Determining high point from flight recorder data

[Sam Discusflyer]

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?

[Doug Haluza]

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.

[5Z]

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/Elbert_Mountain.html?bannertypeclick=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

[Doug Haluza]

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/Elbert_Mountain.html?bannertypeclick=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