What happened
On December 7, 1996, a large, custom-designed model aircraft was being operated at a registered aeromodelling site in Phoenix Park, Dublin. After an initial period of successful flight, the pilot launched the model for a second session. During the final circuit of this session, the pilot suddenly lost all ability to control the aircraft. Despite attempting to use a secondary transmitter on the same frequency, the model remained unresponsive and continued to climb in a north-easterly direction until it disappeared from view.
As the engine's fuel supply was depleted, the model transitioned into a glide. Approximately 30 minutes later, the pilot of a B737 taxiing at Dublin Airport spotted the model resting on a new link taxiway near the threshold of Runway 28. Airport security subsequently recovered the aircraft, which showed damage to the engine, propeller, and fuselage consistent with a heavy landing.
The investigation
AAIU investigators examined the aircraft and its radio control systems to determine why command was lost. The investigation focused on the electrical state of the receiver and the mechanical loads on the servos. Testing revealed that while the radio equipment functioned correctly when powered, the receiver battery was completely depleted.
Further technical analysis showed that the model's design contributed to high power consumption. Specifically, the lack of mass balancing on the ailerons and elevator, combined with the use of servos that were marginally sized for the aircraft's control surfaces, created significant current drains. The investigation also looked into potential radio interference, but found no evidence of signal jamming or interference from other nearby models.
Findings
- The primary cause of the loss of control was the exhaustion of the receiver battery.
- The battery capacity was insufficient for the aircraft's power demands, especially given that the design lacked mass balancing on control surfaces, which increased the load on the servos.
- The use of NICAD batteries contributed to the suddenness of the failure, as these batteries provide a constant voltage until they reach a critical point of rapid depletion.
- The absence of a receiver battery backup device or PCM (Pulse Control Modulation) equipment meant there was no redundancy to maintain control once the primary power failed.
Safety action
- Aeromodellers are advised to use receiver batteries with sufficient capacity to provide a large safety margin and to ensure servos are appropriately sized for the aircraft's loads.
- It is recommended that operators of larger models consider using battery backup devices or PCM equipment to prevent loss of command.
- The investigation highlighted the need for the IAA and MACI to review regulations regarding the operation of models that pose potential hazards to public safety.