PILOT TECHNIQUES

Understanding and equipping for different GPS-based approaches

What you need in the cockpit along with training to take advantage of the latest precision space-based landing systems.


LNAV/VNAV origins

Universal's latest flight management system—the LPV-capable UNS1Ew.

It has been accepted for some time that stabilized approaches are much safer and easier to execute than "dive-n-dive" style NPAs. Initially, the only means available for establishing vertical guidance on an NPA was to use a multisensor FMS with a barometric altimeter input.

While not perfect—the vertical guidance was subject to temperature restrictions—it did allow a stabilized descent to minimums.

Now that WAAS (and other space and ground-based augmentation systems) are available, the GPS can provide better vertical guidance than a baro-aided FMS so all the newer GPS systems can allow the pilot to fly an LNAV/VNAV procedure.

Doing so, however, is a bit like driving a Ferrari to the supermarket 4 blocks away—doable but not much fun. It's better to use the augmented GPS to fly an LPV approach to get the maximum benefit.

Furthermore, if the Aeronautical Charting Forum recommendations are implemented, most of these problematic LNAV/ VNAV procedures will be eliminated.

Another alternative recommended by many experts is to fly the LNAV/ VNAV procedure but to use the LNAV MDA. (It is important to note that this can only be done when there are no step-down fixes between the final approach fix and the runway threshold and the glidepaths are coincident!) Be careful, as the MDA is not a DA. You must arrest your descent before reaching the MDA so as not to go below the MDA. This will provide the desired vertical guidance.

Until this charting issue can be resolved, all pilots should be aware of the potential for the LNAV/VNAV minima to be higher than an associated LNAV-only procedure and the reasons for this. Remember, in the ongoing battle between objects made of aluminum going hundreds of miles per hour and the ground going zero miles per hour, that the ground has yet to lose.

Equipping for GPS approaches

Obviously, you will need a GPS if you want to fly a GPS procedure, but there are many different flavors of receiver.

The first GPS systems were for VFR use only. First-generation IFR-capable GPS navigators were restricted to enroute and terminal use only. The second generation, certified to TSO-C129, permitted nonprecision GPS approaches, but FAA initially restricted them to "overlay" procedures and the under­lying traditional approach aids and onboard equipment (VOR, NDB, etc) had to be functional.

Once the industry and FAA gained some experience, stand-alone nonprecision GPS approaches were allowed. The newest-generation systems allow LPV approaches but equipment certification to a new TSO is required. TSO-C146 is the standard for the equipment that provides the guidance to fly an LPV procedure, such as a panel-mount box.

TSO-C145 is the standard for a WAAS augmented GPS receiver that provides position, velocity and time (PVT) information only, so an FMS is required to compute the guidance to fly the procedure.

If your aircraft has an integrated cockpit, like those based on the Honeywell Primus Epic or Primus Apex or Garmin G1000 systems, you will have to coordinate your GPS upgrade with your aircraft manufacturer.

Most airframers offer an upgrade path supported by their avionics partners. If your cockpit is a federated system with individual displays and black boxes, you may have more options that include going to an independent avionics shop. For all upgrades, the GPS antenna and (usually) its associated cable will have to be changed from a passive antenna to an active one with a built-in low-noise amplifier.

Garmin's panel-mount WAAS navigators.

Honeywell's latest systems based on its Primus Epic platform incorporating WAAS/LPV capability are either shipping now or will be soon, depending on the aircraft you have ordered.

Honeywell Senior Mgr Technical Sales Ed Borger says that existing Primus Epic systems without LPV can be upgraded through the aircraft manufacturer. Other than new antennas, as mentioned above, the GPS and FMS cards just need a firmware upgrade.

Earlier SPZ8000 systems, such as those in the Challenger 601, can fly LNAV and LNAV/Baro-VNAV procedures now, but they need to have a new 24-channel GPS and a new software load in the FMS in order to fly LPV procedures.

A little mode annunciator panel is required as well. Primus 1000 and 2000 systems need a new GPS receiver and a new firmware load for the FMS. The mode annunciators will show up on the PFD as part of the firm­ware upgrade.

These Primus 1000 and 2000 systems and the SPZ8000 systems can usually be upgraded by Honeywell dealers.

Rockwell Collins Principal Market­ing Mgr Adam Evanschwartz explains that his company's newest Pro Line Fusion system, launched on the Bombardier Global Express XRS, will be capable of flying all GPS procedures, including LPV, out of the box.

With very few exceptions, Rockwell Collins' existing Pro Line 4 and Pro Line 21 systems have the GPS4000—a TSO-C129 GPS—providing PVT data to an FMS. Consequently, they can fly the LNAV and LNAV/Baro-VNAV procedures.

The upgrade to fly LPV procedures is fairly straightforward. Replace the GPS4000 with a GPS4000S—a TSO-C145 system—and update the software in the associated FMS. Depending on your aircraft model, the upgrade is available either through the Rockwell Collins dealer network or the aircraft manufacturer, so the best place to start is your friendly neighborhood dealer.

Universal Avionics Mgr Airworthiness and Product Management Paul Damscher says that the company's previous-generation GPS-based FMS units are capable of flying LNAV and LNAV/Baro-VNAV procedures. Flying GPS LNAV/ VNAV and LPV procedures requires an upgraded unit with a built-in WAAS GPS.

In a dual installation, each unit provides a crosscheck for the other when flying LPV procedures and drops the CDI/VDI flags if there is a problem. In a single unit installation, a small hidden unit—the LPV integrity monitor—performs the required crosscheck. Minimal wiring modifications are required in a standard installation.

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