Accuracy of altitude on IFE maps?
Jun 4, 2013, 4:59 pm
#1
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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?
Jun 4, 2013, 10:51 pm
#4
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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?
Jun 5, 2013, 5:57 am
#6
Join Date: Nov 2012
Location: YWG
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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.
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.
Jun 5, 2013, 7:22 am
#7
Join Date: Jan 2009
Location: MCO
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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.
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.
Jun 5, 2013, 8:50 am
#9
Join Date: Oct 2010
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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, 2013 at 8:57 am Reason: forgot the GPS part...
Jun 5, 2013, 10:04 am
#10
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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.
If everyone's a 1000' high, no problem. If some planes are 1000' high you've got big problems.
Jun 5, 2013, 10:15 am
#11
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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.
Edit: See http://en.wikipedia.org/wiki/World_Geodetic_System_1984 for more info than you ever wanted on this.