Date: 201211
Local time of day: 0601-1200
Location: BHM.Airport, AL
Flight conditions: IMC
Weather elements / visibility: Fog; Rain; 10
Light: Night
Aircraft: Beechjet 400
Operator: Fractional
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class C BHM
ATC / advisory: Tower BHM
Aircraft component: GPS & Other Satellite Navigation
Component problem: Malfunctioning
Reporter organization: Fractional
Reporter function: Pilot Not Flying; First Officer
Reporter qualification: Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Engineer
Human factors: Training / Qualification; Situational Awareness; Confusion
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Automation Aircraft Terrain Warning
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Executed Go Around / Missed Approach
Contributing factors: Environment - Non Weather Related; Procedure; Weather; Aircraft
Primary problem: Aircraft

Narrative: We had conducted a GPS approach to BHM in accordance with the given wind and prevailing visibility conditions at the time. The approach was conducted in accordance with all directives and procedures. We had expected to break out much earlier; but failing that continued the approach down toward our minimums. At approximately our MDA; I saw what I took to be the overrun markings of the runway. Within seconds I realized that it was not the runway; but a building! At approximately the same time the GPWS warning sounded and we conducted an immediate go-around; both puzzled by the warning; but nonetheless committed to the go-around. Following all procedures we executed the maneuver and subsequently conducted a successful ILS approach to a different runway. As all professional pilots will attest; we later dissected the unusual event that had potential for catastrophe. After all means available to us we concluded that we had followed all parameters for the approach; but for some reason unknown to us the aircraft was not in the position that it thought it was. It was also apparent that the GPS performed correctly and prevented and incident or worse. A contributing factor was that the weather was not at all what had been reported at the time we began the approach; but had deteriorated substantially; with no update by the Controller. The reason for this deviation from a safe approach still eludes us.

Callback: The reporter stated that the RNAV 36 Approach was very precisely flown based on the aircraft's GPS instrumentation but some unknown error resulted in the aircraft being 200 FT low and about 1/8 of a mile left of the runway when they visually acquired the airport area. What they saw appeared to be the runway end overrun lines but because of the GPWS warning the crew sensed an error and so executed a go around followed by a Runway 24 ILS. The aircraft's navigation systems did not alert an error so the system thought that it was where it was supposed to be. The next morning under clear skies; they drove to the general area where they believe they broke out following the GPS approach and saw small terrain rise and beyond it a series of buildings which at night in the fog and rain could appear to be runway markings; but were in fact large buildings perpendicular to the runway.



Synopsis: A GPS error placed a BE400 200 FT low and 1/8 of a  mile left of BHM Runway 36 where a GPWS alerted; so a go around was executed followed by a Runway 24 ILS approach.