Four eyes are better than two.

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Four eyes are better than two.

This occured just a couple weeks ago.

I often sit right seat when my best buddy who is a ATP, CFI, CFII, MEI flies clients. He flies either a King Air 200 or a 300. We were in a 300 and returning late in the evening from an all day trip to far south Texas. We were going into KGTU at night just about at minimums flying a GPS approach. (This airport does not have a precision approach)

I have had a 11 year break from flying so the only GPS manuvering I know is the one in my car or on my motorcycle. He however, is totally trained on the beast. We flew back at FL 240 and for the most part could see fairly well. As we got farther north WX got crappier by the moment. ATIS confirmed the ceiling at destination was just about 600 feet.

I expected to break out and have the designated runway directly at our 12 O'Clock position. It was not. We are still in the fog when the plane starts telling us we are too low. He at that time breaks out and we see the city, but no airport. Again, we hear "you are too low". At this time we are about 500 AGL and I am doing a constant scan when I see the airport at 2 O'clock. We are a good 1/2 mile off center line and just about the time I stated 2 O'Clock and he looks over confirms seeing the APT the ATC tells him to turn right. We land without bending any metal. It was a little harrowing, but my lesson learned as I am just getting back into flying is if minimums are that low at a non-precision airport maybe I should divert into a airport with an ILS. However, until I get a lot more comfortable flying IFR and especially since I do not fly paying pasengers, I would have kept the plane on the ground until conditions had improved signigicantly.
 
Non-precision night approaches to mins can be some of the more challenging. Even precision approaches can be difficult. Last Friday in the Cheyenne, the forecasted OVC008 turned into OVC002 and 3/4 SM vis. The reality for us was runway lights at 200 AGL, and runway in sight at 100. Right at mins. I was glad to have another pilot with me to look for the runway while I hand flew the approach. As much as I avoid using autopilots on approaches, that might have been a good one to use it on.
 
There is more to this story than what has been described. WAAS GPS accuracy is extremely good...if the needles were centered, you would have been on centerline, not 1/2 mile off the airport. It's why some GPS approaches have 200 - 1/2 minimums.
 
Jim_R said:
There is more to this story than what has been described. WAAS GPS accuracy is extremely good...if the needles were centered, you would have been on centerline, not 1/2 mile off the airport. It's why some GPS approaches have 200 - 1/2 minimums.
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Could have been an overlay o f a circling approach. Even some "straight-in" approaches can seem way off when there is a stiff crosswind.

Sent from my ADR6300 using Tapatalk
 
OP said the airport is KGTU. There are four GPS approaches, and they're all straight-in.

Not sure how the crosswind can make things seem way off if you keep the needle centered.

From the FAA website:
Q. What is the sensitivity of my CDI when using GPS?
A. En route, full-scale Course Deviation Indicator (CDI) deflection is typically 5 miles with an accuracy of +/- 2 miles. Within 30 miles of the arrival and departure airport, GPS CDI sensitivity typically transitions to one mile. When flying an approach (and the approach mode is armed), GPS CDI sensitivity transitions from 1 mile to 0.3 miles approximately 2 miles from the FAWP.
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OP said they were "a good 1/2 mile off" the runway at something near the threshold (airport at 2 o'clock). The CDI needle would have been pegged offscale to the right in that condition (since full deflection = 0.3nm at that range).

Rather than learning a lesson about diverting to a field with ILS, perhaps there needs to be some re-briefing of the approach plate, GPS, and CDI settings / indications.
 
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Last time I safetied for an IFR pilot on a currency run I had to take the airplane on the last approach. He just got tired.
 
Jim_R said:
Not sure how the crosswind can make things seem way off if you keep the needle centered.
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Crab angle. Even though you're dead on centerline, the nose is pointed into the wind so the runway appears on the downwind side of the nose. That's why I teach my trainees to check heading before looking out, and start looking in the direction the runway goes rather than straight ahead, e.g., if it's runway 33 and the "33" on your heading indicator is 10 degrees to the left, look 10 degrees to the left, not straight ahead.
 
Exactly. Fly the KDMW VOR 34 (VOR or overlay) and you're already pointed 20 degrees right of the runway orientation even if you aren't dealing with a crosswind.

Having the runway appear at the 10 - to - 2 o-clock position is common when dealing with strong winds, since non-precision approaches, even GPS, can have you "breaking out" a mile or more from the runway.
 
I am the original poster. And, I am in way over my head on this discussion. Although I have a Instrument rating I am no longer current. In fact, I have not flown PIC in 11 years so I am just learning WAAS GPS or trying to understand this.

What I think one of you said was the GPS is so accurate that the GPS line (purple line on the face of the GPS indicator) to the runway should have been centered (in a no wind situation) as the needle on an ILS or a VOR is centered as the aircraft gets closer to the runway if in fact the plane is in the proper position.

I was so busy looking out of the plane for the airport I did not look at the instruments to verify this, but my friend, the pilot is extremely competent and I was suprised at the fact we were so far left of course. Again, I have practically no experience with GPS in an airplane except in his 172 it has the glass cockpit and I am trying to get back to speed learning to use this method of navigation.

Regardless of a cross wind if the needle (GPS line) is centered shouldn't the aircraft be on or very close to the runway's centerline? I understand crabbing to maintain heading, and in a crosswind I would have to do that, but if I was on course wouldn't the GPS indicate this?

Thanks.

A retrread trying to understand new technology.
 
The GPS understands track (and so does an ILS) and not heading. It knows the airplane's course over the ground and not the direction the airplane is pointed.

A WAAS gps LPV approach is flown very much like an ILS, you keep the needles centered and you are on centerline and on glidepath.

What most likely happened (if your best buddy was keeping the needles centered) is that you had some strong crosswinds and your airplane was tracking the runway centerline, but your heading was different... essentially you were flying a 15-20 degree wind correction angle.

^ The above is assuming a straight in LPV approach
 
If the needle is centered, the airplane should be centered on the final approach course. Looking at the approach plate for the RNAV/GPS 36, you would normally reach the Minimum Descent Altitude (MDA) at 1.5 NM or farther from the runway.

I'm not saying that your pilot was on course or not - only you and he were there. I'm just giving some reasons where he COULD have been on course and you COULD have been surprised to see the runway other than where you expected it. Given the two runways, if 29 was lit and 36 wasn't lit (or not as bright), you might have felt you were WAY left for 29 while in fact you were on course for 36.

Or, your pilot may have been far left of course, and should have executed a missed approach, but was hopeful he'd break out "any second now".

Only he knows for sure.
 
TMetzinger said:
If the needle is centered, the airplane should be centered on the final approach course. Looking at the approach plate for the RNAV/GPS 36, you would normally reach the Minimum Descent Altitude (MDA) at 1.5 NM or farther from the runway.
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Correct - but we all know what the winds do in the last 1000 feet above the ground - it would not surprise me to see a large swing in direction or velocity change a needles-centered approach into a way-off-center approach very quickly during the last 20-30 seconds of the approach, faster than you can catch the change. Whether that would result in a half-mile or not I can't say, I wasn't there obviously - but we've all seen rapid wind shifts on final.
 
airguy said:
Correct - but we all know what the winds do in the last 1000 feet above the ground - it would not surprise me to see a large swing in direction or velocity change a needles-centered approach into a way-off-center approach very quickly during the last 20-30 seconds of the approach, faster than you can catch the change. Whether that would result in a half-mile or not I can't say, I wasn't there obviously - but we've all seen rapid wind shifts on final.
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Yes, that can happen, but it's not nearly common as a steady crosswind resulting in the airplane pointing away from the runway even though it's right on centerline. And if it does happen, you know it very quickly because of the lateral needle's obvious reaction. The crosswind case's indications are more subtle, and based on my instructing experience, often fool the unwary pilot who is craning his/her neck over the glare shield staring straight ahead looking for a runway which would be plainly visible if s/he just turned his/her head 15 degrees left/right.
 
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Hmm. I'll defer to those with more flight time, but so far in my experience, I've had no trouble figuring out where the airport is, crab angle or no. If the needles are centered, when you break out and can see the ground moving (which maybe was hard to do since they were landing at night), you can follow the motion back toward the airport. At any rate, between the DG and CDI you know how much crab you've got, and should know when you break out which direction off the nose to look.

At any rate, if crosswind is what caused the confusion, again the lesson learned here should not be "GPS is inaccurate and it's better to bail and look for an ILS approach", the lesson should be "get practice finding the airport after breaking out with a big crab angle".
 
Unregistered said:
What I think one of you said was the GPS is so accurate that the GPS line (purple line on the face of the GPS indicator) to the runway should have been centered (in a no wind situation) as the needle on an ILS or a VOR is centered as the aircraft gets closer to the runway if in fact the plane is in the proper position.

I was so busy looking out of the plane for the airport I did not look at the instruments to verify this, but my friend, the pilot is extremely competent and I was suprised at the fact we were so far left of course.
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When flying a GPS approach, you should not be flying the magenta line on the moving map. You should be using the CDI (coupled to and driven by the GPS), and fly its needle just like you would with a localizer for horizontal guidance. (GPS approaches may or may not have vertical guidance similar to an ILS glideslope, but I'm not going to get into that here.)

You could also use the GPS screen that has a course deviation indication with the same info available via the round dial CDI, just presented graphically on the GPS display.

Keep that needle centered, and you'll fly right down the runway centerline.
 
Jim_R said:
Hmm. I'll defer to those with more flight time, but so far in my experience, I've had no trouble figuring out where the airport is, crab angle or no. If the needles are centered, when you break out and can see the ground moving (which maybe was hard to do since they were landing at night), you can follow the motion back toward the airport.
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That's another good technique.

At any rate, between the DG and CDI you know how much crab you've got, and should know when you break out which direction off the nose to look.
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Yes, you should, but a lot of IFR newbies like the OP don't fully correlate that.

At any rate, if crosswind is what caused the confusion, again the lesson learned here should not be "GPS is inaccurate and it's better to bail and look for an ILS approach", the lesson should be "get practice finding the airport after breaking out with a big crab angle".
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Agreed.
 
Jim_R said:
Keep that needle centered, and you'll fly right down the runway centerline.
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On ILS, LOC, and RNAV(GPS) approaches, yes. On VOR, GPS, NDB, LDA and SDF (are there any of those left?) approaches, not necessarily. For those, even on a straight-in, the final approach course for Cat A/B aircraft may be up to 30 degrees out of alignment with the runway centerline. Take a good look at the airport picture in the lower corner of the AeroNav chart and see where the little arrow shows you located and heading as you approach the MAP.
 
Ron Levy said:
On ILS, LOC, and RNAV(GPS) approaches, yes. On VOR, GPS, NDB, LDA and SDF (are there any of those left?) approaches, not necessarily.
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Sorry if I was unclear--I was specifically talking about RNAV(GPS) approaches like the one the OP was on (all four of the GPS approaches to KGTU will take you right over the runway centerline).

The OPs original comments made it seem like he questioned the ability of a GPS approach to get you to the airport as accurately as an ILS approach, and I was trying to disabuse him of that misconception.
 
Jim_R said:
Sorry if I was unclear--I was specifically talking about RNAV(GPS) approaches like the one the OP was on (all four of the GPS approaches to KGTU will take you right over the runway centerline).
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Gotcha.

The OPs original comments made it seem like he questioned the ability of a GPS approach to get you to the airport as accurately as an ILS approach, and I was trying to disabuse him of that misconception.
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Careful with the terminology. Not all RNAV(GPS) approaches have LOC-like lateral accuracy (the four RNAV(GPS) approaches at KGTU are all LNAV only, not LP or LPV), and there are a lot of GPS-A approaches out there that don't even have straight-in mins, no less line up with the runway.
 
Ron Levy said:
Careful with the terminology. Not all RNAV(GPS) approaches have LOC-like lateral accuracy (the four RNAV(GPS) approaches at KGTU are all LNAV only, not LP or LPV), and there are a lot of GPS-A approaches out there that don't even have straight-in mins, no less line up with the runway.
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Okay, perhaps this is a teachable moment for me, now. Again, in my limited experience, flying RNAV(GPS) straight-in approaches (is that redundant? Aren't all RNAV(GPS) approaches straight-in?) with LNAV minima has put me right on centerline when I keep the needle centered. Maybe I'm just lucky, or maybe that's the system performance 95% of the time, and I just haven't seen the edge cases yet.

I have had trouble understanding what error tolerance is allowed in the system...the only criterion I consistently see is the "CDI full-deflection equal to 0.3nm on the final approach leg"; I can't seem to find anything like "an error of up to xxxx feet laterally is possible in a correctly-functioning system". Do you know what number xxxx is for LNAV approaches?

I have seen where LP approaches allow up to 40' of lateral error. Do LNAV approaches allow more than that?
 
An LP approach has ANGULAR sensitivity, which means that the tolerance gets tighter as you get closer to the MAP, just like an ILS. And yes, the tolerance at the MAP is much tighter than +- 0.3 NM
 
Yes, on an RNAV(GPS) approach, if you're dead centered on the needle, you'll be on the runway centerline. However, if you're one dot off, you'll be a lot farther off centerline approaching the runway in LNAV mode than in LP or LPV or on a real ILS.
 
Jim_R said:
Sorry if I was unclear--
The OPs original comments made it seem like he questioned the ability of a GPS approach to get you to the airport as accurately as an ILS approach, and I was trying to disabuse him of that misconception.
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That was exactly my point. I did not think the GPS provided the accuracy that a proper ILS approach provides. As my friends Cessna 172 has a newly installed Garmin (think it is a 1000)
Model GPS it looks as if I will be doing a bunch of GPS approaches first to become profecient and also to have the same confidence using that system as I do with the precision when flying the ILS.
 
I find an LPV approach is PREFERABLE to an ILS if they have the same minimums. The GPS doesn't wander all over the place or get funky if a deer walks by the antenna.

To the OP... Trust me, you're going to LOVE Instrument flying with GPS.
 
Unregistered said:
That was exactly my point. I did not think the GPS provided the accuracy that a proper ILS approach provides.
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Sometimes it does, and sometimes it doesn't. Non-WAAS GPS's do not at all -- pretty much at the accuracy of a VOR. WAAS GPS's have several modes of operation. In the LNAV mode, they're about the same as a VOR, but in LPV/LP modes, it's about the same level as ILS laterally and (in LPV mode) vertically. Read the AIM sections on GPS for more on that.
 
TMetzinger said:
I find an LPV approach is PREFERABLE to an ILS if they have the same minimums. The GPS doesn't wander all over the place or get funky if a deer walks by the antenna.

To the OP... Trust me, you're going to LOVE Instrument flying with GPS.
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I'd agree with that. ILS equipment is far from perfect.
 
I am excited. Up to a couple of weeks ago the 172 which the King Air driver owns and is sharing with me had a Garmin 430 glass cockpit. The owner just had that upgraded to a Garmin 1000 (He said he got that radio system at a steal) I believe that as we both are motorcyclists and I watched him get a 2 year old Can Am Spyder with Corbin seats for $8500. And, he bought his wife a 2 year old Suzuki S-40 for $2500. I was there for both transactions.

I told him as I had just bought an Ipad 2 I was going to buy the App for the 430 and he told me about the upgrade. Now, I will have to learn the new system and looking forward to it.

.
 
Huh... Didn't know there were aftermarket upgrades to the G1000. I'm thinking he replaced the plane, or put in a different Garmin system.
 
TMetzinger said:
Huh... Didn't know there were aftermarket upgrades to the G1000. I'm thinking he replaced the plane, or put in a different Garmin system.
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Yeah, that was my understanding as well, no G-1000 retrofits. The retrofit model is the G-500.
And G-600

However, didn't Cessna offer the new 172 with steam gauges for awhile with the G-1000 as an option? Maybe that's how he did it.
 
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I find myself wondering - I have seen the circumstance when a pilot mishandles a GPS approach and it never sequences to the final segment, in which event one can see some seriously unexpected behavior; this, especially when flying vectors instead of the full procedure.

Fact is, I saw exactly this happen on a recent arrival to KADS, and I was very pleased that I broke out high enough to complete as a visual (LOC was inop).
 
Mason said:
Yeah, that was my understanding as well, no G-1000 retrofits. The retrofit model is the G-500.
And G-600

However, didn't Cessna offer the new 172 with steam gauges for awhile with the G-1000 as an option? Maybe that's how he did it.
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Yes, but it was an option ONLY at the time of order, not a "take the airplane back and retrofit it" deal.
 
TMetzinger said:
Huh... Didn't know there were aftermarket upgrades to the G1000. I'm thinking he replaced the plane, or put in a different Garmin system.
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He had a Garmin 430 system in his Cessna 172 B. He told me he got such a great deal that he replaced the 430 with a Garmin 1000. I could be wrong about the model number of the new GPS. He told me so I would not buy the Ipad APP for the 430 as it would no longer be in the plane.
 
Unregistered said:
He had a Garmin 430 system in his Cessna 172 B. He told me he got such a great deal that he replaced the 430 with a Garmin 1000. I could be wrong about the model number of the new GPS. He told me so I would not buy the Ipad APP for the 430 as it would no longer be in the plane.
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Sounds like he probably got one of the new touchscreen GPS units, probably the GTN 650 if he was replacing a 430.

The G500 and G600 glass cockpit displays use an external GPS, so if he was getting that there wouldn't be much point to getting rid of the 430, though it's certainly possible.

And you can't retrofit a G1000 into a 172.
 
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