ERA22LA054

The student pilot was turning from downwind to base in the traffic pattern, with 10° flap extension, when he noticed that the throttle lever was stuck with the engine operating at 2,000 rpm. He turned onto final approach with 25° flap extension and realized that the descent rate was too great and that the airplane was not going to make it to the runway. He retracted the flaps to 0° to reduce drag, turned right to avoid houses at the approach end of the runway, and landed the airplane in an adjacent bay. Metallurgical examination of the throttle cable revealed a worn Teflon liner. When intact, the liner supported the inner cable and prevented metal-to-metal contact with the helical coil of the cable housing. Six of the seven wires of the inner cable had separated due to metal-to-metal contact wear. The seventh (single central) wire did not have the capacity to resist compressive and torsional loading of the cable, which led to collapse and torsional unfurling of the cable. The airplane was manufactured 48 years before the accident and had accrued about 14,000 hours of operation. There were no life-limited components on the make and model airplane and no requirement to replace the throttle cable at a scheduled interval. Review of maintenance records dating back 2 years before the accident did not reveal any problems with the throttle.

On November 9, 2021, about 1605 eastern standard time, a Piper PA-28-151, N40831, was substantially damaged when it was involved in an accident near Sarasota, Florida. The student pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The student pilot reported that he was returning to his home airport at the end of a roundtrip cross-country flight. While turning from left downwind leg to left base leg for runway 14 at Bradenton International Airport (SRQ), with 10° flap extension, he noticed that the throttle lever was stuck at the 2,000 rpm position. He then turned on to final approach with 25° flap extension and realized that the descent rate was too great and that the airplane was not going to make it to the runway. He completely retracted the flaps to reduce drag and checked the throttle friction lock to loosen the throttle but this had no affect. In an effort to avoid houses at the approach end of the runway, the pilot elected to turn right and ditch in an adjacent bay. The student pilot subsequently egressed and the airplane sank. The wreckage was recovered from the bay and examined by a Federal Aviation Administration inspector. The inspector noted substantial damage to the right wing. He also observed that the throttle cable was jammed inside its housing. The throttle cable was forwarded to the National Transportation Safety Board Materials Laboratory, Washington, DC. Metallurgical examination of the cable revealed that the Teflon liner, which supported the inner cable and prevented metal to metal contact with the helical coil of the cable housing, was worn. Six of the seven wires of the inner cable had separated due to metal to metal contact wear. The seventh (single central) wire did not have the capacity to resist compressive and torsional loading of the cable, which led to collapse and torsional unfurling of the cable (for more information, see Materials Laboratory Factual Report for this accident in the public docket). The airplane was manufactured in 1973 and had accrued about 14,000 hours of operation at the time of the accident. There were no life limited components on the make and model airplane and no published schedule to replace them. Review of the maintenance manual for the 100-hour (500-hour, and 1,000-hour) inspections for the make and model airplane revealed a checklist item (item 37) in the engine group section related to the throttle cable. It stated, “Inspect throttle, carburetor heat, and mixture controls for security, travel, and operating condition.” Review of maintenance records dating back 2 years before the accident did not reveal any problems with the throttle.