More acronyms…what’s an STC and TSO?

We live and breath acronyms in aviation. Accept it or else. Two more that you may not be all that familiar with are “TSO”, Technical Standard Order, and “STC”, Supplemental Type Certificate.

TSO vs STC

When looking at approvals for aviation equipment, the two terms are often used interchangeably, but are not the same. By definition, an STC is something that affects the type certificate of the aircraft. A type certificate is approved and issued the the FAA when an aircraft is manufactured and confirms that the aircraft is manufactured according to an approved design and complies with airworthiness standards. The type certificate is very prescriptive and dictates everything related to the design and build.

To alter the the type certificate for the aircraft, one would need to apply for, and receive approval of, a Supplemental Type Certificate (STC). For example, if you want to replace the navigational lights or seat belts on your aircraft, the replacement lights and belts would need to have an STC issued for your specific type of aircraft. The new equipment manufacturer would manage this process and advertise the replacement as compatible with your model. Keep in mind, lights that are STC’d for a Boeing 737, aren’t normally STC’d for a Cessna 172. 

A TSO, on the other hand, is a minimum performance standard for specified materials, parts, and appliances used on civil aircraft. When authorized to manufacture a material, part, or appliances to a TSO standard, this is referred to as TSO authorization. Receiving a TSO authorization is not an approval to install and use the article in the aircraft. It only means that the article meets the specific TSO. There are different TSOs for different parts of the aircraft. The TSO for life vests is of course different than the TSO for headsets. 

TSO for headsets

When evaluating the reams of data available on aviation headsets, you will inevitably encounter models that use TSO – the FAA’s term for indicating it is approved for use on an airplane.  

This might strike you as odd. Why would a portable headset fall under the scrutiny of the FAA? After all, neither your flight bag nor your flashlight needs an FAA approval. Why would any gear that is not permanently attached to the aircraft need TSO?

The answer for most pilots is that it doesn’t matter. Operating under FAR Part 91, general aviation flying, there is no requirement as to what type of headset is used in the cockpit. The FAA comes into play when you look at, or transition, to airline flying (Part 121 air carrier certification). All airlines are required to comply with more stringent FAA regulations. The standards are provided to the airline in the form of Operations Specifications or “OpSpecs” and can require, as part of the communication equipment, for a headset to meet TSO standards.

These standards, and how they are applied, may vary depending on the airline. I’ve heard reports of airline policies requiring a TSO’d headset to be available in the cockpit, yet pilots have the option to use their personal headset instead of the issued headset or headset that was delivered with the aircraft. Other policies might specifically require the use of a TSO’d headset for communication. If you’re worried about violating a company policy, check with your chief pilot prior to purchasing a non-TSO’d headset.

If you’re a new flight student and are looking to transition to the airlines someday, it might be worth considering the TSO’d headset options. Keep in mind that most pilots take years from the time they start, to the time they make it to an airline, and during that time, may transition through several headsets. However, headsets do represent a sizable investment.

What makes a Headset TSO’d?

Why buy a TSO’d headset if you don’t have to? Is there a benefit to having a headset that has the FAA’s stamp of approval? What does a manufacturer of headsets have to do to get the approval? Is this like the “FAA-approved” alternator that can be found for 80% less at the local automotive store?

Not exactly. The FAA won’t grant a TSO to just any headset. There is a long list of requirements that must be met prior to the granting of that designation. Some of it is crossing t’s and dotting i’s, but the testing procedure is pretty intense. It includes:

• Mechanical durability evaluated through the tests such as drop, vibration, cable/connector pull and bending, etc.
• Environmental robustness evaluated through the exposure of high/low temperature, humidity, fluid, altitude, decompression, electrostatic discharge, flame, etc.
• Electroacoustic performance evaluated through various parameters such as sensitivity, frequency response, distortion, and noise for both microphone and speaker, ANR, etc.  
• RF immunity tested in terms of susceptibility to RF signals including lightning, voltage spikes, etc.

In addition to physical testing, a headset manufacturer must convince the FAA that it plans to service and support the headset for many years to come. Clearly-labeled parts that are replaceable, FAA inspections of the manufacturing facility, alerting the FAA to changes to parts, quality testing—the sheer volume of paperwork involved in getting a TSO and maintaining the TSO is extensive. It’s no wonder that only a few manufacturers complete the TSO process. 

If you are occupying the left seat on a major airline flying heavy iron across the pond, you’ll probably need a TSO’d headset. If you’re going to be in a single-engine bug smasher for the next four years, you probably don’t need to check the TSO box. 

If you can’t sleep at night and want to check out the current TSO-C139, you can find the complete document here. Don’t forget the referenced 250+ page document, RTCA/DO-160E: Environmental Conditions and Test Procedures for Airborne Equipment. I’d suggest reviewing the riveting chapter on salt fog testing, or maybe magnetic effect? It’s a real page-turner.