A series of flight tests were conducted to design and evaluate a Combined Vision System (CVS) that integrates a forward looking infrared video image with synthetic vision on a primary flight display. System features included colorizing the video image to mesh with the synthetic terrain background, decluttering the approach symbology to facilitate the detection of the approach lights and runway markings, creating a semi-transparent IR sky to ensure continuous situational awareness of the surrounding terrain, and annunciating the decision height to facilitate the transition to the actual runway environment. Over 100 approaches were flown during three flight test sessions. For the first flight test session pilots reviewed early CVS proofs of concept on Honeywell's Citation Sovereign. During the approach in low visibility conditions, the Pilot Flying remained head-down to 100 ft AGL, at which time he lifted his head and made a subjective judgment of whether he could easily and safely complete the transition to land before making a go-around. In the second flight test session enhancements included IR image coloring, IRS/GPS navigation system integration, and display annunciations. The series of flight tests culminated in a CVS integration on Honeywell's Gulfstream G450 aircraft for a direct head-up display (HUD) versus head-down display (HDD) comparison of the IR imagery. The HUD location is currently the standard for low visibility approaches with IR imagery. The G450 evaluation had three highly experienced pilots with an average of over 12,000 flight hours and over 2,500 hours with a HUD. They flew a total of 46 approaches, most to full-stop landings and many were in high workload conditions - low visibility weather or strong crosswinds. Again the Pilot Flying stayed head-down to 100 ft AGL and then transitioned to the outside view of actual runway environment before landing. Pilot performance with the CVS was equivalent to performance with the HUD on all flight parameters including glideslope deviation, airspeed deviation, configuration to land at the crossing threshold, and the landing footprint on the runway.Workload scores and display ratings were equivalent between the two displays, giving a strong indication that the Honeywell CVS provides equivalent performance and an alternative means to the HUD for displaying the IR imagery.