IDAHO FALLS, ID, USA
N41054
Cessna 425
The accident aircraft had recently had maintenance work performed on its autofeather system pressure sensing switches, due to reports of the left engine not autofeathering properly in flight. The purpose of the accident flight was to verify proper inflight operation of the autofeather system following the maintenance work on the autofeather pressure sensing switches and a successful ground check of the autofeather system. Air traffic control (ATC) communications recordings disclosed that the pilot called ready for takeoff from runway 2 approximately 1207, and requested to orbit above the airport at 8,000 feet (note: the airport elevation is 4,740 feet.) The pilot subsequently reported established in a hold above the airport at 8,000 feet approximately 1213, and was instructed by ATC to report leaving the hold. Approximately 1215, an abbreviated radio transmission, "zero five four," was recorded. The Idaho Falls tower controller responded to this call but never got a response in return from the accident aircraft, despite repeated efforts to contact the aircraft. Witnesses reported that the aircraft banked to the left, or to the west, and that it entered a spiral from this bank and crashed (one witness reported the aircraft was flying at 200 to 300 feet above ground level when it entered this bank, and that it performed a "skidding" or "sliding" motion part way through the bank, about 1 second before entering the spiral.) The aircraft crashed about 2 miles north of the airport. On-site examination disclosed wreckage and impact signatures consistent with an uncontrolled, relatively low-speed, moderate to steep (i.e. greater than 22 degrees) angle, left-wing-low impact on an easterly flight path. No evidence of flight control system malfunction was found, and a large quantity of jet fuel was noted to be aboard the aircraft. Post-accident examination of the aircraft's engines indicated that the left engine was most likely operating in a low power range and the right engine was most likely operating in a mid to high power range at impact, but no indications of any anomalies or distress that would have precluded normal operation of the engines prior to impact was found. Post-accident examination of the aircraft's propellers disclosed indications that 1) both propellers were rotating at impact, 2) neither propeller was at or near the feather position at impact, 3) both propellers were being operated with power at impact (exact amount unknown), 4) both propellers were operating at approximately 14º to 20º blade angle at impact, and 5) there were no propeller failures prior to impact. Post-accident examination of the autofeather pressure-sensing switches disclosed evidence of alterations, tampering, or modifications made in the field on all but one switch (a replacement switch, which had been installed just before the accident flight during maintenance) installed on the aircraft at the time of the accident. All switches except for the replacement switch operated outside their design pressure specifications; the replacement switch operated within design pressure specifications. Examination of the switches indicated that all switches were installed in the correct positions relative to high- or low-pressure switch installations. Engineering analyses of expected autofeather system performance with the switches operating at their "as-found" pressure settings (vice at design pressure specifications) did not indicate a likelihood of any anomalous or abnormal autofeather system operation with the autofeather switches at their "as-found" pressure settings. Also, cockpit light and switch evidence indicated that the autofeather system was not activated at the time of impact. The combination of probable engine power and propeller pitch on the left engine (as per the post-accident engine and propeller teardown results) was noted to be generally consistent with the "zero-thrust" engine torque and propeller RPM settings specified for simulated single-engine practice in the aircraft Information Manual.
HISTORY OF FLIGHT On November 10, 2000, approximately 1215 mountain standard time, a privately owned Cessna 425, N41054, was substantially damaged in a collision with terrain approximately 2 miles north of Fanning Field, Idaho Falls, Idaho. The airline transport pilot-in-command and a second airline transport pilot aboard for the flight were fatally injured in the accident. The local flight, which was a maintenance test flight, departed from Fanning Field approximately 1208, and was conducted under the provisions of 14 CFR 91. Visual meteorological conditions prevailed in the area at the time of the accident, and no flight plan had been filed for the flight. The accident aircraft had recently had maintenance work performed on its autofeather system pressure sensing switches, due to reports of the left engine not autofeathering properly in flight. The maintenance work was performed by Aerohawk Aviation, a maintenance firm based at Fanning Field. According to Aerohawk's owner, the purpose of the accident flight was to verify proper inflight operation of the autofeather system following the maintenance work on the autofeather pressure sensing switches and a successful ground check of the autofeather system. Air traffic control (ATC) communications recordings from the Idaho Falls air traffic control tower disclosed that the pilot called ready for takeoff approximately 1207, and requested to orbit above the airport at 8,000 feet (note: the airport elevation is 4,740 feet.) He was cleared for takeoff on runway 2 at 1208, and his departure was approved by ATC as requested. The pilot subsequently reported established in a hold at the Idaho Falls VOR (collocated with the airport) at 8,000 feet approximately 1213. The Idaho Falls tower controller instructed the pilot to report leaving the hold. Approximately 1215, an abbreviated radio transmission, "zero five four," was recorded. The Idaho Falls tower controller responded to this call but never got a response in return from the accident aircraft, despite repeated efforts to contact the aircraft. At 1227, the Idaho Falls tower controller informed the Boise, Idaho, Automated Flight Service Station (AFSS) that she was receiving reports of an aircraft down about 2 miles north of the airport. A witness to the accident reported that he first observed the aircraft flying south along Interstate Highway 15 (I-15) at an altitude of 200 to 300 feet above ground level. He stated that the aircraft then started banking left, and that part way throught the bank the aircraft then began a "skidding" or "sliding" motion. This witness reported that the aircraft entered a spiral about 1 second later and crashed. This witness reported he did not observe anything wrong with the aircraft at that time, but could not tell whether or not both of the aircraft's propellers were turning. The second witness reported that the airplane was banking west, and that it then "spiraled straight down and hit the ground." A third witness reported that the airplane was northbound, "banking to the west on its wingtip." This witness reported that the aircraft then went out of sight, and that he then saw a smoke cloud. The accident occurred during the hours of daylight at approximately 43 degrees 32.2 minutes North latitude and 112 degrees 3.8 minutes West longitude. PERSONNEL INFORMATION Both pilots were employed as corporate pilots for local companies in Idaho Falls at the time of the accident. The pilot-in-command, who according to the aircraft's owner was highly experienced in the Cessna 425, was retained by Aerohawk Aviation to conduct maintenance test flights on an ad-hoc basis. He held an airline transport pilot certificate with airplane single-engine land, airplane multiengine land, and airplane single-engine sea ratings, and a Cessna 500 type rating (note: according to the provisions of 14 CFR 61.31(a), a type rating is not required for the Cessna 425). He also held a flight instructor certificate with airplane single-engine, airplane multiengine, and instrument airplane ratings, with a date of issue of October 28, 2000. The pilot held an FAA second-class medical certificate dated November 23, 1999, with a limitation requiring the pilot to possess glasses that corrected for intermediate and near vision. No pilot logbooks for the pilot-in-command were located by, or provided to, the NTSB. However, NTSB investigators recovered documentation of the pilot-in-command's completion of a pilot-in-command proficiency check in the Cessna 500 at SimuFlite, Inc. on March 11, 2000. The pilot indicated on his last FAA medical certificate application that he had 8,000 hours total pilot time, with 250 hours in the last 6 months. According to the owner of Aerohawk, the pilot-in-command had invited the second pilot to fly on the accident flight because the company that employed the second pilot was purchasing a Cessna 425. The second pilot held an airline transport pilot certificate with an airplane single-engine land rating, with commercial privileges for airplane multiengine land. He held an FAA first-class medical certificate dated July 27, 2000, with no limitations. No pilot logbooks for the second pilot were located by, or provided to, the NTSB. The second pilot indicated on his last FAA medical certificate application that he had 2,500 hours total pilot time, with 200 hours in the last 6 months. AIRCRAFT INFORMATION The aircraft, a Cessna 425, serial number 425-0172, was manufactured in 1982, and was registered to the then-current owner (who was not aboard the aircraft at the time of the accident) on May 28, 1997. The Hobbs meter reading noted at the accident site was 4,027.1 hours, which according to the aircraft logs also corresponded to the airframe total time. The aircraft was on a manufacturer's approved aircraft inspection program. The last documented inspections were phase 2 and 3 inspections signed off by the owner of Aerohawk Aviation on February 10, 2000, at 3,963.5 airframe hours. The aircraft was equipped with two McCauley 4HFR34C762-H four-bladed propellers, which according to the propeller logs were installed new on the aircraft on March 15, 1996, at 3,216.5 airframe hours. At the time of this new propeller installation, the aircraft was equipped with two Pratt & Whitney Canada PT6A-112 turboprop engines. The aircraft records indicated that the propellers were installed per FAA Supplemental Type Certificate (STC) SA1491GL (held by McCauley Propeller Systems), which authorizes installation of the McCauley 4-bladed propellers on PT6A-112 engines. Two new Pratt & Whitney Canada PT6A-135A engines, serial numbers PCE-PZ0025 (left) and PCE-PZ0026 (right), were subsequently installed on the accident aircraft by Aerohawk Aviation on September 24, 1999, at 3,937.7 airframe hours. The PT6A-135A installation was accomplished per STC SA622CH, held by Duncan Aviation (formerly Kal-Aero) of Battle Creek, Michigan. The McCauley 4-bladed propellers were removed from the aircraft and reinstalled on the new engines at the time of this engine change. A March 1996 amendment to STC SA1491GL added PT6A-135A engines to the eligibility for installation of the McCauley 4-bladed propellers (the original STC, issued in 1990, listed only PT6A-112 engines as eligible for installation of these propellers.) The aircraft was equipped with a propeller autofeathering system that provided for automatic feathering of a propeller in the event of loss of the engine. This system was controlled by an arming switch in the cockpit with ARM, OFF, and TEST positions. In the OFF position, the system was deactivated. During nominal system operation with the switch in ARM, the propeller would automatically feather if engine torque dropped below approximately 185 foot-pounds (ft-lb) at power lever positions of 80% gas generator RPM (Ng) or greater. In nominal system operation, autofeathering of one propeller would disarm the system on the other propeller. L and R AUTO-FTHER ARM annunciator lights, located on the instrument panel, would nominally illuminate when the system was armed. TEST was a momentary switch position used to check out system operation with power below 80% Ng. According to the Cessna 425 Information Manual, the autofeather system on the aircraft is primarily intended for use during takeoff and climb. The Normal Procedures checklists in the Information Manual specify setting the autofeather switch to ARM in the "Before Takeoff" check, to OFF in the "Cruise" check, and back to ARM in the "Descent" check. According to the Cessna 425 maintenance manual, with the autofeather switch in ARM and power levers advanced above 84% Ng, circuits are completed to two oil pressure-sensing switches (a high-pressure switch and a low-pressure switch) on each engine. These switches monitor engine torquemeter oil pressure. The high-pressure switch is designed to actuate under pressures exceeding 9.00 psig, and to deactuate when pressure decreases below 7.75 psig. The maintenance manual states that for serial numbers -0002 through -0186, the low-pressure switch opens with increasing pressure of 4.75 psig, and closes with decreasing pressure of 3.50 psig. For airplanes -0187 and on, the low-pressure switch is specified to open with increasing pressure of 6.00, +0.25, -0.25 psig, and to close with decreasing pressure of 3.00, +0.5, -0.5 psig. During nominal system operation, if an engine fails while the system is armed, engine torque begins to drop off until below the actuating pressure of the high-pressure switch (corresponding to a nominal engine torque of 300, +40, -40 ft-lb). Deactuation of the high-pressure switch on the failed engine closes the circuit to the coil of its respective control relay, which will remain deenergized as long as torquemeter oil pressure exceeds 4.25 psi (because of the open circuit at the low-pressure switch). When the torque of the failed engine falls below the actuating pressure of the low-pressure switch (nominally at an engine torque of 150, +40, -40 ft-lb), the switch returns to its unactuated position, providing a ground return for the control relay of the affected engine. Current through the relay then energizes the autofeather dump valve on the propeller overspeed governor. The autofeather dump valve immediately reduces governor oil pressure to zero, permitting the propeller blades to quickly move to the feathered position on that engine. According to the owner of Aerohawk, the aircraft had been brought in for maintenance because the pilots had been unable to get one of its engines to feather in flight. Aerohawk's owner stated that he ordered two new autofeather pressure-sensing switches from Corporate Aircraft of Fresno, California, and installed them on the right engine in accordance with the owner's instructions. He stated that the aircraft owner subsequently told him that the problem was actually on the left engine, rather than on the right engine. He stated that he then took the new switches off the right engine, installed them on the left engine, and reinstalled the original right engine autofeather switches back onto the right engine. The aircraft was then flown, and the pilots still could not get the left engine to feather properly. The aircraft was brought back to Aerohawk, and the autofeather pressure switches were swapped left engine to right engine and vice versa. Aerohawk's owner stated that after this swap, the problem "followed the switches" (i.e., transferred to the right engine). The owner of Aerohawk stated that he then placed the "old" low-pressure switch that had been removed from the left engine, back onto the right engine. At this point, the owner of Aerohawk stated, the right engine now had an old low-pressure switch (previously installed on the left engine) and a new high-pressure switch installed, and the left engine had two old switches (originally from the right engine) installed. He stated that with this installation arrangement, the system passed three cycles of ground checks on each engine. The owner of Aerohawk reported that after the aircraft passed the ground checks, he summoned the accident pilot to perform the maintenace test flight, on which the aircraft crashed. (NOTE: The Cessna 425 Maintenance Manual contains a procedure to check the autofeather system on the ground, but does not specify any procedure for an inflight check of the system.) According to the Cessna 425 Information Manual amplified Normal Procedures section, "Simulated engine failure takeoff and landing practice is most safely done with both propellers set at 1900 RPM [the maximum allowable propeller RPM, according to the operating limitations specified in both the Information Manual and the STC] and the simulated failed engine set at 150 foot-pounds." METEOROLOGICAL INFORMATION The 1153 Idaho Falls automated METAR observation reported weather conditions there as follows: wind from 040 degrees true at 8 knots; clear skies with 10 statute miles visibility; temperature -6 degrees C; dewpoint -13 degrees C; and altimeter setting 29.80 inches Hg. The 1253 Idaho Falls METAR observation reported the same conditions, except for winds from 010 degrees true at 6 knots and altimeter setting 29.79 inches Hg. WRECKAGE AND IMPACT INFORMATION An on-site examination of the aircraft wreckage was performed by investigators from the NTSB, FAA, Cessna Aircraft Company, and Pratt & Whitney Canada on November 11, 2000. The aircraft had crashed into a level horse and cattle pasture between Interstate Highway 15 (I-15) and the west bank of the Snake River, with the main wreckage coming to rest upright on the river bank on a heading of 143 degrees magnetic. The complete wreckage pattern was confined to an approximately 140-foot area running generally from west to east. The westernmost point of this pattern was marked by a ground scar. A tree with no visible damage was located directly west of this ground scar. Investigators measured the vertical angle from the ground scar to the treetop at 22 degrees above horizontal. The main wreckage comprised essentially the complete aircraft less the left propeller (which was detached from its engine at the propeller mounting flange and located about 30 feet short of the main wreckage), a single right propeller blade, and a 3-4 foot section of left wingtip which was located about 20 feet south of the main wreckage. The separated right propeller blade was found buried approximately 6 to 8 inches into frozen ground approximately 100 feet short of the main wreckage. A slash was observed generally in line with the right propeller plane of rotation on the right side of the aircraft's nose, which was pushed about 30 degrees to the right of the aircraft longitudinal axis. The entire airframe was accounted for at the accident site, and no evidence of flight control system anomalies was noted. A strong odor of jet fuel was evident at the site, and the only evidence of fire was of a fuel fire on the ground approximately 50 feet short of the main wreckage. A large quantity of fuel also leaked from the aircraft during the on-site examination and subsequent wreckage recovery operations, necessitating containment booms to be placed into the river around the immediate vicinity of the main wreckage for environmental damage mitigation, and a fire guard from the Idaho Falls Fire Department to be posted during wreckage removal to ensure personnel safety. No evidence of fire was noted in the main wreckage. All four blades of the detached left propeller were observed in the feathered position at the site. Comparison of engine instrument indications, cockpit engine and propeller control positions, and control lever positions on the engines and propeller governors generally did not yield consistent indications, with the following exceptions: 1) the right Ng RPM indicator was observed at 70%, with the right fuel control lever in the cockpit being found in the RUN position; and 2) the left propeller control in the cockpit was in the full forward position, with the left propellor governor control arm also being found in a high RPM position. The right propeller control in the cockpit was found in the full forward position (the right
The pilot-in-command's failure to maintain adequate airspeed with an asymmetric thrust condition, resulting in a loss of aircraft control. A factor was an asymmetric engine thrust condition, which was present for undetermined reasons.
Source: NTSB Aviation Accident Database
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