Flight Mechanic

Aircraft Mechanic School Study Supplement for Future Aviation Maintenance Technicians




You are here: Home / Basic Aviation Maintenance / Fluid Lines & Fittings / Rigid Fluid Lines – Fluid Line Identification, Fluid Line End Fittings, Universal Bulkhead Fittings
Regretfully, Flight-Mechanic will be turning out the lights after fifteen years. Google, in its infinite wisdom, has chosen to remove Flight-Mechanic from its search results (the claim is that the content on this site is spam). We appealed their decision to shut us down, to no avail.

Unfortunately, since Google has a monopoly over internet search, this means that traffic levels on the site will drop to a level that makes maintaining it uneconomic. As time progresses, we will no longer be able to maintain the server space and will remove the site from the internet. Thanks to all who have supported us over the years.

To fight back against the monopolistic practices of companies like Google, we recommend using a competing search engine such as Brave (you get the added benefit of not being spied on by Big Tech) and advocating for anti-trust legislation from your representatives.

Rigid Fluid Lines – Fluid Line Identification, Fluid Line End Fittings, Universal Bulkhead Fittings

Filed Under: Fluid Lines & Fittings

Fluid Line Identification

Fluid lines in aircraft are often identified by markers made up of color codes, words, and geometric symbols. These markers identify each line’s function, content, and primary hazard. Figure 7-13 illustrates the various color codes and symbols used to designate the type of system and its contents.

Figure 7-13. Identification of aircraft fluid lines.
Figure 7-13. Identification of aircraft fluid lines.

Fluid lines are marked, in most instances with 1″ tape or decals, as shown in Figure 7-14(A). On lines 4″ in diameter (or larger), lines in oily environment, hot lines, and on some cold lines, steel tags may be used in place of tape or decals, as shown in Figure 7-14(B). Paint is used on lines in engine compartments, where there is the possibility of tapes, decals, or tags being drawn into the engine induction system.

 

In addition to the above-mentioned markings, certain lines may be further identified regarding specific function within a system; for example, drain, vent, pressure, or return. Lines conveying fuel may be marked FLAM; lines containing toxic materials are marked TOXIC in place of FLAM. Lines containing physically dangerous materials, such as oxygen, nitrogen, or Freon™, may be marked PHDAN. [Figure 7-14]

Figure 7-14. Fluid line identification using: (A) tape and decals and (B) metal tags.
Figure 7-14. Fluid line identification using: (A) tape and decals and (B) metal tags.

Aircraft and engine manufacturers are responsible for the original installation of identification markers, but the aviation mechanic is responsible for their replacement when it becomes necessary. Tapes and decals are generally placed on both ends of a line and at least once in each compartment through which the line runs. In addition, identification markers are placed immediately adjacent to each valve, regulator, filter, or other accessory within a line. Where paint or tags are used, location requirements are the same as for tapes and decals.

Fluid Line End Fittings

Depending on the type and use, fittings will have either pipe threads or machine threads. Pipe threads are similar to those used in ordinary plumbing and are tapered, both internal and external. External threads are referred to as male threads and internal threads are female threads.

 

When two fittings are joined, a male into a female, the thread taper forms a seal. Some form of pipe thread lubricant approved for the particular fluid application should be used when joining pipe threads to prevent seizing and high-pressure leakage. Use care when applying thread lubricant so that the lubricant will not enter and contaminate the system. Do not use lubricants on oxygen lines. Oxygen will react with petroleum products and can ignite (special lubricants are available for oxygen systems).

Machine threads have no sealing capability and are similar to those used on common nuts and bolts. This type of fitting is used only to draw connections together or for attachment through bulkheads. A flared tube connection, a crush washer, or a synthetic seal is used to make the connection fluid tight. Machine threads have no taper and will not form a fluid-tight seal. The size of these fittings is given in dash numbers, which equal the nominal o.d. in sixteenths of an inch.

Universal Bulkhead Fittings

When a fluid line passes through a bulkhead, and it is desired to secure the line to the bulkhead, a bulkhead fitting should be used. The end of the fitting that passes through the bulkhead is longer than the other end(s), which allows a locknut to be installed, securing the fitting to the bulkhead.

Fittings attach one piece of tubing to another, or to system units. There are four types: (1) bead and clamp, (2) flared fittings, (3) flareless fittings, and (4) permanent fittings (Permaswage™, Permalite™, and Cyrofit™). The amount of pressure that the system carries and the material used are usually the deciding factors in selecting a connector.

The beaded type of fitting, which requires a bead and a section of hose and hose clamps, is used only in low- or medium-pressure systems, such as vacuum and coolant systems. The flared, flareless, or permanenttype fittings may be used as connectors in all systems, regardless of the pressure.

 

Flight Mechanic Recommends

Rod Machado's Private Pilot Handbook -Flight Literacy recommends Rod Machado's products because he takes what is normally dry and tedious and transforms it with his characteristic humor, helping to keep you engaged and to retain the information longer. (see all of Rod Machado's Products).
   
-->