Timing a localizer approach

[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

 

[midlifeflyer]

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.

Wait. Let's makes sure that we're on the same page. There are approaches in the database that are not usable overlays. Load an ILS or LOC approach into the GPS. It's there, complete with magenta line, fixes and a nice drawing of the missed hold. But it also has a nice click-through warning that it is not to be used for primary navigation. The same will be true for an NDB approach that is not an NDB or GPS overlay, but is nevertheless in the database.

(Is there something here that is unit specific? Like a sophisticated GPS that removed all the "situational awareness only approaches" so that the only approaches in the database are ones where the GPS could be used for primary navigation?)

 

[A Squared]

Wait. Let's makes sure that we're on the same page. There are approaches in the database that are not usable overlays. Load an ILS or LOC approach into the GPS. It's there, complete with magenta line, fixes and a nice drawing of the missed hold. But it also has a nice click-through warning that it is not to be used for primary navigation. The same will be true for an NDB approach that is not an NDB or GPS overlay, but is nevertheless in the database.

(Is there something here that is unit specific? Like a sophisticated GPS that removed all the "situational awareness only approaches" so that the only approaches in the database are ones where the GPS could be used for primary navigation?)

Oh, ok. I guess that's shows the limits of my experience. All the approach approved GPS receivers I've used don't have the "situational awareness", "for informational purposes only" things in the database. Either it's an approved GPS procedure, or it's not in the database. Well, if that's what minitour referring to, the "for informational purposes only" data I'll confess to still being mystified. Essentially, we can restate the question as: Why can't we use unapproved data to fly an approach ? The answer to that seems obvious.

 

[midlifeflyer]

Oh, ok. I guess that's shows the limits of my experience. All the approach approved GPS receivers I've used don't have the "situational awareness", "for informational purposes only" things in the database. Either it's an approved GPS procedure, or it's not in the database.

FYI,here's a link to the GNS430 Quick Reference Guide

http://www.garmin.com/manuals/GNS430_QuickReferenceGuide.pdf

You'll find a sample of the Garmin warning for this unit on page 19 with a brief explanation below it.

 

[PURPLEHAZE21]

Murphys Law states...well we know. Just do both and keep a cross check.

P.S. To save room on certain UNS systems, certain approaches are removed or not put in the data base. Usually the NDB is the first to go. Found out the hard way. Good Luck !!!!!!

 

[PositionandHold]

I tested a few VOR approaches into our local airport here and one back in California where I initially trained. One had a hill a few miles east of the field. On a VFR-Clear day, I shut off the GPS and shot the approach with only a timer and the CDI. I had no up to the second wind information, so unless I continued to query ATC as to my groundspeed, I had to rely on my airspeed corrected for the wind report I got on the ATIS and winds aloft data.

Turned out the tailwinds were stronger than reported that day and had I followed my timer to a tee, I'd have ended up way past the runway and not far from the hill.

Lesson learned, if I don't have the GPS, I always cross check the MAP with another VOR, at least something (could be as rudimentary as drawing a line from another local VOR and using that as a reference). At least the GPS gives a groundspeed readout. Especially in those non-radar environments.

 

[MauleSkinner]

I tested a few VOR approaches into our local airport here and one back in California where I initially trained. One had a hill a few miles east of the field. On a VFR-Clear day, I shut off the GPS and shot the approach with only a timer and the CDI. I had no up to the second wind information, so unless I continued to query ATC as to my groundspeed, I had to rely on my airspeed corrected for the wind report I got on the ATIS and winds aloft data.

Turned out the tailwinds were stronger than reported that day and had I followed my timer to a tee, I'd have ended up way past the runway and not far from the hill.

What kind of clearance would you have had from the hill (if any) had you initiated a 200 ft/mile climb from your timed missed approach point?

Also, were the winds substantially different than forecast at your enroute altitude, or just at the altitudes you were at on the approach?

Fly safe!

David

 

[PositionandHold]

What kind of clearance would you have had from the hill (if any) had you initiated a 200 ft/mile climb from your timed missed approach point?

Also, were the winds substantially different than forecast at your enroute altitude, or just at the altitudes you were at on the approach?

Fly safe!

David

I should have done it more scientifically, but I just flew it, came down, and shrugged my shoulders. I never calculated what the winds probably were. I just know that the stopwatch would have taken me past the runway by a non-trivial amount. The MAProcedure would have certainly cleared any obstruction there because the FACourse was perpendicular to the runway and the MAP course involved a turn away from any hill.

My basic point of advice was that if you're going to do a NP approach with no GPS or DME and you're in an area with noteworthy obstructions, use something other than the stopwatch to back yourself up, because those times are never on the dime, and sometimes a bit more than a trivial amount from the dime.

 

[MauleSkinner]

I should have done it more scientifically, but I just flew it, came down, and shrugged my shoulders. I never calculated what the winds probably were. I just know that the stopwatch would have taken me past the runway by a non-trivial amount. The MAProcedure would have certainly cleared any obstruction there because the FACourse was perpendicular to the runway and the MAP course involved a turn away from any hill.

My basic point of advice was that if you're going to do a NP approach with no GPS or DME and you're in an area with noteworthy obstructions, use something other than the stopwatch to back yourself up, because those times are never on the dime, and sometimes a bit more than a trivial amount from the dime.

The point I was trying to make is that a certain amount of error in groundspeed is taken into account when designing these approaches. Sounds like, even with what you considered a substantial error in your time calculations due to unforecast winds, the procedure itself would still have kept you safe. No additional equipment required.

Don't get me wrong...I'm all for backup and verification where possible. Sometimes it's not possible, however, but that doesn't necessarily mean that the procedure can't be done safely.

Fly safe!

David

 

[ksu_aviator]

What I was getting at is that a complete GPS failure, without any of the other avionics failing, is highly unlikely. Even with a RAIM message, your position should be fairly accurate, albeit not within standards. While timing a LOC approach is not as ridiculous as timing an ILS, I am still going to stick with my answer.

Regards,
Devil's advocate

Ok, my initial reaction, after reading, this was anger. But I've calmed down now and I'm going to explain why this statement is dangerous.

RAIM means that the instrument is not working properly. A RAIM message is no different than a NAV flag or Attitude Indicator flag. Going back to steam gauges for a minute, you wouldn't follow a flagged attitude indicator over a working turn coordinator right? Even though the attitude indicator works better normally, once the flag comes out it might as well have tumbled.

The same is true for GPS. RAIM generally means you have at least one less satellite than is required for accurate navigation. The errors can be small or large. You don't know. By far and away, timing is more reliable than a GPS with a RAIM message. I've timed many many approaches, the timing is always accurate if done properly.

The idea that you would rely on a knowingly bad instrument over a perfectly good instrument because "it is probably right" is dangerously stupid.

BTW...time the ILS...then you'll know when to go missed if the glide slope goes bad.

 

[toflyseeme]

Timing the inbound on an ILS from the FAF to the MAP is a great training maneuver and doesn't cost the pilot one thin dime, and if apilot elects to do it that's o.k. with me. However, in the real world of my flying I've never ever had the g/s go belly up on me inside the OM. Perhaps I've been lucky, but in running this scenario past hundreds of professional pilots I've never met one that calculated the time. Maybe they were all just lucky too, but in the real world of flying it's rarely done