Layers of Defense
One of the current concepts used in teaching safety and risk management is the "Swiss Cheese Model". Imagine a stack of cheese slices. Each is a layer of protection between you and a failure. Now imagine that the cheese is Swiss, and has holes. The day that the holes all line up, a failure gets through, and you have an accident. The key, therefore, is to have enough layers of defense to make it unlikely that the holes will line up!
Looking at accident/mishap reports, it is pretty easy to figure out where your defenses need to be. Engine failures happen - but they are statistically insignificant compared to other causes. I don't want to start an argument, but assuming that the aircraft is built using good techniques and practices, as well as reliable components, the most likely place for a failure to occur is with the person in the pilot seat. So the most important thing that I added to the Valkyrie was a well-trained, current pilot with (hopefully) good judgment, and a conservative, questioning bent....yes, I fly IFR, I enjoy aerobatics, fly the odd photo formation mission....but I refrain from getting into situations that I have read about over and over again - maneuvering close to the ground, running low on fuel, trying to go VFR when the weather is bad....the list goes on.
So first thing to add - a Pilot with Good Judgment who recognizes his own mortality and ability to screw up, and is dedicated to continued education.
After that, I go through the list of failures that can happen in the airplane, and make sure that I have a layer of defense for each one. I accept that the engine could quit - then I'd be a glider. Why do engines fail? Rarely does it happen without some warning - we all know the conditions of the engines we own...sometimes we just aren't honest about them. I am thinking of adding a low oil level alarm, cause I have actually ruptured an oil line in a previous plane. When the oil pressure goes away, you don't have long to find a place to land!
Other than that, I have three GPS's. Lose all those? I'[ll call for help! Comm #1 goes down? Go to Comm #2. Do I need to cover more than five failures deep? Statistically...not for my comfort level. Gyros? I have a very reliable AHRS, and an independent Autopilot. Two levels - way better than people have had for many years. In short, most every critical system is redundant in some way. It's easy to design for one or two levels of failure - just do your homework.
Oh, the most common accident I see in perusing the FAA dailys? Damage to the aircraft during landing due to gear collapse (fixed gears collapse too!) or running off the prepared surface. Rairly leads to fatality, but sure bends airplanes.
So I shoot lots and lots of Touch and Goes......
Paul