Sunday, March 14, 2010

IFR Procedures with the G430

The GNS430 is installed and IFR certified, so it is approved to work with the autopilot. I program in what it's supposed to do, and then it will give the autopilot directions on how to proceed. My job then is to monitor that it is giving the right instructions and that the autopilot is carrying them out. It's like you could give your car's cruise control instructions from your dashboard GPS on where the speed limit changed and where your exit was, and it would automatically exit and slow down.

The IFR procedures are all stored on a data card, and yes the cards must be physically replaced every 28 days, which I just know is going to be a pain and a half someday soon when I need to escape IFR from somewhere where overnight delivery service takes three months. I hope it won't lock up and refuse to work at all just because the card has expired. [Update: Paul says the updates can be downloaded over the Internet.] Canadian procedures only change every 56 days, unless by NOTAM. Question: if a NOTAM has temporarily amended a procedure can I modify it in the device? I'm guessing no. I'd have to hand fly it.

To load a procedure into the flight plan, I must first have a flight plan (or it will settle for an active "direct-to" command to an airport). I press PROC and it pops up a menu of types of procedures. I use the large right knob to Select Approach (or Select Arrival?, Select Departure? as appropriate). Enter selects, and it presents a choice of procedures depending on the destination airport in the flight plan or direct-to. Assuming you're selecting an approach, it drills down to allow you to specify the initial approach fix (IAF) or that you will be receiving vectors to final. Once you have selected the procedure with the enter key you can choose to load or activate it. Typically you would load it but not activate it until cleared for it, but it looks like you can activate it any time without restrictions. To do that after loading, you press PROC and then choose Activate Approach.

About 30 nm from the runway the GPS switches to terminal mode, i.e. 1.0 nm CDI scaling each side. It's a gradual transition, so if you were sitting there one dot off centre on the 5 nm scaling it doesn't suddenly whip over to full scale deflection. If you want, you can press FPL and right knob scroll through the list of waypoints to review the approach procedures. Seeing as the autopilot is flying, you could do that the way you look at nav radios as you give the approach briefing.

Approaching the IAF it displays your next desired track (DTK) as the outbound from the nav aid. It's up to you to make sure the OBS is set correctly and to decide which procedure turn to fly. It doesn't direct you left or right but just depicts your position relative to the inbound track. About a minute after passing the final approach fix (FAF), the nav aid you'll fly toward on the way back in towards the runway, it directs Start procedure turn, but again it's up to you to choose and fly the reversal. This could be done presumably by hand flying or by directing the autopilot with the heading bug. Question: Is it always about a minute after the FAF? what about those facilities in tight valleys or over residential areas where you have to fly a longer outbound leg? Halfway through the reversal, the GPS sets your desired track to the inbound and leaves you to intercept it. I'm not entirely clear on how I should be using the heading bug here. The GPS-autopilot interface is quite mysterious, given that they are technologies invented close to fifty years apart in time.

The unit does not accept a barometric pressure setting, and does not display altitude on any of the screens I've looked at in the manual so far, so it would appear that the GPS provides directional guidance only, and that the pilot is responsible for instructing the autopilot in step down altitudes and rates of descent on a non-precision approach. (There is a mode to reach a target altitude at a specified location, but presumably that is GPS altitude and is intended for VFR use). The GPS directs the autopilot to hold a heading and the pilot directs the autopilot to descend to an altitude, while the pilot uses the throttles to instruct the engine regarding power input. It's like CRM between a human and two machines: you have to be very clear who is responsible for what.

On a precision approach, the GPS is not approved to maintain either lateral or vertical guidance: you must switch to the VHF navigation signal from the ground-based ILS. The CDI can be set to automatically switch from indicating deviation from the GPS track to the localizer track, so long as you are tracking within 1.2 nm of the localizer, starting 2.0 to 15 nm back from the FAF. If it isn't set to switch automatically, or if you have a less than 2 nm gate, you can activate it manually by pressing the CDI button. In either case, the correct ILS frequency must be set in the VLOC radio, for the switch to occur. The unit will put the ILS frequency up on standby for you, but you have to toggle it from standby to active yourself.

While you are on an approach, the missed approach point (MAP) is the current waypoint. As you reach the MAP, "SUSP" appears above the OBS key and the pilot is expected to either land or conduct the missed approach. To land I guess you just land: the autopilot is still on and presumably the last heading instruction the GPS issued to the autopilot is still valid, so the airplane continues to the runway and the pilot can disable the autopilot and land at whatever point they prefer. To initiate a missed approach, hit the OBS key (and switch the CDI back to GPS from VLOC if it was an ILS approach) then the unit should give directions into the hold. It is not entirely clear from the documentation whether it will take you step by step through an elaborate missed, but at any rate if the missed called for an immediate hundred degree turn away from the mountain, I would be doing that before messing with the GPS for guidance. It looks like the GPS is a fair weather helper that is not willing to help you with the highest workload portion of the flight. It will tell you the correct entry procedure for a hold, but I'm thinking that unless it is a published hold that the database knows about, it would be quicker to work out the entry myself than to input the hold instructions.

Question: does the approach I fly have to be the LAST WPT, such that I can't pre-include the alternate as part of the flight plan? It appears that that is the case.

It looks like the easiest things to forget are: taking care of the vertical profile; activating the approach; and ensuring the correct CDI source for the phase of flight. Anyone who has any IFR experience with autopilot-linked Garmins is very much encouraged to give me pointers here. I am, for example, stymied as to how I define a hold that is not depicted on the plate as part of an approach, if say there's someone ahead of me on the approach and I just need to hold at the NDB and wait for them, or if ATC suddenly assigns a hold while they sort themselves out.

Tomorrow's post (not skipping a day this time) will be me trying it all out in the Garmin-supplied simulator.

13 comments:

Paul Tomblin said...

Garmin offers an option where you can download options over the internet and install them with a card writer. It's a lot cheaper than swapping cards, and you don't have to hope the cards catch up to you when you're on the road.

fche said...

The absence of barometric settings becomes obvious when one realizes there there is no static pressure input to the widget. It does not care about the atmosphere at all.

The G430 does not specifically assist with uncharted holds.

You can include further waypoints in a flight plan after an approach.

Aviatrix said...

fche: Some GPS units ask for an altimeter setting and use it to calculate a barometric altitude, offsetting appropriately from the GPS altitude.

fche said...

... which ones? (We're not talking about ADAHRS panels, right?)

Sarah said...

Using barometric information to augment the GPS is called "baro-aiding". It predates the WAAS solution, and I'm not aware of new (GA) GPS systems that use barometric data.

Which brings up a point - if you have a Garmin 430W do you have precision approaches available in Canada? In the US, they're called "LPV", for "localizer precision vertical (guidance)" I think. These have minima as good or better than an ILS.

There is a forest of regulation about when it is legal to rely on GPS for primary IFR navigation, and how to do it... and, when you can substitute GPS for a radio navaid.

One last thing I was taught about the miss and the SUSP... don't press it until the initial part of the miss is flown, a straight climb to an altitude or a heading. Pressing it generates a "solution" calling for a turn direct to the published hold... which you may not want to do right away. If you realize that but have already pressed the button, you have a lot of button pushing to do to get GPS navigation set up properly to the hold.

Aviatrix said...

We're talking about aviation GPS units in general, not specific to ADAHRS. I have often skipped the prompt or okayed the default in a unit that I was using for VFR flying. I think the KLN 94 does and that the Garmin 495 doesn't.

According to this page, the GNS430 does use a static source input, so either AOPA is incorrect or yours is installed incorrectly.

I assumed that the altimeter setting was to allow these units to simulate a barometric altimeter. It could be that in each case the altimeter setting was for baro-aiding RAIM detection. Perhaps I'll learn more in company training, and I'll be happy to share.

Aviatrix said...

Sarah, you're going to be my new best friend. I don't think I would have ever figured out the sequence of key presses to select an approach to an intermediate airport. I would have resorted to fche's suggestion and added all subsequent airports after the first approach. The insta-climb in the simulator has masked the need for caution in the use of the OBS key to clear SUSP. Thanks for the heads up.

The chief pilot's e-mail did not include a W on the model number, so we will probably be flying our ILSes the old-fashioned way. I'm not sure.

fche said...

Ah yes, I forgot about the baro-aiding option on the G430. I have only seen a GPS-altitude indication on the screens. (I believe AOPA is wrong about it being compulsory; I'll ask our avionics guys.)

BorealOne said...

On a 430W, baro-aiding is also possible, but redundant when WAAS service is available (which is usually pretty much everywhere but the High Arctic in Canada)...according to TC:

"Unlike BARO VNAV, SBAS [eg WAAS] vertical guidance is not subject to altimeter setting errors, or non-standard temperatures or lapse rates. Vertical guidance provides safer stabilized approaches and transition to visual for landing. This represents one of the principal benefits from SBAS service. The other is lower approach minima at many airports, as a result of greater lateral accuracy. SBAS has the potential to meet CAT I approach standards with the next generation of GPS satellites."

We don't have many RNAV LPV approaches yet, but I've been very pleasantly surprised by how well the 430w I had installed in my plane last year has performed in both the RNAV and conventional ILS approach context.

Sameer said...

About arbitrary holds, you can do one of two things: a) enter the holding fix ID in the fltplan or b) hit direct the holding fix, then hit OBS and turn the course selector on your HSI to the inbound course (cross-check the DTK on the GPS agrees with the course selector). Then, fly the hold using your HSI as you would without a GPS.

Ihab Awad said...

What is it like to rely on such a small-seeming display for so many functions and data? Is this amount of power not more suited to one or more large PFDs?

Critical Alpha said...

Hi Avi,
your comment about the system being a "fair weather friend" triggered a thought about this report: GNSS RNAV Workload from the ATSB. It's an interesting read.
Hope the police aren't after you - the magic word for me today is "coppers".

SteveS said...

Considering the date on your blog entry I'd be surprised if your unit is not WAAS-capable unless it was a used unit when installed. Garmin quit supplying non-WAAS units a while back.