Backup Flight Data Recorder?

[Fox Two]

Hello All,

I'm new to racing, and I have a new D-2b on order from Germany. I'm
considering installing a backup Flight Data Recorder (FDR) to my
Cambridge 302, and I'm curious to see how many other competition pilots
choose to have a backup FDR as well. What kind of backup FDRs do you
use?

Thanks in advance for any advice you have!

Chris Fleming, 'F2'
El Paso, Texas

[Orion Kingman]

Fox Two wrote:
> Hello All,
>
> I'm new to racing, and I have a new D-2b on order from Germany. I'm
> considering installing a backup Flight Data Recorder (FDR) to my
> Cambridge 302, and I'm curious to see how many other competition pilots
> choose to have a backup FDR as well. What kind of backup FDRs do you
> use?
>
> Thanks in advance for any advice you have!
>
> Chris Fleming, 'F2'
> El Paso, Texas

Yes I fly with a backup FDR. I don't think waht you choose to fly with
is a important as how you have it set up. Current;y I am using a CAI
302 as my primary, and a CAI 302A as my secondary (previously I was
using a colibri as my secondary), but I always have my secondary on an
isolated electrical system.

Orion Kingman
DV8

[hans]

I have a backup too.

As primary I use a Zander GP941 and as secondary I use a CAI GPSNAV 10.

It is important that the secondary FR is a different make than the
primary. The most likely reasons, after installation and power supply,
for a FR not working properly are design and production.

Use two independent power supplies for the two FRs. By independent I
mean that there is no common element in the power supplies, not even a
common switch, fuse or .... .

The two antennae should be separated by at least 20 cm.

You ought to be able to use both FR as means of navigation. So you need
also two different navigation displays; again not the same make and not
the same software.





Fox Two schrieb:
> Hello All,
>
> I'm new to racing, and I have a new D-2b on order from Germany. I'm
> considering installing a backup Flight Data Recorder (FDR) to my
> Cambridge 302, and I'm curious to see how many other competition pilots
> choose to have a backup FDR as well. What kind of backup FDRs do you
> use?
>
> Thanks in advance for any advice you have!
>
> Chris Fleming, 'F2'
> El Paso, Texas
>

[Papa3]

Fox Two wrote:
> Hello All,
>
> I'm new to racing, and I have a new D-2b on order from Germany. I'm
> considering installing a backup Flight Data Recorder (FDR) to my
> Cambridge 302, and I'm curious to see how many other competition pilots
> choose to have a backup FDR as well. What kind of backup FDRs do you
> use?
>
> Thanks in advance for any advice you have!
>
> Chris Fleming, 'F2'
> El Paso, Texas

Chris,

I opted to go a slightly different route and purchased a Garmin GPS
Map 76. For well under $200 street price, you get a fully functional
navigation system that is completely self-contained, including the
ability to run on 2 AA batteries for more than 8 hours (I use two
2500mAh rechargables which I swap out after every flight; have recorded
long ridge missions of over 8 hours without any problems). It has
it's own display and good recording and downloading capabilities.
Plus, it doesn't take up any precious panel space or require additional
displays or wiring. Mine is mounted on a home-built bracket, but RAM
and others provide some very slick mounting options. In the US, it
is acceptable as contest documentation. The only problem would be for
a badge flight, where the FAI has not approved it for documentation.

Since you are "new to racing", you might want to consider this simple
approach until you get all of the other stuff (like flying) worked out.

Erik Mann
LS8-18 (P3)

[Orion Kingman]

Papa3 wrote:
> Fox Two wrote:
> > Hello All,
> >
> > I'm new to racing, and I have a new D-2b on order from Germany. I'm
> > considering installing a backup Flight Data Recorder (FDR) to my
> > Cambridge 302, and I'm curious to see how many other competition pilots
> > choose to have a backup FDR as well. What kind of backup FDRs do you
> > use?
> >
> > Thanks in advance for any advice you have!
> >
> > Chris Fleming, 'F2'
> > El Paso, Texas
>
> Chris,
>
> I opted to go a slightly different route and purchased a Garmin GPS
> Map 76. For well under $200 street price, you get a fully functional
> navigation system that is completely self-contained, including the
> ability to run on 2 AA batteries for more than 8 hours (I use two
> 2500mAh rechargables which I swap out after every flight; have recorded
> long ridge missions of over 8 hours without any problems). It has
> it's own display and good recording and downloading capabilities.
> Plus, it doesn't take up any precious panel space or require additional
> displays or wiring. Mine is mounted on a home-built bracket, but RAM
> and others provide some very slick mounting options. In the US, it
> is acceptable as contest documentation. The only problem would be for
> a badge flight, where the FAI has not approved it for documentation.
>
> Since you are "new to racing", you might want to consider this simple
> approach until you get all of the other stuff (like flying) worked out.
>
> Erik Mann
> LS8-18 (P3)

Two words of caution with Garmin units: 1.) they are not IGC approved,
you cannot use them for badges or records. 2.) Some of the newer
Garmin units (the 396 in particular) have XM sattelite weather
capability. This is illegal to use in US contest and can result in a
DQ for that contest and a five year ban from competitive soaring in the
US.

DV8

[[email protected]]

Papa3 wrote:
> I opted to go a slightly different route and purchased a Garmin GPS
> Map 76. For well under $200 street price, you get a fully functional
> navigation system that is completely self-contained, including the
> ability to run on 2 AA batteries for more than 8 hours (I use two
> 2500mAh rechargables which I swap out after every flight; have recorded
> long ridge missions of over 8 hours without any problems). It has
> it's own display and good recording and downloading capabilities.
> Plus, it doesn't take up any precious panel space or require additional
> displays or wiring. Mine is mounted on a home-built bracket, but RAM
> and others provide some very slick mounting options. In the US, it
> is acceptable as contest documentation. The only problem would be for
> a badge flight, where the FAI has not approved it for documentation.

My flight recorder has failed several times (for trivial but fatal
reasons unrelated to installation or power supply) so I'm absolutely
committed to having a backup. Having borrowed Erik's Garmin GPS MAP 76
several times for contest backup, and having evaluated the
alternatives, I bought my own recently ($160 delivered on eBay). It's
completely self contained, produces an IGC file (using SeeYou)--albeit
not a secure one--that is usually indistinguishable from the one in my
primary flight recorder, allows me to upload the contest database, and
has a nice display, with a built in U.S. roads and towns database.

Yeah, it's not IGC approved so you'll need something else as your
primary recorder. But as a backup, it's exactly what I need. It will
also provide a NMEA input to varios and PDAs for a backup moving
map/flight computer, so long as GPS altitude is OK.

The last point is the only real concern I have: if my primary recorder
fails, I'll be relying on GPS altitude for starts, special-use airspace
clearances, etc. So long as I know about it, I just have to pay
attention to the GPS readout. But GPS and pressure altitude vary
anywhere from a few feet to many hundreds of feet.

Garmin makes a similar model with a pressure altitude sensor but it's
unclear whether that would be secure enough to meet the requirements at
a [U.S.] contest...and it costs more. :)

My PDA-and-primary-flight-recorder-based moving map froze at the Uvalde
Std. Nats. a few weeks ago just as I was maneuvering to turn and tap
the last cylinder as I flew by it and head for the finish. I punched up
the TP on the Garmin and used it to navigate while I fumbled with the
PDA, eventually doing a soft reset. I was very happy I had the Garmin!

Chip Bearden
ASW 24 "JB"

[[email protected]]

Papa3 wrote:
> I opted to go a slightly different route and purchased a Garmin GPS
> Map 76. For well under $200 street price, you get a fully functional
> navigation system that is completely self-contained, including the
> ability to run on 2 AA batteries for more than 8 hours (I use two
> 2500mAh rechargables which I swap out after every flight; have recorded
> long ridge missions of over 8 hours without any problems). It has
> it's own display and good recording and downloading capabilities.
> Plus, it doesn't take up any precious panel space or require additional
> displays or wiring. Mine is mounted on a home-built bracket, but RAM
> and others provide some very slick mounting options. In the US, it
> is acceptable as contest documentation. The only problem would be for
> a badge flight, where the FAI has not approved it for documentation.

My flight recorder has failed several times (for trivial but fatal
reasons unrelated to installation or power supply) so I'm absolutely
committed to having a backup. Having borrowed Erik's Garmin GPS MAP 76
several times for contest backup, and having evaluated the
alternatives, I bought my own recently ($160 delivered on eBay). It's
completely self contained, produces an IGC file (using SeeYou)--albeit
not a secure one--that is usually indistinguishable from the one in my
primary flight recorder, allows me to upload the contest database, and
has a nice display, with a built in U.S. roads and towns database.

Yeah, it's not IGC approved so you'll need something else as your
primary recorder. But as a backup, it's exactly what I need. It will
also provide a NMEA input to varios and PDAs for a backup moving
map/flight computer, so long as GPS altitude is OK.

The last point is the only real concern I have: if my primary recorder
fails, I'll be relying on GPS altitude for starts, special-use airspace
clearances, etc. So long as I know about it, I just have to pay
attention to the GPS readout. But GPS and pressure altitude vary
anywhere from a few feet to many hundreds of feet.

Garmin makes a similar model with a pressure altitude sensor but it's
unclear whether that would be secure enough to meet the requirements at
a [U.S.] contest...and it costs more. :)

My PDA-and-primary-flight-recorder-based moving map froze at the Uvalde
Std. Nats. a few weeks ago just as I was maneuvering to turn and tap
the last cylinder as I flew by it and head for the finish. I punched up
the TP on the Garmin and used it to navigate while I fumbled with the
PDA, eventually doing a soft reset. I was very happy I had the Garmin!

Chip Bearden
ASW 24 "JB"

[58y]

wrote:

> Having borrowed Erik's Garmin GPS MAP 76
> several times for contest backup, and having evaluated the
> alternatives, I bought my own recently ($160 delivered on eBay). It's
> completely self contained, produces an IGC file (using SeeYou)--albeit
> not a secure one--that is usually indistinguishable from the one in my
> primary flight recorder, allows me to upload the contest database, and
> has a nice display, with a built in U.S. roads and towns database.

> The last point is the only real concern I have: if my primary recorder
> fails, I'll be relying on GPS altitude for starts, special-use airspace
> clearances, etc. So long as I know about it, I just have to pay
> attention to the GPS readout. But GPS and pressure altitude vary
> anywhere from a few feet to many hundreds of feet.

The GPSMAP 76S has the barometric sensor, at least OLC thinks so,
according to the online flight display.

Or is it the straight 76 that you bought?



Jack

[Papa3]

58y wrote:
> The GPSMAP 76S has the barometric sensor, at least OLC thinks so,
> according to the online flight display.
>
> Or is it the straight 76 that you bought?
>
> Jack

I've noticed that some flight evaluation programs will take the GPS
Altitude and display it as Pressure Altitude if none exists in the
B-Record positions normally reserved for Barometric Altitude. That
may be what you are seeing...

P3

[Papa3]

Orion Kingman wrote:
> Papa3 wrote:
> > Fox Two wrote:
> > > Hello All,
> > >
> > > I'm new to racing, and I have a new D-2b on order from Germany. I'm
> > > considering installing a backup Flight Data Recorder (FDR) to my
> > > Cambridge 302, and I'm curious to see how many other competition pilots
> > > choose to have a backup FDR as well. What kind of backup FDRs do you
> > > use?
> > >
> > > Thanks in advance for any advice you have!
> > >
> > > Chris Fleming, 'F2'
> > > El Paso, Texas
> >
> > Chris,
> >
<snip>. The only problem would be for
> > a badge flight, where the FAI has not approved it for documentation.
> >
> > Since you are "new to racing", you might want to consider this simple
> > approach until you get all of the other stuff (like flying) worked out.
> >
> > Erik Mann
> > LS8-18 (P3)
>
> Two words of caution with Garmin units: 1.) they are not IGC approved,
> you cannot use them for badges or records. 2.) Some of the newer
> Garmin units (the 396 in particular) have XM sattelite weather
> capability. This is illegal to use in US contest and can result in a
> DQ for that contest and a five year ban from competitive soaring in the
> US.
>
> DV8

Chris,

As I mentioned in my original post and as Orion points out, the Garmin
units are not approved as standalone FRs by the IGC. So, they're not
acceptable for badges and records. From your first sentence, I
gathered that you are primarily interested in racing. If so, then I
think the Garmin is a very smart choice for several reasons:

- Reliability. These things are built in large volumes for use in a
lot of demanding environments. My experience is that they are
extremely unlikely to fail, especially since they don't have a lot of
the bells and whistles that some of our FRs have. On the other hand,
I can count over a dozen failures of purpose-built gliding FRs at
contests I've attended in the last 3 years.

- Availability. If worst comes to worst and your Garmin quits on you
(or you run over it with your tow vehicle - don't ask me how I know),
there's almost no place in the US where you can't find a K-Mart or a
Walmart nearby that has a similar unit for sale. You can have your
replacement the same day.

- Self-contained. As long as you are willing to live with an 8 hour
battery life and are careful about rotating batteries before each
flight, the Garmin unit is completely self contained. No extra
displays required. You can still use it as output to a PDA, but it
will get you around the course and home if all else craps out.

- Usability. Because these units are designed for the general public,
their interface is quite simple to learn. In addition, they are well
suited to using for other more mundane pursuits (like finding your
buddy when he lands out), so I tend to use it more frequently than I
thought I would. As a result, I find that I stay current on this unit
even though it's primarily a backup in flight.

- Cost. This one is in the eye of the beholder. When I was young
and single, I wouldn't think twice at having two complete FR and
Navigation systems costing over $1,000 each. Now, with two kids and a
family budget that treasures every penny it can find, saving $800 or
more dollars is a big deal.

Anyway, there are a lot of folks using Garmins as FRs, so you should be
able to find plenty of ideas on how to use them. There are also
newsgroups dedicated to Garmin GPSMAP and ETrex units which have a
wealth of information.

Regards,

P3

[[email protected]]

> The GPSMAP 76S has the barometric sensor, at least OLC thinks so,
> according to the online flight display.
>
> Or is it the straight 76 that you bought?

I bought the GPSMAP 76 (there's also a straight GPS 76 but it's not
suitable for our purposes: not enough trackpoints, for one thing). The
GPSMAP 76S does have the baro sensor (as well as an electronic compass
sensor) but I'm not sure whether it's evident from the track log
whether the user locked out the auto-calibrate feature that slowly
adjusts the altimeter to GPS altitude to account for pressure changes.
I'm also not sure whether the track log records both pressure and GPS
altitudes. If the answer to these questions is yes (i.e., it's evident
if auto calibration is turned off and it records pressure altitude),
then the extra money for the GPSMAP 76C might be worth it. It's still
not an IGC $ecure flight recorder but it's the next best thing...at 1/5
or 1/6 the price.

Erik Mann (Papa3) was a great help during my evaluation of backup
flight recorders and is a lot more conversant with this subject (he's
even interacted with Garmin). As I mentioned earlier, I'm really
impressed with how much functionality you get for $150.

Chip Bearden
ASW 24 "JB"

[Papa3]

wrote:
> > The GPSMAP 76S has the barometric sensor, at least OLC thinks so,
> > according to the online flight display.
> >
> > Or is it the straight 76 that you bought?
>
> I bought the GPSMAP 76 (there's also a straight GPS 76 but it's not
> suitable for our purposes: not enough trackpoints, for one thing). The
> GPSMAP 76S does have the baro sensor (as well as an electronic compass
> sensor) but I'm not sure whether it's evident from the track log
> whether the user locked out the auto-calibrate feature that slowly
> adjusts the altimeter to GPS altitude to account for pressure changes.
> I'm also not sure whether the track log records both pressure and GPS
> altitudes. If the answer to these questions is yes (i.e., it's evident
> if auto calibration is turned off and it records pressure altitude),
> then the extra money for the GPSMAP 76C might be worth it. It's still
> not an IGC $ecure flight recorder but it's the next best thing...at 1/5
> or 1/6 the price.
>
> Chip Bearden
> ASW 24 "JB"

The answers to the two questions are:

- No, the 76S unit records only 1 altitude. All of the recreational
Garmin units (at least through early this year) logged a fairly limited
subset of the available NMEA data (understandably, since the average
recreational user needs only time, position, and altitude).

- As to which altitude it is storing, that depends. The basic
"altitude" in the 76S is the barometric altitude. The auto-calibrate
feature compares the barometric altitude to the GPS altitude and
periodically adjusts the barometric altitude. Exactly how it does
that (ie. average delta over n number of fixes, how it determines
whether to discard a fix, etc.) is not public domain. But, I have had
correspondence with Garmin engineers who pointed out that the algorithm
means that you essentially end up with GPS altitude but smoothed by the
barometric altitude to take care of any individual GPS altitude fixes
that might have been degraded (at least that's my current
understanding). If you turn off auto-calibrate, then only the
barometric altitude is stored. There is no way to tell from the
output record whether auto-calibrate was on or off.

Given the above, I'm not convinced yet that the 76S is necessarily
"better", though I do think it might actually be a long-term solution
if we can come up with a complete understanding of the auto-calibration
algorithm(s).

P3

[Ian Strachan]

Papa3 wrote:
> All of the recreational
> Garmin units (at least through early this year) logged a fairly limited
> subset of the available NMEA data (understandably, since the average
> recreational user needs only time, position, and altitude).
>
> - As to which altitude it is storing, that depends. The basic
> "altitude" in the 76S is the barometric altitude. The auto-calibrate
> feature compares the barometric altitude to the GPS altitude and
> periodically adjusts the barometric altitude.

> Exactly how it does
> that (ie. average delta over n number of fixes, how it determines
> whether to discard a fix, etc.) is not public domain.

>But, I have had
> correspondence with Garmin engineers who pointed out that the algorithm
> means that you essentially end up with GPS altitude but smoothed by the
> barometric altitude to take care of any individual GPS altitude fixes
> that might have been degraded (at least that's my current
> understanding).

>If you turn off auto-calibrate, then only the
> barometric altitude is stored. There is no way to tell from the
> output record whether auto-calibrate was on or off.

>From Ian Strachan, the other side of "the Pond":
Very interesting, Papa 3. Further questions are: to what zero-altitude
datum are the altitude figures:
(1) displayed on screen and
(2) recorded for later download.

I doubt whether recreational GPS units will be set to the ICAO ISA MSL
datum of 1013.2 mb (29.92 inches of Hg) that is the worldwide aviation
pressure setting for all "Flight Levels".

So, is it supposed to be a "mean sea level" datum? If so, how is it
achieved? (another proprietory algorithm, I suppose). The "64kdollar
question" is, how does it relate to spot heights and contours on maps?

The initial datum for GPS altitude is the selected ellipsoid, WGS84 and
local variants. But this is nowhere near MSL except in small areas by
sheer co-incidence. I understand that differences between the WGS84
ellipsoid and local MSL vary by some +65m S of Iceland to -102m S of
India, a total of 167m (548ft).

I suspect that these recreational GPS units have an electronic look-up
table that changes altitudes above the ellipsoid to what the WGS84
source document calls "an equipotential surface". As I understand it,
this worldwide equipotential surface is approximately, but not exactly,
equivalent to local mean sea levels. For instance, in UK mapping, Mean
Sea Level is taken as the average tide at a place in the SW of England
called Newlyn, I guess because it has regular and reliable tides. I
imagine that the mapping authorities in other countries have a similar
system, they have to have an MSL "zero datum" on which to base heights.

These are tricky issues if altitude figures downloaded after flight are
to be accurate and meaningful. At least you know where you are with
altitudes based on the aviation-standard 1013/29.9" sea level datum.
These 1013-based altitudes can then be converted after download into
others using public-domain formulas (the balloonists do this, I
understand, to get a sort of true height for the day rather than just
using the ICAO ISA as IGC does).

But with unknown algorithms applied before download, you do not know
exactly what you are dealing with. This is not a problem in the
recreational area, but may be in ours if accurate altitude is to be
used in a performance claim or to defend a pilot against a possible
airspace violation or other situation where a flight record wiuld be
useful. Then there is the issue of the general reliability of pressure
altitude compared to the sometimes erratic figures for GPS altitude
recorded in a proportion of IGC flight data files ...

Ian Strachan
Lasham Gliding Centre, UK

[Papa3]

Ian Strachan wrote:
> Papa3 wrote:
> > All of the recreational
> > Garmin units (at least through early this year) logged a fairly limited
> > subset of the available NMEA data (understandably, since the average
> > recreational user needs only time, position, and altitude).
> >
> > - As to which altitude it is storing, that depends. The basic
> > "altitude" in the 76S is the barometric altitude. The auto-calibrate
> > feature compares the barometric altitude to the GPS altitude and
> > periodically adjusts the barometric altitude.
>
> > Exactly how it does
> > that (ie. average delta over n number of fixes, how it determines
> > whether to discard a fix, etc.) is not public domain.
>
> >But, I have had
> > correspondence with Garmin engineers who pointed out that the algorithm
> > means that you essentially end up with GPS altitude but smoothed by the
> > barometric altitude to take care of any individual GPS altitude fixes
> > that might have been degraded (at least that's my current
> > understanding).
>
> >If you turn off auto-calibrate, then only the
> > barometric altitude is stored. There is no way to tell from the
> > output record whether auto-calibrate was on or off.
>
> >From Ian Strachan, the other side of "the Pond":
> Very interesting, Papa 3. Further questions are: to what zero-altitude
> datum are the altitude figures:
> (1) displayed on screen and
> (2) recorded for later download.
>
> I doubt whether recreational GPS units will be set to the ICAO ISA MSL
> datum of 1013.2 mb (29.92 inches of Hg) that is the worldwide aviation
> pressure setting for all "Flight Levels".
>
> So, is it supposed to be a "mean sea level" datum? If so, how is it
> achieved? (another proprietory algorithm, I suppose). The "64kdollar
> question" is, how does it relate to spot heights and contours on maps?
>
> The initial datum for GPS altitude is the selected ellipsoid, WGS84 and
> local variants. But this is nowhere near MSL except in small areas by
> sheer co-incidence. I understand that differences between the WGS84
> ellipsoid and local MSL vary by some +65m S of Iceland to -102m S of
> India, a total of 167m (548ft).
>
> I suspect that these recreational GPS units have an electronic look-up
> table that changes altitudes above the ellipsoid to what the WGS84
> source document calls "an equipotential surface". As I understand it,
> this worldwide equipotential surface is approximately, but not exactly,
> equivalent to local mean sea levels. For instance, in UK mapping, Mean
> Sea Level is taken as the average tide at a place in the SW of England
> called Newlyn, I guess because it has regular and reliable tides. I
> imagine that the mapping authorities in other countries have a similar
> system, they have to have an MSL "zero datum" on which to base heights.
>
> These are tricky issues if altitude figures downloaded after flight are
> to be accurate and meaningful. At least you know where you are with
> altitudes based on the aviation-standard 1013/29.9" sea level datum.
> These 1013-based altitudes can then be converted after download into
> others using public-domain formulas (the balloonists do this, I
> understand, to get a sort of true height for the day rather than just
> using the ICAO ISA as IGC does).
>
> But with unknown algorithms applied before download, you do not know
> exactly what you are dealing with. This is not a problem in the
> recreational area, but may be in ours if accurate altitude is to be
> used in a performance claim or to defend a pilot against a possible
> airspace violation or other situation where a flight record wiuld be
> useful. Then there is the issue of the general reliability of pressure
> altitude compared to the sometimes erratic figures for GPS altitude
> recorded in a proportion of IGC flight data files ...
>
> Ian Strachan
> Lasham Gliding Centre, UK

All good questions. What we know at present:

- Garmin describes the altitude feature as an "altimeter". In fact,
it can be set to local station pressure OR local Altitude. The manual
actually encourages this. So:
- If Auto-Calibrate is off, then subsequent altitude readings assume
ISA . So, if Altitude is initially referenced to QNH, subsequent
readings are against the datum used for the local survey. In the US,
I think (don't know for sure) that this is WGS84.
- If Auto-Calibrate is on, then subsequent altitude readings are, in
effect, GPS Altitude. From a closer read of the manual, it appears
that the unit compares the GPS Altitude to the Pressure Altitude
continuously and changes the displayed "altitude" when the delta
between Pressure Altitude and GPS Altitude changes based on some moving
average. Thus, the reference is the chosen ellipsoid (WGS84 by
default in the Garmin)

What's still not 100% clear is what value(s) are in the track log.
There's a tantalizing reference in the manual to the Pressure Plot
which states that this is maintaned separately from the Track Log.
In the ideal world:

- We could set the built in "altimeter" to local station pressure.
- We could turn off Auto Calibrate.
- The primary track log would contain GPS Altitude.
- The pressure plot would contain Barometric Pressure.
- Marrying the two would provide a complete B Record (more or less
:-))

I'm still hazy on what level of precision we're trying to achieve. In
the "real world" of gliding barometry, we live with a lot of
uncertainty. Whether it's accurate cross-reference to local station
pressure (and just how local is local), adjusting for pressure
changes over time during a flight, field elevation changes (15M
difference between the SW and NE ends of my home airport), etc. it
seems to me that we already live with a 10M-20M variability between two
identical flights documented by different OOs on the same day. I
think that the Garmin may provide enough data for post-flight review to
be at least as good as what we did with an old-fashioned barograph.

Erik Mann
LS8-18 P3

[Ian[_1_]]

On Tue, 22 Aug 2006 03:53:42 -0700, Ian Strachan wrote:


>>From Ian Strachan, the other side of "the Pond":
> Very interesting, Papa 3. Further questions are: to what zero-altitude
> datum are the altitude figures:
> (1) displayed on screen and
> (2) recorded for later download.

> But with unknown algorithms applied before download, you do not know
> exactly what you are dealing with. This is not a problem in the
> recreational area, but may be in ours if accurate altitude is to be
> used in a performance claim or to defend a pilot against a possible
> airspace violation or other situation where a flight record wiuld be
> useful. Then there is the issue of the general reliability of pressure
> altitude compared to the sometimes erratic figures for GPS altitude
> recorded in a proportion of IGC flight data files ...

Is this really a problem?

In the old days we were happy to accept a paper baragraph trace without
any attempt to "zero" the altitude. The procedure required the pilot to
scribe a "base line" on the chart at the known local airfield altitude
before start and the official observer measured all other significant
altitudes (start height etc) relative to this this height. Of course
for height gain tasks you only need to measure the difference between a
"low point" and a "high point".

So all we are left with is knowing whether the recorded altitude is from a
GPS engine, a pressure sensor or a mangled proprietary combination of
the two.

Pressure altitude is obviously acceptable. I believe GPS altitude, without
SA, is probably just as useful but it does not comply with the current
rules. The combination is probably the most accurate, but as the algorithm
is undefined and hence not reproducible, the data may not be acceptable.

The are only two significant difference between these GPS units and an
approved Flight Recorder:

1) The the mechanical tamper proof switch is missing. This the security of
the "private key" required to encrypt the trace before it is downloaded
is not protected. I am sure the only reason Garmin and others don't
include encrypted downloads in their software is because we would not
accept them without being assured of the security of their keys.

2) The price difference. Approved flight recorders being about 5x more
costly.


Regards

Ian

(another Ian, from the other side of the equator)