STC Procedures

by Ed Cameron, Director, Flight Engineering

In the STC Update column in the December issue of The S-TEC Approach we commented that one of the major strengths of our dealer program is the availability and ever increasing number of STC's done here at the factory. We aggressively pursue STC's for aircraft that are equipped with flight control systems needing replacement due to obsolescence caused by high maintenance or poor performance, as well as aircraft which have no flight control systems approved.

We do this since our core business is product design and marketing efforts within the retrofit flight control system market. Since the very beginning, S-TEC has had this as our mission. That mission has controlled our product design so that we can pursue STC's in a very cost effective manner. It has controlled our procedures and policies as well, contributing to the ability to accomplish STC's much less expensively than our competition can.

We constantly search for ways to keep our costs in line so that we can continue to pursue STC's aggressively in the future.

Knowing a little about the STC process and what it takes for us to add a new STC to the list of over 1250 we currently offer, should help you sell even more S-TEC flight control systems.

DESIGN CONSIDERATIONS
We first accomplish a design review of the specific aircraft which looks at:

Our review is done with an installers future efforts taken into consideration.

A structural analysis is accomplished which evaluates component locations and the supporting structure available. We then design installation hardware which incorporates all design criteria, including a choice of the exact alloy which will provide the necessary strength with the least possible weight.

System performance parameters are set including clutch slip settings and gain settings to control the servos at the proper speeds. If existing components cannot be adapted from our current product line we redesign existing equipment or design new equipment as necessary.

CONFORMITY INSPECTION
Prior to any flight test activity, the FAA reviews each and every item in the system to assure conformity with the design. It has been our practice to design and test the largest and most sophisticated flight control system for the most complex of the aircraft in the family. This allows us to follow the original STC with additional STC's approved by the FAA through similarity. Since all our systems have very similar designs and performance we often can get approval for the smaller, less sophisticated systems once we have been approved for the more complex system.

FLIGHT PROFILE TESTING
Once we have completed the design portion of the STC effort we enter the flight testing portion of the process. Normal and extraordinary flight profiles must be flown to prove to the FAA that our system will perform in normal operations, and in the worst case scenarios, will not cause a hazardous condition that could not be controlled by a reasonably alert pilot.

Most of our 28 volt servo motors operate at maximum levels of 3 to 4 volts in normal operations. During malfunction flight testing however, we must apply full system voltage, (a voltage that represents the maximum voltage the system can produce during a malfunction), to the pitch, roll and yaw axis servos for a full 3 seconds before the pilot executes a normal recovery. During the 3 second period or during the recovery we cannot exceed the limits specified in F.A.R. Part 23. That is, we cannot exceed a "0 G" to "2 G" envelope during a pitch hardover, and we cannot exceed a bank angle of 60 degrees in a roll hardover during the 3 second test.

These tests are performed in 2 aircraft configurations. First, we fly the aircraft loaded to its maximum gross weight and at the aft CG limit and perform the malfunction testing with the servo clutches set at the high end of their environmental tolerances. Next, we fly the aircraft in approach configuration at full gross weight with the CG at the forward limit with the clutches set to the low end of their environmental tolerances. This way we can determine if the system properly controls the aircraft and performs its intended function under these conditions. Flight testing is conducted to the altitude that will be approved for autopilot operation.

After we have sufficiently proven that we can fly the aircraft without compromising safety in the event of a system malfunction, we then must prove that the system can fly the aircraft smoothly and efficiently in normal flight, climbs, cruises, descents, and approaches to landing.

These flight tests are done with the aircraft in the Experimental Category and usually take less than 10 hours of flight time. At the completion of the flight testing the aircraft is returned to the normal category.

HOW AIRFRAME MODS AFFECT EXISTING STC's
The key consideration in the approval of an STC for a modified airframe is whether or not the modification has affected the flight characteristics of the aircraft during that portion of the flight when a flight control system would be operated.

As an example, we recently completed an STC on a Volpar Tri Gear modified Twin Beech D-18S. The STC's we got on this aircraft will also apply to the conventional gear equipped aircraft since the landing gear on both versions is normally retracted when the aircraft is in the flight profiles where the autopilot is controlling the aircraft.

The STC process may seem complicated, but we make it easy for you. Just call anytime to ask an STC question. We're part of your staff. As we did in previous issues, we want to encourage you to inquire about STC's you don't see in our STC Directory. Since we can accomplish STC's at relatively low cost, we can tackle approvals which may not seem feasible for other manufacturers. 

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