluid line?: A bend that is too tight flattens or kinks the tube. That reduces the cross-sectional area and restricts flow, and it leaves a stress concentration where fatigue cracks start under vibration and pressure cycling. Keeping the radius as large as practical also gives the line the flexibility it needs to absorb vibration and thermal expansion instead of transmitting the load into the fittings.
How is aircraft rigid tubing sized?: By outside diameter in sixteenths of an inch, paired with a wall thickness. A dash eight line is eight sixteenths, or one half inch, on the outside, and the wall thickness is called out separately because two tubes of the same outside diameter can have very different pressure ratings. Fitting sizes use the same sixteenths convention, which is why a fitting dash number matches the tube it serves.
Can I substitute a different tubing material or size for a replacement line?: Not on your own judgement. A replacement line has to match the material, diameter and wall thickness called out in the manufacturer's maintenance data, because alloy and wall thickness set the pressure rating of the installation. Any deviation has to be supported by approved data, and the work and the parts used are recorded in the maintenance record required by 14 CFR Part 43.

All 40 ACS areas

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