Rod Machado's take on it...
Glideslopes galore
By Rod Machado
Aviation writer and humorist Rod Machado knows WAAS
up — most of the time.
There are a few folks out there who believe that the Wide Area Augmentation System (WAAS) involves liposuction to help manage a pilot's waistline. I hope you're not one of them. Why? Because if you're WAAS-wise, IFR-rated, and properly equipped, the chart minimums section will have a lot more meaning for you.
What is WAAS? It's a means of continuously correcting a GPS signal to account for tiny oddities in satellite orbits, satellite clock drift, and transmission delays caused by atmospheric conditions, using information transmitted from ground-based stations. This additional degree of accuracy is demanded by the FAA in order to permit a GPS-guided precision (i.e., with a glideslope) approach (see "
On Display: Going the WAAS Way," page 139).
With WAAS, your average GPS approach typically offers one of three different varieties of software-generated glideslopes for you to fly. It's glideslopes galore for those who know WAAS up.
The figure below is the minimums section for the NACO (National Aeronautical Charting Office) RNAV (GPS) Runway 24 approach to Carlsbad, California's Carlsbad/McClellan-Palomar Airport. At first glance, the far left column of the horizontal minimums equipment section (i.e., LPV, LNAV/VNAV, LNAV) is a little different from the standard format to which you've become accustomed. Let's examine each line of minimums from the bottom up.
Line by line
The bottom line shows the circling minimums for this approach. This is the minimum descent altitude (MDA) and required visibility for an approach to an airport instead of a specific runway. There's nothing new here. I'm sure you're already very familiar with this line.
The next line up is the LNAV MDA. This is the MDA for lateral navigation. In other words, these are the minimums for flying an ordinary nonprecision GPS approach, just like the ones you've probably been flying with your non-WAAS GPS receiver for some time. If you were cleared for the approach and crossed JABAL at 3,100 feet, then you could descend to 2,060 feet until reaching GUGEC, 1,240 feet until reaching ZUXAX, then 860 feet to the missed approach point (the intersections are all named after the aliens that crashed near Roswell, New Mexico.
If, on the other hand, you have a WAAS receiver (assume this to mean a WAAS-capable, IFR-certified GPS receiver), the possibility exists for you to fly a precision approach (one with lateral navigation and a glideslope) to a runway. This will be the first of three possible software-generated glideslopes you might fly.
Precision approach minimums are found in the next lineup, next to the letters LNAV/VNAV, meaning, lateral navigation with vertical navigation (a glideslope). If you flew this as a precision approach, your decision altitude (DA) would be 1,000 feet and the required visibility would be one and three-quarter miles. If LNAV/VNAV minimums were allowed for this approach (based on the GPS receiver meeting stringent horizontal and vertical accuracy and integrity requirements), the letters L/VNAV would automatically appear in your WAAS receiver's approach annunciator window. You'd now see a software-generated glideslope on your omni display (your VOR or horizontal situation indicator [HSI] display, which is slaved to your GPS). The glideslope would typically become active before (or near) the intermediate fix (KANEC). The slope of the descent path would be 3.20 degrees, with a threshold crossing height of 52 feet. In short (or in pants), you'd fly a glideslope just as you would on an ILS, except this glideslope is generated by your WAAS receiver.
In this instance, you'd probably intersect the glideslope somewhere before JABAL, at a minimum of 3,100 feet, and fly it down to a DA of 1,000 feet. At DA, if you didn't have the visibility and the runway environment in sight, you'd execute the missed approach.
Take notice that the LNAV/VNAV DA is 140 feet higher than the LNAV MDA, and the required visibility for landing is one and three-quarter miles. Isn't a precision approach supposed to get you down lower and put you in a good position for landing in low visibility? WAAS up here?
Baro VNAV
The answer lies in something called baro VNAV, or barometric vertical navigation capability.
When the FAA began designing RNAV (GPS) approaches and before WAAS was online, the airlines wanted a piece of the approach action. So, in addition to the LNAV minimums, the FAA created LNAV/VNAV minimums for use by airlines that have baro-VNAV equipment. This is a navigational system that doesn't require GPS/WAAS. It creates a vertical glidepath between two waypoints (or a descending angle from a single waypoint) based on barometric altitude, as measured by a special barometric altimeter in the airplane.
Unfortunately, baro VNAV uses an altimeter, and as with all barometric altimeters, it's affected by extremes in temperature. Higher LNAV/VNAV minimums are partially the result of compensating for a full range of possible temperature errors. Also, LNAV/VNAV approach minimums were created under TSO-c129 (FAA technical standard order) specs (before WAAS was in use) and, therefore, require stricter obstacle clearance criteria, often resulting in unusually high LNAV/VNAV decision altitudes.
Much of the airline fleet is stuck with baro VNAV, and that's probably because the airlines didn't keep their receipt when they bought it. As a result, they can use only LNAV or LNAV/VNAV minimums. You'll be hard-pressed to find even one small airplane that has baro-VNAV equipment. You'll be hard-pressed to find a small airplane pilot who'd want it, either. That's because with a WAAS receiver, you can fly to something known as LPV approach minimums that offer near-ILS performance. This is the second of three possible WAAS software-generated glideslopes.
The very top line in the Carlsbad minimums section shows the letters LPV DA. LPV means "localizer performance with vertical guidance." LPV minimums require that you have a WAAS receiver on board (not at home or on your Christmas wish list). Just to be clear here, you need a WAAS receiver to fly any software-generated glideslope (except for those of you with baro VNAV, who can fly to crummy LNAV/VNAV minimums). An ordinary GPS unit won't do. That's because an ordinary GPS unit can't generate a glideslope, leaving the glideslope needle on your HSI inactive, or off.
If you were cleared for the RNAV Runway 24 approach at Carlsbad, you'd know that LPV minimums were allowed if you saw the letters LPV appear in your WAAS receiver's approach annunciator window. If you saw the letters LPV, then you could descend to a DA of 700 feet if you wanted. The visibility minimums for this approach are 4,000 feet or three-quarter-mile visibility. This approach brings you 300 feet lower and a lot closer to the threshold than the LNAV/VNAV minimums do.
This shows how much improvement WAAS was over baro VNAV in terms of lower minimums. So, see if your chief pilot can find that baro-VNAV receipt. As an aside, most airliners can't take advantage of these lower minimums since they don't have GPS- or WAAS-capable receivers on board. Of course, you probably can't carry 400 passengers.
Keep in mind that for an RNAV precision approach to have minimums as low as 200 above the touchdown zone and one-half-mile visibility, there must be approach lighting available. Not all RNAV approaches are made to runways (or airports) with approach lights (MALSR, SSALR, ALSF), despite the fact that these same approaches may have LNAV/VNAV or LPV minimums. Even without approach lighting, it's possible to have LNAV/VNAV or LPV minimums of 200 feet above the touchdown zone and three-quarter-mile visibility, and that's not bad at all. Without approach lights, however, it's a lot more difficult to actually identify the runway environment for landing.
continued...
