Friday, March 09, 2007

Flying the GNS480

Last week was a bit slow, which provided the perfect opportunity to fly a Piper Arrow that recently became available and is equipped with a CNX80 (which Garmin now calls the GNS480 since they acquired Apollo a few years back). I have given several hundred hours of dual instruction in the venerable Piper Arrow, but this was my first opportunity to fly RNAV approaches with vertical guidance in a real aircraft.

It was my first time flying an Arrow equipped with an air conditioner. We didn't need nor did we use the AC, but its presence required careful weight and balance considerations: With two pilots in the front seat, half fuel, and insufficient ballast in the baggage compartment, you're guaranteed to exceed the forward center of gravity limit in this aircraft because of the additional weight of the air conditioner compressor up front.

This flight allowed me to log some instrument approaches and a hold, so I brought along a friend to act as safety pilot. He flies for a major airline, but has been sitting at home on reserve for several weeks and was more than willing to perform safety pilot duties if it meant a chance to fly. I had experimented with the Garmin GNS480 simulator about a year ago, but hadn't really plumbed its depths. It was just an academic exercise at that time, but I was going to be spending actual dollars and burning real fuel on this flight. The threat of debt, poverty, or credit card interest is an amazing motivator and I spent about four hours in front of my computer creating flight plans, loading departure procedures, loading approach procedures, flying missed approaches, and going to an alternate destination. I couldn't afford to not have my mojo on this flight.

I don't want to start a debate, but I feel a brief sidebar on PC simulation is in order. I have a desktop PC that I use for running simulators, but for my day-to-day use I'm a Mac aficionado, though I'm more likely to be mistaken for John Hodgman (the guy on the left) than Justin Long. I've used a variety of computers from IBM mainframes, to Unix workstations (Sun, HP, et al.), PCs running many flavors of operating systems, and Macs. As I've gotten older, I have less patience for the care and feeding of PCs. So for me, using a Mac is a no-brainer since it requires less effort on my part. But when you want to run a simulator, like the Garmin GNS480 sim, you need a PC. Or do you?

In the pre-Intel Mac world, a product called VirtualPC was introduced. It still exists, if fact. I used VirtualPC in the past and the problem was the performance was abysmal. As a former software guy with operating system development experience, I found the fact that it worked at all to be an amazing achievement. Now if you have a newer Mac with an Intel processor, you have several options for running Windows.

One solution is to install Boot Camp, a free product you can download from Apple that allows you to create a separate disk partition, install Windows XP on that partition (you have to provide Windows XP software yourself), and re-boot your Mac into Windows. That's cool, but you are still in the world of either-or: If you want to switch back to MacOS, you have to reboot.

I think a better option is to use VMware Fusion, a soon-to-be released product currently in beta that lets you run Windows XP in a window under MacOS X (or Linux). This is just what I always wanted. I can run the few Windows XP apps I need to run and when I've had enough, I can close or minimize the window and make the unpleasantness go away. There's at least one other product out there that provides similar capability, but I'm very happy with VMware Fusion.

Meanwhile, back in the air ...

The plan was to depart Oakland, head east, do a practice RNAV RWY 30 approach at Byron, fly the published missed to a hold, then back to Oakland for a couple of RNAV RWY 27L approaches. Since both approaches have LNAV, LNAV/VNAV, and LPV minima defined, this would give me a chance to see which approach sensitivity I would get and what it is like to not know which minima you'll get until a few miles from the FAF.

Under the hood shortly after takeoff, I was working to adapt to the somewhat ponderous control feel of the Arrow. It was a bit of a shock after flying a Cirrus the day before, but adapt I did. The approach into Byron started with a couple of hiccups with Norcal. I made my approach request and they asked where I wanted to start the approach.

Not wanting to take all day (this was on my dime, after all), I asked for vectors to EKIYU and got the annoyed, bored response: "Where?" After repeating the waypoint name twice, the controller took the easy way out and said "proceed on your own navigation, report established on a segment of the approach, maintain VFR." Whatever ... I vectored myself to EKIYU, which happens to be essentially on top of the Tracy Airport, where the course sensitivity changed to LNAV/VNAV.

Look at the Jeppesen version of this approach chart and you'll see a note saying that WAAS NOTAM service is not available for Byron. On the FAA version of this chart, there's a small W in a black rounded square, which means the same thing. The FAA's IAP chart legend says:
... WAAS NOTAMs for vertical outages are not provided for this approach. Use LNAV minima for flight planning at these locations, whether as a destination or alternate. For flight operations at these locations, when the WAAS avionics indicate that LNAV/VNAV or LPV service is available, then vertical guidance may be used to complete the approach using the displayed level of service. Should an outage occur during the procedure, reversion to LNAV minima may be required. As the WAAS coverage is expanded, the [W symbol] will be removed.

So for the time being, you won't know in advance if WAAS will be available at airports like Byron. Oakland, however, does have WAAS NOTAM service and I'd not seen any NOTAMs for Oakland in my DUAT pre-flight briefing. Here's what a WAAS NOTAM looks like, in case you're wondering:


The approach into Byron was otherwise uneventful, though I got a bit low on the VNAV glideslope toward the end. This was due to some spirited, thermal updraft and downdraft activity combined with my mistake in putting in flaps 10 a bit too early - a habit I've gotten into with both the Cirrus and DiamondStar. Reacquainted with the Arrow's wing loading, power loading, and drag characteristics, I vowed to just put the gear down at the FAF on the next approach and save the flaps for later.

Then it was back to Oakland for an RNAV RWY 27L and I was pleased to see the course sensitivity LPV (localizer precision with vertical guidance). So what's it like to fly an LPV approach? It's pretty much like an ILS. You descend to the depicted altitude at the FAF, intercept the glideslope from below, then ride it down to the decision altitude defined for the approach. I did think the LPV gildeslope was just a bit more twitchy than a conventional ILS glideslope.

One big difference between the GNS480 and the Garmin GNS430 or GNS530 units is that you don't need to take any action to begin the missed approach procedure. There are no buttons to push, the GNS480 just starts telling you how to fly the missed. If you're circling or landing straight in, you'll ignore it anyway. I think this is a good design, but unfortunately the new WAAS-enabled GNS530/430W GPS receivers have kept the requirement that the pilot press the OBS button to re-enable waypoint sequencing after passing the missed approach point. A shame about that, really ...

I found I missed seeing the 10 second countdown for turn anticipation more than once. This might have been due to my higher workload as I was getting accustomed to the airplane or it could have been the placement of the unit in this aircraft's panel put it too far from my primary field of view. Or the sunlight could have been too bright and the screen too dim. When I reminded myself to be on the lookout, I found it a big improvement over the terse, few second turn warning that the Garmin 430/540 provide.

The flight planning and direct-to navigation features of the GNS480 are ... different. The unit knows about airways and this can make it easier to translate an IFR clearance to a GPS flight plan, but it's a bit of an acquired taste. Once you get used to it, the interface is logical and makes sense. The flight plan even allows you to enter an alternate airport - a big shortcoming in the Garmin 430/530 world where the last airport you enter in a flight plan is the one the unit thinks is your destination when it comes to loading approach procedures. I'll probably be doing more flights in this aircraft as an instructor, so yours truly will have to become proficient with yet another GPS.


Eric said...

I know what you mean about transitioning back to the Arrow from other aircraft - this winter when I went from our Grumman Yankee back to UND's Arrows was kind of a mind****. Piper's newer models also seem like they're rigged a lot 'heavier' than the old Cherokees.

The GNS 480 sounds like a pretty solid unit - hopefully when I start instructing this summer I'll get the chance to see some aircraft with it installed. For now, we've got an LPV approach in to KGFK, but nobody can fly it with university equipment. On that note, John, do you know if the WAAS upgrade for the GNS 430 allows it to fly LPV or LNAV/VNAV approaches? That's probably more likely here than a batch of 480s.

John said...


Yes, the Garmin 430W and 530W are certified under TSO 146A, so it can be used for LNAV/VNAV and LPV approaches with the ususal caveats: If WAAS NOTAM service is not available for you departure, use the LNAV minima for planning purposes. When you arrive at your destination, you can use the minima that the GPS unit says is available.

The 430W and 530W have the same old flight planning features (and warts) as the older 430/530 units. They do offer improved turn anticipation features, but the same missed approach behavior - you have to push a button (at least one) to start navigating to the missed approach.

I suspect the GNS480 will be discontinued in the near future, though it will continue to be supported. That's my theory, anyway ...

John said...


Although I have a 530W, I think I would have appreciated not having to press OBS to activate the missed approach navigation. The new version of DO-229D is now out and it has eliminated the requirement for automatically providing missed approach navigation, so I doubt future WAAS GPS receivers will implement it.

You can use the prediction program before the flight to help determine if adequate WAAS coverage will be available at your eta at the destination. Also, once in flight, if you check the Satellite page, it will display the current value of HPL and VPL. So long as the VPL is below 50 meters and the HPL is below 40 meters, the 480 will not downgrade the approach to LNAV minimums for a LPV approach with a DH of 250 feet. You need a VPL better than 35 meters in order to not be downgraded on a LPV with a 200 foot DH. I would be interested in knowing what typical values are for VPL in your area.

The Garmin 530W has similar values displayed on the Satellite page, HFOM and VFOM (Horizontal Figure of Merit and Vertical Figure of Merit). In the pilots guide they are defined as the 95% confidence level and are expressed in feet. To convert these values to a HPL or VPL value, which are 99.99999% probability, involves using the relative areas under the normal curve and then convert to feet. My calculation is that a VFOM value of 62 feet corresponds to a VPL of 50 meters and a VFOM of 40 feet corresponds to a VPL of 35 meters. I only converted VPL values to VFOM values as these are the critical ones that will either allow or downgrade a LPV approach. I am in the Charlotte area and typically see values of VFOM around 23 to 26 feet. I would be interested in knowing what others are seeing in other parts of the country.


John said...

Thanks for the pointers. While I'm familiar with the FDE prediciton application, I'm not sure if I'll bother to commit those HFOM/VFOM numbers to memory.

It seems that the FAA has created a monste: They've created standards and a certification process that has resulted in companies like Garmin creating some products that are not at all straightforward or easy-to-use. And when the mistakes in the design are realized, it's too expensive to fix or upgrade to rectify many (most?) design flaws.

Any task on a navigational system that takes more than three steps to perform indicates to me a poor user interface design or a poorly designed TSO.

Many pilots, instructors, designated examiner, and FAA inspectors insist on knowing all the innards of a aircraft or navigational system. My somewhat radical position is that pilots should be required to know less trivia so they can concentrate on the task at hand - flying the aircraft.

But maybe that's just me ...

Thomas said...

Now that is's 2009, what is your opinion on the 480 vs the 430W?

I am upgrading from GX 60, and the 480 does look superior to 430W, but I am a bit nervous on the discontinued status...

How worried shoud I be?

Comanche pilot: Pa 24-260

John Ewing said...


I have mixed feelings. On one hand the GNS 480 (nee CNX 80) is a pretty good unit and Garmin seems to be continuing to release software upgrades for it.

On the other hand, the 480 doesn't have much to recommend it when compared to a 430W. About the only advantage of the getting the 480 is the ability to program ad hoc holding patterns, which are rarely issued anyway.

I've found the 480 has several annoying features, but I may be biased by the fact that I have flown more hours in G1000-equipped aircraft which has a flight plan interface similar to the 430/530. The 480's flight plan and other interfaces are unlike anything else I've used and frankly it seems a bit arcane.

If you can get a good deal on the 480 and you can live the interface, I'd say it's probably an okay choice. It's certainly a big step up from the GX series. My crystal ball says that Garmin will probably continue to support it for a few more years, but I don't know for sure what their plans are.