[N395V]

There is another thread that I do not want to hijack "Why 2 comms for IFR" that had some comments that I think should be explored.

They were (sort of) you cant use the GPS function on a GR EFIS for an approach because it is nt an "approved device".

and (sort of) if alll you are left with is a hand held Garmin (GPS) to do an approach you are screwed" These aren't the exact quotes but close enough to make a point.

The first comment is true and I take no issue with it and the second comment is a point well taken, I would not want to have a portable GPS as my only option for an actual approach, but let me pose a question ( what if scenario)

You are flying to KMCB (McComb Mississippi) , approach plate seen below, from KPNS (Pensacola Florida) IFR in your RV8,\. You are equiped with a Wiley E Coyote super duper EFIS (experimental with super duper vision m, moving map, GPSS roll steer etc and a single comm and a Nav with DME and ILS (in one unit). Everything in the plane has been working fine and you didand logged a successful VOR check 12 days ago. It is winter and there is a stationary weather system over the Southeast.

You file direct KMCB with and alternate of KJAN( Jackson MS) forecast has KMCB 150' above minimums at arrival and KJAN at 1000' above minimums for your flight planned arrival time. You have legal IFR fuel reserves on board plus 30".

You make the flight and all is well. You are enjoying the view on top at 6000' and Houston center says " radar contact lost, cleared for the VOR/DME-A approach into McComb. You have the VOR DME tuned to the MCB Vortac the aural ID is correct, your Garmin 496 has the approach waypoints loaded.

You descend to 2000 feet. You check weather and it has seriously deteriorated. KMCB, KJAN, and every other airport within fuel range is now at mins for altitude and 2 miles visability. The ILS at KMCB is notamed out of service but the ILS at KJAN is functional.

You continue the approach into KMCB and can see the vortac directly below when the to/from indicator swings. You start your descent to 1500" needle centered and just before you get to egpew you notice your EFIS shows you about 1 mile North of course, DME and your EFIS agree on distance and the Garmin handheld just died. The Vor needle says you are where you should be.

Or as an alternative scenario your EFIS has no moving Map but the Garmin 496 disagrees with your VOR.

What would you do and why?

 

[tturner]

I am going with the GPS position info. I also feel safer with an error that may take me to the south, to the north are towers above the DH. A one mile error that close to the VOR is excessive. Have you verified the course setting on your course indicator?

 

[apkp777]

You execute a missed approach and fly the published missed approach procedure. Then contact Houston center and request the nearest ASR or PAR approach.!

500 feet about the ground in crappy weather is no time to try to figure out what piece of electronics has taken a ****.

Once on the ground, slap yourself in the face and sit down and write out a new set of personal minimums that exclude instrument approaches in poor weather on recreational flights.

 

[Ironflight]

I was thinking ASR or PAR as well (which is why I want Comm redundancy if I am serious IFR....and the weather you are talking about is serious IFR!).

"A man with two watches knows not the time" is a good argument against simple Dual redundancy. Yup, I flew around IFR with a single vacuum pump and dual Nav Com's for years....but that was the standard then. I choose to have more redundancy today.

Given the imaginary situation in which you put us, I'd say I had a pretty significant emergency, and would hope I had enough fuel to get somewhere for a Radar approach. If I had no choice but to flip a coin between the VOR and GPS? I'd ask ATC if there were any NOTAMS of GPS outages, and if not, I'd choose to believe that. But again - if I didn't have th fuel to get somewhere with minimums, I have foolish left myself with no viable "out".

Good thought experiment though!

(During my IFR Checkride Oral, I planned a flight from Houston to New Orleans. the examiner asked what I would do enroute if I lost ALL electronics - Comm, nav, the works. I aid I'[d turn south until I was sue I was out over the gulf and start letting down until I saw water.....she liked that answer. Definitely not in the book, but then, I shouldn't put myself there.)

Paul

 

[jferraro17]

I don't have enough firsthand knowledge of the Garmin units, but do they have a tendency to "map shift"? Or do they have the fault to a lower performance/accuracy state?

These are real problems that go along with other certified GPS units I have flown. My point is you can't, and shouldn't, just take the GPS data at face value because it's a GPS, not ground based, and simply assumed more accurate.

My course of action would certainly be the missed, or vectors & climb away from any terrain threat, once I'm safely clear, try re-tuning navs, and restarting GPS (if it's not completely TU.

Back in the day (), I think a PAR or ASR was a good call, now, not sure you're going to find one within a reasonable distance...maybe Columbus AFB or Navy N.O.?

Bottom line for me, and I'm guilty of it on nearly a daily basis: don't become a slave to the magenta line just because it's there. And especially if it's a non-ifr certified tool.

Joe

 

[apkp777]

Your scenario does illustrate how the situational awareness provided by a handheld gps can really cause you to question the FAA "approved" nav device. In the olden days you would fly the approach using the dud VOR and when you finally saw the airport (a mile from where it was supposed to be) you'd utter a few choice words and turn toward the runway. All the way being "legal".

By the way, Baton Rouge has Approach Radar. About 36nm away.

 

[Paul Thomas]

Using past experience, I'd likely continue. The club archer VOR's consistently disagree on an intersection I frequently use and we're near the stations. I've missed an airport by a couple of miles but the station was 30+ miles back IIRC.

Since the VOR passed the check and worked correctly in en route, being a mile off 10 miles from the station is within reason and I'm still in a safe location (nothing to hit at that alt.). I'd triple check I did everything correctly, monitor the situation and continue inbound even more spring loaded to go missed if required than normally.

Link to the plate: http://204.108.4.16/d-tpp/0913/05753VDA.PDF

 

[apkp777]

The more I think about it, I think that I would continue the approach using the VOR and expect to end up on the left base RW15.

I had an older "Anywhere Map" that had a pretty bad refresh rate and it always sticks in my mind as representative of a "hand held". I have done approaches with a 496 and it is much more smooth. Still unless it's driving an HSI or located directly in my "scan", I prefer the VOR indication over a handheld. The EFIS is a question mark for me as I don't know if I would treat it like a g1000 or handheld. I just got my SkyView in the mail, so you can be sure I am looking forward to discovering the limits of its performance.

You have a tower at 955 to the north, I would plan to descend no lower than 1100 until DME 12 or so.

 

[GJHeck]

Are hand helds ever off that much?

I fly w/ the majors and never used hand held portable GPS such as the Garmin 196, 496, 696....etc and would like to know before I use them, whenever I get my RV-6a finished, are the portables off that much?

I've been told they are pretty darn accurate and if you have a VOR or ILS backed up w/ say a map displayed on your GRT EFIS and a portable and they all three mesh up pretty well..........well you should feel pretty darn good.

With that said, if there is any doubt in your position you better not descend below any safe altitude until you do feel sure of your position.

 

[scard]

Missed, climb back above, and call Houston for help. There is no way at all that I'm going to plan to show up with even the forecast weather at that airport with only that approach with ONLY 30 minutes of fuel.

 

[L.Adamson]

I fly w/ the majors and never used hand held portable GPS such as the Garmin 196, 496, 696....etc and would like to know before I use them, whenever I get my RV-6a finished, are the portables off that much?

I've owned "handheld" Garmin moving map GPS's since the beginnings of aviation moving maps in the early 90's. My last three have been the 296, 496, and now the 696. In my experience, they are very accurate, and I'd trust them just just as much as anything else.

L.Adamson ---- RV6A

 

[N395V]

Lots of good and thought provoking comments relative to the scenario. Probably no right or wrong answere just the answer that gets you safely on the ground.

Like any what if scenario you do not have enough info to make the required decision but that is the way it is in real life. There were a couple of things I put in the scenario to help drive the responses in a particular direction and thought I would comment on the comments. And then tell you what I did in the real situation.

I do not claim to be an IFR guru, just a pilot that has been around for awhile and made his share of mistakes and dumb decisions.

I specifically mentioned the VOR had passed its last check and implied that it worked well enough along the way to get to the MCB VOR.

I also put you off course to the North because there were towers there.

I added radar contact lost because that is what happens at KMCB below 2300'

to the north are towers above the DH

You have a tower at 955 to the north, I would plan to descend no lower than 1100 until DME 12 or so.

Good pickup. This should be a concern and prompt a minimum descent and is the single biggest reason to take prompt initial action and by no means continue the approach using any of the devices on board.

In my opinion you need to "go missed" (more on this under another comment)at this point or at least CLIMB.

You execute a missed approach and fly the published missed approach procedure.

Probably a wise choice but if the VOR is incorrect can you really fly the published missed. (That is why I stated the VOR had passed a check and had gotten you this far).

Using past experience, I'd likely continue.
Since the VOR passed the check and worked correctly in en route

This (I think) is the logic you would use to fly the published missed approach. But in the very least it is time to climb.

500 feet about the ground in crappy weather is no time to try to figure out what piece of electronics has taken a ****.


My point is you can't, and shouldn't, just take the GPS data at face value because it's a GPS, not ground based, and simply assumed more accurate.

Bottom line for me, and I'm guilty of it on nearly a daily basis: don't become a slave to the magenta line just because it's there. And especially if it's a non-ifr certified tool.

does illustrate how the situational awareness provided by a handheld gps can really cause you to question the FAA "approved" nav device

Yep, don't try to figure out and correct a serious problem 500'agl, in the clag with towers nearby.

One of my main points with the scenario is we alll get complacent following the magenta line and we alll think OUR EQUIPMENT is better than the certified stuff. But what do you believe when it is your life on the line and the certified stuff disagrees with the gee whiz stuff.

"A man with two watches knows not the time" is a good argument against simple Dual redundancy

As usual Mr techno got my main point. Alot of recent threads about "dual"coms, dual navs, dual gyros etc. But what do you do when you can't see out the window and the dual whizbangs disagree with each other.

Then contact Houston center and request the nearest ASR or PAR approach.!


I said I'[d turn south until I was out over the gulf and start letting down until I saw water.....

In my opinion an immediate climb followed by one of these 2 courses of action is probably the best option. As mentioned BTR is close enough to fly to and shoot an asr/par as is the Gulf coast letdown followed by VFR into GPT.

Once on the ground, slap yourself in the face and sit down and write out a new set of personal minimums that exclude instrument approaches in poor weather on recreational flights.

It was my intent to convet that after proper preflight planning the planned flight was legal and doable but weather did not follow forecast.


In the real scenario I was flying KMOB (Mobile Al) to KMCB, alternate JAN with weather as noted above
Notams at KMCB where Glideslope disabled and construction equipment near the end of RWY 15.

Had plenty of fuel everything looking good. The plane had a Blue Mountain EFIS (2 of em) an SL30 and a Garmin 396.

Started the Localizer approach to 15 but the needle centered well South of where the 2 Blue Mountains said the IAF was. Looked at the 396 for verification and it clearly had locked up 20 miles ago.

I thought of all the above, really wanted to believe the SL30 and continue the approach. No real serious obstructions South of course but by now I was scared, so I went missed. Thought about BTR and radar guided approach and the over the gulf down to the water approach but over the MCB VOR all of my nav devices agreed. Held there and hard booted the 396 and it to agreed. So I talked to Center and flew the VOR approach at 4000' and radar as well as my nav devices agreed so I shot the VOR approach and all devices agreed along the entire approach.

Runway appeared where it should.

Turns out at the end of the day one of the construction workers had parked his bulldozer right next to the ILS shack and it skewed the localizer 15 degrees South.

Bottom line no matter how welll planned things can go wrong that are beyond control and alll the redundancy you put into your plane during the planning stages may not provide the info you need when the stuff hits the fan. The primary instrument in an emergency is between your ears.

 

[N395V]

Missed, climb back above, and call Houston for help. There is no way at all that I'm going to plan to show up with even the forecast weather at that airport with only that approach with ONLY 30 minutes of fuel.

Scott if youn go back and read the scenario it says

You have legal IFR fuel reserves on board plus 30".

This I thought should imply you could arrive at the destination with enough fuel to go missed fly to the alternate, shoot an approach and still have the required 45" minute reserve plus an additional 30".

That means that at the time you identify you have a problem there is still
about 2 hours of fuel on board.

Sorry if I wasn't more clear on this point.

 

[dmaib]

I have nothing to add to Milt's comments, except that I was truly impressed and gratified by the responses to this scenario. Lots of level headed and logical thinking. As always, fly the airplane. 30 some years ago, an old pro gave me this advice. "Don't hit the ground unless it is intentional". Words to live by.
Merry Christmas and Happy New Year to all.

 

[Kevin Horton]

I've had one real occurence where the GPS was off by about a mile. We were shooting a practice GPS approach on a VFR flight. As we approached the FAF, the IFR GPS gave a RAIM warning message, and flagged the HSI. There was another message that said to cancel the approach to regain nav info, so we cancelled approach mode, which put the system in terminal sensitivity, and changed the threshold for RAIM warnings from 0.3 nm to 1.0 nm. The GPS nav info came back into view on the HSI, so we followed the GPS data out of curiousity, wanting to see how accurate it was. It brought us in about one mile off the normal approach track.

I don't know why the GPS data was bad. The IFR GPS detected the error, gave us a RAIM message, and flagged the nav data, as it was designed to do. A handheld GPS does not have RAIM, so it would not have warned us about the bad GPS data.

Modern handheld GPSs with WAAS might be a bit less likely to have an error, but there are no design standards that require them to warn you about any anomalies in the GPS data. Bottom line - don't automatically assume that a handheld GPS is always right if it disagrees with other nav data. Just because it hasn't lied to you in the past is no guarantee that it won't lie to you in the future.

If there is any question about the validity of the nav data, the best answer is to do a go-around and sort it out.

 

[L.Adamson]

I've had one real occurence where the GPS was off by about a mile. We were shooting a practice GPS approach on a VFR flight. As we approached the FAF, the IFR GPS gave a RAIM warning message, and flagged the HSI. There was another message that said to cancel the approach to regain nav info, so we cancelled approach mode, which put the system in terminal sensitivity, and changed the threshold for RAIM warnings from 0.3 nm to 1.0 nm. The GPS nav info came back into view on the HSI, so we followed the GPS data out of curiousity, wanting to see how accurate it was. It brought us in about one mile off the normal approach track.

I don't know why the GPS data was bad. The IFR GPS detected the error, gave us a RAIM message, and flagged the nav data, as it was designed to do. A handheld GPS does not have RAIM, so it would not have warned us about the bad GPS data.

Modern handheld GPSs with WAAS might be a bit less likely to have an error, but there are no design standards that require them to warn you about any anomalies in the GPS data. Bottom line - don't automatically assume that a handheld GPS is always right if it disagrees with other nav data. Just because it hasn't lied to you in the past is no guarantee that it won't lie to you in the future.

If there is any question about the validity of the nav data, the best answer is to do a go-around and sort it out.

I've asked about GPS reliability questions from a friend who is a first officer on a Boeing 737 800 which uses GPS as one of three nav systems. He claims that the GPS always has the best accuracy out of the three. Another 737 800pilot that I know, has said the same thing.

When talking to this friend recently, he stated that the GPS has only failed on one occasion in the six years he's been flying the 800 series. It was near a military installation, and another aircraft inquired of the problem too. The failure was of short duration.

As to WAAS, I know that it takes care of numerous problems that can crop up between GPS satellites. My handhelds do use WAAS, and accuracy has been excellent. For the ability of warnings such as RAIM in regards to WAAS, I'm in the dark, and would like to know myself.

L.Adamson --- RV6A/ Garmin 696

 

[jferraro17]

Being Christmas Eve and all...I can't unequivocally deny that eggnog may have been consumed. So I won't embarrass myself doing this off the top of my head , but this is from my current Ops manual (obviously an IFR certified set-up):

"Required Navigation Performance (RNP) / Actual Navigation Performance (ANP)
Required Navigation Performance (RNP) is the required accuracy of the system (in nautical miles) to fly in a particular area, airspace, route or to accomplish a particular procedure. RNP values originate either from default values based on the navigation phase of flight or from manual entries.
There are four default RNP values:
? 0.3 Approach ? within 2 nm of FAF and 70? of the final approach course
? 1.0 Terminal Area ? within 30 nm of airport reference point
? 2.0 Enroute Domestic ? within 130 nm of nearest navaid, but outside the terminal area
? 12.0 Oceanic / Remote ? more than 130 nm of nearest navaid

Actual Navigation Performance (ANP) is the system?s assessment of the accuracy of the current GFMS computed position.
When ANP values exceed the automatic default or manual entered RNP, the crew will be alerted to a potential degraded navigation accuracy."

AND

"In order to detect GPS position errors, the system implements a fault detection function called Receiver Autonomous Integrity Monitoring, or RAIM.
Using redundant measurements (more than 4 satellites, or 4 satellites and an altimeter), the RAIM function can measure the accuracy of the GPS position solution. Depending on the geometry of the available satellites, the system can annunciate errors which exceed the accuracy requirements for the current flight mode."


AND

"Complete GFMS Failure
? Use ADF, VOR / DME, and / or Radar Fixes to update position.
? Advise ATC that primary long range navigation system is inoperative."


GFMS is a GPS based FMS system. This is Part 121 guidance, but I believe the general idea is conveyed. Obviously, I've distilled a lot of data into a few lines here, but again, general idea.

Surfing VAF sure beats assembling toys all night!

Merry Christmas--

Joe

 

[breister]

The first thread in this post suggests it is a derivative of "why 2 comms." So, your answer really depends on "what you have left" and "where you are when it happens."

Hopefully you had weather reports from many places, and you had some idea of weather conditions at alternates when you lost comm. If you were VFR when it happened, stay VFR and land. If you know there is VFR close by, go there and find an airport to land.

If you only have one comm (or you are so far in the boonies that your hand-held comm raises nothing but static), I personally would feel perfectly comfortable continuing to my intended destination and shooting a nonprecision approach with a hand-held GPS.

If the weather was worse than expected, I would search for a long runway, shoot the non-precision approach to it, and then let down quickly to precision minimums as I approach the approach end of the runway. On a long strip, that leaves plenty of space to land in our small planes.

Face it folks, when you have equipment failure you do what it takes to get down.

 

[IowaRV9Dreamer]

496 gps error confirmed

I have a 496 in my aircraft, with the sucton cup gps antenna stuck to the side of the windscreen.

I ususally use the "ground track predictor" display (not sure what the real name is) - it is a line that sticks out the front of the little aircraft and shows where you will be 1,2,3,4,5 minutes out.

I had occasionally noticed big angluar excursions of the predictor line - like swinging around +/- 100 degrees. It would dance around and then it would work fine. I didn't pay a lot of attention since I fly VFR.

Then one day I noticed that the position solution was bouncing around - about +/- 0.5 miles. Sometimes it would gave up and told me "no GPS signal". But even without any error message the position was moving around a lot. I tried another antenna and got similar results.

One fine day it happened on the ramp and I determined that my VOR receiver was causing GPS interference. You could see the signal strength of the GPS signal go up and down as I tuned the VOR reciever. Of course the worst VOR frequency was the local one! BTW this is with a TKM MX170 nav/com radio.

The long term solution was to change to an external GPS antenna (which was alrady on the airplane). Now there is no measurable GPS interference.

I guess the point here is that the uncertified GPS was getting interfered with, and was presenting misleading information a lot of the time, instead of annunciating a failure. I don't know, but I'd hope that a certified GPS would not have done this.

Just a datapoint for you,

 

[Danny7]

.......

Modern handheld GPSs with WAAS might be a bit less likely to have an error, but there are no design standards that require them to warn you about any anomalies in the GPS data. Bottom line - don't automatically assume that a handheld GPS is always right if it disagrees with other nav data. Just because it hasn't lied to you in the past is no guarantee that it won't lie to you in the future.

If there is any question about the validity of the nav data, the best answer is to do a go-around and sort it out.

Many people are saying "handheld GPS". Do the garmin 296/396/496//5 count as "handhelds" if they are in a panel dock with an airframe mounted antenna? I don't really wonder about the docking but the antenna performance.

 

[Kevin Horton]

I have a 496 in my aircraft, with the sucton cup gps antenna stuck to the side of the windscreen.

I ususally use the "ground track predictor" display (not sure what the real name is) - it is a line that sticks out the front of the little aircraft and shows where you will be 1,2,3,4,5 minutes out.

I had occasionally noticed big angluar excursions of the predictor line - like swinging around +/- 100 degrees. It would dance around and then it would work fine. I didn't pay a lot of attention since I fly VFR.

Then one day I noticed that the position solution was bouncing around - about +/- 0.5 miles. Sometimes it would gave up and told me "no GPS signal". But even without any error message the position was moving around a lot. I tried another antenna and got similar results.

One fine day it happened on the ramp and I determined that my VOR receiver was causing GPS interference. You could see the signal strength of the GPS signal go up and down as I tuned the VOR reciever. Of course the worst VOR frequency was the local one! BTW this is with a TKM MX170 nav/com radio.

The long term solution was to change to an external GPS antenna (which was alrady on the airplane). Now there is no measurable GPS interference.

I guess the point here is that the uncertified GPS was getting interfered with, and was presenting misleading information a lot of the time, instead of annunciating a failure. I don't know, but I'd hope that a certified GPS would not have done this.

The signals received by the GPS receiver are never perfect, so there is always some error in the calculated GPS position. IFR-approved GPS systems are required to monitor the position calculation, and to alert the pilot if the calculated position may have an error that is too large for the current flight phase. Older (non-WAAS) IFR-approved GPS systems have a Receiver Autonomous Integrity Monitoring (RAIM) algorithm. The RAIM algorithm works like this:

The GPS needs good signals from at least four satellites to calculate a position. If there are more than four satellites available, the RAIM algorithm performs a number of position calculations using different combinations of satellites, and it compares how far apart the various calculated positions are. E.g, if the GPS is receiving five good satellite signals, it uses various combinations of four satellites to calculate a position - satellites 1, 2, 3 & 4, then satellites 1, 2, 3 & 5, then satellites 1, 2, 4, & 5, etc. If all the calculated positions are close together, life is good, and the GPS provides you navigation data. If the various calculated positions differ by more than X.X nm, the GPS gives you a RAIM warning, and flags the nav data. The value of X.X that it uses varies with the phase of flight - the errors are allowed to be larger in enroute flight than in the terminal area or on approach. Note that the RAIM algorithm cannot determine which satellite(s) is providing the bad data, so once it determines that there is a problem it stops providing nav data.

IFR-approved WAAS GPS units have a smarter algorithm called Fault Detection and Exclusion (FDE). The FDE algorithm needs more satellites to work, but it can detect a problem, determine which satellite is sending the bad data, and stop using the data from that satellite. Thus it can continue to provide a navigation solution, even if one satellite is giving bad data.

I am not aware of any handheld GPSs that have either RAIM or FDE, so they will happily provide you with bogus navigation info if they are getting bad data.

Many people are saying "handheld GPS". Do the garmin 296/396/496//5 count as "handhelds" if they are in a panel dock with an airframe mounted antenna? I don't really wonder about the docking but the antenna performance.

Yes, the antenna performance might be a bit getter if you have an airframe mounted antenna. But, the unit still does not have RAIM or FDE (see above), so it may still provide you with misleading navigation info if something is interfering with the GPS info. Note that the occurrence I reported earlier where the GPS gave erroneous nav info was a GPS with an airframe-mounted antenna. The RAIM algorithm did detect the error, and it removed the nav signal from the HSI. The various handheld GPSs would not have provided any warning about this problem.

I know that some folks will have the view of "it has never happened to me, so this is a non-issue". Well, we won't live long if we ignore every issue until it happens to us personally.

Handheld GPS systems are a wonderful thing, and they usually give very accurate nav info. But they don't always give accurate nav info.

 

[David-aviator]

Milt,

It is a VOR/DME approach, you have no choice but to continue on the VOR radial. If you choose to ignore the VOR signal or have an indication the VOR has failed, it is no longer a legal approach and you have no business being there. Execute a miss and procede to an alternate. GPS is not a factor. I don't see anything on the approach plate that indicates it is a GPS approach.

I've said it at least a dozen times here, these airplanes are not suited for serious IFR. A circle approach to minimums is a risky deal with 2 qualified pilots. To arrive at the MDA in the senario you set up is in and out of the clouds and further descent is not legal until the runway is clearly in sight and that may not happen until you pass over it. And then what? Make a descending turn to a down wind, still in and out of the clouds? That manuever has to be done on the gages as there will be no horizon.

I could go on and on why this is very bad place to be - in any airplane. I've done a few of these in the real world with everything working and when its all over you are not wishing for another one for a while.

The company I worked for flat out discontinued doing circle approaches because of the risk factors and the cost of keeping pilots current to do them.

With the scene you set up, I would be off to a VFR airport, an airport with a straight in approach, preferably an ILS, or a military field with a GCA if there is such a thing any more. Those guys at one time to could talk anyone down if you had a working 6 pack.