Date: 201305
Local time of day: 1801-2400
Location: BWI.Airport, MD
Flight conditions: VMC
Weather elements / visibility: Thunderstorm
Light: Night
Aircraft: HS 125 Series
Operator: Fractional
Operating under: Part 135
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B BWI
ATC / advisory: Tower BWI
Aircraft component: ILS/VOR
Component problem: Malfunctioning
Reporter organization: Air Taxi
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Confusion; Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; General Maintenance Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: We were cleared for a night visual approach to RWY 33R at BWI.  I was the flying pilot (SIC) in the left seat. The FMS was properly programmed for the 'back up' ILS 33R instrument procedure.  NAV 1 and NAV 2 autotuned to the correct localizer frequency of 111.95.  Due to showers in the vicinity of the airport; we were on a dogleg left base flying direct to ORIOL; a fix on the FAC [Final Approach Course] about 6 NM out.  Descending from 3;000 feet to 2;000 feet; we acquired the airport visually and received clearance for a visual approach to RWY 33R.  Approach was armed.  We were approximately 9 miles from the airport.  I disengaged the autopilot and began hand flying the aircraft.  At 2;000 feet overhead ORIOL the Approach Mode captured on the FD.  We acquired the localizer and the glide slope.  Just inside ORIOL; both PFD's showed that we were on course and on glide slope.  We were also stable on the RWY 33R PAPI with our heading very close to the FAC (little wind).  Shortly thereafter; the localizer indicated that the aircraft was very slightly right of course.  Visually we appeared OK; but I altered our heading slightly to the left.  The localizer showed us tracking further to the right of course.  The PIC commented that it was night and that we had better honor the localizer.  I concurred.  I turned further left to make a more aggressive correction.  The localizer continued moving left and we continued correcting left.  We doubled checked that the localizer was tuned for RWY 33R and that RWY 33R was loaded in the FMS.  The instrumentation set up was correct.  Localizer & glide slope reception were green.  There were no yellow or red fail indications.  At approximately 3 miles from the field we visually confirmed that the localizer was taking us toward RWY 33L.  As we began visually maneuvering back to the right and disregarding the localizer; the BWI Tower requested that we confirm our landing runway as 33R.  We did.  The remainder of the approach and landing were uneventful.  We were baffled.  After our passengers were escorted to the FBO; we triple checked the instrumentation.  The set up was correct.  We even took pictures of the PFD; FMS Legs Page and the Tuning Page.  Again; all was proper.  I called Ground Control to inquire about the working status of the localizer equipment and any interference issues.  Ground Control reported that the RWY 33R localizer was operating normally and that the Tower was unaware of any interference issues.  The Controller did state that it had been awhile since an aircraft had flown the ILS 33R Approach.  The PIC initiated maintenance action for the NAV1 and NAV2 equipment.  The next day; my PIC also discussed the matter with the Chief Pilot.  The PIC and I discussed several potential issues/causes:  aircraft equipment failure; ground equipment failure; interference from aircraft or vehicles moving on the ground; unintentional frequency interference; FMS software malfunction; weather; deliberate third party disruption of the localizer signal; etc.  The combined flight deck experience of the crew is over 50 years.  Neither of us had ever seen a localizer behave in this manner before.  Fortunately; we were VMC and no other aircraft were on the parallel approach to RWY 33L.  Had we been actual IMC with other traffic on the parallel approach; it is hard to contemplate how poorly this situation could have turned out.  Since we were VMC; we should have more quickly sought outside assistance from the Tower to help resolve the conflict between our instrumentation and our visual reference.  The situation was complicated by the PM performing the Before Landing Check during this phase of flight.  His head was inside and outside as he executed his 'flow' and completed the checklist.  Initially; both our instincts were to follow the localizer.  Since I was flying and looking outside; a conflicting mental picture more rapidly appeared in my mind.  Quite naturally; the PM accepted the localizer fora longer period of time.  Good and open communications rapidly flowed back and forth between both pilots.  After some convincing; the PIC/PM agreed that the visual reference was correct.  Night time and overcast skies prolonged the conflict between instrumentation and visual references.  Please look into this matter.  For the first time in my flying career; I chose to disregard what appeared to be properly operating instrumentation in an aircraft.  Many times I have abandoned approaches for equipment failures in the aircraft; malfunctions with ground based gear or 'identifiers' that became inaudible.  In this case; there were no failure indications.  From day one; pilots are taught to believe the instruments.  Both of us are wondering what in the world caused this event.




Synopsis: Hawker 900 Captain experiences a false localizer signal during a night visual approach to Runway 33L at BWI.  The Tower reported no faults with the localizer and the the crew could find no faults with the aircraft or the set up.