Sticky DG and DME distance question

[A Lieberman]

Couple of questions.

Went up to practice some ILS approaches. While being vectored for my
approaches, my DG kept sticking through the 270 heading. First time on my
initial approach and the second approach. Third and fourth approaches, DG
acted normal, though seem a little off compared to the magnetic compass.
(gave me good practice for partial panel approaches!).

Also, my header bug seem to be frozen, couldn't move it at all. I ended up
using my magnetic compass and VFR GPS for my primary navigation and
tracking.

I had this happen once on the ground a long time ago, and never gave any
thought as it performed flawlessly in the air since. Vacuum gauge was
exactly where it should be.

Last thing I want to worry about in the clag is the DG. Is the DG worth
overhauling or just getting a new one???

I went to http://www.aircraftspruce.com/menus/in/gyros.html to see what
prices I am looking at, and it seemed all over the page in price. This
would be a part obviously I don't want to cheap out on, but I don't want to
buy a part that is has something in it I never will need.

Suggestions much appreciated. I figured to post this time b4 going out on
the limb.

DME question. While on my VOR Alpha approach back to MBO, ATC reported me
3 miles from the VOR, and my DME reported me 4 miles. This is the first
time I ever had a discrepancy that much. Anybody know what gives. My DME
seemed correct as it reflected 5 miles to my airport.

Allen

[john smith]

A Lieberman wrote:
> DME question. While on my VOR Alpha approach back to MBO, ATC reported me
> 3 miles from the VOR, and my DME reported me 4 miles. This is the first
> time I ever had a discrepancy that much. Anybody know what gives. My DME
> seemed correct as it reflected 5 miles to my airport.

DME is slant range, ATC may be horizontal. How high were you?

[A Lieberman]

On Sat, 09 Jul 2005 16:11:22 GMT, john smith wrote:

> A Lieberman wrote:
>> DME question. While on my VOR Alpha approach back to MBO, ATC reported me
>> 3 miles from the VOR, and my DME reported me 4 miles. This is the first
>> time I ever had a discrepancy that much. Anybody know what gives. My DME
>> seemed correct as it reflected 5 miles to my airport.
>
> DME is slant range, ATC may be horizontal. How high were you?

Hi John,

I was at 2000 feet, which is about 1700 AGL.

Allen

[john smith]

A Lieberman wrote:
>>DME is slant range, ATC may be horizontal. How high were you?

> I was at 2000 feet, which is about 1700 AGL.

Certainly not enough to give you an extra mile of slant range.

[A Lieberman]

On Sat, 09 Jul 2005 16:53:57 GMT, john smith wrote:

> A Lieberman wrote:
>>>DME is slant range, ATC may be horizontal. How high were you?
>
>> I was at 2000 feet, which is about 1700 AGL.
>
> Certainly not enough to give you an extra mile of slant range.

That was my thought in flight, in which I queried approach and asked him to
verify my distance.

Never noticed a problem before, and wondered if DME's can lose their
accuracy.

I didn't have the time to reprogram my GPS as I had to fly the plane, but
if I had the time, that would have been my next step.

Allen

[Jon Kraus]

Allen,

I can't remember if you ave an auto-pilot or not but if oyu don't we are
having a S-TEC30 installed in 3 weeks and will ahve an "extra" DG when
we are done. Works fine for us now but it doesn't have a heading bug.

Jon Kraus
4443H @ TYQ

A Lieberman wrote:
> Couple of questions.
>
> Went up to practice some ILS approaches. While being vectored for my
> approaches, my DG kept sticking through the 270 heading. First time on my
> initial approach and the second approach. Third and fourth approaches, DG
> acted normal, though seem a little off compared to the magnetic compass.
> (gave me good practice for partial panel approaches!).
>
> Also, my header bug seem to be frozen, couldn't move it at all. I ended up
> using my magnetic compass and VFR GPS for my primary navigation and
> tracking.
>
> I had this happen once on the ground a long time ago, and never gave any
> thought as it performed flawlessly in the air since. Vacuum gauge was
> exactly where it should be.
>
> Last thing I want to worry about in the clag is the DG. Is the DG worth
> overhauling or just getting a new one???
>
> I went to http://www.aircraftspruce.com/menus/in/gyros.html to see what
> prices I am looking at, and it seemed all over the page in price. This
> would be a part obviously I don't want to cheap out on, but I don't want to
> buy a part that is has something in it I never will need.
>
> Suggestions much appreciated. I figured to post this time b4 going out on
> the limb.
>
> DME question. While on my VOR Alpha approach back to MBO, ATC reported me
> 3 miles from the VOR, and my DME reported me 4 miles. This is the first
> time I ever had a discrepancy that much. Anybody know what gives. My DME
> seemed correct as it reflected 5 miles to my airport.
>
> Allen

[[email protected]]

Regarding the DG:

Sounds like a mechanical interference issue, which could account for both
the "snagging" on the 270 hdg and the bug being "frozen". The DG may only
need repair rather than full overhaul. Take it to a competent instrument
shop for diagnosis.

If it does need overhaul, whether to do it or not, in my opinion, depends
upon the quality of your unit. Some of the newer "cheapies" are not worth
overhauling while older, more robust designs are. Again, a good shop should
be able to advise you.

Regarding the DME issue: If your DME doesn't read to tenths of miles but
instead rounds to the nearest mile, the discrepancy may not exist. If the
DME read exactly 3.0 miles from the VOR and ATC told you 4, that could be
just a somewhat sloppy guesstimate by the controller. My understanding is
that they can bring up range rings on their new screens, but if a controller
is busy he/she might just "eyeball" it. In any event, I wouldn't worry too
much about it. If the DME indicates distance it is most likely functioning
properly. Measurement errors are conceivable but not common. Of course, as
you suggest it will be simple to do a check against your GPS

--
-Elliott Drucker

[Newps]

john smith wrote:
> A Lieberman wrote:
>
>>> DME is slant range, ATC may be horizontal. How high were you?
>
>
>> I was at 2000 feet, which is about 1700 AGL.
>
>
> Certainly not enough to give you an extra mile of slant range.

You have a digital readout that gives exact distance. The controller
looks at the little space between you and the VOR symbol on the radar
scope. That symbol, a circle, is about a mile in diameter. The
controller looks at that space and he came up with 3 miles. He was off
a mile. No big deal.

[Newps]

wrote:

.. My understanding is
> that they can bring up range rings on their new screens, but if a controller
> is busy he/she might just "eyeball" it.

The controller almost always eyeballs it. As for range rings they are
alway displayed and are drawn around the radar site, not around an
individual airplane.

[john smith]

>>>> DME is slant range, ATC may be horizontal. How high were you?

>>> I was at 2000 feet, which is about 1700 AGL.

>> Certainly not enough to give you an extra mile of slant range.

> You have a digital readout that gives exact distance. The controller
> looks at the little space between you and the VOR symbol on the radar
> scope. That symbol, a circle, is about a mile in diameter. The
> controller looks at that space and he came up with 3 miles. He was off
> a mile. No big deal.

Another factor is the physical location of the DME site with respect to
the airport reference point.

[[email protected]]

On 9-Jul-2005, Newps > wrote:

> The controller almost always eyeballs it. As for range rings they are
> alway displayed and are drawn around the radar site, not around an
> individual airplane.


Of what possible use would range rings centered on the radar site be? From
my memory of a visit a few years ago to a TRACON facility, I seem to recall
that the controllers could get a reasonably accurate reading of distance
from given fixes (e.g. VORs, OMs, airfields, etc.) by calling up range rings
from that fix to appear on his/her scope. Given the level of digital
sophistication they have available on the newer technology radar displays,
that would be a piece of cake. Still, as you state, my guess is that most
controllers, particularly experienced ones, usually eyeball it unless more
precision is required.

--
-Elliott Drucker

[A Lieberman]

On Sat, 09 Jul 2005 20:47:41 -0600, Newps wrote:

> You have a digital readout that gives exact distance. The controller
> looks at the little space between you and the VOR symbol on the radar
> scope. That symbol, a circle, is about a mile in diameter. The
> controller looks at that space and he came up with 3 miles. He was off
> a mile. No big deal.

Newps,

Thanks for your reply.

It may not be too big a deal from the ATC side, but one mile would be a big
deal for a spam can like myself.

As you know, some approaches require DME and if my equipment happens to be
one mile off, then I may have problems if I was down to minimums and didn't
see the airport because I was one mile short. Missed at MBO is just short
of crossing mid field.

Since my DG had gone south on me, I was even more concerned after passing
the initial approach fix that maybe I could not rely on the DME since he
"reverified" my distance from the VOR as 3 miles when I was reading 4.

I realize that I was operating under VFR, so in my specific circumstances
for today, it's no big deal, but had I been IMC, it would be more critical
to know that my DME is working correctly.

Seems that with airplanes, more then one part seems to go south at the same
time.....

Allen

[Brad Zeigler]

"A Lieberman" > wrote in message
...
> On Sat, 09 Jul 2005 20:47:41 -0600, Newps wrote:
>
>> You have a digital readout that gives exact distance. The controller
>> looks at the little space between you and the VOR symbol on the radar
>> scope. That symbol, a circle, is about a mile in diameter. The
>> controller looks at that space and he came up with 3 miles. He was off
>> a mile. No big deal.
>
> Newps,
>
> Thanks for your reply.
>
> It may not be too big a deal from the ATC side, but one mile would be a
> big
> deal for a spam can like myself.

Its not a big deal if you realize that ATC isn't going to give your exact
distance.

1) ATC is not responsible for calibrating your DME.
2) ATC assumes you will be able to figure out when you cross the VOR
3) This specific approach can use time to identify the MAP

As a CFII, I always seem to have students with GPS and DME who feel
compelled to question ATC for distance discrepancies of about a mile. After
we visit an ATC approach control facility (its on my instrument syllabus)
and see how controllers determine distance, that annoying habit usually goes
away:)

[A Lieberman]

On Sun, 10 Jul 2005 11:45:48 -0400, Brad Zeigler wrote:

Hi Brad,

See below comments. Keep in mind, I consider myself still new at
instrument flying. Followups set to rec.aviation.ifr to maintain on topic.

> Its not a big deal if you realize that ATC isn't going to give your exact
> distance.
>
> 1) ATC is not responsible for calibrating your DME.

Agree 100 percent

> 2) ATC assumes you will be able to figure out when you cross the VOR

Agree 100 percent. That's what the to and from flags are for. So, the
distance to the VOR did not concern me as I had the flags to help me
identify the fix nor did I need a timer to verify the fix as I was being
vectored.

> 3) This specific approach can use time to identify the MAP

This is where I have additional questions. The time is based on a distance
factor. The actual approach is the VOR A at MBO. I need 3 minutes 20
seconds to fly 5 miles from the JAN VOR to the MAF.

Add in a headwind on the approach IF I have a discrepancy between what ATC
is reporting and what my DME says, then I would think there is a potential
problem, especially if ATC is reporting that I am closer then what my DME
says.

I do use all possible tools in my plane, and I do have a VFR GPS to help me
determine ground track, so I can tell whether I should arrive to the MAP
sooner or later then 3.20, but if didn't have that, wouldn't the accuracy
of the DME be more critical since time would be skewed by wind correction?

Now, by posting my concern to the newsgroup, I think I am learning that the
DME would be an overriding factor on "who to trust" for distance
determination. As I posted before, I never had come across a discrepancy
before between my DME and what ATC reported.

Hopefully I am right on my assumption that the DME is the overriding
decision for accuracy??

Allen

[Mike Noel]

I have a 70's vintage Mitchell DG (with autopilot bug) that was drifting off
course rather quickly. After snooping around with Google groups to see
which overhaulers had the best reputation, I sent mine off to Ducasse
Aircraft Instrument Service in Pennsylvania for overhaul. It's been working
great for the last 10 months. Expect it to be out of service for about 3
weeks even with air shipping.


"A Lieberman" > wrote in message
...
> Couple of questions.
>
> Went up to practice some ILS approaches. While being vectored for my
> approaches, my DG kept sticking through the 270 heading. First time on my
> initial approach and the second approach. Third and fourth approaches, DG
> acted normal, though seem a little off compared to the magnetic compass.
> (gave me good practice for partial panel approaches!).
>
> Also, my header bug seem to be frozen, couldn't move it at all. I ended
up
> using my magnetic compass and VFR GPS for my primary navigation and
> tracking.
>
> I had this happen once on the ground a long time ago, and never gave any
> thought as it performed flawlessly in the air since. Vacuum gauge was
> exactly where it should be.
>
> Last thing I want to worry about in the clag is the DG. Is the DG worth
> overhauling or just getting a new one???
>
> I went to http://www.aircraftspruce.com/menus/in/gyros.html to see what
> prices I am looking at, and it seemed all over the page in price. This
> would be a part obviously I don't want to cheap out on, but I don't want
to
> buy a part that is has something in it I never will need.
>
> Suggestions much appreciated. I figured to post this time b4 going out on
> the limb.
>
> DME question. While on my VOR Alpha approach back to MBO, ATC reported me
> 3 miles from the VOR, and my DME reported me 4 miles. This is the first
> time I ever had a discrepancy that much. Anybody know what gives. My DME
> seemed correct as it reflected 5 miles to my airport.
>
> Allen

[Newps]

john smith wrote:
>>>>> DME is slant range, ATC may be horizontal. How high were you?
>
>
>>>> I was at 2000 feet, which is about 1700 AGL.
>
>
>>> Certainly not enough to give you an extra mile of slant range.
>
>
>> You have a digital readout that gives exact distance. The controller
>> looks at the little space between you and the VOR symbol on the radar
>> scope. That symbol, a circle, is about a mile in diameter. The
>> controller looks at that space and he came up with 3 miles. He was
>> off a mile. No big deal.
>
>
> Another factor is the physical location of the DME site with respect to
> the airport reference point.

How is that a factor?

[john smith]

Newps wrote:
> How is that a factor?

If the DME antenna is at the end of the runway farthest from you on a
long runway, the distance is greater.

[Newps]

john smith wrote:
> Newps wrote:
>
>> How is that a factor?
>
>
> If the DME antenna is at the end of the runway farthest from you on a
> long runway, the distance is greater.

The controller will give a distance from a fix, almost always the OM or
the FAF. It isn't relavant where the DME is actually located or how
long the runway is, you still have to do some math to figure out how far
you are from whatever the referenced fix is.