WAAS Up?

Your hand-held GPS works great and gets you where you want to go. Why would you want to take up precious panel space with a version that costs three to five times as much? Those panel-mount versions are just bigger, heavier computers, so shouldn’t they be cheaper?

Understanding starts when you turn on your GPS and get a cute little map of GPS satellite positions; Is that important? Does it matter where the satellite is positioned? You need only three, right?

The answers are yes, yes, and sort of. For now…

We sat in on a “Wings” seminar recently where the subject was WAAS; what is it, why is it, why you should know, what does the future hold, and why should you, the day VFR, don’t-go-more-than-an-hour-or-two-from-home pilot care.

The speaker, Larry Oliver from FAA Flight Standards was very clear on a couple of things: one, VOR stations are expensive to build and maintain hence they’re not being repaired as they fail; two, GPS is here to stay but portable units will never be certified for IFR use.

It’s that last one that caused some mutterings of “Well, why aren’t they?” from the audience.

The answer lies in three factors: antenna position, satellite position, and clock accuracy.

First there’s the unpredictability of antenna position; On your lap, tied to the yoke, or mounted to the side of the window frame all look nice and work fine for day-VFR, but the antenna is the key; without it having a clear shot at the sky there’s no way to ensure that it’s receiving as many sat signals as possible.

When your vanilla GPS sees three satellites it can give you a position fix within about 100 meters. And that’s good enough to get you to that two-hundred dollar hamburger (that’s the price of AvGas for you). But to get the accuracy needed for night flight to a cloudy airport you need better than that.

Second, if the satellites are grouped together it’s not going to be a high resolution picture. And if they’re low on the horizon the signal degrades in the slant-distance through the atmosphere.

And third there’s the clock factor; Contrary to popular opinion, the clocks on the satellites are not those hyper-accurate atomic models that keep time to one second in a gazillion years. Your portable GPS takes the signal from several satellites and averages the answers to give you a position. But because it’s an answer with a relatively fuzzy key factor in the calculation, you get a relatively fuzzy position of somewhere in a 100 meter circle.

The WAAS system, however, uses those satellite signals and compares it to ground based super-clocks. Mind, this is so precise that even at the speed of light, the distance to the satellite is a factor. Indeed, without Einstein’s Theory of Special Relativity factored in, the system wouldn’t work.

The WAAS system takes those satellite signals in on a net of ground stations, corrects them, and broadcasts to a pair of geo-synchronous satellites over North America, which then sends them to the WAAS-enabled receiver on the panel of your airplane.

The result is that where your vanilla GPS is accurate to 100 meters, a WAAS-capable GPS refines your position to within 7 meters.

Additionally, the more expensive WAAS certified GPS on your panel is heavier because it has the circuitry to receive that signal and check it for errors. If something is amiss it will then inform you within six seconds that all is not right and you can pull up for a go-around or whatever you need to do, but you know better than to continue thinking that you’ve got good information.

This will allow airliners to use parallel runways in poor weather; shorter transcontinental routes with closer spacing; and shorter approaches to landing. And all of this is done without airport-based hardware thereby enlarging the number of IFR-capable airports. With this in mind it makes sense to spend money on new WAAS stations instead of maintenance on VOR and Loran stations.

So, keep those VORs for a few more years, but it won’t be long until they’re sitting on the shelf with the sextant. But should you buy that big-bucks panel-mount GPS for your baby? Only if IFR is in your future.

If you’d like to know more, visit the FAA web site.

Friday at the Golden West Fly In

Friday's attendance at Golden West was small, only about 30% of last year's Saturday attendance. But then I didn't see last year's Friday, so here's hoping that it's up today.

Two Harriers and two F/A-18s gave a good show, as always, and you had to keep one eye on the sky for the frequent fly-by of something out of the ordinary including a U-2 and a C-130.

The real interest on the ground, though, was the number of LSAs being shown. There were quite a few new and interesting aircraft even though the on-the-wheels versions were all in the $120,000 range. That's still too pricey for my pocket and, probably, a lot of others' given the news from Wall Street et al.

If civil aviation is to avoid becoming un-civil, the path will be led by the homebuilders.

The expansion of computers and glass got a boost with the display by Vertical Power. Where the market is nearing glut-stage vis-à-vis flight instruments, Vertical Power is looking inward to examine the health of the aircraft. The display goes beyond listing the voltage in each circuit; it starts by displaying a check list tailored to each portion of the flight and, similarly, displays the appropriate system. For instance, when starting the engine, oil pressure and rpm take the fore. Taxi mode puts those away and displays cylinder head temperature. Takeoff mode brings up manifold pressure.

Similarly, all the electrical parameters are displayed with diagnostics and alarms that certified aircraft can only dream of.

What the customer receives is a package consisting of a display and a black box (it's actually red) weighing only about 5 pounds. While not entirely plug and play, it does make the installation substantially easier in that the circuit protection is entirely within the system and is all solid state.

If you're at that stage of building where the fear of wires is looming large, surf on over to Vertical Power to take a close look.

Also of interest, even if you're not scratchbuilding, is the display by Stewart Systems. They're showing a method for both covering a fabric aircraft and then painting it. No stitching for the former and no smell for the latter make this an especially easy procedure.

We'll be testing this paint in an upcoming issue of KITPLANES.

Batteries: Got a Question? Get an Answer!

We're working on an interview with some battery experts, and this is your chance to ask the tough questions. Have you wondered if dropping an aspirin in will help recover a dead battery (or was that chicken soup)? Then there's the story that charging a battery with it on a concrete floor is a bad idea. Really?

Some of the questions we've thought of are below. Send us yours and if we use it, you'll get the credit for asking.

  1. Why are aircraft batteries so expensive?
  2. Besides Gill and Concorde, who makes aircraft batteries?
  3. What’s the story of these batteries that have multi-cylinders for sides?
  4. Why does a car battery last so much longer than an aircraft battery?
  5. Are Li-ion (lithium-ion) batteries suitable for aircraft?
  6. Is there an easy way to know which batteries should never be run to flat?
  7. What’s the “old” technology vs. the “new”? If we’ve updated the panel, how about the battery?
  8. Is there a way to recover a dead battery?
  9. What are the failure modes and how do I avoid them?
  10. Is a solar-powered trickle charger a good idea?
  11. How do de-sulfators work (and what's sulfation)?
  12. What should I do with the battery if I don’t fly but once a month? I’m not going to pull it out and store it every month, so is there some easy way to care for it that will only cost me 10% of its life?
  13. Is there really any difference between an auto-parts store battery and an "official" aircraft battery? Can I use the cheaper version in my Experimental airplane?
  14. Vents, screw-on caps, totally sealed. Why the difference?
  15. How can I check the condition of the battery if I can’t get a hydrometer sample? See question 14.
  16. Overcharging is “bad”, but is undercharging any better?
  17. I left the charger on too long and boiled out the liquid; is it a goner?
  18. (your question here!)

Bob's Jabiru: Building a Solid-State Circuit Breaker Panel

With instrument panels now being populated by flat screens, digital readouts and all manner of 21st century silicon wizardry, it seems odd to rely on 60-year-old technology to protect them from power spikes. Specifically, the mechanical circuit breaker. Have you ever seen one on a computer?

Wondering what was new in the realm of electrical protection, we did some research the modern way, i.e., on the Internet, and came across an article by our own Jim Weir on the subject of polyfuses.

These solid-state circuit breakers are a matrix of plastic and carbon encapsulated in a dime-size wafer. When cool, they pass power through the carbon; when hot, the plastic melts and the carbon filaments are no longer connected. Take away the cause of the excess heat, the filaments reconnect and the circuit is reset.

With that in mind I decided to use this technology on my current project, a Jabiru J250. It turns out to be an exceptionally easy task; if you can figure out the wiring for the panel, you can build this with just a bit of help.

My assistance came in the form of a good friend, Rob, also a pilot, who is rather an expert in electronics.

Rob got a bit fancy, though. Your old mechanical circuit breaker has a satisfaction factor in that when it pops, you can see it. Granted, satisfaction doesn’t extend far if it does so in flight, but you see what I mean.

The polyfuse, however, is worse than a DOS error message for information. You don’t even get Gates speak; it just opens up. Rob decided to rectify (electrical pun intended) that by recognizing that the polyfuse continues to pass a minute amount of power even when tripped. An LED requires little power, so putting one in parallel with each circuit means that when it trips, the light goes on.

Now that the circuit is built, I’ll be installing it and reporting on its performance.

Seaplane Splash-In at Lake Tahoe

Twenty-four seaplanes were expected at the Mike Brown Splash-In on Memorial Day. However seaplane pilots prefer their water under the fuselage, not on the windshield, and only three local aircraft were able to make it to Obexer’s Boat Yard on the west coast of Lake Tahoe, California.

It was interesting, though, to compare the takeoff at 6225 feet elevation of a C180 turning a turbocharged O-540, a DeHavilland DH-2 Beaver and a SeaRey, well known to KITPLANES readers (cover subject, May 2007). John Spratt’s turbocharged N48CJ was off in less than 15 seconds; the others took as long as 90.

Good food and the beauty of the region lifted the spirits of the attendees above the 44°, 10 knots of wind and the occasional rain shower. You might want to consider making plans to attend the event in 2009.