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Determining high point from flight recorder data
So you fly a great XC and download the flight recorder
data. See You shows a trace and the question arises.... DID I go over 17,999'? Who has the procedure to use to determine this from flight recorder data? Does the procedure take into consideration instrument error? Yes! But how? Does the procedure correct for non-standard pressure? Yes! But how? |
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Determining high point from flight recorder data
Sam Discusflyer wrote: So you fly a great XC and download the flight recorder data. See You shows a trace and the question arises.... DID I go over 17,999'? Who has the procedure to use to determine this from flight recorder data? Does the procedure take into consideration instrument error? Yes! But how? Does the procedure correct for non-standard pressure? Yes! But how? SeeYou allows you to set QNH under Edit/Flight properties. This is the same as setting your altimeter to field elevation at takeoff. Note that this adds the logger calibration error at field elevation to the pressure correction. So you should adjust this by removing the error from your calibration trace, It also does not account for pressure changes over time and distance. So setting an official altimeter setting is better. You can check the amount of pressure change over the flight by looking at the landing altitude compared to field elevation. You can split the difference by adjusting the pressure correction value. Your peak altitude recorded is also affected by logger calibration error, so you need to remove this as well. This gives your best estimate of peak altitude using the logger data. Your error budget will vary depending on whether your logger uses a static port or cockpit static pressure, the temperature at altitude, quality of the logger pressure sensor, etc. As far as instrument error, your altimeter can be off by 200-300 feet at 18,000 MSL, even if it is calibrated to IFR standards and set to a current altimeter setting. If it is not, the error can be much greater. So you need to account for this in your flight planning, and allow for altimeter error by maintaining an error budget *below* 18000 MSL when you fly. If you do this, your logger error should be covered as well, and you won't have these worries. I have heard more than one person argue that if your flight reference altimeter reads below 18000 MSL, you are legal. The recent mid-air near Minden is an excellent example of why that logic does not fly. The accident investigation will use ATC logs and flight recorder data from the IFR certified jet to determine the actual altitude of the event. The glider pilot's recollection of the altimeter reading will be highly discounted. Even if you are only involved in a near miss incident, you will not be arguing from a position of strength with this. So maintain an appropriate buffer from Class-A. |
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Determining high point from flight recorder data
On Oct 17, 4:36 am, "Doug Haluza" wrote: It also does not account for pressure changes over time and distance. So setting an official altimeter setting is better. You can check the amount of pressure change over the flight by looking at the landing altitude compared to field elevation. You can split the difference by adjusting the pressure correction value. 1. Make sure your altimeter is properly calibrated. Where BFSS flies, for example, we have an official AWOS at the field that provides an altimeter setting. If setting the Kollsman to the reported value doesnot indicate field elevation +/- about 20' then get your altimeter calibrated. 2. When you fly, reset your altimeter to nearby station values. In Colorado, I very often see changes of several hundred feet over the course of a day, or between stations that are several hundred miles apart. (Warning: some of the mountain pass AWOS sites in CO seem to be reporting pressures much higher than nearby airports - I'm still waiting for a response from CO Department of Transportation about this - so use an airport reading). You can get this info either from AWOS, ATIS, tower, FSS or ATC. 3. After you land, get the daily pressure reading from stations along your route. I use Weather Underground (http://www.wunderground.com/US/CO/El...=miniWeather04) then go to the history link ner the left bottom of the page. They accept airport identifiers for location. 4. The FR reports altitude based on standard pressure of 29.92. So if you were flying over a station that reports 30.22, then you will need to ADD 300' to that reading, and then add or subtract the calibration offset for your FR at that altitude. 5. I suppose it would also help if the IGC trace could include an OUTSIDE air temperature (the CAI 302 *could* do this, but doesn't), as that will also affect the altitude readings. This is the process I use prior to submitting a flight claim for a state or national record. For the OLC, I typically just trust my memory of what I saw as indicated altitude, so won't go into an analysis unitl there's a high point shown my SeeYou of at least a couple hundred feet. I'm also sure one of our more rigorous readers can provide us with some more detailed formulas as well -Tom |
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Determining high point from flight recorder data
5Z wrote: On Oct 17, 4:36 am, "Doug Haluza" wrote: It also does not account for pressure changes over time and distance. So setting an official altimeter setting is better. You can check the amount of pressure change over the flight by looking at the landing altitude compared to field elevation. You can split the difference by adjusting the pressure correction value. 1. Make sure your altimeter is properly calibrated. Where BFSS flies, for example, we have an official AWOS at the field that provides an altimeter setting. If setting the Kollsman to the reported value doesnot indicate field elevation +/- about 20' then get your altimeter calibrated. The AIM allows up to 75 feet: 7-2-3. Altimeter Errors a. Most pressure altimeters are subject to mechanical, elastic, temperature, and installation errors. (Detailed information regarding the use of pressure altimeters is found in the Instrument Flying Handbook, Chapter IV.) Although manufacturing and installation specifications, as well as the periodic test and inspections required by regulations (14 CFR Part 43, Appendix E), act to reduce these errors, any scale error may be observed in the following manner: 1. Set the current reported altimeter setting on the altimeter setting scale. 2. Altimeter should now read field elevation if you are located on the same reference level used to establish the altimeter setting. 3. Note the variation between the known field elevation and the altimeter indication. If this variation is in the order of plus or minus 75 feet, the accuracy of the altimeter is questionable and the problem should be referred to an appropriately rated repair station for evaluation and possible correction. 2. When you fly, reset your altimeter to nearby station values. In Colorado, I very often see changes of several hundred feet over the course of a day, or between stations that are several hundred miles apart. (Warning: some of the mountain pass AWOS sites in CO seem to be reporting pressures much higher than nearby airports - I'm still waiting for a response from CO Department of Transportation about this - so use an airport reading). You can get this info either from AWOS, ATIS, tower, FSS or ATC. True altitude varies from indicated altitude based on the mean difference between actual and standard temperature in the atmosphere between the aircraft and the altimeter setting station. If you are trying to clear a mountain pass, you want an altimeter setting from a station up near the pass because it will eliminate non standard atmosphere errors below the pass. Your indicated altitude will then be equal to true altitude at the station elevation, which helps maintain proper terrain clearance. But this means the altimeter setting at the pass will be different if the temperature/altitude profile is non-standard, which it normally is. For enroute use, you should use the altimeter setting from the ATC center. Use airport settings for takeoff/landing only. And use mountain pass settings for terrain clearance in the mountains. 3. After you land, get the daily pressure reading from stations along your route. I use Weather Underground (http://www.wunderground.com/US/CO/El...=miniWeather04) then go to the history link ner the left bottom of the page. They accept airport identifiers for location. Note that sea level pressure is different than altimeter setting. The SLP corrects for an assumed atmosphere between the station and sea level. The altimeter setting makes your altimeter read correctly on the ground. If you are using an AWOS or airport station, it will give altimeter setting, and may give SLP in the remarks. If you are using a NWS site not on an airport, it will probably only give SLP, and this is not what you need. 4. The FR reports altitude based on standard pressure of 29.92. So if you were flying over a station that reports 30.22, then you will need to ADD 300' to that reading, and then add or subtract the calibration offset for your FR at that altitude. Again, only if the altimeter setting is 30.22. If the SLP is 30.22, you need the 12 hour temperature average to convert it to an altimeter setting, but this is way too complicated. 5. I suppose it would also help if the IGC trace could include an OUTSIDE air temperature (the CAI 302 *could* do this, but doesn't), as that will also affect the altitude readings. I really wish one of the logger mfg would add temp to the IGC flight log. The IGC spec does allow adding parameters to the required data fields. Not only would it make it possible to construct soundings post flight for analyzing strength of thermals, inversions, wave, etc., It would also make it possible to reduce flight test data to sea level standard conditions. This is the process I use prior to submitting a flight claim for a state or national record. For the OLC, I typically just trust my memory of what I saw as indicated altitude, so won't go into an analysis unitl there's a high point shown my SeeYou of at least a couple hundred feet. I'm also sure one of our more rigorous readers can provide us with some more detailed formulas as well Most of the formulas you could ever need are in the WIlliams Avation Formulary (including the true altitude formula under altimetry): http://williams.best.vwh.net/avform.htm |
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