The International Cessna^® 170 Association

gahorn wrote:The 196 also updates once every second, unless it's in WAAS mode in which case it updates every 5 seconds like the 496.

Is that every 5 seconds or 5 times/second?

WAAS slows it down.

Really? So the price you pay for increased accuracy is the fact that it tells you exactly where you WERE 5 seconds ago??? Somehow that doesn't exactly seem like a benefit...

doug8082a wrote:Really? So the price you pay for increased accuracy is the fact that it tells you exactly where you WERE 5 seconds ago??? Somehow that doesn't exactly seem like a benefit...

But remember Doug with WAAS you know alot more accurately where you were 5 seconds ago.

Dave, the point you make is certainly important. No one should be out there flying with gyros without understanding how to use secondary information instruments to confirm/support the use of the primary gyros. This is an important element to all instrument training.
The seeming nit-picking that has been discussed in this thread is regarding the first thing to look at when attempting to determine attitude. It is not the ASI.

doug8082a wrote:Really? So the price you pay for increased accuracy is the fact that it tells you exactly where you WERE 5 seconds ago??? Somehow that doesn't exactly seem like a benefit...

WAAS utilizes one of two independent satellites to supplement/confirm information that the usual constellation provides. This takes additional computer power/time and that is why it slows things down. Five seconds is not a critical period in any case. It took you that long to read this. It took Ol' Gar even longer. (He thought that's how one spells the past tense of what "is ...is".)

(I turned the WAAS feature OFF on my GPS 196. Without WAAS the unit typically provides 15-foot accuracy. I didn't feel I needed the 7'-9' provided by WAAS.)

This isn't the complete story. I don't know about the 196, except that it isn't certified for instrument navigation or approaches. TSO-C146A governs the units that can take full advantage of WAAS for instrument flight, and it requires them to update at least 5 times per second. Units built under the older TSO-C129A can update once per second.

For example, the Garmin GNS430 is a TSO-C129A box, and the WAAS upgrade to it makes it conform to TSO-C146A with a 5 per second refresh rate. With TSO-C146A you can use new kinds of approaches, e.g. LPV approaches, that give you a synthesized glideslope that can be used down to 250 feet AGL decision height.

So: in IFR certified WAAS GPS units, the refresh rate is faster than on non-WAAS units, not slower.

TSO-C146A also allows the GPS unit to be used as the primary navigation device, so you're no longer required to have a real VOR receiver on board.

My information comes from here: http://www.avionicswest.com/myviewpoint/1500_waas.htm

Best Regards,

John

The Garmin 430/530 boxes are not yet upgradeable to TSO C-146a, and the originally projected price of that upgrade ($1500) is not projected to be over $6,000....when, if ever, it becomes available.
Meanwhile, the only GA GPS that can utilize Vnav is the Garmin 480,...not the 430. Only LNAV (no psuedo-glide slope) approaches may be made using Garmin 430/530 units.
The info I gave earlier re: the 196 is correct. It updates more slowly (as do most non-IFR units) in WAAS mode.

My local avionics shop is telling me that GNS430W and WAAS upgrades to the 430s have become available within the last month. He's got upgrades to three aircraft scheduled into his shop now, the first one on March 17th.

The 430W costs $1500 more than a new 430, but depending on where the radio is situated on the panel, it may require a separate annunciator at additional cost. I'm considering having a new 430 installed, and for a new installation, it sounds like a no-brainer to get a 430W instead. We'll see though -- the avionics guy says the WAAS upgrades are having RF interference issues with other installed avionics, so he'll probably want to factor in some additional cost for sorting that out.

Follow-up: my GNS430W installation was completed this week. The shop didn't seem to have any particular problems with it. They removed an old VFR GPS, a COM radio, an ADF that didn't work any more, and replaced the old VFR audio panel. The new W&B shows the airplane is 8 pounds lighter.

The layout of the stock 170 panel meant that a separate annunciator device was required (http://www.midcontinentinstruments.com/pdf/WAAS.pdf), because the GPS unit isn't mounted close enough to the primary scan. The 430W lists for $1500 more than a 430 (non-WAAS), but with the annunciator, my shop charged me $2700 over the price of a straight 430.

I think it's worth it, for two reasons: TSO C146A means the GPS can be the primary navigation device, and you get an active glide slope needle with every GPS approach, it appears. That doesn't necessarily mean lower minimums, but they can be lower on LNAV+VNAV and LPV type approaches.

I haven't had much time to play with it yet; my 170 is down now for an annual that was due in May. But from what I've been able to see so far, this is a rock-solid navigation device with features we could only have dreamed of 10 years ago. I'm going to be completely spoiled now.

So John, what are you putting in the Cub?

John, is the 430 now actually approved for VNAV approaches? Previously only the 480 was approved.... The 430 could only be used to LNAV minimums (despite VNAV reference (Psuedo-GS) being available.)

jrenwick wrote:The layout of the stock 170 panel meant that a separate annunciator device was required (http://www.midcontinentinstruments.com/pdf/WAAS.pdf), because the GPS unit isn't mounted close enough to the primary scan. The 430W lists for $1500 more than a 430 (non-WAAS), but with the annunciator, my shop charged me $2700 over the price of a straight 430.

John,
I'm considering putting a Garmin 430 in my '52 C-170B. I am disappointed to hear that an annuciator is required. Where did you mount your 430? Any chance of posting a picture of your panel? I'm curious what area is considered to be in the pilot's "primary scan". AC 20-138, which deals with the airworthiness approval of GPS navigation equipment, says, "Each display element (i.e., the cross track deviation display (CDI), horizontal situation indicator (HSI), map display, etc.), used as a primary flight instrument in the guidance and control of the aircraft, for maneuver anticipation, or for failure/status/integrity annunciation, shall be located where it is visible to the pilot (in the pilot's primary field of view) with the least practicable deviation from the pilot's normal position and line of vision when looking forward along the flight path." The AC doesn't give any hard and fast measurements for this "primary field of vision". I've seen some reference elsewhere to 16.8" from the centerline of the yoke. The left radio compartment in the 170 is easily within that distance.

N9149A wrote:So John, what are you putting in the Cub?

For Cub "IFR," I should probably have a Streetmap with a current database of roads to follow!

And a CB radio, to talk to the truckers who are passing me.

Or a fishfinder, for when it's on floats!

John

gahorn wrote:John, is the 430 now actually approved for VNAV approaches? Previously only the 480 was approved.... The 430 could only be used to LNAV minimums (despite VNAV reference (Psuedo-GS) being available.)

George,

I believe the 430 is approved for VNAV if it's updated to TSO C146a. Garmin's web site information for the 430W is here: https://buy.garmin.com/shop/shop.do?cID=194&pID=298. The reference manual says "The GPS 400W has a WAAS GPS engine and is TSO C146a certified for primary domestic, oceanic, and remote navigation including en route, terminal and non-precision approaches, and approaches with vertical guidance, such as LPV and LNAV/VNAV...." So I think the answer is yes.

The unit will tell you, on every GPS approach, if it has a good enough signal for vertical guidance, with an annunciator that will say either "LNAV" or "LPV." If it says "LPV," the glideslope needle will come alive. Then it's up to the pilot to look at the approach plate and decide what minimums to use. If the unit says "LPV" and the approach has LPV or LNAV/VNAV minimums, then you can use them.

There's also a predictive FDE/RAIM feature in the PC simulator for this unit. You enter a flight plan into the simulated device, supply departure time and groundspeed information, and it will tell you if the stars will be in the right alignment. Because WAAS effectively gives you one additional satellite, it's supposed to be very rare that you won't have vertical guidance.

That's my current understanding anyway, subject to correction by the CFII who's going to check me out on all of this. Garmin has a syllabus that you can download from the link above. I'll be going through that, too.

Best Regards,

John