A B737-800 Captain reported that several new aircraft equipped with the HGS 4000 HUD have a Flight Path Vector projection error which indicates the aircraft is on a path laterally displaced from the actual track to the selected runway.

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The Reporter stated that within his Company this HGS 4000 HUD problem has been previously reported but he personally is not aware of the frequency of erroneous Flight Path Vector (FPV) approaches. The FPV Mode uses multiple system inputs to calculate an aircraft projected path toward the selected runway that is subsequently displayed on the combiner and the outside view. A Company directive indicated that if -999' were entered in the FPV airport altitude the error would be corrected; but that procedure has not been valid in the reporter's experience although the error is significantly reduced. On a non precision approach (VOR/LOC/GPS) the horizontal flight path vector point will be over the VASI or about 200' left. The horizontal error during a precision approach is minimal. In both cases the HUD commands and the PFD flight director presentation are correct so the system is not sensing an error large enough for it to display the APCH WARN message which would be indicated when either Approach Monitoring tolerances are exceeded or AIII capability (NO AIII) is lost. The Reporter believes that this HUD should be flight tested for FPV validity and if errors are found; prohibit its use in the situations where errors are discovered. He is very concerned that a crew will not know about the possible errors and use the inaccurate HUD data to attempt; or worse; complete a landing using it.