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Here is a look at a very simple panel for our Wagabond. A few days ago it was a brown grocery bag of instruments, some old, some new, and a 8.5″ x 37″ sheet of 6061 T-6 .060″ thick. I am going to stick it in the plane later tonight. The actual time to do the layout, cut the holes out, paint it and assemble it was about 4 hours. Truth be told, I spent several times this long thinking about doing it, contemplating having it cnc routed, trying to think of some clever way of doing it. In the end, the correct answer was just getting out the flycutter, the ruler and a fine point sharpie and going to work on it. I did this after Dan pointed out that I always talk about old school craftsmanship, so I might as well exercise some of it. It pays to have friends who call you out from time to time.
Above: I put all the instruments on the pilots side. The plane has dual controls, but I just wanted things organized in front of the pilot. The plane has been flying with stick controls and a center throttle. I am leaving the sticks alone, but moving the throttle to the left hand side. For some reason, I like throttle on the left with sticks and throttle in the right hand with yokes. I can fly either configuration either way, but it feels awkward.
From the left: The button is the starter. I do not like key starter switches, they are prone to bad contacts, and having your key chain hanging from it will lead to bumping the switch off in turbulence at one hundred feet AGL (ask me how I know this.) The button in this position can be hit with the left index finger without the hands moving from the stick or throttle. I wire this to be capable of being cranked only when the master is on. (the master is on a lower subpanel.)
The next column is topped by the volt meter. I do not like nor condone the use of ammeters. In this case, if the volt meter reads 12.8, just the battery is working, anything above this shows that the charging system is on line and ahead of the power demand. In operation this shows 14.5 volts or so. Below this is the hole for an on-off-on snap action, MS rated switch for the ignition. This does not run through the master. It runs directly from the battery (with a 30 amp fuse 2″ from the battery, protecting the line from burning in a dead short, but never allowing the fuse to blow from running the ignition.) Again no key here, the plane is parked with this switch and the master in the off position. This has the least connections and contacts, and thus is the most reliable way of doing it. (I do have a remote, very loud security warning system for the plane, it has a three digit security code: 3 – 50 – 7.)
Next column, a 12 volt turn coordinator. I only put this in because I had it laying around and it was the easiest way to put a skid slip ball in the plane. I look at the ball more than any other thing in a plane. Dan tells me he never looks at one, and wouldn’t bother to put it in. I really doubt that not having one is the secret to Dan’s airmanship, so I will just keep putting them in my planes. Under it is an altimeter from a 1970s Cessna.
The next row is a used airspeed with a range that is close to the planes envelope. I had removed a 200 mph true airspeed that was in the Wagabond. Under it is the Stewart Warner 82636 Tach.
The next row is three autometer mechanical gauges. Top is oil pressure, oil temp and EGT. There is a 1/4″ hole below the EGT for a mini DPDT switch so I can have two senders, one in each side of the exhaust.
Next column starts with a suction gauge that is only plugging the hole until I buy a micro radio to put in the hole. Beneath it is a mechanical MAP gauge, and under it is a CHT from a certified plane, probably something WWII vintage. Notice that none of the gauges for the engine have electrical sending units. Yes, there are probes, but these are non powered bi-metal senders, not the kind of senders associated with electrical instruments or glass cockpits.
Williams axioms of instrumentation:
A) It is only as reliable as the sender; In the last 10 years, every instrument error I have seen on builders projects or worked on, was related to poor electrical senders. It doesn’t matter how great your glass cockpit is if the specified senders suck eggs. I am not suggesting electrical things are bad, but they are not fool-proof.
B) I would rather have no instrumentation than have ones that occasionally gave false information. Anything that cries wolf often will have you doubt the warning when something really is going wrong. One of the things I hate about automotive O2 sensors running lean/rich gauges is that they read “green” (normal operation) when the sensors fail or lose their ground, which happens a lot. As an absolute design requirement, nothing in real aircraft instrumentation reads OK when it fails. IFR Flight instruments actually display flags when they fail, it’s that important.
C) It is better to have little information about a reliable part than the reverse. By deleting the TC, EGT and using less expensive gauges I could make an even simpler panel that would cost $275 or so. I am setting the Wagabond up with a $1,000 overhauled MA3-SPA carb. These have excellent mixture distribution, and they don’t ever run lean unless you command them to do so. I don’t need 6 CHTs and 6 EGTs for my engine. I believe that anyone who has a $1,000 6 egt/cht monitor keeping tabs on a $275 motorcycle carb is making a serious judgement error.
D) Mechanical gauges with needles offer unique information. I can watch a mechanical oil pressure gauge and if the needle flickers while going up I can tell the bore of the oil pressure relief isn’t smooth enough. You can see rate of change with a needle. Watch the hand on an altimeter in climb. If it is moving as fast as a second-hand on a clock, the plane is climbing 1,000’/min. human eyes are very good at watching rates of change. If your watching climbing EGT on a needle, as it slows you can guess where its going to peak far better than looking at a number displayed on a screen.
E) Electrical instrumentation lies depending on voltage and grounds. If you have electrical instrumentation, warm up the engine, run it at a steady rpm and note the oil pressure. Then take the charging system off line. You need to be aware that many systems will read different numbers depending on if the system is seeing 12.8 or 14.5 volts. Digital electronic systems run between 0-5 volts, and they are particularly susceptible to poor grounds, mediocre crimps and very slight corrosion on connections, three things that happen is homebuilt aircraft. More than one pilot has been on the edge of a heart attack aloft when watching his oil pressure sink to zero, only to find out later it was just a bad ground on an electrical sender.
F) Mechanical instruments are Bad-Ass. On my workshop shelf I have a manifold pressure gauge that reads to seventy five inches of manifold pressure. (22 pounds of boost) It is from a Lockheed P2V-7 Neptune which had 3,700 hp turbo compound radials. It glows in the dark because the numbers are painted on with radioactive paint. There is a pretty good chance that this gauge flew in the cuban missile crisis or attacked the Ho Chi Minh trail. If it could talk, it would tell you that the cold war wasn’t always cold, and it would remind you to think about the people who fought it, but it can’t say anything. It just sits out there, night after night, its faint green glow quietly remembering thousands of hours aloft, in the company of men, men now mostly gone…. In another 15 years, all of the glass cockpits of today, all the MGL stuff from South Africa, all the I-Pads built by virtual slave labor in China, all the garbage like Blue Mountain and Archangel will all be lining the bottoms of landfills accompanying used diapers and copies of People magazine featuring the Kardashians. 15 years from today, my MAP gauge will still be quietly glowing, trying to remind people that there was a time when being an aviator was about skill, reliability under pressure and courage.-ww