What is a flareless fitting and how does it work?

A flareless fitting creates a leak-free connection in hydraulic or fuel lines by using a sleeve (also called a ferrule) and a B-nut rather than requiring the tube end to be flared. As the B-nut is tightened, it drives the sleeve onto the tube, causing the sleeve to bite into and grip the outer surface of the tubing to form a secure, pressure-tight seal. This method is commonly used in aircraft hydraulic, fuel, and oil systems where a reliable connection must be achieved without special flaring tools. Proper installation technique is essential because incorrect assembly can result in leaks or fitting failures under pressure.

How do you correctly install a B-nut and sleeve on a flareless fitting?

According to FAA guidance in Advisory Circular 43.13-1B, the tube is first inserted into the fitting body and the B-nut is finger-tightened until resistance is felt, then turned an additional one-sixth to one-third of a turn (as specified by the manufacturer) to seat the sleeve without over-torquing. A presetting procedure is often required before final installation, which involves assembling the fitting in a vise-mounted preset tool to allow the sleeve to properly bite into the tubing. Over-tightening the B-nut is one of the most common errors and can damage the sleeve or tube, ultimately causing a leak. Always consult the applicable manufacturer's maintenance manual for the specific torque values and turn increments required for the fitting being installed.

What's the difference between a flareless fitting and a flared fitting used in aircraft fluid lines?

A flared fitting requires the tube end to be mechanically deformed into a cone shape — typically 37 degrees for AN fittings used in most aircraft systems — which mates against a matching seat in the fitting body to create the seal. A flareless fitting, by contrast, relies on the sleeve biting into the outer wall of the tube as the B-nut is torqued, so no tube-end deformation is needed. Flared fittings are generally preferred for lower-pressure fuel and instrument lines, while flareless fittings are widely used in high-pressure hydraulic systems. Both types are covered on the FAA Aviation Mechanic General Knowledge Test, and understanding the correct installation and inspection procedures for each is an ACS-aligned study objective for aspiring aviation maintenance technicians.

B-Nut and Sleeve Installation for Flareless Fittings

Flareless fittings are a common and reliable method of connecting metal tubing in aircraft fluid systems — hydraulic, fuel, oil, and pneumatic lines alike. Unlike flared fittings, which require the tube end to be mechanically spread outward before assembly, flareless fittings grip the outside of the tube with a precision-machined sleeve, also called a ferrule or cutting ring. When properly installed, the sleeve bites into the tube wall and creates a mechanically strong, pressure-tight seal. Because the seal depends entirely on correct presetting and final torquing technique rather than the shape of the tube end, knowing the step-by-step procedure is essential — both for safe aircraft maintenance and for the FAA Aviation Mechanic (General) knowledge test.

This article walks through the complete installation procedure for the B-nut and sleeve on a flareless fitting, explains the metallurgy and mechanics behind why each step matters, identifies the correct inspection points, and highlights the exam questions that trap the most students.

Components of a Flareless Fitting

A flareless fitting assembly consists of three parts: the fitting body, the sleeve (ferrule), and the B-nut (also called the coupling nut). The fitting body is threaded into the aircraft structure, component, or union and has an internal conical seat. The sleeve slides over the tube before assembly and is not permanently attached to either the nut or the tube at the start. The B-nut threads onto the fitting body and, as it is tightened, drives the sleeve forward into the conical seat of the fitting body. This axial force causes the leading edge of the sleeve to bite into — or preset into — the outside diameter of the tube, creating the seal.

Sleeves are typically made from the same material as the tubing: aluminum sleeves for aluminum tube, steel sleeves for steel tube, and so on. Mismatching sleeve and tube materials can cause galvanic corrosion or improper bite depth and is never acceptable.

The Presetting Process

Before a flareless fitting can be installed in an aircraft, the sleeve must be preset onto the tube. Presetting is a bench operation that forms the sleeve into its final gripping shape. The FAA Aviation Maintenance Technician Handbook — General describes this as a mandatory step; attempting to preset a sleeve by simply tightening the B-nut in the aircraft is both inaccurate and potentially damaging.

Presetting is accomplished using a special presetting tool (sometimes called a presetting die or fixture) that has the same internal seat geometry as the fitting body. Here is the correct procedure:

Prepare the tube end. Cut the tube squarely using a proper tube cutter — never a hacksaw alone. Deburr the inside and outside of the cut end completely. A burr left inside the tube can break loose in service and damage downstream components such as pumps and valves.
Slide the B-nut and sleeve onto the tube. The B-nut slides onto the tube first (open end facing the tube end), followed by the sleeve. The sleeve must be oriented correctly: the tapered or beveled end faces the tube end, toward the fitting. Reversing the sleeve is one of the most common installation errors.
Insert the tube into the presetting tool. Push the tube firmly into the presetting tool's seat until it bottoms out — this ensures the tube is fully seated before any torque is applied.
Tighten the B-nut onto the presetting tool. Using a wrench (always use two wrenches — one to hold the presetting tool body, one to turn the B-nut), tighten the B-nut until you feel a firm resistance indicating the sleeve has seated. At that point, advance the nut an additional one-sixth to one-third of a turn, as specified by the applicable manufacturer or AC 43.13-1B guidance for the tube size and material. Do not over-tighten; excessive tightening cuts through the tube wall rather than biting into it.
Remove and inspect. Back off the B-nut and remove the tube from the presetting tool. Inspect the sleeve: it should show a uniform ring-shaped impression (the bite mark) around the tube, and the sleeve should not rotate freely on the tube. A correctly preset sleeve will have a slight resistance to rotation and will not slide axially. The tube end should not be crushed or visibly deformed inward.

Final Installation in the Aircraft

Once the sleeve is properly preset, installation into the aircraft fitting is straightforward but must still be done carefully.

Insert the tube into the fitting body so the sleeve seats fully against the fitting body's conical seat. You will feel a slight stop as the sleeve contacts the seat.
Thread the B-nut onto the fitting body by hand until snug.
Using two wrenches, tighten the B-nut. The standard procedure is to tighten until you feel an increase in resistance (the sleeve is re-seating), then advance the nut an additional one-sixth to one-third turn, consistent with the increment used during presetting. Some references describe the final tightening as bringing the nut to the same position it reached during presetting, plus a very slight additional snug — the key point is that excessive torque is destructive, not beneficial.
Check for leaks during the first system pressure test. A correctly installed flareless fitting should be leak-free without further tightening. If a fitting leaks, do not simply continue tightening — investigate the cause, which may be a damaged sleeve, improper presetting, or a scratched fitting seat.

Why This Technique Matters

The flareless fitting's entire sealing ability depends on the precise deformation of the sleeve during presetting. Over-tightening at either stage — presetting or final installation — can sever the tube wall, creating a hidden crack that fails under vibration or pressure. Under-tightening leaves the sleeve improperly seated, resulting in leaks or the fitting pulling off the tube under pressure. In a hydraulic system operating at 1,500 to 3,000 psi or more, a failed fitting can result in immediate loss of system function and, in a flight control system, loss of the aircraft.

Because flareless fittings are used extensively in high-pressure hydraulic systems, brake lines, and fuel systems, an incorrectly assembled fitting can cause fire, structural failure, or complete loss of controllability. The technique is not merely a test topic — it is a genuine airworthiness requirement enforced through the aircraft's maintenance records and airworthiness standards under 14 CFR Part 43.

Key Numbers and Rules

One-sixth to one-third turn — the standard additional tightening range applied after the sleeve seats, both during presetting and final installation, with the specific amount depending on tube size and material per manufacturer or AC 43.13-1B guidance.
Sleeve material must match tube material — aluminum sleeve on aluminum tube, steel sleeve on steel tube.
Sleeve tapered end faces the fitting — reversed sleeves will not seal correctly.
Always use two wrenches — one to hold the fitting or presetting tool body, one to turn the B-nut. Using only one wrench risks rotating the fitting body and cracking welds or brazed joints.
Tube must bottom out in presetting tool before any tightening begins — failure to seat the tube fully results in incorrect sleeve positioning.
Do not re-tighten a leaking flareless fitting without first inspecting it — continued tightening past the design limit damages the tube and fitting seat.
Presetting in the aircraft is prohibited — always preset on the bench with the correct presetting tool.

Common Test Traps

Confusing the tightening increment. The FAA knowledge test frequently asks how much additional tightening is applied after the sleeve seats. The answer is typically one-sixth to one-third of a turn, depending on tube size and material per the applicable guidance. Students often guess one-quarter or one-half turn without qualification, or confuse the range with flared fitting torque procedures.
Reversing sleeve orientation. Questions may describe an assembly and ask what is wrong. A sleeve installed backward relative to the correct orientation shown in the fitting manufacturer's or handbook diagram is a classic distractor — it will not seal correctly and represents an improper installation. Always verify the specific sleeve geometry (tapered edge, cutting edge, or counterbore) against the applicable diagram for the fitting type in use.
Presetting in the aircraft. The test may ask whether it is acceptable to preset a sleeve by tightening it in the aircraft fitting. The answer is no — presetting must always be done with the proper presetting tool on the bench.
Over-tightening as a leak fix. A question may describe a fitting that still leaks after installation and ask what to do. Simply continuing to tighten is incorrect; the fitting must be disassembled and inspected for damage.
Material mismatch. Students sometimes assume any sleeve can be used with any tube. The test exploits this — the sleeve must always be of the same or compatible material as the tubing to prevent galvanic corrosion and to ensure the sleeve bites correctly into the tube wall.