Falcon 20FJF/20C/20D/20E/20F
Flight Crew Multiengine
Flight Crew Last 90 Days 25; Flight Crew Total 9575; Flight Crew Type 1307
Aircraft Equipment Problem Critical
Flight Crew Landed As Precaution; General Maintenance Action
Aircraft
After the passengers departed the aircraft at their destination; I shut and latched the cabin door; and confirmed the door to be secure by pulling the lever to make sure it was snug and verified cabin door light was extinguished. All the latches were visually engaged.  At this point there was no reason to question whether the door was not latched securely.  We then departed VFR on a 16 minute flight back to base.  The takeoff was uneventful and the weather was clear skies with a light wind.  After 10-12 minutes we had the airport in sight visually and started our descent.  Soon after the descent was initiated; I felt a change in cabin pressurization.  A quick scan of the instrument panel revealed no indicator lights; but the cabin VSI was showing a higher than usual descent rate.  Suddenly there was an extremely loud 'boom.'  At this point; I assumed it was the cabin door without any way to verify.  The aircraft remained stable.  I verified all flight controls were normal. I made the decision to land at destination considering it was the nearest airport.  I landed the aircraft without incident. After the door was located; a visual inspection was done and it was determined that the door had unlatched due to the locking mechanism for the exterior door handle not being fully engaged which in turn; released the exterior door handle from the stowed position. Because of air load on the exterior door handle it opened the door and departed the aircraft. Let me be very clear; all the door latches were fully engaged. It was the exterior door handle that was not fully latched. In talking with a Mechanic he told me he has seen this more times than he cares to mention with Falcons coming into maintenance with their exterior door handle in the unlocked position. I was one of the unlucky ones that it actually opened the door.  The cabin door indicator light switch was also found faulty. There are two switches for the door.  One for the lower door latches and one for the exterior door handle.  This was the one at fault.  My corrective action would be to check to make sure the light is working properly and illuminated when the door is open as a prevented measure.
We departed VFR for a normal 16 minute VFR flight. Approximately 10 minutes into the flight; we were starting our descent and somewhere between 7;000 - 9;000 MSL we felt the pressure in our ears start to change. At the time I started to evaluate the cabin pressurization gage; I noticed a higher than normal rate of descent within the cabin. Within 30 seconds after noticing the pressure within my ears start to change we heard a very loud explosion; which was the main cabin door departing from the aircraft. We never had a door light or a cabin light signaling a issue on the caution panel. After evaluating that the airplane was still flyable; we finished the approximate 15 mile flight where we had a non eventful landing.
DA20 flight crew experiences the loss of the main cabin door in flight at low altitude and continues to destination for landing. When the door was recovered it was determined that the external door handle had popped out in flight and had been activated by the slipstream.