iFly GPS Forum

I'm not talking about the fancy ever-scrolling gadget you see on X-Plane 9, simulators and fancy avionics; just a simple "Altitude Tape" to put along the side, similar to the "Course Tape". A visual profile view, in MSL, of my altitude. It would be used on instrument approaches where before the approach I can quickly tap the tape at my personal minimum, and it sets a colored bar there. Then as I descend, another bar representing my actual altitude would slowly slide down the tape and if/when they meet, the whole tape turns red. It'd be a great backup to the altimeter for those times when minimums are 1311 but after scanning your instruments for the 53rd time your mind somehow transposes 1311 to 1113 on the altimeter, a difference small enough to just barely kill ya. Another use could be when flying for fun - it can be set to the FAA legal "1000' above populated areas" to keep you in good graces with The Man. Or when cleared to another altitude, it could remind you when to level off. Not that I ever get distracted during a climb or descent but, you know, I can see where other people might. (wink wink).

IIRC, the maximum error permitted from an FAA approved "sensitive" altimiter is 70 feet. This is a pretty big piece of the 200 foot obstacle clearance that is guaranteed on approaches.

Before relying on a GPS-derived altitude tape I would be interested to know what the maximum error that can occur in GPS reported altitudes, which are calculated as altitudes above a theoretical datum surface. This number would be GPS-dependant, since it is the GPS that reports altitude in the NMEA data stream. It is not calculated by the EFB AFIK. This would have to be a non-WAAS number, since we have no way of knowing whether the GPS is getting or using WAAS corrections.

Well, it certainly isn't going to agree with the panel altimeter very often but what I don't know is whether the errors can be large enough that relying on the GPS altitude in IMC could be dangerous. i.e., can the errors be more than 70 feet? It isn't a backup if it is dangerous to use.

Re altitude deviations I guess that is a matter of taste. I have a fair amount of G1000 time and if I could, I would ditch the tapes completely and replace them with analog displays. Same reason most of us wear digital watches with analog displays -- analog displays can be read and interpreted more quickly than numbers. I can watch the big hand and keep it straight up or straight down quite easily. To read and interpret the numbers on a tape inarguably requires a little more mental horsepower.

If the tape includes some kind of audible alarm bugs and the tablet is patched into the audio system then it could be useful, but just visual information would not be of interest to me. I have very effective visual information already in the panel.