N75RM

HISTORY OF FLIGHTOn October 27, 2021, at 1817 central daylight time, a Beech B36TC airplane, N75RM, was substantially damaged when it was involved in an accident near Lena, Wisconsin. The pilot was fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The airplane departed from Fort Worth Meacham Airport (FTW), Fort Worth, Texas, about 1357 and proceeded on a northeasterly course. The pilot’s mechanic stated that the airplane was destined for Camdenton Memorial – Lake Regional Airport (OZS), near Camdenton, Missouri, so that the airplane could undergo an annual inspection.

About 1427, the airplane leveled off near 15,500 ft mean sea level (msl). About 1451, the airplane entered a climb; shortly afterward, its flightpath began to deviate as the airplane approached a line of storms. About 1519, the airplane leveled off about 24,500 ft msl and returned to a northeasterly course.

Air traffic control attempted to contact the pilot after the airplane climbed through 18,000 ft and into class A airspace, but these attempts were not successful. About 1756, the airplane entered a descent from 24,500 ft msl; at the time, the airplane was about 25 miles west of Green Bay, Wisconsin. About 1758, the airplane’s course became slightly erratic as the descent continued; about 6 minutes later, the airplane returned to the northeasterly course and became established in a steady descent of about 1,000 ft per minute, which continued until the data ended. The final data point was recorded at 1817:11. The airplane impacted a cornfield about 480 ft northeast of the final recorded data point. The impact path was aligned with the final portion of the flightpath. PERSONNEL INFORMATIONThe pilot’s logbook was not located during the investigation. As a result, the pilot’s most recent flight experience and flight review could not be determined. At the time of the pilot’s most recent airman medical examination, conducted in October 2017, the pilot reported total civil flight time of 2,465 hrs. The pilot’s third-class medical certificate expired in 2019. In July 2020, the pilot completed a BasicMed comprehensive medical examination checklist and a BasicMed course. AIRCRAFT INFORMATIONThe airplane was not pressurized, but it was equipped with an onboard oxygen system that included a 76.5-cubic ft oxygen cylinder installed below the front seats and fitted with an altitude compensating regulator. The cylinder shutoff valve was controlled by a push-pull knob on the lower portion of the instrument panel. The oxygen supply was routed to receptacles on the left and right cockpit sidewalls for the pilot and copilot/front seat passenger, respectively. Receptacles were also located in the center cabin for any rear seat passengers. A gauge indicating the pressure within the oxygen system was located on the left cockpit sidewall, but the airplane had no independent indication of low oxygen system pressure.

A pulse-demand oxygen delivery module in the airplane was connected to the right sidewall (copilot) oxygen receptacle. The module was fitted with two nasal cannulas, and first responders reported the pilot was found wearing one. The module was set to the “F10” mode, which supplied an oxygen flow rate equivalent to 10,000 ft above the pressure altitude sensed by the unit.

The oxygen delivery module incorporated several annunciations, which included a flow fault, an apnea event, and low-battery warnings. The flow fault provided a red light indication and an aural alarm if no oxygen was flowing to the unit. An apnea event provide an amber light indication and an aural alarm when a “valid inhalation event” was not detected within 30 seconds. Neither warning was intended to indicate to the pilot that the system was out of oxygen. The NTSB did not perform an evaluation of the salience of the audible alarms when presented in the cockpit with background engine noise and when a pilot was using an aviation headset.

According to the mechanic that had performed the most recent annual inspection, the pilot contacted him a few weeks before the accident to inquire about an annual inspection. The mechanic was expecting the pilot to bring the airplane to his facility in Camdenton, Missouri, on the day of the accident.

There was no record that showed when the oxygen system was last serviced, refilled, or used. A specific maintenance record is not required when refilling the oxygen cylinder. Similarly, routine use of the system during flight is not required to be logged.

Although that airplane was equipped with an autopilot, there was no data specific to its use during the flight, or any modes that may or may not have been selected. METEOROLOGICAL INFORMATIONThe National Weather Service composite radar mosaic at 1450 depicted a line of rain showers and thunderstorms across the airplane’s flightpath. The line ran from near Wichita, Kansas; southeast to Tulsa, Oklahoma; and past Hot Springs, Arkansas. Individual thunderstorm cell tops were indicated from 20,000 to 24,000 ft msl. AIRPORT INFORMATIONThe airplane was not pressurized, but it was equipped with an onboard oxygen system that included a 76.5-cubic ft oxygen cylinder installed below the front seats and fitted with an altitude compensating regulator. The cylinder shutoff valve was controlled by a push-pull knob on the lower portion of the instrument panel. The oxygen supply was routed to receptacles on the left and right cockpit sidewalls for the pilot and copilot/front seat passenger, respectively. Receptacles were also located in the center cabin for any rear seat passengers. A gauge indicating the pressure within the oxygen system was located on the left cockpit sidewall, but the airplane had no independent indication of low oxygen system pressure.

A pulse-demand oxygen delivery module in the airplane was connected to the right sidewall (copilot) oxygen receptacle. The module was fitted with two nasal cannulas, and first responders reported the pilot was found wearing one. The module was set to the “F10” mode, which supplied an oxygen flow rate equivalent to 10,000 ft above the pressure altitude sensed by the unit.

The oxygen delivery module incorporated several annunciations, which included a flow fault, an apnea event, and low-battery warnings. The flow fault provided a red light indication and an aural alarm if no oxygen was flowing to the unit. An apnea event provide an amber light indication and an aural alarm when a “valid inhalation event” was not detected within 30 seconds. Neither warning was intended to indicate to the pilot that the system was out of oxygen. The NTSB did not perform an evaluation of the salience of the audible alarms when presented in the cockpit with background engine noise and when a pilot was using an aviation headset.

According to the mechanic that had performed the most recent annual inspection, the pilot contacted him a few weeks before the accident to inquire about an annual inspection. The mechanic was expecting the pilot to bring the airplane to his facility in Camdenton, Missouri, on the day of the accident.

There was no record that showed when the oxygen system was last serviced, refilled, or used. A specific maintenance record is not required when refilling the oxygen cylinder. Similarly, routine use of the system during flight is not required to be logged.

Although that airplane was equipped with an autopilot, there was no data specific to its use during the flight, or any modes that may or may not have been selected. WRECKAGE AND IMPACT INFORMATIONAfter impacting the cornfield, the airplane slid about 150 ft before coming to rest. The landing gear and wing flaps were retracted. The lower fuselage structure was damaged due to impact from the airplane nose to the mid-cabin area. Flight control continuity was confirmed from each control surface to the cockpit. An engine examination revealed no anomalies consistent with the engine’s inability to produce rated power. Both fuel tanks appeared to be intact. About 35 gallons of fuel remained in the left tank; no fuel remained in the right tank. The cockpit fuel selector was found set to the right tank.

Examination of the onboard oxygen system revealed that the oxygen cylinder was intact. The altitude compensating regulator and overpressure relief valve attachment fittings were damaged due to impact. The cylinder valve was in the ON position, and the control cable from the cylinder valve to the instrument panel control knob was intact and continuous. The oxygen lines appeared intact with the exception of a right-angle fitting common to the left (pilot) sidewall receptacle. With the system pressurized, a noticeable leak was identified behind the left sidewall. The fitting was cracked at the supply end.

Metallurgical examination of the fitting revealed that one side of the fracture surface exhibited an area with Teflon thread sealant embedded onto the surface. The sealant was in the area of the first three threads and did not extend along the full width of the crack. A portion of the fracture surface was smooth and showed no fracture features, such as ductile dimples, which was consistent with a casting defect in that area. The remaining portion of the fracture surface exhibited ductile dimples, consistent with an overstress fracture.

Examination of the oxygen cylinder valve revealed no anomalies. Examination of the altitude compensating regulator revealed that the regulator exhibited minor deviations from the test requirements; however, none of these discrepancies were consistent with the system’s inability to provide the required oxygen.

Examination and testing of the pulse-demand oxygen delivery module and associated in-line pressure regulator revealed no anomalies from the required test parameters. Testing of the oxygen system pressure gauge located on left cockpit sidewall revealed no anomalies. When oxygen pressure was applied, the gauge indicated the correct supply pres...