N1653UCessna 207 · 1974

On May 30, 2015, about 1130 Alaska daylight time, a Cessna 207, N1653U, sustained substantial damage after impacting trees about 40 miles southeast of Bethel, Alaska. The airplane was being operated by Yute Air, Bethel, as a visual flight rules (VFR) post maintenance flight under Title 14 CFR Part 91 when the accident occurred. The commercial pilot, the sole occupant, sustained fatal injuries. Visual meteorological conditions were reported in the area of the accident, and company flight following procedures were in effect. The accident flight originated at the Bethel Airport about 0830, with an expected return time of 1200.

About 1415, flight coordination personnel from Yute Air in Bethel notified the director of operations (DO) that the accident airplane was overdue. About 1435, the DO notified the Federal Aviation Administration (FAA) who issued an alert notice (ALNOT). About 1532, an aerial search was initiated by Yute Air, Alaska State Troopers, Alaska Air National Guard, as well as other air operators and Good Samaritans. On May 31, about 1730, searchers discovered the airplane's submerged and fragmented wreckage in a swift moving river slough.

The pilot, age 47, held a commercial pilot certificate with an airplane single and multi-engine land rating. The most recent second-class airman medical certificate was issued on April 14, 2015, and contained no limitations. According to company records, the pilot had about 7,175 total flight hours; about 6,600 flight hours were accrued in the accident airplane make and model.

In the preceding 90 and 30 days prior to the accident, the company listed the pilot's flight time as 362 and 94 hours, respectively. The pilot was hired on June 9, 2010.

The pilot completed an airman competency/proficiency check flight under Title 14 CFR Part 135.293 (Initial and Recurrent Testing), and 135.299 (Pilot-in-Command Line Check), with the chief pilot for the operator in a Cessna 207 airplane on January 20, 2015. In the remarks section of FAA form number 401-07 (airman competency/proficiency check form), the chief pilot wrote: "IPC (instrument proficiency check) satisfactory."

The accident flight was the pilot's first flight of the day, on the second day of a two-week on-duty rotation.

The Cessna 207, a seven-seat high-wing, tricycle landing gear-equipped airplane, serial number (S/N) 20700253, was manufactured in 1974. It was powered by a Continental Motors IO-520-F engine, serial number 810024-R, rated at 300 horsepower. The airplane was also equipped with a Hartzell model PHC-C3YF-1RF, controllable pitch propeller. According to maintenance records, the last inspection performed on the airplane was a 100-hour inspection dated May 29, 2015; at that time the airframe had accumulated 28,211.4 total hours. The engine had accrued 6,296.9 hours, 537.8 hours since overhaul.

In addition to the 100-hour inspection, all six of the engines cylinders were replaced just before the accident flight. The purpose of the flight was to break in the new cylinders in accordance with Continental Motors guidelines, which recommends a normal takeoff and a shallow climb to gain airspeed and cooling. Level flight should be at 75% power and richer mixture for the first hour of flight. The second hour power settings should alternate between 65% and 75% power, varying power every 15 to 20 minutes. Descents should be made at low cruise power settings, avoiding long descents with cruise power.

On September 23, 2013, a "Winter Heat Kit" was installed on the accident airplane. The modification included the installation of an additional air induction box on the right side of the engine, additional ducting and scat tubing, as well as a heat shroud on the right muffler, and a Y duct to join the airflow from the left and right heat shrouds to increase cabin heat. According to the operator's director of maintenance (DOM), the air induction box was fabricated onsite, the Y duct was a specialized order to accommodate the modification, and the right exhaust pipe was shortened to fit the larger muffler and heat shroud.

Examination of the exhaust system and records revealed that it was not in compliance with either the engine or the airframe manufacturer's specifications. This type of modification requires an FAA 337 Major Repair and Alteration per CFR 43, Appendix A. After a submitting a Form 337 describing the alteration, the Flight Standards District Office determines what data is needed to approve the alteration as a field approval or a supplemental type certificate (STC). Once the alteration is approved the 337 is signed by an FAA inspector and the airplane is considered airworthy.

Modifications for the accident airplane were not in compliance with the manufacturers, or any known STC, nor was there an accompanying FAA 337 Major Repair and Alteration documentation.

During an inspection of Yute Air maintenance operations in May 2013, a Cessna 207 with the exhaust modification was in the hangar and there was discussion between the PMI and DOM. Based on that discussion, the PMIs understanding of the modification was that it was a "work in progress" and the final product and required paperwork would be submitted for inspection and approval. The DOM believed that a verbal agreement had been made and that a logbook entry of the modification would meet the requirement.

The closest weather reporting facility was Bethel, about 40 miles northwest of the accident site. At 1053, an aviation routine weather report (METAR) reported, in part: Wind 210 degrees at 10 knots; visibility 10 statute miles; clouds and sky condition few at 12,000 feet, scattered at 2,000 feet; temperature 16 degrees C; dew point 9 degrees C; altimeter 30.12 inHg.

The Cessna 207, a seven-seat high-wing, tricycle landing gear-equipped airplane, serial number (S/N) 20700253, was manufactured in 1974. It was powered by a Continental Motors IO-520-F engine, serial number 810024-R, rated at 300 horsepower. The airplane was also equipped with a Hartzell model PHC-C3YF-1RF, controllable pitch propeller. According to maintenance records, the last inspection performed on the airplane was a 100-hour inspection dated May 29, 2015; at that time the airframe had accumulated 28,211.4 total hours. The engine had accrued 6,296.9 hours, 537.8 hours since overhaul.

In addition to the 100-hour inspection, all six of the engines cylinders were replaced just before the accident flight. The purpose of the flight was to break in the new cylinders in accordance with Continental Motors guidelines, which recommends a normal takeoff and a shallow climb to gain airspeed and cooling. Level flight should be at 75% power and richer mixture for the first hour of flight. The second hour power settings should alternate between 65% and 75% power, varying power every 15 to 20 minutes. Descents should be made at low cruise power settings, avoiding long descents with cruise power.

On September 23, 2013, a "Winter Heat Kit" was installed on the accident airplane. The modification included the installation of an additional air induction box on the right side of the engine, additional ducting and scat tubing, as well as a heat shroud on the right muffler, and a Y duct to join the airflow from the left and right heat shrouds to increase cabin heat. According to the operator's director of maintenance (DOM), the air induction box was fabricated onsite, the Y duct was a specialized order to accommodate the modification, and the right exhaust pipe was shortened to fit the larger muffler and heat shroud.

Examination of the exhaust system and records revealed that it was not in compliance with either the engine or the airframe manufacturer's specifications. This type of modification requires an FAA 337 Major Repair and Alteration per CFR 43, Appendix A. After a submitting a Form 337 describing the alteration, the Flight Standards District Office determines what data is needed to approve the alteration as a field approval or a supplemental type certificate (STC). Once the alteration is approved the 337 is signed by an FAA inspector and the airplane is considered airworthy.

Modifications for the accident airplane were not in compliance with the manufacturers, or any known STC, nor was there an accompanying FAA 337 Major Repair and Alteration documentation.

During an inspection of Yute Air maintenance operations in May 2013, a Cessna 207 with the exhaust modification was in the hangar and there was discussion between the PMI and DOM. Based on that discussion, the PMIs understanding of the modification was that it was a "work in progress" and the final product and required paperwork would be submitted for inspection and approval. The DOM believed that a verbal agreement had been made and that a logbook entry of the modification would meet the requirement.

The main wreckage was submerged in the Kwethluk River, at 244 feet mean sea level (msl), and at latitude N60.20.123 and longitude W161.03.256. An area believed to be the initial impact point was marked by a broken treetop, atop an estimated 30-foot-tall birch tree. A portion of the right wing tip was found on the ground beneath the tree. The fuselage came to rest inverted on its left side, along the opposite side of the river, submerged in fast flowing water, about 270 feet from the initial impact point, on a magnetic heading of 010 degrees. The engine separated from the airplane; the engine and propeller were located submerged upstream and in the main river channel.

On August 5, 2015, under the supervision of the NTSB investigator-in-charge (IIC), the wreckage was examined at a private hangar in Bethel. Flight control system cable continuity was established from each control surface to the point of impact-related damage.

The throttle, propeller, and mixture controls were in the full forward position. The cowl flaps were in an intermediate position. The fuel selector was selected to the left tank, the fuel valve was also indicating the left fuel tank. The fuel pump was in the off position. Both the left and right fuel tanks were bladder tanks. The vent system was unobstructed; the vented fuel caps were on and secure.

On August 19, the NTSB IIC, and the parties noted in this report, completed an engine teardown and inspection after it was retrieved from the river and transported to Palmer, Alaska.

The engine was submerged in a river for 2 months and showed signs of water corrosion. The engine was placed on a stand for examination. The exam revealed impact damage to the oil sump, and a portion left exhaust system was missing. The right side exhaust system was removed and retained for leak testing. Only the risers remained of the left side exhaust system.

Rotational continuity was established throughout the engine and valve train. The crankcase, crankshaft, and camshaft did not show any signs of operational distress, and the bearings showed normal wear.

An Aviation Supplies & Academics, Inc., (ASA) carbon monoxide detector was mounted on the instrument panel, the date opened was recorded as 09/2013. This type of detector is a spot detector, which turns dark in the presence of carbon monoxide. Instructions on the back of the detector include "Write date opened on front and replace every 90 days."

The examination of the airframe and engine revealed no evidence of mechanical malfunction or failures that would have precluded normal operation.

The accident airplane was equipped with automatic dependent surveillance-broadcast (ADS-B) technology. In typical applications, the ADS-B capable aircraft uses an ordinary Global Positioning System (GPS) receiver to derive its precise position from the Global Navigation Satellite System (GNSS) constellation, and then combines that position with any number of aircraft parameters, such as speed, heading, altitude, and aircraft registration number. This information is then simultaneously broadcast to other ADS-B capable aircraft, and to ADS-B ground, or satellite communications transceivers, which then relay the aircraft's position and additional information to Air Traffic Control (ATC) centers in real time.

The data retrieved from the accident airplane's ADS-B information shows the airplane was operating at less than 500 feet msl for the majority of the approximate 3-hour flight. The last return signal from the accident airplane was when it was 23 miles southeast of the accident site, at an altitude of 1,075 feet msl.

A flight track map overlay is included in the public docket for this accident.

A postmortem examination was conducted under the authority of the Alaska State Medical Examiner, Anchorage, Alaska, on June 8, 2015. The cause of death for the pilot was attributed to blunt force, traumatic injuries.

The FAA's Civil Aeromedical Institute performed toxicological examinations for the pilot on August 10, 2015, which was negative for ethanol. The toxicological examination revealed 21 percent carboxyhemoglobin (carbon monoxide) in the pilot's blood.

Carbon monoxide is an odorless, tasteless, colorless, nonirritating gas formed by hydrocarbon combustion. Carbon monoxide binds to hemoglobin with much greater affinity than oxygen, forming carboxyhemoglobin; elevated levels result in impaired oxygen transport and utilization.

Nonsmokers may normally have up to 3 percent carboxyhemoglobin in their blood; heavy smokers may have levels of 10 to 15 percent. Family members and friends reported that the pilot was a nonsmoker.

The NTSB's chief medical officer reviewed the pilot's autopsy, toxicology report, personal medical records, the FAA blue ribbon medical file, and the NTSB IIC's reports.

A copy of the NTSB's medical officer's factual report is available in the public docket for this accident.

When the airplane failed to arrive in Bethel, company personnel initiated a telephone and satellite phone search to see if the airplane had diverted to another village. Unable to locate the airplane, company management personnel contacted the Air Force Rescue Coordination Center, Anchorage to report the missing airplane.

The airplane was not equipped with, nor required to be equipped with, a digital, 406 MHz ELT that instantly transmits a distress signal to search and rescue satellites, thereby alerting rescue personnel within minutes of the location of the crash. As of February 1, 2009, analog, 121.5 MHz ELT's stopped being monitored by search and rescue satellites, and the installation of the 406 MHz has been voluntary.

The fragmented and submerged wreckage was spotted in a river on May 31, about 1730, by former Yute Air pilots who had volunteered to participate in the search and rescue efforts.

Portions of the right exhaust system were retained for further examination and testing. A pressure test was done on the right muffler/heat exchanger, which revealed no leaks or factures. As previously noted, the accident airplane had been retrofitted with an additional muffler/heat exchanger, but a search at the accident site failed to find the additional muffler/heat exchanger, and it remains missing.