Shortly after take-off the pilot noticed a 10 to 15 centimetre gap between the forward ends of the upper and lower cowlings on the left engine. He elected to continue the flight at reduced airspeed and engine power. About 15 kilometres from the destination the upper cowling became detached and struck the horizontal stabiliser. It remained wrapped around the stabiliser and resulted in severe vibration and a temporary loss of elevator control. The pilot was able to regain control, and during the turn onto a long final approach elevator control returned to normal when the engine cowling fell free. The cowling did not become detached until well after DEPARTURE. It was likely that an uneventful landing could have been carried out had the pilot returned to the DEPARTURE aerodrome as soon as he noticed the problem. The detached cowling was not found and the reason for the failure of the latches to hold it in place was not determined. However, the surface of the DEPARTURE strip was reported as rough and the aircraft had flown about 12 hours since the cowlings were last disturbed.