Bell 412 EP Cargo Door Window Detachment During Ferry Flight (ZS-HNB)

Incident Overview and Flight Details On 20 October 2021, a Bell 412 EP helicopter, registration ZS-HNB, was conducting an international ferry flight from Upington International Airport (FAUP) in South Africa to Oranjemund International Airport (FYOG) in Namibia. The aircraft was carrying a pilot and two crew members. The flight was operated under visual meteorological conditions (VMC) during daylight and in accordance with Part 91 of the Civil Aviation Regulations (CAR) 2011. The ultimate destination for the ferry flight was N’djili/Kinshasa International Airport (FZAA) in the Democratic Republic of the Congo. ## Event Sequence Approximately ten minutes into the flight, with all doors closed, the aft left cargo door window suddenly detached from its frame. The pilot reported no pre-warning indications to the crew. The pilot descended to the approximate area where the window fell but was unable to locate it. The helicopter then returned to FAUP and landed without further damage. No occupants were injured. ## Maintenance and Inspection Findings The last maintenance inspection prior to the incident was a 5-year/5000-hour service, certified on 30 September 2021 at 13,003.7 airframe hours. This service included the installation of new helicopter windows. The Aircraft Maintenance Organisation (AMO) installed Parts Manufacturer Approval (PMA) windows fitted according to installation instructions and approved by the FAA under STC No. SH3316SO. Post-incident inspection revealed that the helicopter had flown 4.5 hours since the window installation. The window frame was inspected, and the retainer seal and filler seal remained in place, indicating the detachment was not due to improper bonding. The PMA window installed was thinner and more flexible than the Original Equipment Manufacturer (OEM) window. OEM windows were 0.150 inch thick, while the PMA window was 0.125 inch thick. Technical Bulletin No. 412-86-54 noted that the installed window was within minimum thickness specifications but thinner than the OEM windows. ## Probable Cause and Safety Actions The probable cause was likely that the installed window was thinner and more flexible than the OEM part and, most probably, detached from the door structure during cruise due to helicopter vibration. Following the incident, the operator decided to install all windows for their entire fleet with OEM approved parts. No safety recommendations were issued.