N9883R

On April 1, 2020, about 1310 central daylight time, a Beech M35 airplane, N9883R, sustained substantial damage when it was involved in an accident near Sequin, Texas. The pilot and passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

According to the pilot, on the initial climb, the No. 3 cylinder indicated a cylinder head temperature (CHT) about 60° cooler than the other five cylinders. At 1,000 ft above ground level, the first power reduction was made from takeoff power to 2,500 rpm and 25 inches manifold pressure. The pilot reported a “normal reduction” in fuel flow was made with the mixture control. Immediately, the pilot heard and felt a loud, continuous “rhythmic banging” noise. The pilot noted the No. 3 cylinder CHT had reduced to 220° F; however, all other engine instrument indications remained in the normal range, and the engine still produced power.

The pilot decided to return to the departure airport and land. The pilot assessed that, if a total loss of engine power occurred, there was sufficient altitude and airspeed to turn base and final legs for a landing on the departure runway. He also planned a touchdown point on the grass runway beyond the normal touchdown point.

Once established and set up on the final approach for runway 14, the pilot concluded that the airplane was "high and fast" and there was insufficient altitude to perform any descending turns. The pilot reported he was committed to the landing even though he knew he would land "long and hot." After the touchdown on the dry grass, the rollout was “completely under control,” but the groundspeed was “fast.” The airplane crossed the end of the runway and traveled past the runway overrun area. The airplane impacted a small metal farm fence and came to rest upright.

The outboard portion of the right wing impacted a small tree trunk and rotated the airplane 180° back toward the runway, which caused substantial damage. The pilot reported the engine continued to run until he shut it down. The pilot and passenger were able to egress from the airplane without further incident.

The pilot reported he departed with 52 gallons of fuel onboard; there was fuel present in all four fuel tanks, and there were no fuel leaks after the accident. A postaccident examination of the engine was performed and a small amount of unidentified debris was present underneath the fuel manifold valve screen. No other preimpact anomalies were noted with the engine. The airframe was released to salvage before the investigation was able to complete a detailed examination.

This engine’s No. 3 cylinder is located on the right side of the engine, in the middle, between the No. 5 cylinder (forward), and the No. 1 cylinder (aft). The Federal Aviation Administration (FAA)-approved Continental Motors Operator’s Manual for the IO-470 series engine discusses CHT and states in part:

Cooling of cylinders depends on aircraft baffles being maintained properly fit to the cylinder heads and barrels and other joints in the pressure compartment being tight so as to force air between the cylinder fins. Proper cooling also depends on operation practices. The recommended power setting (RPM and manifold pressure) should not be exceeded for long periods. Fuel and air mixture ratio also affects cylinder temperature.

The FAA Airplane Flying Handbook (FAA-H-8083-3B) discusses three likely probable causes of a low CHT and states

Excessive cowl flap opening.

Excessively rich mixture.

Extended glides without clearing engine.

This document also lists corrective actions for the three likely probable causes of a low CHT and states:

Adjust cowl flaps.

Adjust mixture control.

Clear engine long enough to keep temperatures at minimum range.