What happened
On October 30, 1971, at approximately 15:20 GMT, a Piper Super Cub PA-18 (CS-ACD) operated by the Coimbra Academic Aviation Center was conducting local training flights at Cernache Aerodrome in Portugal. The aircraft, registered as CS-ACD and powered by a Continental C90-12F engine, was carrying two occupants: a 26-year-old pilot with over 100 hours of total experience and a 28-year-old passenger.
During the flight, the pilot executed a low pass over Runway 33. Mid-pass, the passenger reported feeling unwell, prompting the pilot to abort the maneuver and initiate an immediate landing on the remaining runway length ahead. The approach was conducted at a higher-than-normal airspeed due to the momentum from the previous low pass. Upon touchdown, the aircraft rolled down the hard-packed grass strip. The pilot observed an unexpected acceleration of the aircraft's movement, suspecting that the passenger had inadvertently contacted the throttle lever while leaning forward, causing momentary engine power application.
Attempting to stop within the limited remaining distance, the pilot applied the brakes. However, the right brake was noted to be weak. As the aircraft neared the end of the runway, it veered off to the right to avoid overrunning into a slope and trees. During this evasive maneuver, the aircraft entered a ground loop. The right rudder tip struck small bushes, causing the control surface to break at the hinge. Centrifugal force during the turn caused the right wing to drop and strike the ground, bending the main spar of the right wing.
The investigation
The DGAC (Direcção-Geral da Aeronáutica Civil) conducted a technical investigation into the incident. Key factual findings included:
- The aircraft was airworthy prior to the event, with its last 100-hour inspection completed on October 23, 1971.
- The center of gravity was within limits at both takeoff and landing.
- Weather conditions were favorable: unlimited ceiling, good visibility, light wind from 030 degrees, and normal temperature.
- The runway (Runway 33) was 950 meters long with hard-packed soil and light grass. The pilot utilized the final 200 meters for landing.
- A pre-existing defect was identified: weak brakes, particularly on the right side.
The investigation determined that the pilot’s decision to land was made precipitously. The aircraft was not in a position or at a speed suitable for a safe landing on the available runway length. However, the investigator noted that the pilot’s subsequent actions in the emergency situation were appropriate, successfully preventing the aircraft from overrunning the runway and crashing into the adjacent terrain.
Findings
The accident was caused by the pilot’s poor assessment of the situation when deciding to land under unsafe conditions. Contributing factors included:
- High approach speed: The momentum from the low pass made the landing speed excessive for the remaining runway.
- Inappropriate decision point: The choice to abort the low pass and land immediately was made too late and without adequate safety margins.
- Deficient braking system: The weak right brake prevented effective deceleration during the rollout.
- Passenger interference: It is likely that the passenger inadvertently advanced the throttle lever, exacerbating the speed issue.
The structural damage resulted from these operational errors. The right elevator was fractured because it struck bushes during the ground loop. The right wing spar was bent due to the impact of the wingtip on the ground, caused by centrifugal force during the turn.
Safety action
The GPIAAF issued several safety recommendations:
- Operators and pilots must ensure aircraft are in good operational condition and act according to their limitations, particularly regarding known defects like brake efficiency.
- Pilots should plan maneuvers with adequate lead time and avoid last-minute changes that compromise safety. The report noted that it would have been preferable for the passenger to vomit than to cause structural damage.
- No punitive measures were taken against the pilot, but he was formally reprimanded for the error in judgment.
- The Coimbra Academic Aviation Center was advised to ensure better operational maintenance of its fleet.