Brad Zeigler
July 10th 05, 11:39 PM
"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
...
> 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