Sticky DG and DME distance question

"A Lieberman" wrote in message
...
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.

The 3:20 is based on groundspeed. Shorten the time as necessary for a
tailwind. The approach designers build in a lot of wiggle room for
obstruction clearance based on the imprecise nature of timed MAPs. I teach
the method of using the first notification of the MAP to go missed...i.e. if
DME says five...climb; handheld GPS says five from the VOR...climb; times
says 3:20...climb...you get the idea. If you go missed a little early...no
problem.

Don't worry about what ATC says. If they're grossly off, by say three or
more miles...question them as they may be mistaking you for another target.
You're better off cross-checking your DME with GPS than the controller.
Between DME slant range, radar refresh delay (3 seconds), and ATC eyeball
error, they're likely to be a mile off from your DME readout.


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?

No, see comment above about apprach design. Remember, this is a
NON-PRECISION approach. You don't have to be exact.


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.

There's always a first for everything


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

USE YOUR JUDGEMENT! Cross check with cross radials, time, marker beacons,
GPS, whatever. If you just tracked a radial right over the VOR, watched the
flag flip from TO to FROM, and the DME reads 10, you might want to consider
crosschecking the instrument unless you're flying along at FL600.


Allen