Altitude tape, similar to the Course tape

We have a new Forum! Go here to get started: https://adventurepilot.community.forum.
The new forum is easier to use and much more capable than the old, we hope you will join our community!

Below is a copy of the old forum. This will remain available for a short period so you can access and review the information contained here. To continue a conversation, or start a new one, please register and create a post at our new forum location.

Search

Home

Discussions

iFly Wish-List

Altitude tape, similar to the Course tape

3/3/2015 9:41 AM

citabriabob

Joined: 4/14/2010

Posts: 156

Re: Altitude tape, similar to the Course tape

I find the GPS altitude helpful in correcting an error in real altimeter setting (it happens!)

Many, many moons ago, I had a controller give an erroneous settting in IMC, which persisted through several "say agains" until he finally caught himself the fourth time. One glance at my iFly will give me an order of magnitude.

Robert Szego
Bellanca-Champion Club http://www.bellanca-championclub.com
Aeronca Aviators Club http://www.aeronca.org

3/3/2015 11:00 AM

Walter

Joined: 7/8/2009

Posts: 2778

Re: Altitude tape, similar to the Course tape

GPS Altitude is an interesting beast. It will commonly be more accurate than pressure altitude. But then you fly over a noisy power line, or you bank hard and your wing blocks a key satellite, or a solar storm hits, etc - and suddenly your altitude accuracy skips a beat with no warning.

Pressure altitude is calibrated based on a sample at ground level, so can be inaccurate at higher altitudes. But at higher altitudes, accuracy isn't important...what's important is that everybody is using the same scale. So 8500 MSL in airplane A = 8500 to airplane B = 8500 to the controller. It doesn't matter so much if an unstable air mass means that 8500 reading is, in reality, 8250 MSL. Pressure altitude (properly calibrated) becomes accurate again as you approach 0 AGL, where you need it to be accurate.

So GPS Altitude - even when it's perfect - may not reflect the "properly inaccurate" pressure altitude you should be using.

And GPS Altitude - when it's not perfect - can easily be hundreds of feet off. And this error is not relative…it will be equally wrong at 8500 AGL or 50 AGL.

IMO for these reasons GPS Altitude, and any altitude related instruments or displays in the iFly or any other app, should be treated for what it is: An interesting bit of information. It should never be considered a replacement or backup to a pressure based altimeter.

My 2 cents ;-)

-Walter

Walter Boyd
President, Adventure Pilot

3/3/2015 3:44 PM

Tinker

Joined: 2/9/2014

Posts: 109

Re: Altitude tape, similar to the Course tape

Walter Boyd wrote: It should never be considered a replacement or backup to a pressure based altimeter.

My 2 cents ;-)

-Walter

Yes. You are in a tough spot: You have to deliver feature after feature in order to remain in the competiton, but some of the features are things that many people really shouldn't have. For example the comment above about checking the approved alimeter with a GPS altitude reading -- not dumb if the altimeter setting is off by an inch, maybe, but seriously dumb to adjust the altimeter by 0.1"/100 feet based on a GPS altitude.

Different subject: Congrats on having the discipline to not release code untill it is completely tested and ready. No one will remember if a release is a week or a month late, but everyone will remember if you release something that is not ready for prime time. Years in the technology business have taught me that it is the late adopters that sleep peacefully and it is the early adopters that were out front and consequently have arrows in their chests. There are fewer arrows with a quality-driven release schedule than there are with a schedule-driven relase schedule.

3/4/2015 6:40 AM

Cobra

Joined: 4/20/2011

Posts: 2602

Re: Altitude tape, similar to the Course tape

OldPilot wrote:

That is location, not altitude. Again, think about the geometry. You're asking a couple satellites 22,000 miles overhead to triangulate your altitude by looking almost straight down on you.

To be clear, the issue is not about accuracy of the system in general. Panel-mounted WAAS GPS systems (i.e., Garmin 430W, Avidyne IFDt540, etc.) are certified to support LPV approaches to minimums equal to an ILS (200' and 1/2 mi vis) because the WAAS-supplemented GPS altitude is accurate enough to support those minimums.

The issue is about how accurate a non-certified, portable solution like an iFly or an iFly + a non-certified GPS source like from a SkyGuardTWX receiver is.

Part of the certification criteria for the panel-mounts involves lots of testing with specific GPS antennas, specific antenna placement on the aircraft, specific antenna cable requirements, etc., etc. to ensure consistently reliable signal integrity.

As Walter pointed out, the portable installations don't involve any of that, and so the signal is not as reliable.

So signal integrity of portable GPSs is really the primary concern, not any inherent weaknesses in the WAAS GPS system itself.

3/4/2015 9:28 AM

Tinker

Joined: 2/9/2014

Posts: 109

Re: Altitude tape, similar to the Course tape

To amplify Cobra's point: TSO GPS navigators also continuously monitor satellite geometry and availability and will not offer the pilot an approach if the approach's performance requirements cannot be achieved. Consumer boxes do none of this. Look up "RAIM" for more ...

Page 3 of 4

Altitude tape, similar to the Course tape

iFly GPS Forum

Adventure Pilot LLC