The cannon effect is triggered by the FSX APU START command. Have you assigned a key or key combination to this command?
Also, you say this happens “from start to engine off”. You should not be able to use the effect when the landing gear is down, so something is wrong here. Are you using any other third-party fx?
Win 10 Pro | P3D v4.5 | AMD Ryzen 3 3.5 GHz | 32 GB RAM | NVIDIA GT1030 | TM HOTAS Warthog | TM HOTAS Cougar