A pre-buy inspection on an aircraft is your chance to research and investigate every aspect of your potential investment. The prebuy should include a thorough inspection of the mechanical as well as cosmetic condition of the aircraft.

In addition, the legal status of the aircraft needs to be looked at. This includes ensuring that the FAA records on the aircraft are up to date. An often overlooked step of the prebuy is investigating which STC’s are legally installed on the aircraft.

STC stands for Supplemental Type Certificate. This is a type certificate that is issued when the FAA approves an aircraft modification that was not part of the original design. It could be something as small as an oil filter or something as big as adding a set of canards to your Cessna.

An STC is usually owned by the individual or company that is selling the aircraft modification. If you purchase an item with an STC, it must be installed in accordance with the procedure outlined in the STC. A logbook entry is made in the aircraft maintenance logbook and a 337 form must be filed with the FAA.

The 337 form notifies the FAA that the aircraft has been modified. The person filing the form must include information that shows that the modification has been tested and approved. The STC noted on the 337 represents that approval.

If a search on a tail number does not contain an installed STC, it cannot legally be on the aircraft until the 337 form is refiled. Because the 337 form must reference the applicable STC, you’ll also need to find the original STC paperwork as well.

As we heard in this phone call to the Pilot Information Center, it may be difficult to find the owner of the STC. The FAA maintains a searchable database of STC’s on its website but it is possible that the STC owner is no longer in business, no longer maintains the STC, or sold it to another company. If the FAA database offers no results, the aircraft owner may obtain a field approval through the updated FAA process or remove the modification in question, replacing it with an updated STC’d product if one is available.