Layering STCs: Understanding and managing the risks

In early 2010, a Cessna T337G crashed at Farmingdale, N.J., during climbout from a low pass over the runway, resulting in five fatalities. Witnesses observed the airplane’s nose pitch up just before the outboard six-foot section of the right wing separated, and the airplane descended uncontrollably and impacted the ground. The airplane was modified under 22 different supplemental type certificates (STCs), which included separate STCs for a short-field takeoff and landing (STOL) kit, an extended wingtip fuel tank, and winglets.

The investigation found evidence that the combined effects of the multiple STC modifications on the accident airplane were not accounted for and may have adversely affected the airplane’s wing structure. Recorded GPS tracks indicate that the aircraft exceeded maneuvering speed (V_A) during the low pass and subsequent pull-up. However, one of the open questions in this case is, "What is V_A for this highly modified aircraft?"

Adding a series of modifications to an aircraft poses a challenge to owners and mechanics. This is because it is the installer's responsibility to ensure that the new modification will safely integrate with previous modifications made to the aircraft. However, the aircraft owner is ultimately responsible for the airworthiness of the aircraft. So, the chain of events begins as early as the planning stage for the aircraft improvement.

Contrary to the owner and the installer, STC holders have only one responsibility: to make sure that their modification can be safely implemented on an aircraft conforming to its original type certificate (as it left the factory). Therefore, the STC holder for a cowl modification has no responsibility to ensure that it works with a different exhaust system, etc. There are exceptions, but not many.

The concept of implementing STCs on previously modified aircraft is known as "layering STCs," and doing it properly is paramount to safety. When adding an STC to a previously modified aircraft, consider the following in your analysis:

Establish a good baseline. Be sure you are making installation calculations based on the modified, not stock, aircraft. Before making a new modification, it is your responsibility to review the previous mods to ensure that they were properly documented. Has the weight and balance been updated? Pilot’s operating handbook changes noted? Any operating limitations changed from stock?

Map the affected systems. Begin by reviewing every system that the STC touches: electrical, fuel, airframe, power, etc. Knowing what has been altered is a critical first step in the process.

Find all the overlaps. Investigate both the obvious and not-so-obvious interactions between STCs. For example, it’s obvious that tip tanks would affect the fuel system. However, they can also affect the electrical system with transfer pumps and new gauges. Did they change the gross weight of the aircraft? Any operational changes?

When in doubt, ask. You can get a lot of information to help you determine compatibility from the STC holders. It’s unlikely that they’ll make any final determination for you, due to the fear of liability. However, they can answer important questions. Has the aircraft been tested in the new configuration? Are any installation deviations permitted? Are other aircraft flying with the combined modifications?

STC interactions are not always obvious. Adding fuel capacity may invalidate an STC for mogas. Adding power means more heat to dissipate and may affect cowl or baffling mods. And, most important, adding two STCs that alter similar operating limitations, such as V_A, should raise red flags.

Layering STCs is not necessarily a bad thing. If carefully thought out, it can really increase the performance of the aircraft. However, it's crucial to understand what each modification is actually doing, so that we can ensure that we're helping, not hurting, the safety and performance of the aircraft.

Jeff Simon is an A&P mechanic, pilot, and aircraft owner. He has spent the last 14 years promoting owner-assisted aircraft maintenance as a columnist for several major aviation publications and through his how-to DVD series: The Educated Owner. Jeff is also the creator of SocialFlight, the FREE mobile app and website that maps over 5,000 aviation events. Free apps available for iPhone, iPad and Android, and on the Web at www.SocialFlight.com.