Post Field Approval Data used for STC
Part 23 performance base is justification
Part 33 the basis for Engine STC
The aerospace industry worked long hard hours to convince the FAA, through Congress, too, to implement a better certification process– both standards and policies. The end result of the regulatory and legislative work is a new Part 23. The rules are now less prescriptive and more performance-based.
This magazine has cited an unusual STC application of proof that the FAA gets the policy shift that drives certification towards a more performance case. The facts, as reported, are as follows:
- The initial step involves the engine of Helio Courier H-295s, a 1949 STOL aircraft used by the military and for missions requiring a very short take-off-
- The Wright Air Service operates 4 of the Helios and years ago sought FAA field approval of modifications of two engines.
- Since then Wright has flown the aircraft a total of 22,000 hours.
- Field approvals, even under the more generous Part 23 approach, are not favored, sometimes not granted by ACOs.
- The advantage of getting an STC to modify aircraft is that once a company obtains a specific STC, the same modifications can be done to multiple aircraft without approving each modification individually. Additionally, the STC process ensures that the installation has passed scrutiny by the certification branch of the FAA
- Wright decided that it wanted to perform the same modification of two H-295s and contacted Momentum Aeronautics of Saint Paul, Minnesota, to obtain two supplemental type certificates (STCs).
- The project was unique because of the old field approvals Wright Air Service obtained did not really involve a certification. In order to prove that the Wright engine modifications were airworthy in the context of an STC application, Wright and Momentum sought to use much of the existing data and experience to get STCs for these modifications.
- The ACO agreed that no STC testing was required. It would accept of Wright’s extensive time spent in which the airplanes with the engines. FAA staff is usually not comfortable with service data as the basis for compliance data.
- The standard Helio engine has a complicated pressure carburetor system. Its system imposes a lot a lot of maintenance. The field approved engine minimizes that MX time by replacing the carburetor system with an off-the-shelf fuel injection system, which makes the engine much more efficient, easier to maintain, smoother to run, and often easier to start.
- This improvement saves a couple of gallons of fuel per hour in a standard cruise configuration. Parts are also easier to find for the newly installed fuel injection system, which can be critical when trying to keep a legacy aircraft flying in commercial service.
- The second STC was for the installation of the modified engine into the airframe. This STC required modification of fuel lines, installation of fuel pumps, and changing gauges/instrumentation in the airplane for the new engine configuration.
- Momentum took the existing field approvals and the Wright operational data to put in a format that fit the STC form and policies. The paperwork produced included continued airworthiness documentation, flight manual supplement updates (explaining to the pilot how to operate the modified systems), and addressed the structural concerns for the installation of the engine. The Anchorage ACO issued two STCs for the modifications to the Helio H-295 aircraft and the engines.
- The value of these certificates is that Wright may not only apply them to the company’s other Helio H-295s but can be merchandised to others who have the same aircraft.
What is impressive is the ACO acceptance of a field approval for the Part 23 STC and even more notable for the Part 33 STC (for which there is no statement of liberalization as with small aircraft).One wonders whether the Engine and Propeller Directorate[i] reviewed this innovative application of the performance principles to an STC.
The General Aviation News headline praises this advance; hopefully after one STC step forward followed by one STC step backwards.
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