Taking Synthetic Vision To The Next Level

By John Croft
Source: Aviation Week & Space Technology

Carrico says the certification process is “complicated” as certain elements of testing may not have to be shown since the company has already certified its head-up guidance system (without synthetic vision) for 150-ft. minimums and 1,400-ft. visibility via FAA Order 8400.13D for airline customers. The company is using system performance standards created by RTCA Special Committee 213 in 2011 (DO-315B) for reduced minimums to Cat. 1 ILS runways. RTCA plans to publish standards for reduced minimums for LPV approaches by the end of 2014.

“We believe we can get through the process in the next year or two,” he says of the timing. Beyond that, he says, the company will “likely” seek landing credit for LPV approaches. “Once we establish performance requirements, then we will evaluate which technologies and sets of technologies can get us further credit. It's premature to be picking those technologies just yet.”

The typical issue with the LPV approach in terms of synthetic vision for credit is that “you're using the same source for navigation and positioning,” says Sarah Barber, a principal engineering manager with Rockwell Collins and member of RTCA's Special Committee 213.

Honeywell is confident it has already come up with the necessary solutions for using SVLM on both the ILS and LPV approaches. The company officially started its one-month proof-of-concept project with the FAA's Long Beach, Calif., aircraft certification office on May 22 using N966E.

The pilot's interface to SVLM on the primary flight display is the Runway Approach Indicator, a suite of primary-flight-display cues that use first principles and visuals to help navigate to the runway end, and a “level of service” monitor to indicate the health of the SVLM system. Running in the background are five new monitors that validate the information and allow the synthetic scene to be used for navigation and lower minimums.

En route to Tucson International at 15,500 ft., we set up the Dassault flight-management system to fly the LPV approach to Runway 11 Left. The level-of-service monitor in the top right corner of the display has green text that lists the type of approach, the unique identifier for the approach, and a label that reads either “SVLM” or “NO SVLM.” In this case, “SVLM” was written in green text, meaning the approach was usable. An audible “No SVLM” or its accompanying text in an amber box in the level-of-service monitor means the approach must either be abandoned or flown as a normal ILS or LPV. Below the normal ILS or LPV minimums, the box turns red and pilots must fly the missed-approach procedure.

As Wyatt puts it, there are “five ways to make the SVLM go amber or red”—a fault in any of the five monitors. The runway-data-integrity monitor compares runway coordinates from two disparate databases to make sure the runway approach indicator box is correctly aligned to the runway; the altitude monitor compares barometric-, radio- and GPS-based altitude to protect against a bad barometric or GPS altitude; the missed-approach monitor identifies the missed-approach point by its distance from the runway rather than a barometric altitude; the approach-deviation monitor protects against crew flight technical errors, and the delta-position monitor compares the path along the ILS or LPV approach being flown with the aircraft's inertial reference system (IRS) baseline path.

The IRS reference path is key to Honeywell's approach to gaining lower minimums. With LPV and Cat. 1 approaches, the FAA allows for a 6-sec. lag between the time the ground or satellite-based approach guidance has a fault and becomes unusable to the time the aircraft must be alerted. For minimums lower than 200 ft. the FAA requires a 2-sec. notification time however, meaning LPV or Cat. 1 ILS beam data cannot be used. Honeywell solves the problem by decoupling the GPS data from the IRS after calibrating and monitoring the IRS system starting at about 4 nm from the runway end for period of 12-23 sec. After that time, the runway-approach-indicator guidance is based on the decoupled IRS output.

Although the autopilot system will continue to use the ILS or LPV data for its reference, the delta position monitor as an independent check will catch errors within the 2-sec. requirement and alert the pilot with a “NO SVLM” message.

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