When he was distracted during the preflight; the First Officer of a Lear 35 failed to secure all cowling fasteners on the right engine. Flight diverted to the nearest airport to properly secure the cowling.

Narrative

After takeoff we noticed that there continued to be a sign of airframe vibration. We rechecked the flaps and gear to make sure that they were in fact in the stowed position. Both verified to be stowed. While trying to figure out what else it may be the First Officer remembered that he may have forgotten to latch the right hand engine cowl after checking the oil. An in flight visual inspection by the medical crew determined that the cowl was in fact open. We landed at the nearest airport. The after landing inspection showed that all three latches on the inboard side of the engine cowl and only the rear of three latches on the outboard side of the cowl were holding the cowl together. In our preflight duties I file the flight plans; do weight and balance; file the customs paperwork; while the First Officer tows the aircraft to the fuel farm and transfers fuel and does the preflight inspection. When I am done with paperwork I come help fuel the aircraft. In our post flight brief it was determined that the First Officer was checking the oil during the preflight. He stated that he was distracted by me when I showed up at the airplane. When I arrived at the aircraft he was on the other side of the aircraft doing the preflight inspection. I asked if he was ready for my help fueling. He stated that his thought was to answer me and come back to latch the cowl later. We fueled the airplane and towed it back to the hangar for our departure. This situation may have been avoided if I had done a follow up inspection. In the future it would a good idea to have two people do the preflight inspection to try and catch anything that may have been missed.