A POLICEMAN who masterminded the escape of a suspect from custody and instructed an armed gang to carry out the well-thought out plot was given a heavy jail sentence in the High Court yesterday. Deputy Judge Cheung sentenced Wong Chi-yun, 28, to 18 years' jail. During sentencing, the judge said it was the Hong Kong police's duty to maintain law and order, yet Wong chose to associate with criminals and carried out criminal activities. Wong not only disgraced himself, the judge said, but the entire police force had been brought into disrepute through his behaviour. Wong, who joined the force in 1984, was found guilty after trial by a jury on five conspiracy charges - one of conspiracy to help Lo Sai-on escape from custody, two of conspiracy to rob two police officers of their revolvers and two of conspiracy to falsely imprison two officers. The jury acquitted him of a third charge of conspiracy to falsely imprison a policeman. During evidence led by the prosecuting counsel, Stevan Cole, the court heard Wong often socialised with Lo, who was arrested on July 20, 1991, for breach of a supervision order. While Wong did not actively take part in the actual escape, he planned it using inside information, the court heard. Wong suggested Lo feign illness so he would be taken to hospital, as it was easier to free him from there. Lo was taken to the Prince of Wales Hospital. The first escape attempt took place the day after Lo's arrest, but it failed because the gang Wong had organised were not properly prepared. The gang decided to try again the next day, and all went according to plan, the court heard. In the early hours of July 22, the gang stood in wait for two policemen who were escorting Lo to the toilet. Two members of the gang accosted the policemen, took the revolver from one and freed Lo, the court heard. They handcuffed the officers together and tied them to a water pipe. A third member of the gang confronted another policeman in a waiting room and stole his service revolver, Mr Cole told the court. The four men then fled to a safe house in Shui Tau village, New Territories. The court heard police launched a raid on the house five days later. Shots were exchanged and a gang member and Lo were shot dead. Three accomplices, Mak Chan-pui, Nguyen Van Trong and Man Hung-biu, gave evidence against Wong, saying he plotted the escape. Wong, represented by Andrew Kan, did not testify in his defence.