Window detachment during Bell 412 EP ferry flight

Casualties unknown • 17 nautical miles (nm) west of FAUP, Northern Cape, ZA

A helicopter window detached from its frame during an international ferry flight, prompting an immediate return to Upington International Airport.

What happened

On 20 October 2021, a Bell 412 EP helicopter, registration ZS-HNB, was performing an international ferry flight from Upington International Airport in South Africa to Oranjemund International Airport in Namibia. The flight was part of a larger mission intended to deliver the aircraft to the Democratic Republic of Congo.

Approximately ten minutes into the flight, while operating under visual meteorological conditions, the pilot experienced a sudden event where the window of the aft left cargo door detached from its frame. There was no prior warning or indication to the crew before the window fell away. Following the incident, the pilot descended to the area where the window was lost in an attempt to locate it, but the item could not be found. The crew subsequently returned to Upington International Airport, landing safely without further damage to the aircraft or injuries to the three occupants.

The investigation

SACAA AIID examined the maintenance history and the physical components of the aircraft following the incident. The investigation found that the helicopter had recently undergone a major 5-year/5000-hour maintenance service, which was completed in late September 2021. As part of this service, new windows had been installed.

Technical inspections of the aircraft revealed that the installed windows were Parts Manufacturer Approval (PMA) components. While these parts were FAA-approved under an existing STC and met minimum thickness specifications, they were notably thinner and more flexible than the Original Equipment Manufacturer (OEM) windows. Specifically, the PMA windows measured 0.125 inch in thickness, compared to the 0.150 inch thickness of the OEM parts. Post-incident inspections confirmed that the window frame's retainer and filler seals remained intact, indicating the failure was not caused by improper bonding.

Findings

  • The primary cause of the window detachment was likely the increased flexibility and reduced thickness of the PMA window compared to the OEM part.
  • It is believed that standard helicopter vibrations during cruise flight caused the thinner window to detach from the door structure.
  • The aircraft had flown only 4.5 hours with the new windows installed prior to the incident.

Probable cause

The window likely detached from the door structure due to vibrations during cruise, facilitated by the fact that the installed PMA window was thinner and more flexible than the original manufacturer's specification.

Frequently asked questions

What happened in the 2021-10-20 Bell 412 EP accident near 17 nautical miles (nm) west of FAUP, Northern Cape, ZA?

A helicopter window detached from its frame during an international ferry flight, prompting an immediate return to Upington International Airport.

What aircraft was involved and where did it happen?

The accident on 2021-10-20 involved a Bell 412 EP, registration ZS-HNB, at 17 nautical miles (nm) west of FAUP, Northern Cape, ZA.

What was the probable cause of the accident?

The window likely detached from the door structure due to vibrations during cruise, facilitated by the fact that the installed PMA window was thinner and more flexible than the original manufacturer's specification.

Loading the flight search…