Figure 4.  A diagram of the EADI.
Note: The diagram shows the glideslope and the fast/slow indicators on the right and left side of the EADI, respectively, which is opposite of the accident airplane‘s configuration.
The EFIS source controller is used to select attitude, heading, navigation, and bearing sources for display on the EADI. The NAV button on the source controller is used to control the source of the very high frequency (VHF) NAV information. When a frequency is entered in the VHF NAV receivers, it is stored in the standby position. The standby frequency can be activated instantly by flipping a toggle switch located next to the frequency display. The display data include the selected course and the course, localizer, and glideslope deviations.
The airplane was equipped with an autopilot/flight director mode selector panel. The APR mode is used during an ILS approach. An ILS frequency must be selected as the NAV source and valid ILS signals must be received for the APR mode to activate. When these conditions are met and the APR button is pressed, both the NAV and APR modes are armed to capture the localizer and glideslope, respectively. When the APR mode is activated, the flight director‘s lateral and vertical modes, including the glideslope and localizer captured and armed modes, are annunciated on the top of the EADI display. According to CVR information, neither the first officer nor the captain was initially able to activate the APR mode on their flight directors.
The airplane was equipped with an FMS that had a multifunction display (MFD), which was located on the center instrument panel and displayed navigational approach waypoints13 that are entered into the FMS by the flight crew. As the airplane proceeds toward the airport, the displayed waypoint numbers change position on the MFD relative to a fixed airplane symbol at the bottom of the screen. An FMS does not provide the minimum altitudes for each navigational approach waypoint and supplements the airplane‘s other navigational guidance instruments.

Ground Proximity Warning System
The airplane was equipped with a Honeywell Mark VI ground proximity warning system (GPWS), which provides aural alerts when thresholds are exceeded in several operational modes.14 Specifically, an aural —too low terrain“ alert will be generated when the airplane reaches a specific allowable altitude for a given airspeed and the landing gear is down. Also, when a valid glideslope frequency has been selected on the captain‘s NAV receiver and the landing gear is down, an aural —glideslope“ alert will be generated and repeated every 3 seconds if the airplane deviates below the glideslope by more than 1.3 dots and if the altitude is below 1,000 feet. The —glideslope“ alert gets louder as the airplane descends below 300 feet. If a valid ILS frequency has not been selected on the NAV receiver, the —glideslope“ alert is inhibited.
In addition, when the airplane descends below 500 and 200 feet and deviates below the glideslope by more than 1.3 dots and a valid ILS frequency has been selected, —500“ and —200“ foot aural alerts will be generated. The GPWS also generates a —minimums“ alert when the airplane descends to the decision height (DH) selected by the flight crew. The CVR did not record any GPWS alerts during the accident flight.
An avionics functional check of the GPWS, which consists of a self-test and audio level check, is conducted by maintenance personnel every 12 months. The last maintenance functional check of the accident GPWS occurred on April 2, 2004, and no discrepancies were noted. According to the director of training at Business Jet Services, company pilots also perform preflight avionics functional checks. The director, who had flown a series of flights in the accident airplane during the 4 days before the accident, indicated that the airplane‘s GPWS tested normally during the preflight checks. He also stated that, within the last year, he had received a valid in-flight —glideslope“ alert while flying low on the glideslope during a visual approach. Other company pilots who had recently flown the accident airplane reported no problems with the GPWS during the preflight checks. According to Honeywell, the only common failure that could prevent activation of the GPWS glideslope and altitude callouts is a radio altimeter failure. No evidence was found indicating that there were any problems with the radio altimeter.
　
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