What happened
On July 23, 2015, an Israel Aircraft Industries 1125 Westwind Astra, registration N765A, was performing a business flight from Baltimore-Washington International Thurgood Marshall Airport (BWI) to Chicago Executive Airport (PWK).
Prior to departure, the pilot monitoring closed and locked the main entry door (MED), visually verifying that the handle pin was engaged in the handle lock. During engine start, taxi, and takeoff, the cockpit's red cabin door warning light and amber door seal caution light remained extinguished. However, immediately after takeoff, the flight crew noticed that the red CABIN DOOR light, amber CABIN DOOR SEAL light, and the MASTER CAUTION light had illuminated.
As the pilot flying maneuvered the aircraft to return to BWI, the MED opened in flight. The crew declared an emergency but was unable to hear the air traffic controller's response due to the high noise levels caused by the open door. The crew was eventually directed to land via light gun signals. Upon landing, the open door made contact with the runway, though it remained attached to the airframe. There were no injuries to the two pilots or the five passengers.
The investigation
Investigators examined the airframe and found substantial damage to both the MED and the surrounding structure.
Testing of the door mechanism revealed that while the door remained secure when the locking pin was fully engaged in the stirrup, the self-locking air springs failed to function. These air springs were designed to provide an over-centering force to pull the handle into the locked position. When tested with mock-up air springs, the self-locking feature operated normally, but the original springs exhibited no output force.
Additionally, testing of the cockpit annunciator system showed that the CABIN DOOR light would not illuminate if the locking pin was merely resting on the stirrup rather than being fully engaged. This meant the crew was not alerted to the unsecured door status during pre-flight checks.