Report on the serious incident involving a Boeing 747-200 Aircraft, 5N-JRM

On 4th October, 2013 at about 20:59 h, a Boeing 747-200 aircraft with nationality and registration marks 5N-JRM, operated by Kabo Air Limited as a charter (Hajj) flight with call sign QNK617, departed Mallam Aminu Kano International Airport, Kano (DNKN) Nigeria, en-route King Abdul-Aziz International Airport Jeddah (OEJN); Saudi Arabia via Sultan Abubakar Airport, Sokoto (DNSO); Nigeria, on an Instrument Flight Rules (IFR) Flight Plan.
There were 512 persons on board, inclusive of 18 crew members with seven hours’ fuel endurance. The Captain was the Pilot Flying (PF) and the Co-pilot was the Pilot Monitoring (PM).
The flight was originally scheduled to route DNKN to OEJN. While in the cockpit preparing for the flight, the company advised the crew of a new routing from DNKN to OEJN via DNSO to pick additional passengers.
At 21:10 h, QNK617 contacted DNSO Air Traffic Control (ATC) and passed their traffic information. ATC acknowledged and cleared QNK617 as follows: QNK617 cleared Sokoto FL180, no delay. Expect ILS Approach Runway 08. Sokoto 20:00, Surface Wind calm, visibility 10km, Nil Weather, Few cumulonimbus North West at 750 m, QNH 1012.
At 21:12 h, Kano Control released QNK617 to Sokoto Control. At 21:24 h, QNK617 requested descent and was cleared to FL65 and instructed to report 25 DME SOK. At 41 DME SOK, QNK617 was re-cleared to 3500 ft on QNH 1012 and further cleared for ILS approach runway (RWY) 08 on reaching overhead SOK.