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.

How is aluminum alloy tubing identified so the mechanic can confirm that the correct alloy is being used for an application?

A stamped code and color band identifying the alloy

A band of yellow paint near each end of the tube

A red stripe running the full length of the tube

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

Flattening or wrinkling of the tube wall

Excessive weight of the finished line

Loosening of the installed fitting nuts

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

Between the flare cone and the fitting seat

On the threads of the coupling nut

Against the sleeve's outer shoulder

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

remove burrs from the inside and outside of the tube

anneal the cut end with a torch flame

coat the cut end with anti-seize compound

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

A ferrule or sleeve that cuts into the tube

A rubber O-ring seated in the nut

A flared cone formed on the tube end

On a fluid line that carries a hazardous or specific medium, the identifying decals or tape primarily tell the mechanic the

contents and function of the line

torque value for the end fittings

Which practice is correct when routing and supporting a fluid line near a moving part or another line?

Support the line with clamps to prevent chafing

Allow the line to rest directly against the structure

Tighten the line so it cannot move at all

In aircraft hydraulic systems, a flared-tube connection using a two-piece sleeve and nut is used in place of a one-piece fitting because it eliminates

the possibility of thinning the flare by wiping or ironing during tightening.

all wrench damage to the tubing during tightening.

the need to flare the tubing prior to assembly.

What does the dash number stamped on aluminum tubing fittings and on the size of rigid tubing represent?

The outside diameter in sixteenths of an inch

The inside diameter in eighths of an inch

The wall thickness in thousandths of an inch

What is the primary purpose of forming a tube bend or a loop in a rigid fluid line near a fitting?

To absorb vibration and allow for expansion and contraction

To reduce the internal fluid pressure in the line

To increase the flow velocity through the line

When a rigid tube has been flattened, dented, or kinked during bending, what is the correct repair action?

Discard the bend and replace the damaged section of tubing

Heat the kink and tap it round with a soft mallet

Install a larger sleeve to reinforce the flattened area

What is the maximum allowable flattening of a tube bend, expressed in terms of the original outside diameter of the tube?

Flattening that reduces the diameter to less than 75 percent of the original OD is not acceptable

Flattening that reduces the diameter to less than 90 percent of the original OD is not acceptable

Flattening up to 50 percent of the original OD is acceptable

A scratch or nick in aluminum alloy tubing is generally acceptable for continued service if it does not exceed what depth of the tube wall thickness?

10 percent of the wall thickness

50 percent of the wall thickness

25 percent of the wall thickness

What is the purpose of a flareless (MS) fitting compared to a flared fitting?

It seals by a sleeve biting into the tube, eliminating the need to flare the tubing

It requires a 45-degree flare instead of a 37-degree flare

It is used only on low-pressure instrument lines

When presetting (preforming) the sleeve on a flareless fitting, how is the tube end prepared before assembly?

Cut square, deburred, and bottomed firmly in the fitting

Threaded externally to engage the fitting nut

Flared to 37 degrees before the sleeve is added

In addition to color codes, what is also placed on aircraft fluid line identification tape to aid mechanics who may be color blind?

Geometric symbols and the name of the system

The manufacturer's serial number only

The maximum operating temperature only

When installing flexible hose, why should it be installed with a slight slack (about 5 to 8 percent of the total length)?

Because hose shortens in length and expands in diameter when pressurized

To increase the flow rate through the hose

Because hose lengthens when pressurized and would buckle without slack

How should flexible hose be installed with respect to twisting?

It must be installed without twisting, indicated by a straight lay line

It should be twisted one-quarter turn to preload the fittings

It should be twisted to follow the natural curve of the routing

The lay line (a stenciled stripe running lengthwise) printed on a flexible hose is used primarily to determine what?

Whether the hose has been twisted during installation

The flow direction through the hose

The cure date of the rubber compound

What is the function of the support clamps used along a run of rigid tubing?

To secure the line to the structure and prevent vibration and chafing

To increase the pressure rating of the tubing

To provide an electrical ground path for the system

When tightening a flared tube nut on an AN fitting, what is the result of overtightening?

It can split or crack the flare and cause leakage

It causes the sleeve to expand and seal more tightly

It is required to seat the flare and is recommended

When making up a flared-tube connection, what is the purpose of using a backup wrench on the fitting while tightening the tube nut?

To prevent the fitting body from turning and twisting the tube

To allow the flare to spin freely as it seats

To double the torque applied to the nut

What thread sealant or lubricant practice applies to tapered pipe-thread (NPT) fittings used in fluid systems?

Apply approved sealant to the male threads but keep it off the first two threads

Never use any sealant on tapered pipe threads

Apply sealant generously to all threads including the first two

Aluminum alloy 1100 (1/2 hard) or 3003 tubing is generally used for fluid lines that carry fluid under what condition?

Relatively low pressure, such as instrument and vent lines

Only fuel under high pressure to the engine

The highest hydraulic pressures in the aircraft

After cutting rigid tubing with a tube cutter or hacksaw, what step is essential before flaring the tube?

Deburr the inside and outside of the tube end

Anneal the entire length of the tube

Coat the tube end with thread sealant

Why should a fine-tooth hacksaw or a tube cutter, rather than a coarse-tooth blade, be used when cutting aircraft tubing?

To produce a square, clean cut with minimal burr and distortion

To heat-treat the tube as it is cut

To form the flare automatically during cutting

When proof-testing a fabricated flexible hose assembly, the test pressure is generally set at what value relative to the hose's normal operating pressure?

Twice the maximum operating pressure

Exactly the maximum operating pressure

The rated burst pressure of the hose

What does a TEFLON (PTFE) flexible hose require special care to avoid after it has been installed and taken a set in a bent position?

It should not be straightened or rebent, because the liner can crack at the set

It should be straightened completely before storage

It should be flexed back and forth to keep it pliable

Which statement about routing fluid lines relative to electrical wiring is correct?

Fluid lines should be routed below and away from electrical wiring

Fluid lines should be bundled together with wiring for support

Fluid lines should be routed directly above electrical wiring

Bonding clamps used on metal fluid lines serve what purpose in addition to support?

They provide an electrical bonding path to the airframe for static charges

They raise the operating temperature limit of the line

They add internal pressure capacity to the line

When a section of damaged rigid aluminum tubing must be repaired in the field and a replacement tube cannot be fabricated, what fitting may be used to splice in a repair section?

A standard flared-tube union joining a new flared repair section

A soldered copper sleeve over the damaged area

A threaded pipe coupling brazed to the tube

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.

Written by the AMTprep editorial team · Published July 27, 2026 · Last reviewed October 21, 2026

Written against the FAA primary sources cited at the foot of this page.

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 1,553 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.

37 free sample questions from ACS AM.I.D

AM.I.DTap an answer
How is aluminum alloy tubing identified so the mechanic can confirm that the correct alloy is being used for an application?
Show the answer and explanation
Answer: A stamped code and color band identifying the alloy
Why: Per FAA-H-8083-30, aluminum alloy tubing is identified by the alloy designation marked on the tube and by a color code band for each alloy (for example, 1100 white, 3003 green, 2024 red, 5052 purple, 6061 blue and yellow). A mechanic must verify the correct material before installation because each alloy has different strength and corrosion characteristics. A single color stripe or band on its own does not identify the alloy without the matching code.· FAA-H-8083-30 — Fluid Lines and Fittings (tubing materials and identification)
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?
Show the answer and explanation
Answer: Flattening or wrinkling of the tube wall
Why: 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.· FAA-H-8083-30 — Fluid Lines and Fittings (tube bending)
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A flared tube fitting forms its fluid-tight seal at which location?
Show the answer and explanation
Answer: Between the flare cone and the fitting seat
Why: 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.· FAA-H-8083-30 — Fluid Lines and Fittings (flared tube fittings)
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Which flare angle is standard for AN (Army-Navy) flared tube fittings used in aircraft fluid lines?
Show the answer and explanation
Answer: 37 degrees
Why: 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.· FAA-H-8083-30 — Fluid Lines and Fittings (flared fittings, AN vs AC)
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After cutting a length of metal tubing with a tube cutter, the next step before flaring should be to
Show the answer and explanation
Answer: remove burrs from the inside and outside of the tube
Why: 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.· FAA-H-8083-30 — Fluid Lines and Fittings (cutting and deburring tubing)
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A flareless (MS) tube fitting depends on which component to bite into the tube and form the seal?
Show the answer and explanation
Answer: A ferrule or sleeve that cuts into the tube
Why: 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.· FAA-H-8083-30 — Fluid Lines and Fittings (flareless fittings)
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A flexible hose marked with a lay line (a stripe running along its length) is checked at installation to confirm the hose is not
Show the answer and explanation
Answer: twisted
Why: 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.· FAA-H-8083-30 — Fluid Lines and Fittings (flexible hose, lay line)
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On a fluid line that carries a hazardous or specific medium, the identifying decals or tape primarily tell the mechanic the
Show the answer and explanation
Answer: contents and function of the line
Why: Aircraft fluid lines are identified by color-coded tape, decals, or bands with words and symbols that indicate the contents (fuel, hydraulic, oxygen, etc.) and the function of the line. This lets a technician quickly recognize what a line carries and handle hazardous fluids correctly. The identification markings do not specify fitting torque or the tubing manufacturer.· FAA-H-8083-30 — Fluid Lines and Fittings (line identification)
AM.I.DTap an answer
Which practice is correct when routing and supporting a fluid line near a moving part or another line?
Show the answer and explanation
Answer: Support the line with clamps to prevent chafing
Why: Fluid lines must be secured with the proper clamps (often cushioned) at the recommended intervals so they cannot chafe against the structure, moving parts, or each other—chafing wears through the tube and causes leaks or failure. A line resting unsupported against structure will rub, and clamping a rigid line under tension induces stress; lines should be supported, not pulled tight.· FAA-H-8083-30 — Fluid Lines and Fittings (line support and routing)
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In aircraft hydraulic systems, a flared-tube connection using a two-piece sleeve and nut is used in place of a one-piece fitting because it eliminates
Show the answer and explanation
Answer: the possibility of thinning the flare by wiping or ironing during tightening.
Why: In a two-piece sleeve-and-nut flare connection the separate sleeve bears against the flare while the nut turns freely on the sleeve, so tightening no longer rubs and thins the flare by wiping or ironing as a one-piece nut would. The flare must still be formed first, so it is the wiping of the flare, not the flaring operation, that is eliminated.· FAA-H-8083-30 — Fluid Lines and Fittings
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What does the dash number stamped on aluminum tubing fittings and on the size of rigid tubing represent?
Show the answer and explanation
Answer: The outside diameter in sixteenths of an inch
Why: Per FAA-H-8083-30, the size of rigid tubing and fittings is given by the outside diameter in sixteenths of an inch. A dash number such as -8 indicates 8/16 inch (1/2 inch) outside diameter.· FAA-H-8083-30 — Fluid Lines and Fittings
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Which material is most commonly used for rigid fluid lines carrying hydraulic fluid under high pressure in modern aircraft?
Show the answer and explanation
Answer: Corrosion-resistant steel
Why: Per FAA-H-8083-30, corrosion-resistant steel tubing (either annealed or 1/4-hard) is used extensively in high-pressure hydraulic systems because of its strength, and it does not have to be annealed for flaring or forming.· FAA-H-8083-30 — Fluid Lines and Fittings
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What is the standard included angle of an AN (Army-Navy) flare on aircraft tubing?
Show the answer and explanation
Answer: 37 degrees
Why: Per FAA-H-8083-30, AN standard fittings and the corresponding tubing flares use a 37-degree flare. This distinguishes AN flares from the 45-degree flares used in automotive and some plumbing applications.· FAA-H-8083-30 — Fluid Lines and Fittings
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What is the primary purpose of forming a tube bend or a loop in a rigid fluid line near a fitting?
Show the answer and explanation
Answer: To absorb vibration and allow for expansion and contraction
Why: Per FAA-H-8083-30, rigid tubing should not be cut or installed without some slack or a bend to absorb vibration and allow for expansion, contraction, and structural movement; a straight tight run is prone to cracking and fatigue failure.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
When a rigid tube has been flattened, dented, or kinked during bending, what is the correct repair action?
Show the answer and explanation
Answer: Discard the bend and replace the damaged section of tubing
Why: Per FAA-H-8083-30, a kink or flattening from a poorly made bend weakens the tube and restricts flow; the damaged section should be removed and replaced rather than reused, because such a bend cannot be safely straightened.· FAA-H-8083-30 — Fluid Lines and Fittings
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What is the maximum allowable flattening of a tube bend, expressed in terms of the original outside diameter of the tube?
Show the answer and explanation
Answer: Flattening that reduces the diameter to less than 75 percent of the original OD is not acceptable
Why: Per FAA-H-8083-30, the minimum diameter through a bend (the small diameter of the flattened bend) should not be less than 75 percent of the original tube OD; flattening greater than 25 percent is cause for rejection.· FAA-H-8083-30 — Fluid Lines and Fittings
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A scratch or nick in aluminum alloy tubing is generally acceptable for continued service if it does not exceed what depth of the tube wall thickness?
Show the answer and explanation
Answer: 10 percent of the wall thickness
Why: Per FAA-H-8083-30, scratches or nicks not deeper than 10 percent of the wall thickness in aluminum alloy tubing, and not in the heel of a bend, may be repaired by burnishing; deeper damage requires replacement.· FAA-H-8083-30 — Fluid Lines and Fittings
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What is the purpose of a flareless (MS) fitting compared to a flared fitting?
Show the answer and explanation
Answer: It seals by a sleeve biting into the tube, eliminating the need to flare the tubing
Why: Per FAA-H-8083-30, flareless fittings (MS) eliminate the need to flare the tubing; instead a sleeve with a sharp cutting edge bites into the tube OD to form the seal, which saves time and is suited to high-pressure systems.· FAA-H-8083-30 — Fluid Lines and Fittings
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When presetting (preforming) the sleeve on a flareless fitting, how is the tube end prepared before assembly?
Show the answer and explanation
Answer: Cut square, deburred, and bottomed firmly in the fitting
Why: Per FAA-H-8083-30, the tube end must be cut square and deburred, then bottomed firmly in the fitting (or presetting tool) so that the sleeve's cutting edge bites into the tube as the nut is tightened a specified amount.· FAA-H-8083-30 — Fluid Lines and Fittings
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Which of the following identifies the color coding traditionally used to mark aircraft fluid lines for a HYDRAULIC system?
Show the answer and explanation
Answer: Blue and yellow
Why: Per FAA-H-8083-30, color-coded tape and decals identify line contents. Hydraulic lines are marked with a blue and yellow color band combination, distinguishing them from fuel, oil, and pneumatic lines.· FAA-H-8083-30 — Fluid Lines and Fittings
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What color is used in the standard color code to identify a FUEL line on an aircraft?
Show the answer and explanation
Answer: Red
Why: Per FAA-H-8083-30, the standard fluid-line identification color code assigns red to fuel lines. The bands of colored tape with the appropriate symbol are applied at intervals and near connections along the line.· FAA-H-8083-30 — Fluid Lines and Fittings
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In addition to color codes, what is also placed on aircraft fluid line identification tape to aid mechanics who may be color blind?
Show the answer and explanation
Answer: Geometric symbols and the name of the system
Why: Per FAA-H-8083-30, the line identification marking includes geometric symbols (and the word naming the system) in addition to color, so the system can be identified by personnel who are color blind or when working in poor lighting.· FAA-H-8083-30 — Fluid Lines and Fittings
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When installing flexible hose, why should it be installed with a slight slack (about 5 to 8 percent of the total length)?
Show the answer and explanation
Answer: Because hose shortens in length and expands in diameter when pressurized
Why: Per FAA-H-8083-30, flexible hose contracts in length and expands in diameter when pressurized, so it must be installed with slack (approximately 5 to 8 percent of its length) to prevent it from being pulled taut and stressing the fittings.· FAA-H-8083-30 — Fluid Lines and Fittings
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How should flexible hose be installed with respect to twisting?
Show the answer and explanation
Answer: It must be installed without twisting, indicated by a straight lay line
Why: Per FAA-H-8083-30, flexible hose must not be twisted on installation because pressure applied to a twisted hose can cause failure or loosen the connections. The lay line (identification stripe) printed along the hose should not spiral, indicating no twist.· FAA-H-8083-30 — Fluid Lines and Fittings
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The lay line (a stenciled stripe running lengthwise) printed on a flexible hose is used primarily to determine what?
Show the answer and explanation
Answer: Whether the hose has been twisted during installation
Why: Per FAA-H-8083-30, the lay line is a stripe printed along the length of the hose; if it spirals around the hose after installation, the hose has been twisted and must be reinstalled correctly.· FAA-H-8083-30 — Fluid Lines and Fittings
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What is the function of the support clamps used along a run of rigid tubing?
Show the answer and explanation
Answer: To secure the line to the structure and prevent vibration and chafing
Why: Per FAA-H-8083-30, support clamps secure tubing lines to the airframe structure to prevent excessive vibration and chafing; clamps with cushioned (synthetic rubber) inserts are used to protect the tubing.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
When tightening a flared tube nut on an AN fitting, what is the result of overtightening?
Show the answer and explanation
Answer: It can split or crack the flare and cause leakage
Why: Per FAA-H-8083-30, overtightening a flared tube nut can split the flare or damage the fitting cone, causing leakage; the nut should be tightened to the specified torque using a torque wrench whenever possible.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
When making up a flared-tube connection, what is the purpose of using a backup wrench on the fitting while tightening the tube nut?
Show the answer and explanation
Answer: To prevent the fitting body from turning and twisting the tube
Why: Per FAA-H-8083-30, a second (backup) wrench is held on the connector or fitting body to prevent it from turning while the nut is torqued; this avoids twisting and stressing the tube and the mating part.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
What thread sealant or lubricant practice applies to tapered pipe-thread (NPT) fittings used in fluid systems?
Show the answer and explanation
Answer: Apply approved sealant to the male threads but keep it off the first two threads
Why: Per FAA-H-8083-30, an approved thread lubricant or sealant is applied to the male tapered pipe threads, but never to the first two threads, to prevent the sealant from entering and contaminating the system.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
Aluminum alloy 1100 (1/2 hard) or 3003 tubing is generally used for fluid lines that carry fluid under what condition?
Show the answer and explanation
Answer: Relatively low pressure, such as instrument and vent lines
Why: Per FAA-H-8083-30, soft aluminum alloys 1100 and 3003 are used for lines carrying fluid at relatively low pressures, such as instrument lines and ventilation, while stronger alloys (2024-T, 5052-O) or steel are used for higher pressures.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
After cutting rigid tubing with a tube cutter or hacksaw, what step is essential before flaring the tube?
Show the answer and explanation
Answer: Deburr the inside and outside of the tube end
Why: Per FAA-H-8083-30, the cut tube ends must be deburred (inside and outside) to remove sharp edges and burrs; failure to deburr can restrict flow and cause a poor flare that leaks.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
Why should a fine-tooth hacksaw or a tube cutter, rather than a coarse-tooth blade, be used when cutting aircraft tubing?
Show the answer and explanation
Answer: To produce a square, clean cut with minimal burr and distortion
Why: Per FAA-H-8083-30, a tube cutter or a fine-tooth saw produces a square, clean cut with minimal burr and distortion; a coarse blade tends to tear the metal and leave excessive burrs that are difficult to remove.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
When proof-testing a fabricated flexible hose assembly, the test pressure is generally set at what value relative to the hose's normal operating pressure?
Show the answer and explanation
Answer: Twice the maximum operating pressure
Why: Per FAA-H-8083-30, a newly fabricated hose assembly is proof-tested at twice (200 percent of) the maximum operating pressure to verify the integrity of the assembly before installation, without reaching the burst pressure.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
What does a TEFLON (PTFE) flexible hose require special care to avoid after it has been installed and taken a set in a bent position?
Show the answer and explanation
Answer: It should not be straightened or rebent, because the liner can crack at the set
Why: Per FAA-H-8083-30, once a Teflon hose has been pressurized and has taken a permanent set, it should not be straightened or rebent, because the PTFE liner can crack or kink at the set; such a hose must be reinstalled in the same configuration.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
Which statement about routing fluid lines relative to electrical wiring is correct?
Show the answer and explanation
Answer: Fluid lines should be routed below and away from electrical wiring
Why: Per FAA-H-8083-30, fluid lines (especially fuel, oil, and hydraulic) should be routed below and away from electrical wiring so that any leaking fluid will not drip onto wiring and create a fire or electrical hazard.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
Bonding clamps used on metal fluid lines serve what purpose in addition to support?
Show the answer and explanation
Answer: They provide an electrical bonding path to the airframe for static charges
Why: Per FAA-H-8083-30, bonded clamps are used to secure metal hydraulic, fuel, and oil lines. Besides supporting the line, they provide an electrical bonding path to the airframe so static charges and stray currents are conducted to structure. The contact surfaces must be clean and free of paint or anodizing to ensure conductivity. Unbonded clamps are used only for securing wiring.· FAA-H-8083-30 — Fluid Lines and Fittings
AM.I.DTap an answer
When a section of damaged rigid aluminum tubing must be repaired in the field and a replacement tube cannot be fabricated, what fitting may be used to splice in a repair section?
Show the answer and explanation
Answer: A standard flared-tube union joining a new flared repair section
Why: Per FAA-H-8083-30, a damaged tube section can be replaced or spliced using standard AN/MS unions (such as a flared-tube union) together with a new length of tubing flared at each end; the union connects the repair section to the existing line.· FAA-H-8083-30 — Fluid Lines and Fittings

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Related subject areas

The areas that share the most ground with AM.I.D — study them alongside it.

Hydraulic and Pneumatic Systems (AM.II.F)
Aircraft Fuel Systems (AM.II.J)
Engine Fuel and Fuel Metering Systems (AM.III.I)
Aircraft Materials, Hardware, and Processes (AM.I.E)

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

General

Airframe

Powerplant

Primary sources

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