The crew reported for duty at Belfast City Airport at 05:50 hrs UTC to carry out a four-sector day flying to London City Airport and Inverness Airport. The first two sectors to London City and returning to Belfast City were flown without incident and with all the aircraft equipment and systems operating normally. Following a normal turnaround and flight preparation, the aircraft was refuelled and the passengers boarded. The weather was good with a surface wind of 260ø at 12 kt, CAVOK with a temperature of +8øC and a dew point of +4øC. The aircraft was started and taxied for runway 22, from which it departed at 11:18 hrs. Shortly after takeoff, the landing gear selector lever was selected to the up position. On the Landing Gear Control and Indication Panel (LGCIP) the crew observed that the landing gear green lights extinguished, the three red lights and the amber landing gear door lights as well as the amber light in the landing gear selector lever all illuminated. After a short time, all three red lights extinguished, the left and right door lights extinguished but the nose door light and the light in the landing gear selector lever remained illuminated. The crew kept the airspeed below the 185 kt landing gear limit speed and climbed to 4,000 feet, routing to waypoint Magee to take up the hold whilst they assessed the problem. The aircraft entered the hold at 1132 hrs and the total fuel onboard was 2,800 kg. Whilst in the hold, the crew initially actioned the ‘LANDING GEAR FAIL TO RETRACT’ abnormal checklist followed by the ‘LANDING GEAR FAIL TO INDICATE LOCKED DOWN’ abnormal checklist. The MLG lowered and indicated locked down, but the NLG showed the unsafe indication with the amber nose door light still illuminated. The landing gear inoperative (ldg gear inop) caption also illuminated on the Caution and Warning Annunciator Panel. They actioned the ‘ALTERNATE LANDING GEAR EXTENSION’ abnormal checklist and made several attempts to operate the nose landing gear alternate release handle but the indications remained the same. They reviewed the ‘EMERGENCY LANDING – ONE OR BOTH ENGINES OPERATING’ abnormal checklist and decided to divert to Belfast International Airport. At this stage, the crew did not know if the NLG was up or down. The crew sought advice from their company and this confirmed their decision to go to Belfast International Airport. They reviewed the fuel and decided to leave the hold with 1,100 kg which would minimise the fuel onboard at touchdown but ensure sufficient fuel to carry out a go-around if required. The Senior Cabin Crew Member (SCCM) was called to the flight deck and the situation explained to her along with a briefing which included the procedure for an emergency evacuation, should it be required. Passengers seated adjacent to the propellers were moved to other seats away from the possible arc of any debris in case the propellers should contact the runway. Following this, the commander briefed the passengers on the problem and his intentions using the PA system. The cabin crew played the pre-recorded passenger emergency briefing and then walked through the cabin ensuring the passengers were all aware of what was required and that their restraint harnesses were secure. The passengers at the rear of the aircraft were warned of a possible increased drop to the ground from the rear doors due to the nose-down attitude if the NLG was not lowered. At Belfast International Airport, runway 25 was in use, the surface wind was 250ø at 12 kt, visibility more than 10 km, clouds few at 1,800 feet and scattered at 2,900 feet, temperature +12øC, dew point +3øC, and the QNH 1020 hPa. The crew briefed for a radar vectored ILS approach for runway 25 with full landing flap and an approach speed of 110 kt. They also reviewed the emergency landing actions and reminded themselves where the CVR/FDR circuit breakers were located in order to ensure they were pulled after landing. The aircraft left the hold at 13:20 hrs with 1,100 kg of fuel as planned and commenced the approach. All the normal checks were carried out and at about 4 nm, ATC informed them that the nose landing gear had not extended. The approach was continued as briefed and at 200 ft, the co-pilot gave the ‘BRACE’ command over the PA and all the passengers were seen to adopt this position. The aircraft touched down at 13:32 hrs on the main wheels and the nose was held off as the speed decayed and gently lowered onto the runway. As the aircraft came to a stop, both engines were shut down and when stopped, the commander ordered the evacuation. Conclusion The investigation concluded that mechanical damage within the electrical harness of the primary ‘nose gear lock’ proximity sensor caused an intermittent and erroneous sensor state change during landing gear retraction. The measured inductance value associated with the sensor state change was not sufficiently high for the sensor to be flagged as faulted, and the erroneous state change was therefore considered valid. This had the effect of interrupting the NLG retraction sequence by causing the forward NLG doors to close prematurely while the NLG was still retracting, such that the tyres came into contact with the doors. When the NLG finally retracted, the tyres became jammed in the NLG bay, preventing it from extending when subsequently commanded. The flight crew followed the appropriate procedures for dealing with the incident, which led to the safe landing and evacuation. Prior to this accident, the failure mode of an erroneous sensor state change below the threshold at which the sensor would declare itself faulted, had not previously been identified. The investigation determined that the harness had been secured by a non-flexible cable tie, which restricted it from flexing during normal operation of the nose landing gear. This created loading cycles sufficient to create a cyclically-driven fatigue failure mechanism in the two conductor wires within the harness. The aircraft manufacturer has taken action to clarify nose landing gear proximity harness routing and attachment instructions in relevant AMM tasks, and has published inspection requirements. The aircraft and landing gear manufacturers are working to identify a more flexible harness design.