When flying at 5,500 feet, the FlightAware track log reports a consistent 5,400 along the way (autopilot engaged showing 5,500). Today, the track log was reporting 6,200 the entire flight while I was at 6,500. I know there might be some variance due to difference in pressures along the way, but not 100 to 300 feet for 50 or so nautical mile trips. Is FlightAware incorrect or per chance could it be something wrong with my plane?
I see that a lot while flying. We have TCAS and go past aircraft that are reporting 100 feet higher or lower than they should be, it is a difference betweenin the mode C reporting and their altimeter. An avionics guru told me that you are dealing in microvolts and adjusting it to the nearest 100 feet is often as close as they can get especially in older light aircraft systems.
You’re probably looking at a position from a source that reported pressure altitude uncorrected.
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Note that Mode S / ADS-B transponders (and TCAS) always report uncorrected pressure altitude. If you’re comparing those to an altimeter that’s corrected for local pressure then that’s where the difference comes from.
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Thank you. However, it’s a 2015 Cirrus with the G1000.
Thank you, but it’s a 2015 Cirrus with the G1000.
Even up to 300 feet difference?
Sweet airplane but could still have very minor pitot-static errors that are putting the reporting off by 100 or 200 feet. Don’t forget your mode C puts out a flight level that is then corrected by ATC radar depending on whether you are above or below 18,000 feet. rounding and microvolt differences could be the difference.
frostfam5
September 10 |
Thank you, but it’s a 2015 Cirrus with the G1000. Visit Topic or reply to this email to respond.
In Reply To
porterjet
September 10 |
I see that a lot while flying. We have TCAS and go past aircraft that are reporting 100 feet higher or lower than they should be, it is a difference betweenin the mode C reporting and their altimeter. An avionics guru told me that you are dealing in microvolts and adjusting it to the nearest 100 fee… Visit Topic or reply to this email to respond. To unsubscribe from these emails, click here.
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Sure, 300ft is around 10hPa so an altimeter setting of <1003hPa or >1023hPa would cause that much difference.
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I realize the question was asked a year ago. Here is an update.
We were watching an aircraft making an instrument approach today and Flightaware showed it well below descent minimums before it made a missed approach. This led to research and discussion about Flightaware capabilities and a search of this forum. The answer was not found here.
We researched and came up with the following: All aircraft that report altitude report it based on a fixed 29.92 \1013.2 standard sea level pressure . However, below 18,000 feet, a pilot flies based on the aircraft altimeter readout that has been set to the current local pressure (QNH) for the area (It now shows indicated altitude). When set to the local pressure at their departure airport the altimeter should read close to the published airport elevation. Once airborne the pilot resets the pressure correction based on info passed by ATC or by monitoring airport weather transmissions.
ATC computers are updated with the local pressure and the information is used to correct the 29.92-based reported altitude (pressure altitude) for the reporting aircraft. Thus, in theory, when the pilot reports being at 5,000 feet, the controller should see a 5,000 foot altitude on the screen. If the pressure in the area was 30.22 then the difference (30.22 - 29.92) of .3 inches (of mercury) would result in a transponder reported altitude of 4,700 feet which would then be corrected by the computer and sent to the controller’s screen as 5,000.
So all planes flying at the same pressure altitude within a given airspace should, in theory, show the same indicated altitude.
However, altimeters can have allowable errors of between 20 feet and 120 feet from the surface to 18,000 feet. The encoders that transmit the altitude data to the ground need only agree with the altimeter +/- 125 feet. Further, the older encoders only report in 100 foot increments (newer in 25 foot). So, ignoring other factors, a plane actually flying at 5,049 feet could report 5.000 and one at 5,5051 report 5,100 even though they were just two feet apart in actual altitude.
Flightaware’s minimum altitude reporting interval (at least here in the USA) appears to be 100 feet. When you combine some of the factors above you can see that if one aircraft has a set of positively biased factors and another has a series of negatively biased numbers both could be reporting altitudes that differ greatly.
Combined with Flightaware using both ATC (corrected) altitudes and raw (Flightaware only sources) and even greater than normal tolerance differences might be seen.
ATC in the USA cannot accept an electronic altitude that differs more than 300 feet from a pilot reported altitude.
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I have the same issues and just had 2 year cert done with no issues!! So more of a system and reporting glitch not aircraft related as far as I can tell