Background
On 11 June 2015, at about 18:25 h, an HS-125-800XP aircraft with registration N497AG, operated by SWAT Technology Limited, departed Nnamdi Azikiwe International Airport, Abuja (DNAA) for Port Harcourt International Airport (DNPO) as a charter flight under an Instrument Flight Rule (IFR) flight plan. On board were five persons: three flight crew and two passengers. The Captain served as Pilot Flying (PF) and the Co-pilot as Pilot Monitoring (PM).
Flight Sequence
At 18:48 h, N497AG established contact with Lagos and Port Harcourt Air Traffic Control (ATC) units while cruising at Flight Level (FL) 280. At 18:55 h, the aircraft was released by Lagos to continue with Port Harcourt. Port Harcourt cleared N497AG for descent to FL210. At 19:13 h, the crew reported the field in sight at 6 nautical miles to touch down to the Tower Controller (TC). The TC cleared the aircraft to land with a caution that the runway surface was wet.
Approach and Landing
The crew experienced light rain at about 1.3 nautical miles to touch down; runway lights were on for the ILS approach. At about 1,000 ft after landing gear extension, the PM remarked, 'Okay...I got a little rain on the windshield,' and the PF responded, 'We don’t have wipers sir... (Laugh) Na wa o (local parlance for expression of surprise).' On the Cockpit Voice Recorder (CVR), at Decision Height, the PM called out 'minimums' and the PF responded 'landing.' The PM reported that runway edge lights were visible on the left side, but on the right side they were missing to a large extent, appearing only for about a quarter of the way from the runway 03 end. The PM observed the aircraft was slightly left of the centerline and directed 'right, right, more right.' At 50 ft, the PF retarded power and turned left. At 40 ft, the PM cautioned, 'keep light in sight don’t go to the left.' At 20 ft, the PM again said 'keep on the right,' to which the PF replied, 'Are you sure that’s not the centre line?'
At 19:16 h, the aircraft touched down with the left main wheel on the grass and the right main wheel on the runway, then was steered back onto the runway. The PF later stated that on touchdown the right-hand lights were out, and in attempting to align with the lights, the aircraft veered off the runway to the left. The nose wheel landing gear collapsed, and the aircraft stopped on the runway.
Aftermath
Engines were shut down, and all persons on board disembarked without any injury. From CVR recordings, the PF told the PM he mistook the brightly illuminated left runway edge lights for the runway centerline and apologized for the error; the PM responded 'I told you.' The aircraft was towed off the runway and parked at GAT Apron at 21:50 h. The accident occurred at night in light rain.
Official Probable Cause
The investigation determined that the accident was the consequence of a black hole effect disorientation causing a low-level manoeuvre into the grass verge. Contributing factors included most of the runway 21 right edge lights being unserviceable at landing time and inadequate Crew Resource Management during the approach.