[prkaye]

Can someone explain to me what, technically, the difference is between an IFR-GPS and a non-IFR GPS? Is it just a matter of having all the GPS approach coordinates programmed into its memory? Or is it just a matter of the legal status of its certification?

 

[brian]

it's the latter

brian

 

[jcoloccia]

Can someone explain to me what, technically, the difference is between an IFR-GPS and a non-IFR GPS? Is it just a matter of having all the GPS approach coordinates programmed into its memory? Or is it just a matter of the legal status of its certification?

The 2 biggies are the database and RAIM.

 

[kentb]

RAIM - receiver autonomous integrity monitoring

The 2 biggies are the database and RAID.

This is a critical feature of the legal IFR - GPS.
Another is that on the approach the scale of the CDI is stepped down automatically.
Another is that the annunciator indicate the mode of the GPS (armed, approach,etc).

There may be other difference.

One thing to keep in mind is that if the unit was not IFR certified, it is not likely that the manufacture took the time or expense to make sure the the database and functionality are up to the standards of the certified units.

I don't know if it is for real or a OWT, but I have heard of a plane the use using a handheld GPS to make an approach and crashed 2 miles right of coarse on the approach.

Kent

 

[dan]

Google for "TSO C129a" and you can read up on what makes a GPS an IFR GPS. There are different classifications based on functionality, i.e. enroute, terminal, and non-precision approach. Class A1, A2, B1, B2, etc.

 

[Yukon]

Many technical differences but the big difference is price. Garmin 496 2700 bucks. Garmin 430 - $8500 with indicator.

 

[kentb]

Is it worth it???

Many technical differences but the big difference is price. Garmin 496 2700 bucks. Garmin 430 - $8500 with indicator.

$8500 - $2700 = $5800

VFR not worth $5800.00, besides 496 weather feature is a cheap add-on.
IFR $5800.00 is cheap life insurance.

Kent

 

[fliier]

An IFR GPS is TSO'd to C-129a. This involves both a level of hardware testing (DO-160) and that the software was developed in accordance with RTCA DO-178/B, probably to level "C" for non-precision approach.

The DO-160 testing includes things such as lightning testing, interference testing, etc.

RTCA DO-178/B Level "C" requires that the software be developed in an approved environment using an approved method and approved tools. The FAA has determined that software has become too complex to test after it has been developed (i.e. that no testing scheme can reasonably be developed to test all failure modes of the hardware/software "system") Therefore they get involved in verifying that the actual process of software development meets certain standards. Basically that means the FAA is "in your shorts" during the entire development process. It's very expensive and explains why manufacturers of electronic flight equipment can't typically take non-TSO'd equipment and test them to the TSO.

On top of that, the FAA has called out in the TSO the major software features of a TSO'd box, which is why all IFR GPS's operate the same way with respect to the behavior of the external annunciator box, needle sensitivity at various stages of the approach, the use of an external button to disable sequencing, etc.

The TSO also calls for a RAID scheme. RAID is a method by which, using 5 satellites, you can calculate multiple 4 satellite fixes and figure out if one satellite is giving bad data.

John Allen

 

[hevansrv7a]

It's also the installation

I agree with all of the above; this is just a little more information based on my frustrating experiences of recent years.

Even a TSO'd IFR GPS such as a 430 or a 530 is not "legal" unless installed in a way that conforms to the "standards" which are not really standards, but that's a nit to be picked elsewhere. It's about the CDI and the annunciators - stuff like that. On a certified aircraft, the installation can add significantly to the total cost of putting one in. The licensed installer has to OK the whole thing on certified aircraft.

Just a note in passing: until now, there were no manufacturers that I know of selling a GPS that met the TSO standards without actually being TSO'd. GRT is now selling or about to sell a $750 add-on GPS for their EFIS that they will say meets the standard but will not be formally TSO'd. For an experimental airplane, that puts you into a gray area of regulation. You can fly it IFR, I think. The cost and update process for the database is another, related and necessary aspect to this if one were to file "/G" with that equipment.

 

[hevansrv7a]

...
I don't know if it is for real or a OWT, but I have heard of a plane the use using a handheld GPS to make an approach and crashed 2 miles right of coarse on the approach.

Kent

I don't know either, but I'd trust my '496 more than I would trust my VOR, let alone an ADF. If a handheld puts you 2 miles off course, there is more wrong than a TSO would fix IMHO. I've seen more ILS outages than I'm comfortable with and I've never seen my Garmin 295, 296 or 496 off by more than 30 feet. Can it happen, yes. Is it a big risk? Not in my estimation. I use VOR and ILS mostly for practice and I use handheld GPS on every flight, no exceptions, since 2000. Just for fun, I've made practice WAAS simulated ILS approaches with the Garmin down to pavement and it's always perfect. You can only get it to do that (lower than 500 AGL) by making an overlay waypoint and assigning a lower altitude to it.

 

[pierre smith]

Clarify please

I You can only get it to do that (lower than 500 AGL) by making an overlay waypoint and assigning a lower altitude to it.

Could you kindly explain this. I think I know what you're doing...adding another waypoint. I've acquired an as new 496 just today and would like to know.

Thanks,
Pierre

 

[Ron Lee]

"RAID" should be "RAIM"

RAIM is an integrity functionality and if a GPS satellite clock run-off drives your position off hundreds of miles you want to know about it. Another TSO is TSO 146 for WAAS units IIRC.

 

[hevansrv7a]

Making 496 do what you want

Could you kindly explain this. I think I know what you're doing...adding another waypoint. I've acquired an as new 496 just today and would like to know.

Thanks,
Pierre

I'm doing this from memory; the 496 is not here. Email me if this is not enough or if I get something wrong. I'm glad to help with the understanding that Garmin does not support this and you are PIC!

Step 1 is to create the waypoint in the same location as the airport. You can do that with Lat-Long or if you are at the airport, wake it up, wait until it says "you are here" and press enter, go from there. You can even go to the foot of the runway for this if you like. If there is a real ILS, use the FAF to start the approach. If there is no FAF, you may want to create on so as to get a straight-in path.

Step 2 is to put in an altitude. I have not done this in a while but I recall that you can do it. Look for the right field or tab. Make the altitude 500' lower than the ground you are on and the pseudo-glideslope will take you right down to the ground. Otherwise the vertical guidance will vanish at 500 AGL. This way it is 500 Above Virtual Ground Level. I don't think the altitude matters for the FAF.

Step 3 is optional. Save it with a name and it will appear in your personal favorites, "user" or something like that and you can even pick an appropriate symbol to display on the map. Now you will be able to "goto" it or select it on a flight plan.

Using it: On the moving map display as you approach the target, there will be a little symbol on the far left and the far right that will appear as your VNav profile is reached and that offers vertical guidance. You can also see it very clearly on an HSI view which is offered on at least one page, but that page is either-or with the panel display. For precision, the HSI page will take you there as well as an ILS. Set your CDI distance scale to 1/4 mile. For this to work with any destination, User or Jeppeson Database, the destination must be the final one on the flight plan (or just a goto). In otherwords, it has to know you intend to land. On my map display, I programmed one of my four data fields to show "time to Vnav". That will work nicely in conjunction with the appearance of the vertical guidance display.

I should mention that if you have a GRT EFIS and it is receiving GPS data from any valid source, that it will take you down to the ground and along the center of the runway using the HITS display. You can merely select the runway and arm that when in the air, even while using the ILS if you have configured it right. It's VERY nice and in an X-Wind, even better than the '496 alone and you don't need to plan ahead. The HITS, once you figure out how to fly it, helps you intercept the correct heading and course; the display incorporates the crosswind for better situational awareness and the HITS boxes show horizontal and vertical guidance overlaid on the Artificial Horizon. If you are getting ILS signals, it can show that either instead or in addition. And no, I don't miss steam guages even though I thought I would. This helps explain why.

 

[kentb]

H Evan, your scaring me...

If I understand your directions correctly (and I may not), there could be people telling there 496 to "goto" a point 500 feet below the runway from anywhere they happen to be. This would be regardless of any obstructions between them and that point and also regardless of what would be in front of them when they reached that point.
I know that if you understand where you are and only ask for that kind of guidance when it is correct you will be OK, but people don't always do what they should.
If I understand from you previous post you are using your 496 as a backup to the NAV radio, just to see how well it would do. If so I think that this is fine, but the 496 does not understand (IE will force you) how to fly a correct approach.

Kent

 

[hevansrv7a]

Responsible or Irresponsible?

If I understand your directions correctly (and I may not), there could be people telling there 496 to "goto" a point 500 feet below the runway from anywhere they happen to be. This would be regardless of any obstructions between them and that point and also regardless of what would be in front of them when they reached that point.
I know that if you understand where you are and only ask for that kind of guidance when it is correct you will be OK, but people don't always do what they should.
If I understand from you previous post you are using your 496 as a backup to the NAV radio, just to see how well it would do. If so I think that this is fine, but the 496 does not understand (IE will force you) how to fly a correct approach.

Kent

Kent, thanks for your post. I expected that at least one person would say something like this and I don't mind.

But first, let's clarify that the '496 (295, 296,396) terminates vertical guidance at what it thinks is 500 AGL, so it won't take you underground if you tell it an altitude of -500 from reality. I also agree (it's clear in Garmin's book) that it's just a hypothetical slope (you determine the descent rate, not the angle) and does not guarantee terrain nor obstacle clearance even though both sets of data are in its database and will provide warnings.

Using it that way has a couple of valid uses. For example, the airport that I was based at when I did that had no approaches, but was in very flat land and I knew where the towers were, had them on the moving map. It was something I never used "for real" but wanted to have it for emergencies. Practicing with it was useful for many obvious reasons including proving how accurate it is. I always use it to back up my ILS CDI and it's especially good on a back-course localizer approach. BTW - the GRT's HITS provides a 3 degree slope which is not equal to Garmin's based on feet-per-minute. The GRT will also only do it at real airports for which it knows the ends of the runway. It still is a virtual slope and not an approved, published approach. Anyone with an IFR rating knows the difference between an approach and a 500 FPM descent. Use at your own risk.

Now to personal responsibility. Even though I did not need to, I pointed out that Garmin does not support this and that whoever does this is PIC. We all know, or should know exactly what that means. If I teach you to operate a .357 Magnum that doesn't make me responsible when you play Russian roulette with it. Don't do nuthin' dumb, please.

Now, back to my original point: notwithstanding the possibility that someone somewhere crashed while (because of?) using some unspecified handheld GPS, the '496 is a very accurate and very reliable navigational tool. In my experience, it is more accurate and more dependable than VOR's, ADF's and sometimes ILS's. I've also won in-air disputes twice with controllers about whether I was or was not in their airspace. We all know that it's only legal for "situational awareness". Many of us also know that if you put "VFR GPS" in the remarks section of your IFR flight plan that ATC will often clear you direct and be happy to do it. Lastly, since it is WAAS enabled and warns you if it loses WAAS, it's a very good backup and verification for the aneroid altimeter. Am I the only one who has ever set the Kollsman window wrong? Probably not.

 

[gmcjetpilot]

Info is good with common sense

In my experience, it is more accurate and more dependable than VOR's, ADF's and sometimes ILS's.

Appreciate the GPS enthusiasm, but lets get real, more accurate than an ILS? ILS and outages? The utility of a single VOR/LOC is still pretty good, or good enough. The price of the equip and the paper plates is a bargain to the IFR GPS with electronic updates. However with VFR GPS on board used as a back-up you can improve IFR safety, no doubt.

ILS still King of the Approach:

The ILS (cat I) gets you down to 200ft/1800 RVR w CL lights (1/2 stat miles otherwise). I don't do auto lands or CAT II or III(A/B/C) approaches (but have in the sim). Each Cat gets you to progressively lower decision heights of 150ft, 100ft, 50ft and zero decision, with RVR's of 1000 to zero feet. The GPS mins right now are about NDB/VOR non-precision mins (500ft/1 mile typ). I would not put ground based nav down too much. Ground based nav is going to be around way past the time we stop flying (breathing).​

I don't know either, but I'd trust my '496 more than I would trust my VOR, let alone an ADF.

Be that as it may, not disagreeing, but its a moot point; I agree its a GREAT back up, its just not legal for IFR and you can bust your a$$ both physically and legally in ways you or I may never have thought of.

As far as messing with VNAV using a VFR GPS in IMC conditions..... that sounds dangerous, WAAS or not. If you have a trick to get a lower alt waypoint for a "Glide Slope" to the runway, great, but you said it, "We all know, or should know exactly what that means."

I could see flying a VOR approach at night, using the VFR GPS VNAV for a "how goes it" to the runway and a pseudo VDP (visual descent point). It can add safety. It could also distract you if you let it. However, if a pilot uses it to make a "FAKE ILS" in IMC, that is crazy; we all can agree I'm sure. I know we are not talking about that. If we are, this famous last phrase might follow: "What is that mountain goat doing in this cloud" "splat"

My feeling is if you want to fly GPS en-route and approaches, follow the rules and get the darn IFR GPS. You can buy a first generation IFR GPS unit (panel mounted of course) for less than 1/2 the price of a state of the art Garmin 496 handheld. Granted the Garmin 496 is more cool and does stuff the plain-Jane IFR panel mount can never dream if, not the least of which is a color map display, obstacle alerts and weather, but no amount of wishing will ever make make that cool handheld GPS IFR rated, even though we think it can do the job (better).

I am not that smart. I have found when I follow the rules, I don't out smart myself. However using all avaiable INFO is a smart thing. Clearly a VFR GPS brings lots of info a IFR pilot can use, with a big helping of common sense.

 

[hevansrv7a]

I said only what I said

George, I have no argument with your points, which reflect your much greater expertise and good common sense.

But you were picking on a straw man, not me. I never said to use it in IMC except as an aid to situational awareness. I wouldn't, either. I yield on the point about ILS accuracy because I was only talking about CAT 1 and the accuracy/dependability issue arises when it is not working well which is common in these parts. I have often helped local towers determine whether it was working well. If they can't tell, how can we, when it counts? There is no ILS equivalent to RAIM. I still maintain that the GPS is much more accurate and dependable than a VOR, let alone ADF. And OK, WAAS is not suitable for precision descents except with an instrument certified for that - no argument. It's just good, useful data within the known limits and good to use to compare to the altimiter. Unfortunately, neither is quite right with respect to height above ground!

I did point out, twice, that a slope is not the same thing as an approach. Yes, I do carry and use "paper plates". I also don't file "/G". It's legal to go direct enroute without the GPS, so it's legal with it and ATC understands this. But that doesn't change the rules for approaches.

I have done practice approaches with the GRT EFIS's HITS and the ILS displayed simultaneously. The HITs used that way is perfect and it is a terrific way to deal with the changing wind conditions as you come down. This is using data from the '496 as interpreted by the GRT. I think that when the ILS is present, the GRT uses the true glide slope rather than the synthetic one. The GRT will soon have a non-TSO'd GPS receiver that they will say meets the IFR standards and Jepp's data. I'd consider using that, depending on things not yet known.

We all know what PIC means. Even if I did advocate something dumb and/or illegal, I would not be responsible for another pilot doing it.

 

[pierre smith]

Thanks...

Thanks a lot H Evans,
I'll agree with the accuracy of a good Garmin. Last month I did an IFR refresher with a CFII friend of mine and shot a GPS Loc approach with my velcro-mounted Garmin 195 using the HSI and it brought me down the centerline! As good or better than my old ILS equipped Cessna 310 used to do.

Regards,

 

[az_gila]

Well, not quite....

Your thought process regarding IFR navigation is seriously flawed. GPS is NOT more reliable than ILS and VOR. That's why we are still using them. ILS's have no undetectable failure modes. When they fail, you loose ident and you get a flag. Same for VOR. They have been in continuous use for 50+ years and are not considered to be a source of accidents....
.

Well not quite... that is assigning too high a reliability to any equipment...

The ILS can transmit erronous data...

http://www.tc.gc.ca/CivilAviation/SystemSafety/Newsletters/tp185/3-02/406.htm

gil in Tucson - Electronics are good, but not perfect....

 

[hevansrv7a]

Keeping it civil?

Your thought process regarding IFR navigation is seriously flawed. GPS is NOT more reliable than ILS and VOR. That's why we are still using them. ILS's have no undetectable failure modes. When they fail, you loose ident and you get a flag. Same for VOR. They have been in continuous use for 50+ years and are not considered to be a source of accidents.

GPS is operated and or controlled by the military, and is subject to their needs and whims. GPS outages are random and uncontrollable by the civilian community. Even when the military is not screwing with the system, satellites go bad and transmit errant signals, hence the need for complex error-checking signal comparator hardware and software not obtainable in VFR units. I repeat, cheap GPS's are only accurate enough for VFR usage.

The only way SAFE and LEGAL IFR usage of GPS is a certified unit utilizing RAIM and the testing and verification which led to it's approval. You are doing a serious disservice to the experimental community by encouraging our
fellow RV'ers to circumvent the law and the safety it provides. If you can't afford the hardware, do the rest of us a favor and stay out of the clouds.

1. Hand held GPS is just as accurate as certified but does not have RAIM. This is fact, not opinion. They are both usually good to about ten feet. They also tell you how accurate they are, should you care to look at that page.
2. I should not have to repeat myself about personal responsibility, but I'm not "encouraging" anyone to do anything unsafe nor illegal and PIC means what it means. It's no disservice to tell the truth and the community is full of people who are smart enough to make their own decisions about what to use and when to use it. Keep in mind this discussion was all about an accident that may have happened to a pilot that may have had some unknown GPS and may have used it in such a way as to crash a couple of miles from the runway. I'll say it again - I'll take GPS accuracy and accept the risks that are inherent with all flying.
3. Don't, please, attack me personally on the basis of what you think you know about me or my ability to afford stuff. You don't add value to the discussion that way. My other critics may share your low opinion of me (or not) but at least they are keeping it civil and on topic.
4. Your comparison of GPS to VOR/ILS is not the full truth and anyone who has had conversations with towers about whether it's working right knows that. The flag is not a 100% reliable indicator and NOTAMS are not timely like RAIM. The GPS outages are also NOTAM'd, now. The military "screwing" with it unpredictably stopped several years ago by presidential order.
http://gps.faa.gov/gpsbasics/SA-text.htm
http://gps.faa.gov/FAQ/faq-gps.htm
Twelve channel receivers are capable of dealing with satellite outages. When things go a little bad on a localizer, glideslope or VOR, it's a single point of failure. When a satellite goes bad, in most cases, there are still four or more sending valid data. It's common to see NOTAM's about certain radials being unreliable, for instance.

Let's not get into a p...ing match about competing technologies when any reasonable person will use both when he can. They all have their strengths and weaknesses, TSO'd or not. My new SL-30 is digital and thus much more accurate than my old VOR/LOC/GS. The allowable error for VOR/DME is 4 degrees! If you take off with a 3 degree error at 180 degrees on the VOT, what is your position error on the 240 degree radial at ten miles? You do not and cannot know. I notice nobody is claiming ADF is better; at least that's a non-issue. But that's legal, isn't it? I don't encourage anyone to make an ADF approach in IMC.

OK, you are doing a legal VOR approach and you use your handheld to monitor your descent. Legal? Yes. Safe? Certainly not more dangerous than CDI and altimeter alone. Assume no DME. Would you use only your timer and an assumed ground speed or would you also sneak a peek at your GPS for groundspeed and, oh yes, distance to the airport. Safe and legal. Smart, too, IMHO.

Your point about why we are still using them is not completely candid. As good as they are, they are slowly going away. The slowness of the change is due to practical and economic forces much more than because of technical superiority. We are still using LORAN and ADF, too. Aircraft-centered navigation is the direction being taken over ground-centered (ADSB, for example), but the fleet's economic and training needs must be taken into account.

Yukon, George and others are entirely right to point out that there is no legal way to use a Garmin '496 or any handheld for primary navigation IFR/IMC. But I never said it was! It is legal as an aid to situational awareness.

I am saying what is obviously truthful about the accuracy of the thing and I also mentioned that Garmin's arbitrary limits notwithstanding, you can use the handheld below 500' AGL for vertical guidance if you choose to do so. In any event, the horizontal guidance keeps working. I've never done it except as an experiment in VMC. I also rarely do accelerated stalls. But I want to know where the limits are and I do encourage others to know that. I've seen discussions on these pages about running Lycoming engines beyond 2,000 hours and beyond 2700 RPM. Is that not a parallel situation? Why is this open discussion of facts drawing so much more censure than others? Does anyone really disrespect his fellow pilots so much that he thinks that my revelation of a technical work-around makes them suddenly stupid and unsafe?

 

[az_gila]

Perfect

OK Gil, you searched the internet and found on instance in a foreign country where the ILS was left in "test" by some aussie numbnuts and the monitoring equipmnet was inoperative as well. Does this invalidate my assertion that VFR equipment is unsuitable for IFR operation??? I think not.

You actually are confiming my assertions due to the fact that the ILS system, when flown by competent aviators, can be QC'd by checking vertical descent rate, power requirements and FAF height against the know glideslope angle. Try that with a Garmin 496.

Actually, I was trying to say no system (including the human elements) is perfect.... some are better than others...

Apparently the recent LAX ILS problems were partly caused by construction equipment distorting the beams... no accidents were caused (VFR weather?) but the system was shut down. I didn't see a report on how the error was actually found, but I don't think the monitoring system found it... The FAA report to the Senate talked about 5 seperate problems occuring in a close time frame.

gil in Tucson

 

[gmcjetpilot]

GPS v ILS v MLS v LPV

Well not quite... that is assigning too high a reliability to any equipment...

The ILS can transmit erronous data...

gil in Tucson - Electronics are good, but not perfect....

Not to pile on, but what Yukon said. I am very familiar with the ILS test incident, studied it in re-current. Yep stuff happens, it was in test. **

Since last years FAA policy approval of GPS/WAAS LPV (Localizer-precision with Vertical Guidance) approaches (with mins as low as 200 & 1/2), its probably a matter of time when ILS will be at least partially phased out. Certainly new ILS approaches will be commissioned less frequently in favor of GPS/WAAS, at least at smaller airports. ILS will probably still be at bigger fields and used by commercial operators for a long time to come.

I am no IFR/GPS expert, but one thing that must be emphasized and grasped, GPS is not infallible. From what I read and talking to pilots that use GPS for IFR routinely, they get at least one or two failure/abort messages a year from some weirdness of the equip operation or GPS signal itself. It's a low powered signal coming from space and through lots of atmosphere. I have never had an approach system failure while about to enter or in the final approach segment, or any segment really.**

The main goal of the FAA is to stuff more planes into the sky and airports. The plan is to have autonomous flight, no ATC. Planes will fly coast to coast, pretty much when and where they want (direct) with computer controlled commands. GPS position fits into that star wars plan best. I just think of 2001 space odyssey when HAL says, "Sorry Dave I can't let you do that", WTF.

Typical accuracy of GPS without WAAS is about 15 meters, whereas even a "CAT I" approach, the least demanding ILS, requires a min vertical accuracy of 4m. Not to get into a Pee-match GPS is a complicated alphabet soup of minutia to learn. I can teach someone basics of ILS and how to setup. Now look at a Garmin 430 or 530. Progress, sure, but more pilot work load.

ILS is pretty ridged, straight in deal; actually that's the charm of the ILS and reason for the safety. It's simple, fairly fool proof. Just dial the freq, point it in the right direction and descend, fly the needles than make a happy landing.

If you think about GPS: ionospheric billows, clock shift, WAAS, RAIM, LPV, LNAV/VNAV and data base, the mind boggles with details; an instrument-rated GA pilot who flies single-pilot IFR, work could end up being more complicated than ever with a GPS.

**PS: The worst ILS "incident" I had was early in the morning landing at Indy; cleared to land they forgot to turn the parallel ILS on. It was pretty obvious. "Is the ILS on", I asked, and magically the flags went away with not a word. As a new-bee rookie IFR rated pilot in Cessna's years ago, ILS cross checks where drummed into my thin head. Check your crossing altitude at G/S intercept or OM and on final note airspeed, rate of descent and distance (if you had DME). Regardless of what "system" you use know it well. ILS can do bad things and so can an ILS or VOR/LOC/NDB, beware.

 

[Yukon]

Mr Heavensrv7,

Don't mistake disagreement with a lack of civility. Be more careful with your assertions if you want the respect of your fellow aviators.

 

[jcoloccia]

Why is this open discussion of facts drawing so much more censure than others?

I was just thinking the same thing.

re: reliability
The reliability of the VOR/ILS itself is not the only thing we need to consider. What we're interested in is SYSTEM reliability. The pilot is part of that system. I've got to think that GPS is easier to use and follow than VOR. It is for me, at any rate. Situational awareness is vasty improved as well.

re: using a Garmin x96 for IFR flight
Y'all are entitled to any opinion you want but if your opinion is that you can't use a handheld for GPS flight, then you're opinion is WRONG. There's no problem using anything you wish for supplemental navigation and a wise pilot makes use of all the resources available to him.

re: trusting a 496 over a VOR
I think this is what started this whole thing. In the end, I'd choose the one that agreed with my compass
If they didn't agree with each other and I was asked to choose one blindly, though, I'll take the 496 (or any GPS) anytime. There's redundancy built into the GPS algorithm. Just because it doesn't support RAIM doesn't mean it flys off drunk when there's a hiccup.

A lot of the arguments I see against GPS basically amounts to "it's newfangled". Well, tough...technology moves on. It's your plane and your comfort level so don't use them if you don't want to. In terms of complexity, though, I find it hard to believe that learning how to turn a few knobs compares with the complexity of flying a hold at an intersection with a VOR. With a GPS, it's trivial. You gain some complexity but you certainly gain simplicity as well. System reliability? I think GPS get the blue ribbon.

 

[Robert M]

Hey Phil.......

.......did any of this answer your question or are you more confused? - I know I got confused though - I'm not IFR but am considering it.

 

[OldAndBold]

In FMSs...

In an FMS, there are several GPS, several IRS, automated tracking of VORs and automatic control of the ILS. While the pilot can make specific selections, normally all of the navigation solutions get combined together into one big solution weighted for the accuracy of each contribution.

What we are talking about here is getting one solution from the Pilot-working-the-VOR/ILS radios and another solution from the VFR GPS and the pilot (not an FMS) makes a decision about which is to be believed. I'd have to believe that in almost any situation the pilot is better off with the added input of the VFR GPS than without it.

Would you trust a VFR GPS to get you down to a runway through IMC in an emergency? I wouldn't want to, but if it's that or no instruments at all, I'd take it. Would it work as well as an IFR GPS, user interface aspects aside? Probably. The difference in software testing is meaningless - either the software works or it doesn't (ahem, I think DO-178B is a crock). It is likely to be the same radio electronis inside, in any event, the two radios would be getting the same signals and computing the same answers in a given situation. The thing I would worry about is how accurate the database is in the VFR GPS. *That* could kill you.

John Babrick
N777XV (empennage about to be riveted)

 

[hevansrv7a]

Mr Heavensrv7,

Don't mistake disagreement with a lack of civility. Be more careful with your assertions if you want the respect of your fellow aviators.

"If you can't afford the hardware, do the rest of us a favor and stay out of the clouds."

John/Yukon, the rest was fine; this part of what you said was, in my perception, uncivil. Have any of my assertions of fact been refuted yet? If so, I mssed it. I've had much worse said to me, of course. No biggy. I actually like a vigorous disagreement and I think that part adds to the value of the forum for those who read what we say. I just don't think that attacking the author instead of his points helps. I'd welcome the views of others on the civility issue. I'm all talked out on the IFR GPS issue.
H Evan (= H Evan's RV7A)

 

[AlexPeterson]

RAIM errors

In about 850 hours of flying my 430 equipped plane, I've seen about three RAIM errors detected by the 430. They are transient, and typically clear in a few seconds. Keep in mind, these are only the ones I have noticed when looking at the display. There is a reason for the RAIM detection algorithms, as has been pointed out earlier.

An interesting anomaly is that somehow, my 430 has recorded a maximum ground speed of something like 260 knots, something which has never happened. I speculate it could have occurred during one of its momentary RAIM fits.

I am humbled to the complexity of the GPS system. Things such as relativity (Einstein sort) are taken into account in the calculations. I wouldn't claim to know 1% of what goes on to tell me where I'm at.

With all due respect, this thread highlights a frequently occurring, and potentially serious, flaw in our thoughts on reliability. Very often on this forum (and throughout our society), practices or designs are deemed acceptable or safe based upon data that simply is not scientifically meaningful. To make up an example, "I've made 100 approaches without a problem using such and such a method, therefore the method is sound", are simply not proof of soundness. Perhaps those 100 approaches could confirm a method if, and only if, it is combined with a thorough understanding of all the theory behind it.

I'm simply urging us to be humble to what we don't know... and believe me, I'm as guilty as the next.

 

[Yukon]

PC...........NOT

"If you can't afford the hardware, do the rest of us a favor and stay out of the clouds."

John/Yukon, the rest was fine; this part of what you said was, in my perception, uncivil. Have any of my assertions of fact been refuted yet? If so, I mssed it. I've had much worse said to me, of course. No biggy. I actually like a vigorous disagreement and I think that part adds to the value of the forum for those who read what we say. I just don't think that attacking the author instead of his points helps. I'd welcome the views of others on the civility issue. I'm all talked out on the IFR GPS issue.
H Evan (= H Evan's RV7A)

After spending 2 years on this forum, I am growing weary of hearing how guys are saving money or gaining a technological edge with non-aviation parts and equipment. Car gas, car parts, car motors, car GPS........ it's got to stop somewhere, and when guys start advocating IFR with uncertified equipment, I have to respond. Much of what you said is wrong AND illegal making a politically correct response difficult.

Since I am not selling anything or placating advertisers, I like to speak directly. You always know what side of an arguement I'm on, agree with it or not. Take a look at the last couple of posts. I can't really say for sure which side of this discussion they are on. What benefit is gained from that type of post?

Follow the rules, and you will live long and prosper.

 

[jcoloccia]

After spending 2 years on this forum, I am growing weary of hearing how guys are saving money or gaining a technological edge with non-aviation parts and equipment. Car gas, car parts, car motors, car GPS........

Get used to being weary. This is what experimental aviation is all about. If you want a certified aircraft why not buy a certified aircraft (other than the fact you want to save a few bucks). Inconsistent logic.

Anyhow, there's NOTHING illegal about using a VFR GPS, a handheld or tea leaves as suppplemental navigation for IFR flight. IFR equipment must still be used for primary navigation but so what? What point are you making and what part of any of this, specifically, do you see as illegal? I'm being persnickety as there are many newbies out there and this thread is full of misinformation and contradictions.

 

[Yukon]

Joe, call me when you buy or build an RV.

 

[trawlergeek]

RAIM and FDE!

One other BIG difference between the IFR and NON-IFR GPS instrument is a REQUIREMENT that the IFR GPS be able to predict based on flight plan data, the known health status of the observable satellite constellation and the ability of the constellation's "future" geometry (for the duration of the flight!)to provide sufficient navigational performance to complete the flight using the GPS. Yes, it "predicts" the future system performance based on where you are going to take the instrument.

Wow. That is a BIG difference and hence why the systems are MUCH MORE EXPENSIVE.

Surely, if you are to begin a flight using this instrument as a primary source of navigational data you will want to know in advance (prior to departure) whether the flight can be completed with that instrument.

I am no expert. But I think this is a "valuable feature" that is certainly worth the dollar difference (if you decide to fly IFR expecting to rely on your GPS for Primary Navigational input.)

All the usual caviats apply about having all the equipment necessary for completing the flight. Surely, you will have alternative methods of completing the flight should the GPS constellation performance become degraded such that it becomes unusable. Backup IFR GPS, VOR, ILS/GS, etc.

Cheers

Jim

 

[Kevin Horton]

RAIM and FDE - One other BIG difference between the IFR and NON-IFR GPS instrument is a REQUIREMENT that the IFR GPS be able to predict based on flight plan data, the known health status of the observable satellite constellation and the ability of the constellation's "future" geometry (for the duration of the flight!)to provide sufficient navigational performance to complete the flight using the GPS. Yes, it "predicts" the future system performance based on where you are going to take the instrument.

RAIM (Receiver Autonomous Integrity Monitoring) and FDE (Fault Detection and Exclusion) are different beasts. Non-WAAS IFR GPSs have RAIM, but not FDE. The RAIM algorithm calculates the position using different sets of the available satellites. E.g., if six satellites are being received, the RAIM algorithm will calculate six different positions, using a different set of five satellites each time. If all satellites are putting out good data, all these positions will be similar. If one satellite is putting out bad data, the positions calculated using different sets of satellites will vary by quite a bit, and the RAIM function will know that there is a problem, but it won't know which satellite is bad. So, it will cause the GPS to stop putting out a position. There is another type of event where there will be enough satellites visible to allow navigation, but not enough satellites to allow the RAIM function to work. In this case the GPS will put out a message like "Unable RAIM", and it will continue to provide navigation info, but it will refuse to switch into approach mode. If the GPS is already in approach mode when the "unable RAIM" event occurs, it will remain in approach mode for a few (five?) minutes.

Predictive RAIM is the function that looks ahead to the destination at the planned arrival time and figures out whether there will be enough satellites visible with acceptable geometry to allow the RAIM algorithm to function at the planned time of the approach. This is important in theory because if there aren't enough satellites visible to allow the RAIM function to work, the GPS will refuse to go into approach mode as you reach the Final Approach Fix, and you cannot legally continue the GPS approach.

But, in practice, the predictive RAIM functions appear to be next to useless. I have had many events where the predictive RAIM said that we would be unable to complete the planned approach at the destination, but in almost every case when we got to the FAF the GPS had enough satellites to allow it to go into approach mode. I have had several events where the GPS refused to go into approach mode at the FAF, but non of those events was preceded by a predictive RAIM warning.

IFR approved WAAS GPSs have Fault Detection and Exclusion, which goes one step further than RAIM by figuring out which satellite is putting out the bad data. It stops using that satellite, allowing it to continue to navigate if there are enough satellites visible.

 

[Steve Brown]

ADF sanity check

The 496 has integrity monitoring. In fact, you can set the limit for the alarm. SO:
-It has RAIM
-It has up-to-date data base
-Accuracy of mine has been verified

My reading is that it meets the requirement for IFR use in an experimental airplane.

Is it safe?

The 496 in my current airplane is much more accurate than the non-WAAS 430 in my previous plane. I am where it says I am, to within a few meters.

Pretty hard to convince me that flying a routine lateral guidance approach with a 496 is somehow more dangerous than flying a VOR approach, let alone an NDB approach.

In fact, flying an NDB or VOR approach with the 496 is super easy because the 496 "approach" function has the two points required. The typical GPS approach has more than two points. You have to manually set up a flight plan with the approach points and manually set the HSI to the sensitive setting.

The "If you can't afford the hardware" argument is total bunk. any jet driver could say the same to any of us about even being in the sky.

Again, just use the ADF/NBD sanity check. Anyone actually flown an NDB in IFR conditions? Some are harder than others. My view is that no one in their right mind would actually fly the ADF when they have a perfectly good 496 sitting in the panel.

 

[Ron Lee]

Non precision approach wuth 496: Feasible vs legal

Steve, I am still trying to download the 496 manual. I am skeptical that it has RAIM but need to read the manual.

I just did a search for RAIM and found nada.

I doubt that your 496 is more accurate than a non-WAAS GNS430..unless you can show me that the 430 uses an unrealistic value for GPS signal accuracy (assumes that neither uses WAAS corrections).

I have made a few homemade approaches (still a VFR approach) to my airport using my 430 in VFR conditions and with a safety pilot. I could do it with my Lowrance Airmap. Would I be legal to do it in IFR conditions? Almost assuredly no.

You can kid yourself that it would be legal and safe to use your 496 that way. And you may get away with it. Or you may be flying an approach when a satellite clock goes bonkers and end up crashing.

 

[Kevin Horton]

The 496 has integrity monitoring. In fact, you can set the limit for the alarm. SO:
-It has RAIM
-It has up-to-date data base
-Accuracy of mine has been verified

My reading is that it meets the requirement for IFR use in an experimental airplane.

Take a look at section 1-1-19 (d)(1)(a) Global Positioning System (GPS) in the FAA Aeronautical Information Manual. It states:

Authorization to conduct any GPS operation under IFR requires that:

(a) GPS navigation equipment used must be approved in accordance with the requirements specified in Technical Standard Order (TSO) TSO-C129, or equivalent, and the installation must be done in accordance with Advisory Circular AC 20-138, Airworthiness Approval of Global Positioning System (GPS) Navigation Equipment for Use as a VFR and IFR Supplemental Navigation System, or Advisory Circular AC 20-130A, Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors, or equivalent.

The Garmin 496 has not been approved in accordance with TSO-C129, nor has it been installed in accordance with AC20-138, so it is not legal for IFR navigation.

 

[Scott DellAngelo]

I wonder how many IMC accidents have occured because the pilot was using a non-IFR GPS that would not have happened with an IFR GPS? Why do controllers not seem to care and send people direct even with VFR GPS (is that not technically illegal too)? Those two things make me wonder. Can someone point me to some accident data that suggests airplanes were bent up because of the non-IFR GPS versus IFR GPS in it? I also wonder how many otherwise legal setups are illegal because of expired databases? My guess is LOTS.

Scott

 

[Ron Lee]

Scott, my guess is that the cases are RARE. Frankly this issue is not even on my radar screen should you want to reduce aircraft accidents/fatalities.

Go after the pilot error and poor equipment installation choices on RVs and you can effect a real change.

 

[Steve Brown]

RAIM by another name

Steve, I am still trying to download the 496 manual. I am skeptical that it has RAIM but need to read the manual.

I just did a search for RAIM and found nada.

I doubt that your 496 is more accurate than a non-WAAS GNS430..unless you can show me that the 430 uses an unrealistic value for GPS signal accuracy (assumes that neither uses WAAS corrections).

I have made a few homemade approaches (still a VFR approach) to my airport using my 430 in VFR conditions and with a safety pilot. I could do it with my Lowrance Airmap. Would I be legal to do it in IFR conditions? Almost assuredly no.

You can kid yourself that it would be legal and safe to use your 496 that way. And you may get away with it. Or you may be flying an approach when a satellite clock goes bonkers and end up crashing. Today 07:57 AM

It's not called RAIM, but does the same thing. Start from alarms

As far as accuracy. I've flown both and can tell you with certainty that the 496 is more accurate than a non-WAAS 430. I'm not talking about specs, I'm talking about putting you over a runway threshold.

The 496 does have WAAS. Also has a faster engine than the non-waas 430. 200ms update verses 1 sec.

As far as TSO, read what the EAA has written on the subject:
http://members.eaa.org/home/homebuilders/faq/Equipping a Homebuilt for IFR operations.html#TopOfPage

I'm no lawyer, but what they wrote is logical and that's how I reached my conclusion. I probably need to do a better job of "documenting" the accuracy.

If you want to "kid yourself", start with the assumption that TSO or certified equates to safe.

Did the 430 pass testing that the 496 hasn't? Absolutely. So also have Piper Warrior's passed testing that my RV9 hasn't. I think the people that access this site, for the most part, have reached a conclusion that expensive government mandated testing does not necessarily produce a superior product.

The fact that somebody died using a hand held GPS is as relevant as the fact that somebody died flying an RV. It means nothing.

I've read about plenty of IFR accidents and I can't remember one of them that was due to a satellite going bonkers. I may in fact get killed flying IFR, but if I do, I predict it will be due to one of the same dumb errors that have killed countless other IFR pilots.

 

[Ron Lee]

Steve, if you are using this to indicate that the 406 provides the required integrity to perform a non-precision approach (the accuracy alarm on page 107), then if it uses DOP (probably HDOP) as the criteria, it does not protect you against a GPS clock failure.

Look at what Kevin posted. Using a 496 for an instrument approach is not approved.

I used my non-WAAS 430 for five years before upgrading to the WAAS version. Although I never did a scientific study of its accuracy compared to any surveyed benchmark, the notion that a 496 is orders of magnitude better in accuracy...even twice as good...is questionable. Even if I accept your assertion as fact, any difference is trivial for a non-precision approach.

 

[flytoboat]

"...poor equipment installation choices..."

Scott, my guess is that the cases are RARE. Frankly this issue is not even on my radar screen should you want to reduce aircraft accidents/fatalities.

Go after the pilot error and poor equipment installation choices on RVs and you can effect a real change.

Ron,
At the risk of starting a heated debate, could you elaborate on the poor equipment comment. I am in the beginning stages (tailkit) and am trying to learn as much from the smart builders as I can. (as well as what not to do from the other category of builders )
Thanks