Page 1 of 5 123 ... LastLast
Results 1 to 10 of 42

Thread: Instruments for Home Built

  1. #1

    Join Date
    Apr 2018
    Posts
    66

    Instruments for Home Built

    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.

  2. #2
    rwanttaja's Avatar
    Join Date
    Jul 2011
    Location
    Seattle
    Posts
    2,035
    Quote Originally Posted by wmgeorge View Post
    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

  3. #3

    Join Date
    Apr 2018
    Posts
    66
    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?

  4. #4
    Auburntsts's Avatar
    Join Date
    Jul 2011
    Location
    Dumfries, VA
    Posts
    250
    GPS altitude might become the standard one day, but I donít think it will be anytime soon. Until it does, the difference is significant enough (I typically see a difference of hundreds of feet) that youíll have to stick with barometric simply because itís the current standard and as Ron pointed out, all of the current rules and procedures are based on that standard.
    Last edited by Auburntsts; 05-10-2018 at 01:02 PM.
    Todd Stovall
    PP ASEL - IA
    RV-10 N728TT - Flying
    My builder's log (which is woefully out of date): www.mykitlog.com/auburntsts
    WAR DAMN EAGLE!

  5. #5

    Join Date
    Apr 2018
    Posts
    66
    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!!

  6. #6
    rwanttaja's Avatar
    Join Date
    Jul 2011
    Location
    Seattle
    Posts
    2,035
    Quote Originally Posted by wmgeorge View Post
    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

  7. #7
    Auburntsts's Avatar
    Join Date
    Jul 2011
    Location
    Dumfries, VA
    Posts
    250
    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)?
    Last edited by Auburntsts; 05-10-2018 at 02:15 PM.
    Todd Stovall
    PP ASEL - IA
    RV-10 N728TT - Flying
    My builder's log (which is woefully out of date): www.mykitlog.com/auburntsts
    WAR DAMN EAGLE!

  8. #8

    Join Date
    Apr 2018
    Posts
    66
    So for navigation and proper air space between air craft the standard barometric altimeter is used, but for true how far above the ground am I for sure, the GPS comes in handy. Got it. Thanks guys.
    Last edited by wmgeorge; 05-10-2018 at 03:41 PM.

  9. #9
    Gary.Sobek's Avatar
    Join Date
    Jul 2011
    Location
    Gold Hill, NC
    Posts
    9
    I have an IFR Approach Certified GPS in my airplane and have more more than a decade. One thing I can tell you about GPS altitude is the it is never correct. There have been many times that I am sitting on the ground adjusting my altimeter only to have my IFR Approached Certified GPS tell me that I am over 200-feet below ground level.
    Gary A. Sobek
    EAA Lifetime Member
    A&P, Homebuilder, Pilot

    When once you have tasted flight,
    you will forever walk the earth
    with your eyes turned skyward,
    for there you have been,
    and there you will always long to return.
    - Leonardo da Vinci

  10. #10
    Auburntsts's Avatar
    Join Date
    Jul 2011
    Location
    Dumfries, VA
    Posts
    250
    Quote Originally Posted by wmgeorge View Post
    So for navigation and proper air space between air craft the standard barometric altimeter is used, but for true how far above the ground am I for sure, the GPS comes in handy. Got it. Thanks guys.
    That's not really true either. GPS height is based upon a flat reference ellipsoid modeled to approximate the geodetic mean sea level. The GPS receiver uses this theoretical sea level estimated by the World Geodetic System (WGS84) reference ellipsoid in conjunction with the GPS constellation to establish its height above the ellipsoid model, not the actual ground. In simple terms, the reason for this is the model and Earth don't match because the Earth doesn't have a geometrically perfect shape.
    Last edited by Auburntsts; 05-10-2018 at 04:48 PM.
    Todd Stovall
    PP ASEL - IA
    RV-10 N728TT - Flying
    My builder's log (which is woefully out of date): www.mykitlog.com/auburntsts
    WAR DAMN EAGLE!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •