DFW07TA089

The 6,127-hour commercial pilot discovered that the throttle was locked and would not advance when he attempted to add power after leveling off during an enroute descent. The airplane continued to descend, and the pilot tried to "wiggle the throttle back and forth" but the throttle would not advance beyond 1,000 RPM. During the subsequent forced landing, just prior to touch down, the pilot pushed the throttle forward very hard and felt the throttle pop loose, and the pilot was able to move the throttle aft and forward; however, with no affect to the engine RPM. The airplane landed on rough and uneven terrain and the airplane nosed-over during the ground roll. A review of the airframe logbooks noted the aircraft had previously been repaired after previous mishaps. Additionally, the airframe had completely been rebuilt in July 2003, including replacement of the firewall during the scheduled overhaul for the airplane. A newly overhauled engine had been installed approximately 3.5 hours prior to the accident. The investigation revealed that a throttle cable support angle was missing from the firewall. No evidence was found to confirm that the support angle was installed when the engine firewall was last replaced.

History of flight

On March 29, 2007, about 1115 central daylight time, a single-engine Piper PA-18 airplane, N744, was substantially damaged during a forced landing following the loss of engine power shortly after takeoff from the Avenger Field Airport (SWW), near Sweetwater, Texas. The commercial pilot, sole occupant of the airplane, sustained minor injuries. The airplane was owned and operated by the U.S. Department of Agriculture. Visual meteorological conditions prevailed and no flight plan was filed for the 14 Code of Federal Regulations Part 91 public-use flight.

The pilot initially reported to the Federal Aviation Administration (FAA) inspector, who responded to the site, that the throttle cable broke leaving him with reduced power. The FAA inspector confirmed that the airplane received structural damage during the forced landing.

The pilot reported in the pilot/operator aircraft accident report (NTSB form 6120.1), that while in level flight at 4,000 feet, with the engine operating at 2,500 RPM, he reduced power to 1,600 RPM and applied carburetor heat. He then further reduced the engine power to 1,400 RPM as he slowed airplane to 70 MPH, and extended the flaps to the 10-degrees setting. The pilot added that he then initiated a decent to 3,000 feet. As the pilot attempted to increase engine power as he leveled-off the airplane, he noted that the throttle "was locked and would not go forward." The airplane continued to descend, and the pilot tried to "wiggle the throttle back and forth", it would pull back but it would not move forward. The pilot stated that with the engine operating at approximately 1,000 RPM, he decided to attempt an off-airport landing. Just prior to touch down the pilot pushed the throttle forward very hard and felt the throttle "pop" loose, it would then move forward and aft, however, the throttle position would have no effect on the engine RPM. The airplane touched down in rough and uneven terrain and the airplane nosed over during the landing roll. The airplane came to rest in the inverted position and the pilot was able to egress the airplane without assistance.

Personnel information

The 6,127-hour pilot held a commercial pilot certificate for airplane single-engine land and instrument airplane. His last second class Federal Aviation Administration (FAA) medical was issued on March 27, 2007.

Aircraft information

The airplane was a 1982 model Piper PA-18-150, which is a tailwheel-equipped single-engine, high-wing airplane, normally configured with 2 tandem seats. The airplane was powered by 160-horsepower Lycoming 0-320-A2B reciprocating engine. The engine was driving a single McCauley 2-blade fixed pitch propeller.

The aircraft had a standard airworthiness certificate and had been heavily modified with several supplemental type certificates (STC) including tundra tires, 160 horsepower engine, L-18 windows, auto-gas, shoulder harnesses, auto flagger, and others.

A review of the airframe logbooks noted the aircraft had previously been repaired after previous mishaps. Additionally, the airframe had completely rebuilt in July 2003, including replacement of the firewall during the USDA's scheduled overhaul for the airplane. The airplane received a newly overhauled engine on March 20, 2007, and had accumulated approximately 3.5 hours prior to the accident. The airplane's maintenance logbooks revealed that the last 100-hour inspection was performed on March 20, 2007. The airframe had accumulated a total of 11,955 hours at the time of the accident.

Meteorological information

At 1105, the automated weather station at SWW reported winds from 250 degrees at 6 knots, temperature 59 degrees Fahrenheit, dew point 50 degrees Fahrenheit, visibility 10 miles, ceiling 8,500 feet overcast, and an altimeter setting of 30.02 inches of Mercury.

A review of the carburetor icing chart reveals that the temperature/dewpoint spread places the flight in the "serious icing-any power" operating realm.

Aerodrome information

The Avenger Field Airport (SWW) is a public use airport, located near Sweetwater, Texas. The airport does not have a control tower, but has a local area common traffic advisory frequency (CTAF). SWW features two asphalt runways. Runway 17-35 is 5,840-feet long and 100-feet wide. Runway 04-22 is 5,659-feet long and 75-feet wide. The field elevation is 2,380 feet mean sea level (msl).

Wreckage and impact information

The fuselage of the airplane came to rest inverted approximately 5 miles southeast of SWW. There was no post-impact fire. All major components of the airplane were accounted for at the scene.

The wreckage was recovered from the accident site on April 1, 2007, and was relocated to the US Department of Agriculture hanger, located at Avenger Field Airport (SWW), near Sweetwater, Texas, for further examination.

On May 7, 2007, under the supervision of the NTSB, an examination of the airplane was accomplished in the U.S. Department of Agriculture hangar, with representatives from Lycoming and Piper Aircraft Company. The engine was rotated by hand at the propeller flange. Crankshaft and valve train continuity were verified through to the accessories at the back of the engine. Thumb suction and compression was obtained on all cylinders. The ignition system tested normal when the propeller flange was turned by hand. Spark was confirmed at all cylinder spark plugs. Color was consistent with normal operation when compared to a Champion Check-A-Plug chart. The carburetor was found to have been removed and sent to Precision Airmotive LLC, near Marysville, Washington, for examination and testing. A throttle cable support angle was missing from the firewall. There was no evidence that the support angle had been installed since the firewall was replaced. During the examination, when the throttle control in the cabin was manipulated, the cable housing would move instead of the throttle clevis, preventing proper control and movement of the carburetor throttle arm.

The propeller displayed leading edge polishing on one blade, and the other blade was bent aft at mid-span and displayed leading edge scoring and polishing.

The examination of the engine did not reveal any preimpact mechanical anomalies that would have prevented normal engine operation.

On May 15, 2007, at Precision Airmotive LLC, the float carburetor was examined. The carburetor sustained impact damage during the accident sequence. The unit was too badly damaged to administer a flow test. The throttle shaft had been broken out of the body and was separated, but would rotate with finger pressure. The throttle shaft bearings measured below minimum diameter. The throttle lever was bent and attached to the throttle shaft. The mixture lever was separated and the index wire was bent.