Date: 201509
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Weather elements / visibility: 10
Light: Daylight
Aircraft: BAe 125 Series 800
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: INS / IRS / IRU
Component problem: Malfunctioning
Reporter organization: Corporate
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Flight Engineer
Human factors: Troubleshooting; Confusion; Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Diverted; General Maintenance Action
Contributing factors: Procedure; Human Factors; Aircraft
Primary problem: Procedure

Narrative: HS-125/800.  The first leg of the flight was ZZZ to ZZZ1 to pick up passengers. We taxied out of the ramp. We then noticed headings were not in agreement on taxi out. As we could not find a slave/Directional Gyro (DG) switch we decided to come back to the ramp and get assistance. The Mechanic came out and helped us by shutting down the IRUs and got the compasses to agree on a proper heading. The aircraft then departed for ZZZ1. The aircraft remained on APU on the ground. On departure ZZZ1; ATC said that we were not tracking directly to waypoint ZZZZZ and we saw that the LNAV mode had not engaged. The flight continued and we used the heading mode to track the course. As we got near ZZZ2; we noticed that the wind correction was becoming substantial. All HSI and Multi-Function Display (MFD) compass headings were in agreement. There were no warning or comparator lights on. The flight was cleared to ZZZ3 from about the ZZZ2 beach area and was heading out over the water.The Controller queried us about our heading for ZZZ3. We made a few corrections and he did not like our headings. Still at this time all HSI and MFD readings were in agreement. He [ATC] said our heading should have been about 170 and we were looking at a heading of about 130. We had a brief cockpit discussion after referencing the whiskey compass and decided to not go out over water with the worsening condition. We told ATC that we wanted to divert to ZZZ4. They turned us and descended us. We could see the coast and were given suggested headings. We continued as a no-gyro approach in visual conditions with vectors to Runway 10L at ZZZ4 and landed. An Avionics Technician met the aircraft and tried to diagnose the problem. He restarted the system and saw 5-6 degrees of drift over 20-minutes on the ramp.The aircraft was powered down overnight and restarted later the next morning by a Honeywell Avionics Technician from ZZZ5. It was powered by Ground Power Unit (GPU) and later the APU. No further anomalies were noted. A flight check was conducted. All compasses; HSI/MFD were in agreement. All LNAV functions engaged normally. It is possible the IRUs did not like the initial set up and began to drift out of alignment over time. We believe that a complete system reset may have resolved the issue. However; neither pilot had ever seen this before and felt that this was nothing we would be able to correct in flight. Thus the decision to land the aircraft was made as a precautionary measure. Both pilots were new to the aircraft as contract pilots for the day and had no history on the aircraft prior to the flight.




Synopsis: After returning to gate for compass headings that were not in agreement and restarting their IRUs; a contract pilot describes an inflight event that followed where a 40-degree difference in their Hawker-Siddeley HS-125-800 track headings were noted by an ATC Controller; even though the aircraft's HSI and Multi-Function Display (MFD) compass headings were in agreement. Flight diverted. Aircraft was powered down overnight and complete system reset appeared to resolve the issue.