Note that, while it is required that the transponder equipment meet the performance and environmental requirements of the applicable TSO, it is not required that the equipment be manufactured under a TSO authorization.

In order to address this question, it’s helpful to understand what a “TSO” is.

TSO stands for Technical Standard Order, which is defined in 14 CFR Part 21, section 21.601(b)(1) as “….a minimum performance standard for specified articles (for the purpose of this subpart, articles means materials, parts, processes, or appliances) used on civil aircraft.” As you can see from this definition, a TSO is actually a performance standard to which an article can be manufactured.

(9) Gyroscopic direction indicator (directional gyro or equivalent).

While much of this regulation is straightforward and self-explanatory, there are a few areas that leave some room for confusion and/or interpretation.

However, the easiest way to be assured that your transponder meets the requirements of 91.215(a) is to install one that has been built under a TSO authorization.

The requirements for altitude reporting equipment associated with the transponder are called out in 91.217(c), which states that, the altimeters and digitizers must meet the standards of TSO-C10b and TSO-C88, respectively.

When someone says an article is “TSO’ed”, what they really mean is that the unit was manufactured under a TSO authorization.

Section 21.601(b)(2) says, “A TSO authorization is an FAA design and production approval issued to the manufacturer of an article which has been found to meet a specific TSO”.

Most of the confusion arises from the requirement for certain “gyroscopic” instruments. The often-asked question is, what constitutes a “gyroscopic” instrument.

Is an instrument containing an actual rotating mass gyro required, or are alternatives such as ring laser gyros or accelerometer-based instruments acceptable?

The Experimental/Amateur-Built (aka homebuilt) segment of general aviation has grown and expanded over the years.