What happened
On 15 April 2022, a Cessna 402 C aircraft, registered as ZS-WRI, departed from Runway 07 at Lanseria International Airport on a charter flight bound for Alldays Aerodrome in Limpopo. The flight was being operated under visual flight rules by a pilot and two passengers.
Following takeoff, the pilot attempted to retract the landing gear. However, a red 'gear unlocked indication' light remained illuminated on the instrument panel, accompanied by a sudden yaw to the left. To manage the imbalance, the pilot adjusted the left engine mixture to restore power and performed the emergency procedures outlined in the Pilot’s Operating Handbook, which involved yaws to both the left and right to attempt to achieve the three green gear-down indications.
To ensure safety, the pilot requested a visual inspection from the Lanseria International Air Traffic Control tower during a fly-past. The controller reported that the landing gear appeared to be in the down position. The pilot subsequently returned to the airport and landed safely on Runway 07. There were no injuries and no damage to the aircraft.
The investigation
An investigation into the incident examined the aircraft's maintenance history and mechanical condition. The aircraft was within its valid Certificate of Airworthiness and had undergone a mandatory periodic inspection less than a month prior to the event. The pilot held a valid Commercial Pilot Licence with the appropriate type rating.
Upon inspecting the main landing gear wheel well after the flight, investigators identified a worn rubber seal on the nut connecting to the left gear hydraulic loaner pump actuator. This degradation allowed hydraulic fluid to escape under pressure, which interfered with the landing gear indication system.
Findings
- The primary cause of the gear indication failure was a failed hydraulic fluid hose seal on the loaner pump.
- The worn seal allowed hydraulic fluid to leak when the system was under pressure, triggering the 'unlocked gear' warning.
- The aircraft's maintenance records showed the 50-hour periodic inspection had been completed recently, and the aircraft was within its service intervals.