Certified logger altitude recording?

[[email protected]]

How do current certified loggers determine altitude? Being some are plumbed to external pitot/static and some are cockpit pressure only, what differences are expected?

Does your mechanical altimeter (especially 57mm size) read close to your "glide caculator" altitude?

Are loggers solely dependent on pressure altitudes or is GPS altitude comparison ever included in the algorithms?

Seems one would be more likely to "bust" min/max altitudes using cockpit pressure loggers.

[John Godfrey (QT)[_2_]]

On Monday, March 5, 2018 at 2:48:06 PM UTC-5, wrote:
> How do current certified loggers determine altitude? Being some are plumbed to external pitot/static and some are cockpit pressure only, what differences are expected?
>
> Does your mechanical altimeter (especially 57mm size) read close to your "glide caculator" altitude?
>
> Are loggers solely dependent on pressure altitudes or is GPS altitude comparison ever included in the algorithms?
>
> Seems one would be more likely to "bust" min/max altitudes using cockpit pressure loggers.

IGC data loggers record an uncorrected (29.92 assumed) pressure altitude in an integer number of meters. They may also record a GPS altitude as a separate item. Scoring software calculates a fixed correction based on the known surface height at takeoff that is applied throughout the flight. A correction computed at landing is used in the US at the finish if more favorable.

In the US competition environment, GPS altitude is not considered unless baro altitude is not available from the logger (i.e. it is not an IGC logger but is an IGC "position recorder").

If the logger is not plumbed to the aircraft static, meaningful differences can occur between your "steam gauge" altimeter and the logger. This is a function of how your cockpit pressure varies up or down relative to outside (mostly) with airspeed.

Differences also occur because the calibration curves of the "steam gauge" and the IGC logger are not the same.

Knowing how to monitor (while flying) the actual baro altitude being recorded in you logger is a very worthwhile endeavor and not always easy depending on your equipment.

This difference bites folks most often in two ways:
1. Busting the 17,500 or 17,900 MSL ceiling of airspace
2. Being too high or too low when crossing the finish cylinder boundary

Hope this helps.

Scorer with the scars to prove it,
QT

[Dan Daly[_2_]]

On Monday, March 5, 2018 at 2:48:06 PM UTC-5, wrote:
> How do current certified loggers determine altitude? Being some are plumbed to external pitot/static and some are cockpit pressure only, what differences are expected?
>
> Does your mechanical altimeter (especially 57mm size) read close to your "glide caculator" altitude?
>
> Are loggers solely dependent on pressure altitudes or is GPS altitude comparison ever included in the algorithms?
>
> Seems one would be more likely to "bust" min/max altitudes using cockpit pressure loggers.

Read chapter 4 "pressure altitude recording" of http://www.ukiws.demon.co.uk/GFAC/documents/tech_spec_gnss.pdf for the first question. If you check the Flight Recorder approval documents, they say, for example, for PowerFLARM: "The recorder case is not pressure-sealed and "cockpit static" pressure is recorded on the IGC file." For CNv-IGC (which you might assume uses plumbed static pressure) it is the same (not pressure sealed, "cockpit static" is recorded. An assertion that some FRs use plumbed sources is incorrect (you can go through the 59 Approval Documents if you want to check your specific one at http://www.ukiws.demon.co.uk/GFAC/igc_approved_frs.pdf ).

Second has nothing to do with flight recorders since you put QNH into glide computer and altimeter (neither are FRs; Flight Recorders use QNE as a reference). Mine agree reasonably closely (50' or so) since I had my winter altimeter (57 mm) recalibrated at the factory. They recommend every couple of years. Before calibration - many hundreds of feet out. I find my V3 vario (set QNH) and CNv Flight Recorder (set QNH) read very closely to each other..

Third: Pressure alt to 15,000m/49,213'; above, approved "High Altitude Flight Recorder", see Sporting Code 3b, Chapter 2.1. http://www.ukiws.demon.co..uk/GFAC/documents/sc3b(2015).pdf .

Last question - mentioned in Grand Prix threads - open cockpit vents at start (max alt), close cockpit vents on Vne finish (min alt). Minor pressurization effect, but harsh penalty after 5m in GP. You could do a couple of tows and do short final glide at Vne with vents open on the first, closed on second, to determine whether it's worth it to you. The effects are essentially the same for all finishes (around same speed, around same altitude) so it balances out somewhat. The altitudes are compared to the baseline altitude after landing too.

Dan

[John Godfrey (QT)[_2_]]

On Monday, March 5, 2018 at 2:48:06 PM UTC-5, wrote:
> How do current certified loggers determine altitude? Being some are plumbed to external pitot/static and some are cockpit pressure only, what differences are expected?
>
> Does your mechanical altimeter (especially 57mm size) read close to your "glide caculator" altitude?
>
> Are loggers solely dependent on pressure altitudes or is GPS altitude comparison ever included in the algorithms?
>
> Seems one would be more likely to "bust" min/max altitudes using cockpit pressure loggers.

IGC data loggers record an uncorrected (29.92 assumed) pressure altitude in an integer number of meters. They may also record a GPS altitude as a separate item. Scoring software calculates a fixed correction based on the known surface height at takeoff that is applied throughout the flight. A correction computed at landing is used in the US at the finish if more favorable.

In the US competition environment, GPS altitude is not considered unless baro altitude is not available from the logger (i.e. it is not an IGC logger but is an IGC "position recorder").

If the logger is not plumbed to the aircraft static, meaningful differences can occur between your "steam gauge" altimeter and the logger. This is a function of how your cockpit pressure varies up or down relative to outside (mostly) with airspeed.

Differences also occur because the calibration curves of the "steam gauge" and the IGC logger are not the same.

Knowing how to monitor (while flying) the corrected-to-takeoff-elevation baro altitude being recorded in you logger is a very worthwhile endeavor and not always easy depending on your equipment.

This difference bites folks most often in two ways:
1. Busting the 17,500 or 17,900 MSL ceiling of airspace
2. Being too high or too low when crossing the finish cylinder boundary

Hope this helps.

Scorer with the scars to prove it,
QT

[Papa3[_2_]]

John beat me to the punch.

But FWIW, the IGC approval document for each recorder gives details of the altitude recording mechanism including what sensors are used and how the system obtains pressure. From what I recall, many (most) FRs do not have a sealed case, so the pressure reading is "cockpit static". Those with the appropriate understanding and mindset might use this for transient advantage in a very specific situation.

P3

https://www.fai.org/page/igc-approved-flight-recorders


On Monday, March 5, 2018 at 3:36:00 PM UTC-5, John Godfrey (QT) wrote:
> On Monday, March 5, 2018 at 2:48:06 PM UTC-5, wrote:
> > How do current certified loggers determine altitude? Being some are plumbed to external pitot/static and some are cockpit pressure only, what differences are expected?
> >
> > Does your mechanical altimeter (especially 57mm size) read close to your "glide caculator" altitude?
> >
> > Are loggers solely dependent on pressure altitudes or is GPS altitude comparison ever included in the algorithms?
> >
> > Seems one would be more likely to "bust" min/max altitudes using cockpit pressure loggers.
>
> IGC data loggers record an uncorrected (29.92 assumed) pressure altitude in an integer number of meters. They may also record a GPS altitude as a separate item. Scoring software calculates a fixed correction based on the known surface height at takeoff that is applied throughout the flight. A correction computed at landing is used in the US at the finish if more favorable.
>
> In the US competition environment, GPS altitude is not considered unless baro altitude is not available from the logger (i.e. it is not an IGC logger but is an IGC "position recorder").
>
> If the logger is not plumbed to the aircraft static, meaningful differences can occur between your "steam gauge" altimeter and the logger. This is a function of how your cockpit pressure varies up or down relative to outside (mostly) with airspeed.
>
> Differences also occur because the calibration curves of the "steam gauge" and the IGC logger are not the same.
>
> Knowing how to monitor (while flying) the corrected-to-takeoff-elevation baro altitude being recorded in you logger is a very worthwhile endeavor and not always easy depending on your equipment.
>
> This difference bites folks most often in two ways:
> 1. Busting the 17,500 or 17,900 MSL ceiling of airspace
> 2. Being too high or too low when crossing the finish cylinder boundary
>
> Hope this helps.
>
> Scorer with the scars to prove it,
> QT

[jfitch]

On Monday, March 5, 2018 at 11:48:06 AM UTC-8, wrote:
> How do current certified loggers determine altitude? Being some are plumbed to external pitot/static and some are cockpit pressure only, what differences are expected?
>
> Does your mechanical altimeter (especially 57mm size) read close to your "glide caculator" altitude?
>
> Are loggers solely dependent on pressure altitudes or is GPS altitude comparison ever included in the algorithms?
>
> Seems one would be more likely to "bust" min/max altitudes using cockpit pressure loggers.

As a sanity check, full pitot pressure at 60 knots is around 0.085 psi. A foot of altitude depends on altitude but call it 0.0004 psi below 18,000 ft. So if you could get full dynamic pressure in the cockpit at 60 knots your recorder would be off by 200 ft. It should actually be a small fraction of that.

[[email protected]]

> Scoring software calculates a fixed correction based on the known surface height at takeoff that is applied throughout the flight. A correction computed at landing is used in the US at the finish if more favorable.

Right, so in the US, the "Scoring Software" has to compute, track and manage 4 altitude values:

1) GPS Altitude
2) Raw Pressure Altitude
3) Pressure Altitude corrected with takeoff roll
4) Pressure Altitude corrected with landing roll

And know when and where each one is applicable, and present those results transparently to the official scorer and contestant.

[John Godfrey (QT)[_2_]]

On Wednesday, March 7, 2018 at 2:58:36 PM UTC-5, wrote:
> > Scoring software calculates a fixed correction based on the known surface height at takeoff that is applied throughout the flight. A correction computed at landing is used in the US at the finish if more favorable.
>
> Right, so in the US, the "Scoring Software" has to compute, track and manage 4 altitude values:
>
> 1) GPS Altitude
> 2) Raw Pressure Altitude
> 3) Pressure Altitude corrected with takeoff roll
> 4) Pressure Altitude corrected with landing roll
>
> And know when and where each one is applicable, and present those results transparently to the official scorer and contestant.

This is true US or otherwise. The only twist in the US rules is the calculation of a second correct number at landing.

Significantly easier in the USA is the absence of airspace ceilings based on flight levels. Everything in the USA is MSL.