FMS limitations leave some operators unable to use LPVs

 - June 2, 2008, 9:33 AM

Although WAAS LPV (lateral precision with vertical guidance) approaches have been popping up at airports around the U.S. at an impressive rate, only a handful of business jets are approved to fly the procedures. That’s because most flight management systems don’t yet support the new type of approach and some airplanes might not be approved to do so without costly upgrades.

At last count the FAA had published 1,051 LPV approach procedures, but operators of many older business jets can’t upgrade their airplanes to take advantage of them. Because only Universal Avionics has so far gained certification for a WAAS-certified FMS, even many airplanes rolling out of the factory aren’t certified to fly the procedures, which provide ILS-like vertical and lateral precision without the need for costly navaids installed on the airport.

What’s frustrating for many operators of large-cabin jets is that VLJs such as the Garmin G1000-equipped Cessna Citation Mustang can fly LPV procedures, and yet their brand-new Gulfstream G550 or Challenger 300 can’t because Honeywell and Rockwell Collins have yet to gain approval for WAAS versions of their respective FMSs. Such approvals are expected to occur eventually, but because each model must be certified separately for LPV, the pace of approvals will be slow.   

The situation is even worse for operators of many older business jets, for which WAAS LPV upgrades might be difficult to certify depending on the type of equipment originally installed. Rockwell Collins, for example, plans to offer WAAS-capable FMSs only in airplanes equipped with its Pro Line 4, Pro Line 21 and Pro Line Fusion integrated avionics systems.

Part of the problem for operators of some older models centers on the fact that the WAAS signal is fed through the localizer and glideslope ports and, consequently, the autopilot thinks it’s flying an ILS approach. On some older EFIS units, ILS would be annunciated on the display even though the airplane is actually flying an LPV approach. The FAA has said LPV and not ILS must be annunciated during the WAAS approach, but on certain EFIS screens that’s not possible because the units were never designed to show LPV.   

“It’s an interesting situation because you’ve got the certification folks at the FAA saying they won’t allow certain airplanes to fly LPV approaches, but then you’ve got the [Aviation Systems Standards] branch that’s putting in LPV approaches as quickly as it can,” noted Dave Pleskac, an avionics salesman for Duncan Aviation in Lincoln, Neb.

Universal Avionics has obtained TSO approval for its line of WAAS-capable flight management systems, as well as STCs in a few aircraft models. Rockwell Collins and Honeywell say they will achieve TSO for their WAAS FMS units later this year and then work with OEMs to bring the capability to production airplanes and, soon after, begin offering individual STC programs for in-service business jets.

Collins has gained TSO approval for its WAAS-capable GPS-4000S receiver, which has been installed in hundreds of aircraft, according to the company’s flight deck systems marketing manager, Adam Evanschwartz. Collins’s first STC program for its WAAS-based FMS-4000A flight management system will be in the FAA flight inspection office’s Challenger 604.

Extending WAAS LPV capability to modern business jets won’t be especially difficult, observers say, and should start happening fairly quickly after more WAAS FMS units are TSO’d. Honeywell and Gulfstream recently secured FAA approval for the PlaneView cockpit system in the G350, G450, G500 and G550 to fly ultra-precise Required Navigation Performance Special Aircraft and Aircrew Authorization Required (RNP SAAAR) approaches. Aircraft flying under RNP SAAAR procedures use global positioning and inertial navigation reference systems to fly predetermined paths that allow for safer, more direct and lower-minimum approaches. “WAAS LPV is far simpler than RNP,” said a Honeywell spokesman, “but like everything else it takes time” to certify.