Friday, January 05, 2007

Flying LNAV Approaches

NOTE: For a more up-to-date discussion, see Understanding RNAV Approaches.

A while back, I took Garmin to task because it seemed their new WAAS-certified GPS units promised to increase pilot workload at a critical moment in flight - just before the final approach fix on an RNAV approach (like they listen to me!). Well, it might not be as bad as I first thought because these new units also provide some new features that could really help a pilot stay ahead of the game. Continuing where I left off with my previous post on FDE prediction, here's an example of flying the Byron RNAV RWY 30 approach using the new 400/500W Trainer software.

There are several new features in the 400/500 WAAS-certified unit, many of which are described in this Garmin document. Some of the features I find most useful don't seem to be mentioned.

One change involves the GPS status screen. Note that there are some new terms used, namely HFOM and VFOM. These stand for Horizontal Figure Of Merit and Vertical Figure Of Merit, which represent the 95% confidence in the horizontal and vertical accuracy. The Garmin 530W Pilot Handbook and Reference simply says that smaller numbers mean greater accuracy. EPU (Estimated Position Uncertainty) is more straightforward - it's the estimated horizontal position error in nautical miles. Lastly, the histogram bars that show the relative signal strength of each satellite will contain a D when that particular satellite has differential corrections (WAAS) applied. This is good stuff to know, but it only indirectly helps you fly an approach.

One annoyance is that when using the Trainer, the MSG annunciation is constantly displayed and when you press the MSG button you're warned to not use the simulator for navigation. This is important if you are using an actual 530W unit in simulator mode, but for the trainer software to display this message continuously it is pointless and annoying.

Back to the Byron RNAV RWY 30 approach, let's say you've loaded the approach and are being vectored to SHARR intersection (the initial approach fix). As you approach SHARR (a fly-by waypoint), you'll begin to see a curved path displayed which shows the path you'll fly if you follow the turn anticipation prompt provided by the 530W. I find this to be a huge improvement because it gives the pilot a visual cue as to the path they'll be flying and helps them visualize and prepare for the change in course.

The prompt for turn anticipation messages in the old 530 units lasted only a few seconds and unless you watched the display like a hawk, they was easy to miss. A new feature in the 430/530W is a 10 second countdown display prior to beginning a turn and prior to any waypoint passage. This message is much harder to miss and the countdown helps get the pilot in step with the approach.

A few miles outside of the final approach fix, note that the course sensitivity annunciation has changed from TERM to LNAV/+V. Now as I understand it, this annunciation is for approaches that have published LNAV/VNAV minima, but haven't yet been identified as such in the Jeppesen database that Garmin uses. The Byron RNAV RWY 30 approach has only LNAV and LPV minima, so I'm concluding that LNAV/+V is also used to provide advisory vertical guidance on approaches that have only LNAV minima. So LPV, LNAV/VNAV, and some LNAV approaches will all display vertical guidance and it's up to the pilot to note the annunciation (LPV, L/VNAV, or LNAV/+V) and descend to the appropriate minima. If the LNAV annunciation appears, they you won't receive any vertical guidance.

When vertical guidance is provided, the behavior similar to an ILS glideslope and you should intercept the glideslope from below, just like you would an ILS glideslope. You then descend using that guidance to the appropriate MDA or decision height. It appears from my use of the simulator that this vertical guidance is provided right down to the runway, so you'll need to know when to call it quits and level off if you don't have the required visual references to operate below the MDA or DH.

You can use the altitude button on the Trainer's simulated autopilot to descend on the advisory glideslope, keeping the needle centered. If you descend at too high a rate and you have selected to simulate a GPS unit with TAWS, you'll even get a terrain warning. Should you need to execute the missed approach, the unit's behavior is the same as in the old 430 or 530 units.

Next installment? LPV approaches with the 530W.


phil said...

they may be listening. i googled "Garmin RNAV approach" and you were #2 behind

Ron said...

I don't think you were too harsh on Garmin with your previous post.

It's very confusing to have only LPV and LNAV minima on an approach plate and then have "LNAV+V" show up on the GPS receiver. Even if you've read about it, it just doesn't make sense.

If it were me, I'd see that, look down at the plate, and figure that something was wrong. I mean, how can it be right when there are no minima called LNAV+V on the plate? There's nothing on the plate that even suggests the presence of a glideslope.

The avionics are getting complex enough that surprises like this can be dangerous distractions in the cockpit.

Santosh Kumar said...

According to the King Schools Cleared for Flying the Garmin G1000 Multimedia Training System, IFR Lab 5, "WAAS Approaches" video, "LNAV + V" is available and appears for those standard (LNAV) GPS approaches that, in addition to step down altitudes, show a diagonal glide path on the profile view (as opposed to drawing in "stepped" paths) and a "<" glide path icon next to the approach path with recommended glide angle in the profile view.