Incident Overview
On 10 June 2018 at approximately 18:30 local time, a Robin DR 400-140 B aircraft, registration HB-KFD, operated by Groupement de vol à moteur Lausanne, departed from Lausanne-La Blécherette Airport (LSGL) for a local flight with a private pilot and one passenger. The pilot held a valid EASA Private Pilot Licence (PPL(A)) and had accumulated 87 hours 47 minutes total flight time, with 29 hours 38 minutes on type. The aircraft was owned by the same group.
During cruise at 5500 ft AMSL, the pilot performed a standard cruise check. While manipulating the mixture control, engine power dropped significantly. The pilot attempted to move the mixture control back to rich but observed no response. He then initiated the engine failure procedure. As engine power continued to decrease, altitude could not be maintained. The pilot identified a grass field ahead and performed an approach followed by an emergency landing. The aircraft touched down in the grass field, which sloped downward, and could not stop before entering a rye crop field approximately 1 meter tall, where it came to rest.
Both occupants exited the aircraft unharmed. The Emergency Locator Transmitter (ELT) did not activate.
Findings
A visual inspection revealed no apparent damage to the aircraft, and no marks were visible on the propeller.
Cockpit Observations
- Mixture control lever in the cockpit was set to the “RICH” position.
- Carburetor heat control was in the “COLD” position and functioned correctly.
- Throttle control was at idle and operated correctly.
- Air filter was clean.
Engine Compartment Observations
- The mixture lever on the carburetor was found in the “LEAN” position.
- The mixture control cable (flexible assembly) was detached from its cable stop.
- The outer sheath (twisted wire with plastic coating) was worn at its end.
- The cable stop was heavily damaged.
- The twisted sheath slid freely within the cable stop without resistance.
- The retaining nut of the cable stop was tightened.
- The mixture control inner wire was jammed against the cable stop screw; the nut was tight.
- Spark plugs showed correct color and wear.
Description of Mixture Control System
The mixture control cable consists of a rigid steel inner wire (1.2 mm diameter) sliding inside a twisted wire sheath (outer diameter 4.5 mm with plastic coating, 3.7 mm without). At the cockpit end, the sheath is crimped to a tube fixed to the aircraft structure; the inner wire is crimped to a threaded rod that connects to the mixture control lever via a fork-shaped connector. At the carburetor end, the sheath is secured to a cable stop mounted on a bracket attached to the carburetor; the inner wire is locked to the carburetor mixture lever by a cable stop screw.
Aircraft Maintenance
No recent maintenance on the carburetor or mixture control cable was documented. The last 100-hour inspection was performed on 6 June 2018, about 4 days and 10.6 flight hours before the incident. At the time of the incident, the engine had accumulated 2245 hours 17 minutes since overhaul, exceeding the manufacturer’s 2000-hour limit. A Lycoming Service Instruction (SI No. 1009BC) permitted operation up to 2200 hours under certain conditions, and the Swiss Federal Office of Civil Aviation (FOCA) technical communication allowed a 10% extension.
Analysis of Failure
During the in-flight manipulation of the mixture control, the worn outer sheath of the mixture cable became dislodged from the cable stop. Excessive play in the cable stop on the carburetor mixture lever had gradually allowed the sheath to slip out. Tool marks (pliers) were observed on the upper part of the cable stop, indicating prior intervention. Although the cable stop retaining nut was tight, it did not prevent the sheath from pulling out.
As a result, the mixture control became free and inoperative. There is no return spring on the carburetor mixture lever to return it to the “RICH” position, so the inner wire remained in the “LEAN” position at the carburetor despite the cockpit control being set to “RICH.”
Conclusion
Attempts to restore engine power were ineffective due to the failure of the mixture control cable assembly in the engine compartment. The pilot quickly recognized that an off-field landing was inevitable. The chosen field was suitable, and the emergency landing procedure was executed correctly, preventing any danger to occupants or third parties. The Swiss Transportation Safety Investigation Board (STSB) decided to conclude the investigation with this summary report and no further measures.