What happened
On August 8, 2012, an EMB-500 (Phenom 100), registration PT-TPY, was landing at the Mossoró Aerodrome (SBMS) in Rio Grande do Norte, Brazil. The flight had originated from Montes Claros, MG, carrying one pilot and six passengers.
Immediately after the main landing gear made contact with the runway, approximately 200 meters from the threshold of runway 23, the Crew Alerting System (CAS) triggered an "ANTISKID FAIL" message accompanied by an aural warning. The pilot noted a significant tendency for the aircraft to yaw to the left. This was caused by the left main gear tire skidding, which eventually led to the bursting of the tire tread. The aircraft continued along the runway for approximately 700 meters before coming to a stop on the right side of the pavement. All seven occupants (one pilot and six passengers) were unharmed, though the aircraft sustained substantial damage to the left main gear tire and wheel hub.
The investigation
CENIPA investigators conducted an extensive examination of the aircraft's braking components, including the Brake Assembly, the Wheel Speed Transducer (WST), and the Brake Control Unit (BCU). Testing was performed at specialized facilities in the United States and at Embraer's service center.
Laboratory tests, including High Accelerated Stress Screening (HASS) and vibration tests, confirmed that the physical hardware components—the brake assembly, transducers, and wiring—were functioning correctly and showed no signs of prior damage or overheating. The investigation focused on the software logic within the BCU. Analysis of the flight data recorder (CVDR) revealed that during the landing roll, the BCU's self-test was active. The software failed to correctly process wheel speed information during a specific 60-millisecond window, leading the system to deem the left wheel speed data unreliable and subsequently disable the antiskid protection for that wheel.
Findings
- The primary cause was a design flaw in the software logic of the Brake Control Unit (BCU). Specifically, the programming of the bias test condition during the self-test phase caused the system to incorrectly identify a failure and disable the antiskid function.
- The pilot applied significant braking pressure (over 80% pedal deflection) following the antiskid failure, which contributed to the wheel locking and the subsequent tire burst.
- A software update in the fifth version of the BCU introduced a logic change where the system would replicate braking pressure from the functional side to the failed side; however, this change was not adequately reflected in the aircraft's technical publications.
- The occurrence of a very soft landing may have contributed to the timing of the self-test interference with the touchdown phase.