Piper PA 28-140 N57DB accident near San Antonio, Texas

Probable cause

A partial loss of engine power after departure due to the collapse of the carburetor air inlet duct, which restricted the airflow to the engine. Contributing to the accident was the overdue annual inspection and the installation of the improper air inlet duct.

NTSB analysis

The pilot reported that, after advancing the airplane's throttle to full power for takeoff, the engine produced "normal thrust," the airspeed reached 60 mph, and the tachometer indicated 2,700 rpm. The acceleration was smooth and all indications were normal when the airplane reached rotation. About 600 feet above the ground, the airplane's nose began to lower, the airspeed remained 60 mph, and the tachometer indicated 2,000 rpm. The pilot pitched the nose down to prevent a stall and ensured that the throttle and mixture controls were full forward and that the electric fuel pump was on. He reported to the air traffic controller that the airplane's engine was not producing sufficient power and that he was returning to the runway. Shortly after the radio transmission, the airplane impacted terrain and a tree.

An examination of the airplane's engine revealed a fuel stain on the carburetor inlet, indicating that the airflow through the carburetor had been decreased. The carburetor induction air inlet duct was single-thickness scat-type ducting, which was not the material described for the part, which should have been installed in accordance with a manufacturer's service bulletin. The material appeared to be beyond its normal service life with frayed edges and loose strings, and most of the wire was loose and separated from the duct. Further, the carburetor air inlet duct appeared to have been compressed due to negative air pressure during high engine rpm, which would have restricted the airflow to the engine and reduced the engine power. The No. 2 cylinder and its associated spark plugs contained dark sooting, likely due to the restricted airflow, which resulted in a rich fuel/air mixture inside the cylinder. The airplane was 12 days overdue for an annual inspection; the improper, damaged air duct could have been identified during the inspection.

Text from the NTSB. Open the NTSB docket for CEN14LA058 for the full record.