Rnav advisory circular brings headaches

 - October 2, 2006, 5:32 AM

The FAA’s grandiose agenda to streamline operations in congested U.S. airspace is causing  headaches for business jet operators with flight management systems (FMS) that do not calculate position to new Rnav standards due out this month.

The FAA recently published an advisory circular aimed at increasing airport arrival and departure capacity around the country– and, more important for many business jet operators, reducing departure delays at busy Northeast and Florida airports–by making changes to Rnav routes, SIDs and STARs that would require FMSs to revert to DME/DME or DME/ DME/IRU calculations in the event of a loss of GPS signal integrity.

NBAA has queried the FAA about the apparent technical short- comings of certain FMS equipment. At issue is the GPS predictive RAIM (receiver autonomous integrity monitoring) requirement of AC 90-100, which calls for continued tight track accuracy using the DME/DME or DME/ DME/IRU techniques (depending on the type of procedure) if GPS satellites experience a loss of signal integrity. Not all FMS equipment can do this.

If a prolonged GPS satellite outage (more than five minutes) is expected to affect the route, the operator would not be permitted to accept an ATC clearance for the type of Rnav procedure, SID or STAR outlined in AC 90-100. Because there is often at least one GPS satellite scheduled to be out of service somewhere at any given time, operators flying the new procedures with FMS gear that does not fully support the AC’s requirements would have the option of using a RAIM-prediction software tool before departure to forecast whether the outage might affect their route of flight.

The FAA said that certain Rockwell Collins and Universal Avionics FMS units fully comply as far as GPS updating is concerned, but they do not do DME/ DME or DME/DME/IRU calculations. Rockwell Collins has told operators flying with its FMS equipment that it will make available a PC-based pre-departure RAIM-prediction tool by year-end, which it will provide to customers at no charge. Universal already offers one.

Precisely what operators flying with Collins gear should do between now and the end of the year has been what’s causing the biggest headaches. Roger Southgate, director of avionics certification for Rockwell Collins, said some 240 SIDs and STARs around the country are subject to the new standards. However, provided that there is GPS installed aboard the airplane and RAIM coverage exists, operators of the affected FMS units will be able to fly the procedures without any trouble, he said.

“All of the equipment out there that I’m familiar with today would annunciate a loss of RAIM,” he said, “and over the years we haven’t seen any huge incidence of reports that RAIM has been lost, so we don’t think, from our perspective anyway, this is a huge issue.”

Grady Dees, director of engineering services for Universal, said he agrees with that sentiment, adding that Universal plans to file comments on AC 90-100 with the FAA to voice the company’s displeasure with the wording of the document.

“Our GPS systems are RAIM compliant,” he said. “As long as GPS and RAIM are available, you’re fine. If the system annunciates a loss of RAIM, that’s when you should be concerned.”