No parts? No problem

Purchasing a new part was out of the question because of the age of our airplane, no serviceable used exhaust systems were available at the time, and having a reproduction exhaust system made was prohibitively expensive.

We ultimately avoided these challenges altogether by installing a Cessna 150 exhaust system instead. Now, if you’re wondering how non-original equipment, like the exhaust system from an entirely different airplane, could be legally installed on our certified Cessna 140, the answer is the supplemental type certificate (STC).

The STC solution

All aircraft certificated under FAR Part 21 have a type certificate. The type certificate data sheet developed by the aircraft manufacturer and issued by the FAA lists the technical data for a specific make and model of aircraft, including weight and balance, installed and optional equipment, operating limitations, and more. One of the requirements for an aircraft to be considered airworthy is that it must match the specifications of its original design as specified by the type certificate. That is, unless the design has been modified via a legal means such as an STC.

An STC allows an aircraft to be modified and/or operated outside of the strict limitations of the type certificate, provided sufficient testing has been done to prove that the modification or operation does not negatively affect safety. Once an STC has been applied to an aircraft, it becomes an equal part of the documentation of the design specifications for that serial number aircraft. While an STC is still issued by the FAA, it can be created by, and issued to, anyone—not just an aircraft’s manufacturer, as is the case with a type certificate. Many STCs are created by companies that produce avionics, engines, and accessories to allow their products to be installed on a wide variety of aircraft. Many more are created by individuals to improve existing aircraft through modifications or the installation of nonstandard equipment. Some aircraft manufacturers will even create STCs for their own aircraft as a way of improving the design without the creation of an entirely new type certificate.

Upgrade benefits

STCs offer innumerable benefits for aircraft owners. They allow the installation of more readily available and modern parts on older aircraft, assist in complying with airworthiness directives (ADs), and enhance aircraft safety, performance, and convenience. In the case of our Cessna 140, installing a relatively modern and readily available Cessna 150 exhaust system allowed us to get flying again without the headache and financial burden of sourcing an original 80-year-old part. It also simplified any future exhaust system work because, now that this STC has been applied to our airplane, we can replace worn exhaust components as easily as any other standard part listed on the airplane’s type certificate. In addition, this installation was an alternate method of compliance (AMOC) for AD 47-43-01, which requires an aluminum channel to be installed around the primer line where it passes over the stock tailpipes to avoid the risk of fire from dripping fuel. The new exhaust system moved the tailpipes to an entirely new location, eliminating this risk.When we rolled our airplane out of the hangar for the first time after a month’s worth of weekends spent assisting our A&P with the installation, outwardly, little had changed.

The largest percentage of STCs are those aimed at enhancing aircraft safety, performance, and convenience by allowing for the installation of aerodynamic improvements, more powerful engines, and creature comforts; increasing gross weight; strengthening airframes and landing gear; or updating avionics. As an aircraft owner, you can obtain a full list of the STCs available for your aircraft via the FAA’s Dynamic Regulatory System website or from your aircraft’s type club. The number of STCs available for a particular aircraft will vary depending on factors like the age and popularity of the design and the culture and mission of its pilots. The Piper PA–18 Super Cub is one example of a design that has a high number of STCs. This is most likely driven by its popularity in bush flying and other utilitarian applications. There was recently a 1958 PA–18 Super Cub A model on the market which, through the utilization of STCs, had been transformed from its original two-place, 150-horsepower configuration into a four-place 160-horsepower airplane with increased gross weight, fuel capacity, and numerous upgrades to safety and practicality. Despite these major changes, this airplane is still a certified Piper PA–18 Super Cub, albeit with a highly modified type design.

In the case of our Cessna 140, over 100 STCs are available, of which six had been installed in our airplane prior to our ownership. These include an aluminum wing covering, a Scott 3200 tailwheel, Cleveland wheels and brakes, a King KX-155 radio, a KT-75 transponder, shoulder harnesses, and an auto fuel STC. We have added three additional STCs since purchasing the airplane: a uAvionix tailBeacon, an LED beacon, and the Cessna 150 exhaust system. Each one of these STCs improves our airplane in its own way. For example, the uAvionix tailBeacon increases safety by providing ADS-B Out capability for greater situational awareness and convenience by allowing unrestricted operation in controlled airspace. The Cleveland wheels and brakes improve braking performance while also simplifying maintenance and parts availability. The metal wing skin replaces fabric and thereby avoids the need for periodic recovering of the wing. The auto fuel STC allows us to run unleaded ethanol-free auto gas in our airplane, which saves us money on fuel and future engine maintenance. The most recent STC we have installed is the Cessna 150 exhaust.

When we rolled our airplane out of the hangar for the first time after a month’s worth of weekends spent assisting our A&P with the installation, outwardly, little had changed. Our cowling now sports two short tailpipes protruding from the bottom, and the ragged crackle of straight pipes has been replaced by the refined rumble of muffled exhaust. While we have found that the cabin heat is noticeably more effective with the new exhaust system, the true benefits of this installation are a cost-effective and efficient return to flying and ease of obtaining replacement parts in the future.

The tradeoffs

As with so many things in life, STCs are a compromise, and each one has its advantages and disadvantages. In the case of our Cessna 140, some of the STCs that have been added have increased the empty weight of the airplane, thereby decreasing its useful load.

Increased empty weight is one of the more common disadvantages of a lot of STCs, but there are others. For example, large tundra tires, an STC that is very common today, allow for operation off of rough terrain but increase drag and reduce cruise speed and fuel efficiency. Another disadvantage, for those of us with older aircraft, is loss of historical accuracy. With owning a 1946 Cessna 140 comes the responsibility of considering the value of historical preservation when making the decision to modify the airplane. Some of the more preservation-minded members of the Cessna 140 community encouraged us to endure the extra cost and headache of sourcing and installing an original exhaust system, arguing that it was worthwhile in the name of historical accuracy. For us, however, the mission to have a practical, flyable airplane was more important, so we chose to give up some historical accuracy in favor of getting back in the air as efficiently as possible.

Know your mission

Considering the advantages and disadvantages each STC presents, it’s important to have a clear sense of the mission that you want your aircraft to fulfill when choosing whether or not to modify. Are you trying to fly quickly over long distances or get in and out of short runways? Are you trying to reduce maintenance costs or improve passenger comfort?

Knowing your mission will allow you to weigh the benefits and drawbacks of the STC you are considering based on whether they improve your aircraft’s ability to fulfill that mission. When we first purchased our Cessna 140, Michelle and I did not yet have a hangar and had to tie it down outside. The heavy aluminum wing covering was a necessity for us in that context because it’s relatively weatherproof when compared with fabric. Low useful load was not a particular issue either, because our mission at the time was flight training. We have since moved our airplane into a hangar, and our mission has changed to include short- and medium-distance cross-country travel. Having the airplane hangared means the metal wing is no longer a necessity, and we often find ourselves wishing we had the extra useful load that a fabric-covered wing would provide. While we may someday return our wing to its original fabric-covered configuration—yes, STCs can be reversed—there are several STCs we plan to utilize in the meantime to increase our airplane’s useful load, such as a lightweight starter and alternator, a lithium battery, and modern lightweight avionics.

One of the best ways to understand if a certain STC will be a good fit for you is to find someone who has it installed on their aircraft and have a discussion with them about the benefits it provides and any drawbacks that come with it. Better yet, go flying with them and experience it firsthand.

License to mod

The options are practically endless when it comes to STCs. They can allow you to change your aircraft’s engine, convert it from tricycle to conventional or conventional to tricycle gear, fly faster or slower, increase gross weight or seating capacity, and so much more. There is similar variety in what it takes to install an STC, which can range from a purely paperwork modification to a plug-and-play installation to a major modification. With careful planning and a thorough understanding of the impact an STC will have on the safety, performance, convenience, and maintenance of your aircraft, you can utilize them to create an aircraft that is uniquely perfect for you and your mission. 

Patrick Walker is a commercial pilot who has been flying for 10 years. He can often be found wrenching away on his 1946 Cessna 140 just outside AOPA headquarters at Frederick Municipal Airport (FDK) in Maryland.

drs.faa.gov