A Couple Of Questions about IFR

[Louie1975]

1.) If Flying a VOR/DME approach without a DME on board, can you use GPS in lieu of DME info(if the procedure is in the database)? Would this be an issue because DME is slant range, and GPS is ATD, so the fixes might be slightly different?

2.) When on a Victor airway, can you use the GPS OBS mode to dial the course(radial) of the airway and track to the VOR waypoint on the GPS, instead of the VOR?

3.) On the oubound leg of a DME fix hold, when do you start timing outbound? When do you turn inbound? I would think it would be abeam the fixes but your not measuring along the holding course when outbound.

[Doug Taylor]

You're bumming me out because I used to know the answer to this question

[Fly_Unity]

Yes, to both 1 and 2.

You should review the plates and caculate (or at least be aware) of the difference of distances because of slant range error.

http://www.aopa.org/whatsnew/air_tra...s_in_lieu.html

[tgrayson]

Originally Posted by Louie1975

1.) If Flying a VOR/DME approach without a DME on board, can you use GPS in lieu of DME info(if the procedure is in the database)? Would this be an issue because DME is slant range, and GPS is ATD, so the fixes might be slightly different?

Yes, but the procedure doesn't have to be in the datbase, only the fix the DME is derived from. The slant range error is trivial at low altitudes.

When on a Victor airway, can you use the GPS OBS mode to dial the course(radial) of the airway and track to the VOR waypoint on the GPS, instead of the VOR?

OBS mode isn't a good idea. You should be in leg mode, making sure that any fixes that denote a course change are in your flight plan.

3.) On the oubound leg of a DME fix hold, when do you start timing outbound? When do you turn inbound? I would think it would be abeam the fixes but your not measuring along the holding course when outbound.

You don't need to time. Start your turn when the DME distance is reached. The error is small when measuring along the outbound vs the inbound.

[Murdoughnut]

Originally Posted by Fly_Unity

Yes, to both 1 and 2.

You should review the plates and caculate (or at least be aware) of the difference of distances because of slant range error.

http://www.aopa.org/whatsnew/air_tra...s_in_lieu.html

I assume that 1 and 2 are only yes if the GPS is RNAV approved - correct?

[Cessnaflyer]

I thought a few months ago that the FAA came out and said that only a few GPS systems are approved. There were a few Garmins and maybe a KLN series in there IIRC.

[MidlifeFlyer]

Originally Posted by Cessnaflyer

I thought a few months ago that the FAA came out and said that only a few GPS systems are approved. There were a few Garmins and maybe a KLN series in there IIRC.

Then they came out and fixed the faux pas.

[MidlifeFlyer]

Originally Posted by tgrayson

OBS mode isn't a good idea. You should be in leg mode, making sure that any fixes that denote a course change are in your flight plan.

I agree, unless it is a situation where you are not already tracking the airways. Such as where you are receiving vectors and then are told to intercept an airway, in which case I would use OBS mode to define the airway-equivalent course to the next fix for interception purposes, just as I would do with a VOR.

[Cessnaflyer]

Originally Posted by MidlifeFlyer

Then they came out and fixed the faux pas.

Ok good I haven't been reading many IFR stuff recently so I must've missed it.

[tgrayson]

Originally Posted by MidlifeFlyer

then are told to intercept an airway, in which case I would use OBS mode to define the airway-equivalent course to the next fix for interception purposes, just as I would do with a VOR.

Depending on how far you were from the next fix, that may not be any better than the original scenario. I'd reserve OBS mode for short segments. And no, I can't define "short."

[ajf005]

The only technicalities for questions 1 & 2 are 1. the GPS must be approved for IFR navigation 2. The database must be current and 3. You must have the GPS operator's manual on board with the updated revisions(if shooting an instrument approach with the GPS you also need the quick reference handbook). I know this is pretty anal but I had a student fail an instrument ride b/c the FAA was in the backseat observing and looked through the AFM/POH and found the revisions weren't up to date and the GPS operators manual was not on board and failed him on that alone! So if you want to be completely legal thats what you need. Personally if im IMC, single pilot and the GPS craps out im not going to worry about reaching in the back and reading how to try and fix it especially on an approach.....but hey maybe the FAA has those kind of skills

[Louie1975]

I was really wondering about the airway stuff because I know GPS gives you great circle route, whereas VOR does not, but this must probably only be a factor over very long distances.

[milski]

Originally Posted by MidlifeFlyer

I agree, unless it is a situation where you are not already tracking the airways. Such as where you are receiving vectors and then are told to intercept an airway, in which case I would use OBS mode to define the airway-equivalent course to the next fix for interception purposes, just as I would do with a VOR.

My concern with doing that too far away from the VOR would be that the 0 radial is not always perfectly aligned with the magnetic North - there are a few airways around here which have about 2-3 degree difference in the radial defining them from the both ends. For example the outbound would be 183 but the inbound to the next VOR is 180. If I remember correctly, they get realigned if they're more than 4 degrees off from the magnetic North.

[MidlifeFlyer]

Originally Posted by milski

My concern with doing that too far away from the VOR would be that the 0 radial is not always perfectly aligned with the magnetic North - there are a few airways around here which have about 2-3 degree difference in the radial defining them from the both ends. For example the outbound would be 183 but the inbound to the next VOR is 180. If I remember correctly, they get realigned if they're more than 4 degrees off from the magnetic North.

You're right about VOR "misalignment" and it's effect. But do you think it's likely to be a significant factor? In the small aircraft I fly, the typical situation in which I've been given "Fly heading XXX to join Victor YYY" is in the departure to enroute or SID transition and not distances where the difference is likely to be a factor (i.e., exceed the acceptable tolerances for OBS and airway centerline error).

[milski]

Originally Posted by MidlifeFlyer

You're right about VOR "misalignment" and it's effect. But do you think it's likely to be a significant factor? In the small aircraft I fly, the typical situation in which I've been given "Fly heading XXX to join Victor YYY" is in the departure to enroute or SID transition and not distances where the difference is likely to be a factor (i.e., exceed the acceptable tolerances for OBS and airway centerline error).

Yes, I've been getting exactly the same thing in a very similar situation. Usually the join point for me is at least 25-30 miles away from the nearest VOR on the airway.
If your VOR receiver in the airplane is off 4 degrees and the VOR is misaligned by 3 degress, at 30 miles away from the VOR you're almost 4 miles off from the center. So I would say that it's enough of a deviation to at least keep in mind. Plus I don't see what additional information are you getting by using the OBS vs the airway as defined by the two nearest waypoints/VORs.

[MidlifeFlyer]

Originally Posted by milski

Yes, I've been getting exactly the same thing in a very similar situation. Usually the join point for me is at least 25-30 miles away from the nearest VOR on the airway.
If your VOR receiver in the airplane is off 4 degrees and the VOR is misaligned by 3 degress, at 30 miles away from the VOR you're almost 4 miles off from the center. So I would say that it's enough of a deviation to at least keep in mind. Plus I don't see what additional information are you getting by using the OBS vs the airway as defined by the two nearest waypoints/VORs.

Good point.

[mojo6911]

Originally Posted by tgrayson

You don't need to time. Start your turn when the DME distance is reached. The error is small when measuring along the outbound vs the inbound.

I think he might be talking about a DME fix, and not a DME leg hold, where the DME fix and the radial define the intersection.

For this, you should start your time wings level outbound, or abeam the fix, whichever occurs later. With no wind, it should happen at pretty much the same time. With a headwind or tailwind, it will change it. You would do this for any intersection hold. For holding over a VOR, you have no accurate means of determining if you are abeam the fix, so you just start time when you are wings level outbound.