Ernest Christley wrote:

The most recent Sport Aviation has an article on make up hoses. The
author states that 'you must use 37 degree fittings' and as is typical,
gives no reason why. Is there a reason for this that involves the laws
of nature, vs the "laws of the FAA"?

There is no FAA rule specifying the type of flared fittings you must
use in your homebuilt aircraft. However, the flared fittings used in
most modern aircraft are 37 degree AN/MS fittings. Also, the flared
fittings used in professional race cars are typically 37 degree AN/MS
fittings. There is no "magic" in the 37 degrees. It is simply an
aircraft standard that has become a de facto performance standard for
several non-aviation applications as well. The flared fittings under
the hood of your family car are likely SAE 45 degree fittings. The
flares used in your home plumbing are also 45 degrees. Someone
mentioned Tony Bingelis' books. Tony specified AN/MS 37 degree flares
in his books. AC 43.13-1b specifies AN/MS 37 degree flares as well.

David O -- http://www.AirplaneZone.com