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bruceflys(at)comcast.net
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 Posted: Sun Jan 15, 2006 10:05 am Post subject: IFR GPS Display |
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AC 20-138A, Airworthiness Approval Of Global Navigation Satellite System
Equipment, sets the requirements for IFR GPS units. Para 18d , Navigation
Display,. requires that the horizontal and vertical deviation display(s) and
failure annunciation be within the pilot's primary field of view. Primary
field is defined as within 15 degrees of straight ahead of the pilot. Other
displays may be anywhere from the airspeed indicator on the left in a
standard six pack to and including an avionics center stack on the right.
One method of compliance is to place an IFR GPS receiver that displays
CDI/VDI, such as the GNS 480, within the primary field of view and eliminate
the requirement for an external display. The geometry of my GlaStar gives a
12" wide zone in the panel for the GPS' display. Placing a Dynon or GRT PFD
above or below the GPS would result in a lot of flight/navigation info
directly
in front of the pilot.
Regards, Bruce McGregor
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mlas(at)cox.net
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| Posted: Sun Jan 15, 2006 8:38 pm Post subject: IFR GPS Display |
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AC 20-138A is not regulatory for experimental aircraft unless You
incorporated this into your limitations.
--
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khorton01(at)rogers.com
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| Posted: Mon Jan 16, 2006 2:45 am Post subject: IFR GPS Display |
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I'll leave the question as to whether AC 20-138A is regulatory or not
to those who know the US regs better than I.
But, just because something may be legal doesn't mean it is a good
idea. If your installation does not match up against AC 20-138A,
then I strongly recommend you should not fly IMC based on the GPS, on
approach, or lower than 1000 ft above any obstacles in the enroute
and terminal phases of flight.
Kevin Horton
On 15 Jan 2006, at 23:35, Mike wrote:
[quote]
AC 20-138A is not regulatory for experimental aircraft unless You
incorporated this into your limitations.
--
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bruceflys(at)comcast.net
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brian
Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA
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| Posted: Mon Jan 16, 2006 5:50 pm Post subject: IFR GPS Display |
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Kevin Horton wrote:
| Quote: |
I'll leave the question as to whether AC 20-138A is regulatory or not
to those who know the US regs better than I.
But, just because something may be legal doesn't mean it is a good
idea. If your installation does not match up against AC 20-138A,
then I strongly recommend you should not fly IMC based on the GPS, on
approach, or lower than 1000 ft above any obstacles in the enroute
and terminal phases of flight.
|
I find this discussion interesting and far more complex than it needs to
be.
How many people have sat in their cockpit and moved across the panel
pointing to each device and asking the question, "what if this failed
and I am IFR?" This is a very simple procedure and it will go a long way
toward figuring out whether you need to attach the specified device to
the e-bus or to add a redundant unit.
For instance, I can think of only one engine instrument I need to safely
complete a flight -- CHT, and that only if the aircraft has cowl flaps.
I can guess a MAP and RPM by sound and aircraft performance. If the
engine is running then oil-P and oil-T are OK. To me, engine instruments
are not critical to safe completion of a flight.
And I don't need an airspeed indicator. If I know my airplane then I
know that a certain power setting, MAP and RPM, is going to give me
known performance.
I do need an altimeter -- but only for IFR flight. If you can't get into
the pattern and safely land the airplane without an altimeter, you need
to go back to school. But if you have a transponder with an altitude
display you have your backup altimeter for IFR flight, at least insofar
as flying an ILS is concerned. (Intercept the glideslope at
approximately the proper altitude and then fly the GS to the
middle-marker and miss if you can't see the runway environment. You
don't need an altimeter for that. And I know that some ILS's don't have
markers but you get to pick one that does.)
If I have a working HI (DG) and a TC or T&SI, I can probably survive
loss of my AI while on instruments. I cannot survive loss of all three.
That means if I have something like one of the current crop of PFDs, I
need some sort of backup.
And I would want my backup to be different as I wouldn't want a common
failure mode. That means that if I have a BMA EFIS-1 I will probably opt
for a 3-pack (ASI, alt, and AI) or a Dynon rather than an EFIS-light.
Why? Well, I bet that the EFIS-1 and EFIS-light share common AHRS and
software technology. That which causes one to fail might take out the
other as well. I would probably backup a BMA unit with a GRT or a Dynon
just so there is less commonality.
But it is pretty hard to beat a 3-pack for simplicity.
But if I had my 3-pack and an SL-30, I could afford to lose my whole
EFIS-1. I can use the built-in CDI display on the SL-30 to fly a VOR or
LOC approach. If I lose my SL-30 I can fly a VOR or NDB approach using
the GPS. It may be non-certified but it will get me safely on the ground
and with probably more accuracy than the VOR or ADF receiver would.
And if all else fails, fly the 3-pack and holler on your handheld for a
PAR at the nearest military airfield.
This isn't rocket science; this is common sense. You don't need an
engineering degree to sit in front of your panel (or in front of the
picture of your panel) to do this.
--
Brian Lloyd 2243 Cattle Dr.
brian-yak at lloyd dot com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery
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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery |
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khorton01(at)rogers.com
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| Posted: Tue Jan 17, 2006 3:18 am Post subject: IFR GPS Display |
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On 16 Jan 2006, at 20:49, Brian Lloyd wrote:
| Quote: |
yak(at)lloyd.com>
Kevin Horton wrote:
>
> <khorton01(at)rogers.com>
>
> I'll leave the question as to whether AC 20-138A is regulatory or not
> to those who know the US regs better than I.
>
> But, just because something may be legal doesn't mean it is a good
> idea. If your installation does not match up against AC 20-138A,
> then I strongly recommend you should not fly IMC based on the GPS, on
> approach, or lower than 1000 ft above any obstacles in the enroute
> and terminal phases of flight.
I find this discussion interesting and far more complex than it
needs to
be.
How many people have sat in their cockpit and moved across the panel
pointing to each device and asking the question, "what if this failed
and I am IFR?" This is a very simple procedure and it will go a
long way
toward figuring out whether you need to attach the specified device to
the e-bus or to add a redundant unit.
|
I agree with all you wrote, but there is one more thing to consider
too. Anyone considering using a non-TSO'd GPS receiver for IFR
flight needs to also ask the question "what if the GPS receiver
provided false position information and I was flying IFR?". TSO C129
units have functions that look for bad satellite data, and either
exclude it, or stop providing position info. Non-TSO C129 units
almost certain don't have this function, and they will quite happily
use bad satellite data to calculate a bad position. If the position
error is large, you will probably notice it. But, I am aware of one
occurrence that happened to a co-worker. They were flying a practice
GPS approach in VMC conditions. The GPS receiver gave a RAIM warning
prior to the FAF, and refused to go into approach mode. They
continued the approach, curious to see what the GPS was going to do.
It ended up bringing them in on a path over 1 mile offset from the
correct approach path. That could put you into the rocks or
obstacles in some places.
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://www.kilohotel.com/rv8
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mlas(at)cox.net
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| Posted: Tue Jan 17, 2006 7:27 am Post subject: IFR GPS Display |
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Question, how do you know it doesn't match up? Think about your answer.
Mike
--
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brian
Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA
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| Posted: Tue Jan 17, 2006 9:41 am Post subject: IFR GPS Display |
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Kevin Horton wrote:
| Quote: | I agree with all you wrote, but there is one more thing to consider
too. Anyone considering using a non-TSO'd GPS receiver for IFR
flight needs to also ask the question "what if the GPS receiver
provided false position information and I was flying IFR?". TSO C129
units have functions that look for bad satellite data, and either
exclude it, or stop providing position info.
|
Which they do wonderfully well as I have discovered. I have had GPS stop
working for me on several flights, including one over water flight.
Fortunately I had an ADF as a backup. Planning for loss of your primary
navigation is part and parcel of flying.
Heck, on one of my Atlantic crossings my LORAN packed it in. I planned
for this and had a recent LORAN fix to use as a jumping-off point for
DR. I had also been checking the winds aloft using the LORAN so I had a
good wind set too. I navigated this way until I could get a good ADF
bearing and used that as a check until I got in range of a VOR. It
happens and it is not the end of the world.
| Quote: | Non-TSO C129 units
almost certain don't have this function, and they will quite happily
use bad satellite data to calculate a bad position.
|
So you are going to bet your life on the RAIM feature of your GPS? You
aren't going to cross check? I have had GPS fail just too often to trust
GPS as my sole source of navigation, RAIM or no RAIM.
In fact, it has happened to me so many times with so many different GPS
receivers (which mysteriously get better later all by themselves) that I
have stopped flying GPS direct for the most part. I now find that I fly
most of my cross-country flights by Victor airway again. I do this
because it makes transition back to VOR much, much easier and the cost
in terms of time is almost always negligible. It also means that I can
cross check my GPS with the VOR/DME/RNAV. I know the GPS is more
accurate but I trust VOR/DME more in the long run.
So if the VOR/DME and GPS are telling me the same thing, I am happy. If
they start telling me different things, I am not happy but I know I have
to figure out who is lying. And usually the GPS just goes toes-up with a
RAIM error followed by loss of navigation information about 20 seconds
later.
| Quote: | If the position
error is large, you will probably notice it. But, I am aware of one
occurrence that happened to a co-worker. They were flying a practice
GPS approach in VMC conditions. The GPS receiver gave a RAIM warning
prior to the FAF, and refused to go into approach mode. They
continued the approach, curious to see what the GPS was going to do.
It ended up bringing them in on a path over 1 mile offset from the
correct approach path. That could put you into the rocks or
obstacles in some places.
|
Sure it will. It breaks like everything else. Actually it breaks more
often as the USAF is still "experimenting" with degrading GPS in some
areas. Most of the time it gets NOTAM'd but sometimes ...
TSO C129 is not a panacea. RAIM helps but it doesn't solve the problem
that GPS has a number of interesting failure modes. You cannot count on
your GPS to always give you accurate information all the time. You need
something else or you need to be VFR so you can fall back on pilotage.
--
Brian Lloyd 2243 Cattle Dr.
brian-yak at lloyd dot com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery |
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