Timing a localizer approach

[FL420]

Many IFR GPS units will give you the same distance as the ILS DME if you enter IUBR

works good, lasts long time.

Yes. But if a localizer does not have a co-located DME, the location of the LOC antenna(Lat/Long) may or may not be surveyed and in the database. I read somewhere recently(sorry, I can't provide a reference at present) that the FAA is working on the problem so eventually GPS users can use the GPS distance for determining the MAP.

 

[minitour]

Yes. But if a localizer does not have a co-located DME, the location of the LOC antenna(Lat/Long) may or may not be surveyed and in the database. I read somewhere recently(sorry, I can't provide a reference at present) that the FAA is working on the problem so eventually GPS users can use the GPS distance for determining the MAP.

Fantastic! The next thing they need to work on is letting us w/o an ADF shoot an NDB approach w/o a GPS overlay provided the procedure is in the database.

-mini

 

[A Squared]

Fantastic! The next thing they need to work on is letting us w/o an ADF shoot an NDB approach w/o a GPS overlay provided the procedure is in the database.

-mini

Hmmm, I'm a little unsure what you're saying here, if the NDB procedure is in the GPS database, it *is* an overlay. If there's an overlay, you can shoot the approach using a GPS (assuming approach approved receiver)

 

[Flysher]

Hmmm, I'm a little unsure what you're saying here, if the NDB procedure is in the GPS database, it *is* an overlay. If there's an overlay, you can shoot the approach using a GPS (assuming approach approved receiver)

I think he meant if you dont have an ADF in the cockpit. If I recall correctly you CANT use the GPS in that case. This is without the FAR/AIM in front of me, but I also think that if you have a WAAS GPS you ARE allowed to use it for an NDB approach, someone correct me if Im wrong though.

 

[minitour]

Hmmm, I'm a little unsure what you're saying here, if the NDB procedure is in the GPS database, it *is* an overlay. If there's an overlay, you can shoot the approach using a GPS (assuming approach approved receiver)

I've seen approaches in the GPS for the "NDB XX" approach and the airport only has one approach plate...it's the "NDB XX" approach.

If I don't have an ADF, I can't shoot the approach...even though the GPS is much more accurate.

Flysher,
I wasn't aware of the WAAS GPS for NDB approaches w/o an ADF thing...I'll check it out.

-mini

 

[Flysher]

yea like i said my FAR/AIM isnt with me right now so i cant check, but i think i remember reading that. I know that with a WAAS GPS you dont have to have any other kind of "supplemental" navigation like you do right now with GPS, you can just have your GPS unit and thats it.

 

[nosehair]

"Timing" is not a 'regulatory' requirement on any non-precision approach. It is a long standing teqhnique by which we make an estimate of our position on the approach. We 'wag' and guess at the effects of the head or tail wind, but as we all know, it is a guess - without DME or other ground measuring equipment.

If you study the characteristics of a localizer needle as you approach the end of the runway, you can make a much more accurate 'wag' at reaching the runway than with timing. If you've ever had an unobserved strong headwind on a localizer approach, you have probably observed that the 'estimated time' ran out a mile or two from the runway, and the localizer needle had not yet become so sensitive. Learn to correlate the needle movement with Middle Marker passage or runway environment and you will know better when to start a missed. It will also prevent an overshoot when you have a strong tailwind.

Always time because that gets you in the ballpark and 'times' your more accurate estimation by the movement of the needle or other means such as a GPS.

This is not saying to make your decision solely on GPS, or needle movement or time, you should take all factors into account.

 

[A Squared]

I think he meant if you dont have an ADF in the cockpit. If I recall correctly you CANT use the GPS in that case. This is without the FAR/AIM in front of me, but I also think that if you have a WAAS GPS you ARE allowed to use it for an NDB approach, someone correct me if Im wrong though.

Ok, I wasn't commenting on the "without a ADF" part of his statement, I was commenting on the second part of statement, the part where he seems to me to be referring to NDB approaches where "the procedure is in the database" but there is no overlay. My point was this: If the NDB approach is in the GPS database (not just the NDB position) that *IS* an overlay, there's no such thing as an approach that is in the database, but there is no overlay. That's what an overlay *is* a procedure based on another navaid which is in the GPS database as a selectable approach procedure.

Now, to take it one step further, whether or not you can fly the approach without an NDB installed depends on the Phase of the approach. (phase of the overlay program) There are no more Phase I approach overlays. If it's a Phase II overlay, the title will say NDB RWYXXX with no mention of GPS and you have to have an ADF installed, If it's a Phase III overlay, the title on the plate will say NDB or GPS RWY XXX, you can fly the approach with no ADF installed.

I don't know how many Phase III approaches there are left, I can't think of any off the top of my head. In my area they seem to have all been replaced with RNAV only procedures. Your results may vary I do know of a few Phase II approaches that are still lurking in the databases

 

[Flysher]

ah ok, sorry about that

 

[MauleSkinner]

As Nosehair said, timing is how we estimate our position relative to estimates of headwind/tailwind component, etc.

Personally, I think that the thought process in timing an approach is important. You have to think about the winds, your airspeed, and how they affect your groundspeed, and while you're thinking about it, your track and wind correction angle become part of the conscious process as well. In other words, it forces you to think about what's going on around you, and you get a bigger picture of what you're doing. The big picture, IMO, is almost always good.

Figuring out the time required forces you to estimate a groundspeed on the approach. If you don't have a groundspeed in mind, you might not notice that the front went through recently, and you're now shooting the approach with a 20-knot tailwind. Yes, in this case your time estimate would be way off. That's why you DO look at your GPS to compare groundspeeds and distance to the MAP, and take appropriate action to either go missed in the "right place", or find out whether or not the surface winds are what you thought they were so that you can take appropriate action (straight-in or circle).

If timing the approach is all you've got, then timing the approach is all you've got. If GPS is all you've got, then GPS is all you've got. But don't throw good tools out of the cockpit simply because they're not as accurate as you might think necessary or desirable.

Fly safe!

David