Most builders of Experimental Amateur-built aircraft are familiar with the requirement to flight test a newly finished homebuilt. By flight test, I mean the 40-hr fly-off, or the Phase I testing, or whatever people (somewhat inaccurately) call it. In fact, those requirements are clearly spelled out in your aircraft’s Operating Limitations, which makes them compulsory to remain airworthy. In short, you must flight test to keep the FAA happy, and you must keep the FAA happy to keep your insurance company happy. But there’s much more to it than that!
What are we trying to accomplish?
Let’s review the purpose and intent of the initial flight test, with regards to the Sonex. After all, the Sonex has been well-engineered, has been proven to have stable & pleasant handling characteristics through hundreds of flying examples, and the airframes are largely built exactly to plans. Areas like CG range, control surface sizing, surface throw and linkage routing, and wing/tail incidences are all standardized... Why go thru all the effort to flight test when we already know the design flies great? The real answer is COMPETENCE and CHARACTER.
Competence and Character
Our goal is not to prove the basic suitability of the design (that’s already been done), but rather to prove that our examples are executed safely and in line with the factory design. We need to verify each plane behaves as expected, and establish a few key performance speeds and metrics along the way. What we’re really doing is proving the CHARACTER of our planes is CONSISTENT with that of the prototype. Most people get this part right in their flight testing, but that’s only half the story.
The part about competence comes in when thinking about the pilot. COMPETENCE is built by the pilot over the flight test period by gaining familiarity with the FULL flight envelope of the plane. It’s worth stressing the FULL part again. It’s not enough to simply fly for extended periods doing basic operations. Instead, you need to explore every part of the envelope, and prove the reliability of every system. Don’t just do incipient straight-ahead stalls, do slow flight, full stalls, power-on and power-off stalls. Do turning stalls, 30 degrees, and 45 degrees, with flaps out, and with various power settings. When you’ve done all that, do some extended slow flight, then practice deep stalls where the plane is held in a mushing-descent using rudder to prevent falling off on a wing and into a spin. This is real spin-avoidance practice! If the thought of doing that worries you, you may have some additional work to do in your plane to build competence.
The point of the extensive testing is to peek into every aspect of the plane’s personality, and train you to understand and anticipate it. Ask yourself: “What speed will I first observe airframe buffeting while in a 45 degree bank?” If you don’t know the answer, then you might need some additional flight testing - both to determine that speed and to ingrain it into the pilot. And there are others: “What are the indications of an impending stall? What do they look, sound and feel like in a Sonex?” The plane is talking to you, but you might not be listening. You may need that experience TOMORROW, and if you don’t already have it, there won’t be time to think your way through it. If you haven’t already done the testing and ingrained the results into your skill set it will be too late. Pilots don’t “rise to the occasion” in an emergency, they “fall back to the level of their training”.
Flight testing should be looked at as an OPPORTUNITY to establish the basics of your airplane, but also should be a courtship of your new airplane. You should get to know every aspect of your plane. You should know where the edges of the envelope are, and where the skeletons might be hiding. You should think through emergency situations in practice, before you need them for real. Flight testing should prove the pilot as much as it does the plane. Someday you might actually depend on the knowledge you gain during your flight test. Don’t subvert the process…. Your life might depend on it.
Part 2 - Flight Test Mechanics