Agency Should Offer Guidance On Compatibility Of Modifications

The NTSB has issued a set of recommendations to the FAA concerning the installation of STC modifications on aircraft. In its recommendation letter, the board cites several examples of accidents in which the STC modifications were made on aircraft but no compatibility check was performed to be sure there was no conflict between the mods.

The board recommends that the FAA develop specific guidance and/or a checklist to help installers performing supplemental type certificate (STC) modifications determine the compatibility  and interaction between a new STC and any previously installed STCs on the aircraft to ensure that the new STC will not adversely affect the aircraft's structural strength, performance, or flight characteristics. If the guidance and/or checklist indicate any adverse effects between the STCs, additional testing and/or an engineering evaluation should be performed before installing the new STC.

It also recommends that the FAA instruct installers to document in the Description of Work Accomplished block of Federal Aviation Administration Form 337 how the installer determined the compatibility and interaction between the new supplemental type certificate (STC) and previously installed STCs on the aircraft to show that the new STC will not adversely affect the aircraft's structural strength, performance, or flight characteristics.

Once the FAA develops specific guidance for installers performing supplemental type certificate (STC) modifications, as requested in Safety Recommendation A-12-21, (1) educate owners and operators of all aircraft with multiple STCs about the potential hazards of incompatible STCs; (2) encourage them to have their aircraft evaluated to determine if the multiple STCs adversely affect the aircraft's structural strength, performance, or flight characteristics; and (3) document any evaluation in FAA Form 337 for that aircraft.

FMI: Read The Recommendation Letter