A common question we get is, “What is the difference between AN and AND fittings?” It can be a particularly confusing area, so you’re not alone. It’s confusing for some because AN fittings are Air Force – Navy Aeronautical Standard fittings and AND fittings are Air Force – Navy Aeronautical Design Standard fittings. Kinda close, right?
Let’s take a look at these fittings, how they came to be, and make some sense out of an otherwise confusing category of fittings.
Quick History of AN/AND Fittings
The term AND fitting is really just a synonym for AN fitting. Many people use them interchangeably. Pre-World War II, the United States Military created standards for the fittings that were used on Air Corps military equipment and were referred to as AN fittings or Army-Navy.
As the military evolved, the standards for the fittings kept the AN abbreviation but changed the full name to Air Force – Navy Aeronautical Standard. After the war, other industries adopted the military standards and thus, we have AN fittings throughout an array of different equipment.
The original fitting created using these standards was a fitting with a 37° flare angle with 3A/3B threads. Because the use of this fitting took off in the private sector, a level of confusion arose regarding the AN fitting and the SAE 37° fitting. As different versions of the flare fitting became available, consumers confusion hit an all-time high.
This disarray prompted the Joint Industry Conference to standardize the AN fitting with the modification of 2A/2B threads to make manufacturing easier. Thus, the SAE 37° flare fitting came into being in 1950, while AN fittings are still used in the private sector today; the military is moving toward AS standards, or Aerospace Standards. These standards use lighter weight, greater strength materials such as titanium and aluminum.
AN/AND Fittings vs. SAE/ISO
While there really isn’t a difference between AN flare fittings and AND flare fittings, there is a difference between AN/AND fittings and their industrial counterparts. AN fittings and SAE fittings function the same, but they are not interchangeable. As mentioned above:
- AN 37° fittings use 3A/3B radiused root threads and
- SAE 37° fittings use 2A/2B series threads.
The 3A/3B radiused root threads allow for fewer fatigue issues on aircraft. If you were to try and use these fittings interchangeably, you could experience failure under pressure. You should keep an eye out:
- The threads will not line up because of different flaring angles
- You will not get a tight connection causing separation and leakage at the flare.
These fittings also come in different materials. While they are both available in stainless steel and carbon steel, only the SAE 37° fittings are available in brass; the AN 37° fittings come in:
For a visual comparison of these fittings, check out this article from Parker. Parker was at the forefront of the flare fitting technology back in the 1920’s and was the inspiration for the 37° flare fittings used today. While there will likely always be confusion over which fitting is the best to use, hopefully, there is now a greater understanding of AN fittings, how they came to be, and what they are used for.