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Accuracy of altitude on IFE maps?

Accuracy of altitude on IFE maps?

Old Jun 4, 13, 5:59 pm
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Accuracy of altitude on IFE maps?

Something I've always wondered about is the accuracy of the altitude readout on flight maps on IFE systems. During cruise, it's always a nice, round number, like 35,000 feet. I know that ATC assigns altitudes in round numbers, but it seems to me like maintaining exactly 35,000 feet is like threading a giant thread through a tiny needle but I've never seen 34,999 or 35,001 on the IFE. Do the IFE systems display what's programmed into auto pilot (instead of actual altitude that may fluctuate a foot or two) or are planes just that good at maintaining a programmed altitude exactly, not an inch more or less?
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Old Jun 4, 13, 7:19 pm
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Moving this thread to the Travel Technology Forum.
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Old Jun 4, 13, 9:56 pm
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Funnily enough, on two of my flights last week, I noticed altitudes of 34999 and 32290 on the IFE.
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Old Jun 4, 13, 11:51 pm
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Originally Posted by t325 View Post
Something I've always wondered about is the accuracy of the altitude readout on flight maps on IFE systems. During cruise, it's always a nice, round number, like 35,000 feet. I know that ATC assigns altitudes in round numbers, but it seems to me like maintaining exactly 35,000 feet is like threading a giant thread through a tiny needle but I've never seen 34,999 or 35,001 on the IFE. Do the IFE systems display what's programmed into auto pilot (instead of actual altitude that may fluctuate a foot or two) or are planes just that good at maintaining a programmed altitude exactly, not an inch more or less?
On the Panasonic IFE systems the data comes from the nav system. But I am not sure if there is a buffer that prevents it from reporting every little change, or if it is smoothed out by software before it is displayed on the IFE screen. Most commercial airliners are allowed roughly a +/- of 100 feet in the altimeter readings.
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Old Jun 5, 13, 5:09 am
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How accurate is the altimeter anyway?
I've seen a fairly significant difference between my GPS logger and IFE altitude.
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Old Jun 5, 13, 6:57 am
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I would think that the system only updates every 10 seconds or so and is not streaming data real time. As such, any minor fluctuations (plus/minus 5-10 feet) wouldn't get displayed because the plane corrected for them.

Or the system has a buffer. The top of the vertical stabilizer is often 20-60 feet higher than the lowest part of the plane. The computer might just take that as an error buffer.
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Old Jun 5, 13, 8:22 am
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Airplanes are also good at holding a precise altitude. When I've flown with a friend in a 172 and he puts the autopilot on, it holds whatever altitude he sets without drifting a foot (as long as the air is smooth).

As for GPS versus altimeter accuracy- altimeters do have to be reset every so often below 10,000 ft., and are set to a standard altitude above that height. As a result of that standard setting, I'm guessing it would make it more inaccurate then a GPS. The reason for the standardization is so all the planes fly at the same flight levels regardless of variations in local atmosphere. At 10k feet, the odds of flying into a land-based obstacle is nearly nil.
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Old Jun 5, 13, 9:50 am
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I took this photo of a GPS I happened to have with me when in a UA CRJ-700 a few years ago over Utah or thereabouts.

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Old Jun 5, 13, 9:50 am
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Originally Posted by kkjay77 View Post
How accurate is the altimeter anyway?
I've seen a fairly significant difference between my GPS logger and IFE altitude.
Air data computers (system that measures the various air pressures and converts it to airspeed/altitude indications) are fairly accurate overall. In doing altimeter scale checks I have had differences of about +/- 5-20 feet at 30kft+ settings between the test box and plane. There are allowable differences in the indication which increase with altitude.

Haven't tried a GPS receiver during flight, but I figure the error radius on the GPS and software settings for IFE altitude display may cause interesting differences.

Last edited by CLTmech; Jun 5, 13 at 9:57 am Reason: forgot the GPS part...
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Old Jun 5, 13, 11:04 am
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Originally Posted by LizzyDragon84 View Post
As for GPS versus altimeter accuracy- altimeters do have to be reset every so often below 10,000 ft., and are set to a standard altitude above that height. As a result of that standard setting, I'm guessing it would make it more inaccurate then a GPS. The reason for the standardization is so all the planes fly at the same flight levels regardless of variations in local atmosphere. At 10k feet, the odds of flying into a land-based obstacle is nearly nil.
This. At high altitude jetliners aren't worried about running into mountains, just other jetliners. It's much more important that they all agree on exactly what altitude the various planes are at than that the altitude is correct.

If everyone's a 1000' high, no problem. If some planes are 1000' high you've got big problems.
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Old Jun 5, 13, 11:15 am
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The big difference is that airplane altimeters measure altitude by using barometric pressure. GPS does it by calculating the difference between the 3 dimensional position of the receiver and the theoretical model of the earth's surface. The model is not completely accurate, and altitude is the least accurate of the receiver's calculations.

Edit: See http://en.wikipedia.org/wiki/World_Geodetic_System_1984 for more info than you ever wanted on this.
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