Sticky DG and DME distance question

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.

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.

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?

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.

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.

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

"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

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