Aircraft fittings connect rigid tubing and flexible hose to each other and to components such as pumps, valves, and actuators. Flexible hose, as the FAA's Aviation Maintenance Technician Handbook explains, is used to connect moving parts with stationary parts, in locations subject to vibration, or where a lot of flexibility is needed [1].
WHAT'S IN STOCK
OneMonroe's AN and MS fittings include nuts, sleeves, unions, tees, elbows, plugs, caps, and grease fittings, from AN818 nuts and MS20819 sleeves to MS21900-series flareless parts. On the hose side, OneMonroe carries MIL-DTL-6000 hose and hose ends, plus Aeroquip and Stratoflex product lines.
The FAA handbook breaks down AN fitting codes (AN744 to AN932) this way [1]:
Dash number - The tube or hose size in 1/16ths of an inch. It measures the outside diameter of tubing and the inside diameter of hose. A -4 fitting is for 4/16 (1/4) inch tube [1].
Pipe-thread fittings - The dash number is the pipe size in 1/8ths of an inch [1].
Material code letter (after the dash number) [1]:
D = aluminum alloy
No letter = steel
B = brass
Z = aluminum bronze (AN819 sleeve)
Color helps confirm material: the FAA notes that AN steel fittings are black, AN aluminum fittings are blue, and aluminum bronze fittings are cadmium plated and natural in appearance [1].
MS flareless parts use a similar pattern. The FAA handbook's example, MS21900-4D, is an aluminum alloy adapter (flareless tube to AN flared tube) for 4/16 inch tube [1].
AN FLARED VS. MS FLARELESS FITTINGS
These two systems look similar but are not interchangeable [1].
AN flared fittings
- A flared fitting uses a sleeve and a nut. Tightening the nut draws the sleeve and the tube flare against the cone of the male fitting to make the seal [1]. The FAA handbook shows the AN818 nut and AN819 sleeve as the typical pair [1].
- You can recognize AN flared fittings by the cone on the end of the fitting [1].
- Aircraft flares are 35° to 37°. The FAA says it is never acceptable to use an automotive flaring tool that makes a 45° flare [1].
- Soft aluminum tubing 3/8 inch OD and under gets a double flare [1].
MS flareless fittings
- MS flareless fittings have a straight end instead of a cone [1].
- They're designed mainly for high-pressure (3,000 psi) hydraulic systems that may see severe vibration or pressure swings [1].
- The fitting has a body, sleeve, and nut. The sleeve cuts into the tube when tightened, so a new flareless assembly must be preset with the proper presetting tool before installation [1].
Avoid mixing alloys in one fitting combination. The FAA notes that this helps prevent dissimilar metal corrosion [1].
TIGHTENING AND TORQUE
Flared fittings
- Never put compound on the fitting faces or the flare. It destroys the metal-to-metal contact that makes the seal [1].
- Align the line before tightening, and don't pull it into place with torque on the nut [1].
- Use a torque wrench and the published values. AC 43.13-1B Table 9-2 lists AN818 nut torque by dash size and tube material. For example, a -4 (1/4 in.) nut on aluminum alloy tube is 50 to 65 in-lb, and on steel tube 135 to 150 in-lb [2].
- If you can't stop a leak with correct torque, don't add more. The FAA warns that flare flaws can't be corrected by extra torque [1].
Flareless fittings
- Run the nut down by hand until resistance increases, then turn one-sixth turn (one hex flat) with a wrench, holding the connector so it doesn't turn [1].
- If a connection leaks during the pressure test, you may tighten up to one more sixth turn, for one-third turn total. Never go past one-third turn [1].
- Overtightening drives the sleeve too deep and can weaken the tube until it shears in flight [1].
Pipe threads
- Use an approved thread lubricant for the fluid, and keep it out of the system. Don't use lubricants on oxygen lines [1].
CHOOSING HOSE
Hose is classified by operating pressure [1]:
Low pressure - Below 250 psi, fabric braid reinforcement.
Medium pressure - Up to 3,000 psi in smaller sizes, up to 1,500 psi in larger sizes, one wire braid.
High pressure - All sizes up to 3,000 psi operating pressure.
Match the hose material to the fluid [1]:
- Buna-N has excellent resistance to petroleum products but should not be used with phosphate ester (Skydrol) fluid.
- Neoprene also should not be used with Skydrol.
- Butyl works well with Skydrol but should not be used with petroleum products.
- Teflon hose runs from -65°F to 450°F and is compatible with nearly every fluid used in aircraft.
The FAA handbook describes MIL-H-6000 type hose as approved for aircraft hydraulic, pneumatic, coolant, fuel, and oil systems, at -65°F to 250°F for sizes 3 through 12 [1]. It also notes that maximum temperatures and pressures should not be used at the same time [1].
When replacing a flexible line, AC 43.13-1B says to use the same type, size, part number, and length of hose as the original [2].
INSTALLING HOSE
These rules come from the FAA handbook [1]:
- Leave 5% to 8% slack. Hose shortens in length and expands in diameter under pressure [1].
- Don't twist it. Use the lay line printed along the hose to check. A twisted hose can rupture or loosen its nuts [1].
- Keep it straight at the ends. The hose should stay straight for at least two hose diameters from each end fitting [1].
- Respect the bend radius. Sharp bends greatly reduce burst pressure. Use elbow fittings where needed [1].
- Support it. Hose must be supported at least every 24 inches, and closer is better [1].
- Never straighten preformed hose. The FAA warns against straightening a preformed hose assembly [1]. AC 43.13-1B adds that Teflon hose takes a permanent set and needs extra care in handling [2].
CLAMPS AND SUPPORTS
Rubber-cushioned clamps are used to secure lines subject to vibration, while plain clamps are for areas without vibration [1]. Teflon-cushioned clamps are used where Skydrol, hydraulic fluid, or fuel exposure is expected [1]. Use bonded clamps on metal fluid lines, and make sure clamps are the correct size so they don't restrict hose flow [1]. Shop line support clamps.
Straight-thread fittings that seal against a port use boss gaskets. Learn how to pick the right boss gasket compound in our O-ring guide.
Never use titanium tubing or fittings in an oxygen system. The FAA warns titanium is oxygen reactive [1].
FAQ
Q: Are AN flared and MS flareless fittings interchangeable?
A: No. The FAA handbook states they are not interchangeable. AN flared fittings have a cone on the end; MS flareless fittings have a straight end [1].
Q: What does the dash number on an AN fitting mean?
A: It's the tube OD or hose ID in 1/16ths of an inch. A -6 fitting is for 6/16 (3/8) inch tube [1].
Q: Can I use an automotive flaring tool on aircraft tubing?
A: No. Aircraft flares are 35° to 37°, and the FAA says a 45° automotive tool must never be used [1].
Q: How much slack should flexible hose have?
A: 5% to 8% of its total length [1].
Q: What does the D in AN818-4D mean?
A: D is the material code for aluminum alloy [1].
Shop: Fittings | Hose | Fitting Kits | Aeroquip | Stratoflex | Clamps
Related guides: O-Rings & Seals | Aircraft Nuts
This guide is part of the Monroe Aerospace Guides library. Browse the full library, or contact our team if you need help cross-referencing a fitting.
References
[1] FAA, Aviation Maintenance Technician Handbook - General, Chapter 9 (Fluid Lines and Fittings), NTSB docket copy: https://data.ntsb.gov/Docket/Document/docBLOB?ID=17451994&FileExtension=pdf&FileName=AMTG%20HDBK%20--%20Chap9_Tubing-Rel.pdf
[2] FAA, AC 43.13-1B, Paragraph 9-30, Hydraulic Lines and Fittings (Table 9-2): https://www.landseaskyspace.com/ac4313/p9-19.html