Introduction
A recent investigation examined the in-flight separation of a VHF antenna. Analysis of the antenna base revealed the presence of aluminium oxide and chlorine on its surface, indicating that the antenna failed due to pre-existing damage caused by corrosion. The corrosion was attributed to moisture ingress into the unit.
Findings
The majority of the antenna was not recovered, making it impossible to determine the exact pathway through which moisture entered. The seal surrounding the base had separated along with the top half of the antenna. Under the seal, some regions showed evidence of moisture, while others appeared to have been protected from corrosion. The moisture under the seal could have indicated a point of ingress, but it is also possible that all observed corrosion originated from moisture already inside the antenna rather than a compromised seal.
Beyond the cracking around six fastener holes, the precise failure mechanism could not be established. Two possibilities were considered: first, corrosion may have weakened the bond between the polyurethane core and the base or skin, reducing stiffness and allowing cracks to develop during service flexing. Second, moisture could have been absorbed by the glass fibre composite skin, reducing its strength and increasing susceptibility to cracking.
Analysis
Without understanding how moisture entered and led to separation, it was not possible to determine whether damage would have been externally visible during the relevant inspection eight months prior. The cracking around the fastener holes might not have started by that time, or damage could have been too small or obscured by paint.
An alternative possibility of a birdstrike or collision with a remotely piloted aircraft (RPA) causing the separation was considered but could not be entirely ruled out. However, birdstrikes above 10,000 feet are rare, and most RPAs are not certified to fly that high. No in-flight RPA loss was reported. Additionally, visual evidence of a birdstrike was absent, and such an event would likely have produced a loud bang rather than the reported whistling sound followed by an impact sound.
The investigation concluded that while the exact failure sequence remains uncertain, the antenna's in-flight failure was due to pre-existing corrosion damage from moisture ingress.