Instruments for Home Built

[wmgeorge]

Right now I studying to get my Student Certificate and passing the written test. So learning about flight instruments and just wondering Altimeters are $400 - $2000 and need to be set correctly before every flight. Since I already own a very nice GPS that when locked on can give readings including altitude to within 12 feet why the need for a barometric altimeter?

Don't know if I am going to buy or build but Light Sport is my first goal and perhaps all.

[rwanttaja]

Right now I studying to get my Student Certificate and passing the written test. So learning about flight instruments and just wondering Altimeters are $400 - $2000 and need to be set correctly before every flight. Since I already own a very nice GPS that when locked on can give readings including altitude to within 12 feet why the need for a barometric altimeter?

Because all the guidance and rules about altitudes are based on barometric altitude, not GPS altitude. You may bust through the bottom of Class B airspace if you use GPS for your altitude reference. And being hundreds of feet off the pattern altitude won't endear you to other folks in the same pattern.

Buying individual instruments is SOOOOOO old school. An Electronic Flight Information System (EFIS) will give all the needed data on a single screen, for less than new individual instruments would total.

http://dynonstore.com/#!/EFIS-D6-Sys...egory=13788228

Ron Wanttaja

[wmgeorge]

I would think a GPS altitude would be more accurate to real ground location than a barometric. So a little online research shows that since everyone else is using barometric pressure they are all going to be off by the same number of feet. But really the GPS is a more true reading of where the ground level is actually at.

The Electronic Flight Information System (EFIS)is nice, but how many air craft actually have them installed?

[Auburntsts]

GPS altitude might become the standard one day, but I don’t think it will be anytime soon. Until it does, the difference is significant enough (I typically see a difference of hundreds of feet) that you’ll have to stick with barometric simply because it’s the current standard and as Ron pointed out, all of the current rules and procedures are based on that standard.

[wmgeorge]

I am wondering how Chris got permission to sail to the new world with those 3 ships equipped with those new style compasses.

So even if the GPS system is good enough for air force bombers and missiles to complete their missions, its not good enough for modern aviation. I was in the USAF mid 60s and our aircraft electrical shop was right next to the instrument shop. Now I wished I paid more attention to what they were doing!!

[rwanttaja]

I am wondering how Chris got permission to sail to the new world with those 3 ships equipped with those new style compasses.

So even if the GPS system is good enough for air force bombers and missiles to complete their missions, its not good enough for modern aviation. I was in the USAF mid 60s and our aircraft electrical shop was right next to the instrument shop. Now I wished I paid more attention to what they were doing!!

If you look on the panel of today's USAF aircraft, you'll see an EFIS with a readout of barometric altitude.

Physical height above the ground is good to avoid contact with said ground, but it's of little worth during actual navigation. My airport is at 57 feet in altitude, one about 80 miles away is at 1500 feet. Between us is a mountain range running to 6000 feet or so. Imagine trying to fly that distance with constant altitude. Your way, I'd be hauling back and forth on the stick to keep the same distance from the rising and falling terrain. With barometric altitude, it's easy to maintain a constant altitude.

Imagine, also, that I'm over a area with ~500 feet ground level near some hills to 1500 feet. Another plane is over that 1500-foot range. If I report I'm at 2000 feet (above the 500 foot ground) and the other pilot is at 1000 feet (above the 1500 foot ground), WE'RE ACTUALLY AT THE SAME PHYSICAL HEIGHT. But if I tell him I'm at 2,000 feet, and he says he's at 1000 feet...it sounds like we're well clear of each other.

Again, the ABSOLUTE altitude isn't important air traffic coordination...nor for normal operations, other that keeping the height in the positive range. What's important is a common altitude standard that all aircraft in the area can comply with. And that's where the barometric altitude comes in.

I'm guessing you aren't a pilot?

Ron Wanttaja

[Auburntsts]

I don’t understand your angst with barometric altimetry. It’s just not that big a deal. You do realize that besides not using GPS for altitude you’ll need to learn to navigate without GPS too (ground based radio Nav aids, ded reckoning, pilotage)?

[FlyingRon]

You're off on your computation of accuracy as well. WAAS GPS vertical accuracy is on the order of 10 meters (32+ feet) and as RonW points out, the GPS altitude is NOT the same as the barometric reason for two reasons:

1. GPS altitude is determined based on a stylized model of the shape of the earth. IFR vertical guidance is predicated on the fact that the individual approach in the database has the GPS altitude in it as well (and still you have to tell the GPS about the baro correction).

2. Everybody else (as Ron W says) is using baro altitude, which may not be accurate but ATC has told them what the common altimeter setting everybody should be using is.

[Frank Giger]

And GPS can play games as well.

This morning's flight, as recorded using GPS data, had my aircraft hovering fifty feet above the ground during landing and while putting her back in the hangar.

[wmgeorge]

Went out to our local airport flying service this morning to pick up a sectional map, and a young pilot was standing there. Turns out he was CFI and he said there a big changes coming from the FAA in the next few years regarding GPS and navigation in general and not to believe everything you read online.