What happened
On April 21, 2005, a Robin CAP 10B, registration I-IZAJ, was conducting a flight training mission at Viterbo Airport. During the takeoff roll on runway 04L, the aircraft reached a speed of approximately 40 knots when it began to yaw pronouncedly to the right. The instructor, recognizing that the student pilot could not maintain directional control, took command and immediately attempted to execute an emergency braking procedure by applying the left brake and pulling the control column.
However, the left brake pedal traveled to the floor without providing any deceleration. The aircraft continued its uncontrolled movement, exiting the runway on the right side approximately 500 meters from the threshold. The aircraft traveled across the grass and struck a temporary maintenance fence made of iron rods and plastic netting, which caused significant structural damage to the wing. Both the student and the instructor escaped the incident without injury.
The investigation
The ANSV investigation focused on two distinct phases of the accident: the initial directional instability and the subsequent failure of the braking system. Investigators examined the flight conditions, noting a left crosswind of 8 knots. The investigation also included a technical analysis of the aircraft's hydraulic braking system, specifically the properties of the Aeroshell Fluid 4 used in the circuit.
Findings
- The initial yaw during takeoff was attributed to the student pilot's lack of experience in managing a tailwheel aircraft during a crosswind takeoff.
- The primary cause of the runway excursion was the formation of vapor bubbles within the hydraulic brake circuit.
- Because the pilot had been using the brakes continuously during taxiing to maintain control, the hydraulic system had undergone significant thermal loading.
- The brake fluid, which is hygroscopic, likely contained small amounts of absorbed moisture. The heat generated during taxiing caused this moisture to evaporate, creating compressible air bubbles that rendered the brake pedal ineffective.
- Post-accident inspections of the braking system showed no mechanical or hydraulic malfunctions, confirming that the issue was transient and caused by thermal-induced vaporization.