Monitors Watch SmartView Lower Minimums
Honeywell’s SmartView Lower Minimums (SVLM) must be able to show precisely where the aircraft is, without the use of additional navigation signals from transmitters on the ground, as well as tell the pilot when a system malfunction makes the lower-minimums approach unsafe. Honeywell uses five monitors to ensure the integrity of the system and the aircraft’s position.
* Runway-data-integrity monitor. “The current runway database in the FMS does not have the integrity to monitor itself,” said Honeywell engineer fellow Thea Feyereisen. Honeywell has an independent runway database (RAS) from its EGPWS. The SVLM compares these databases,” to make certain the runway-approach indicator (see box “SVLM Symbology”) is properly aligned with the runway.
* Delta-position monitor. One of the reasons LPV 200 approaches stop at 200 feet decision altitude is because it takes 6.2 seconds for the satellite to tell you it has a bad signal, Feyereisen said. “So your GPS signal can go bad and it’s going to take six seconds for the GPS to flag it,” she continued. “For a Category II approach, the time to alert is two seconds. So we developed a monitor based on our [Laseref] initial reference system [IRS]. The IRS uses pure inertial signals from the unit to calculate the error. We compare our IRS positions to the GPS position so that we can tell if the positions veer off from each other.” If this happens, a warning appears on the SVLM display telling the pilots they cannot go lower than 200 feet.
* Altitude monitor. Honeywell wants SVLM to be approved for both lower decision altitudes (DA, based on barometric altitude) and decision heights (DH, above the ground), which require a radar altimeter and a special survey. “We know pilots set the wrong barometric setting, or maybe the tower gives them a wrong baro setting,” Feyereisen said. “The altitude monitor compares altitude across three sources: barometric, GPS and radio altimeter versus terrain database. All must align to use 150 feet DA.”
* Virtual inner marker (VIM). “This is the missed-approach-point monitor, which is also a supplemental way to measure DA. So whether you reach your DA or the virtual inner marker, which measures the distance from the runway via GPS, your ‘minimums’ alert comes up,” said Feyereisen.
* Pilot monitor. This approach-deviation monitor makes sure the pilots are staying within a finely defined corridor on their approach path, and “will alert the crew to flight technical errors, or in other words, a poorly flown approach.” After seeing the alert, the pilots can continue the approach, but use the 200-foot DA or DH, instead of 150 feet, or fly a missed approach. “We try to follow the Cat II framework for procedures,” Feyereisen added.