Robinson R44 N632NV accident near Wahiawa, Hawaii

Probable cause

Loss of oil in flight as a result of a loose oil filter adapter which resulted in the pilot initiating an immediate landing to an open field. Contributing to the accident was the pilot's manipulation of the throttle that led to a reduction in rotor rpm, which resulted in his inability to perform a normal landing.

NTSB analysis

The pilot and two passengers were in cruise flight in the helicopter when the engine oil light illuminated. The pilot initiated an immediate descent to land and confirmed that the engine continued to produce power (the oil pressure light is one of the indications of an engine failure). As the helicopter approached the ground, the pilot slightly manipulated collective to maintain rotor rpm within limits. The helicopter impacted terrain with some forward speed and slid about 40 ft before coming to a stop. During his postflight examination of the helicopter, the pilot noted oil on the outside of both lower engine inspection panels and on the inside of the engine inspection panel on the pilot's side. The oil level dipstick indicated oil at about the 1/8-inch mark.

Review of video from the onboard camera indicated that the red "oil" warning light illuminated in flight, and the oil pressure gauge indicated that the oil pressure had dropped to zero. It also revealed that during the deceleration/flare sequence, the engine rpm dropped to about 80%; however, once the helicopter stabilized on the ground, the engine rpm increased and returned to 102%.

According to the R44 Pilot’s Operating Handbook, engine rpm is maintained by a governor, but only above 80%. For the engine rpm to drop to 80% and below, the pilot must manually rotate the throttle. Manual manipulation is not typically required except during start up, shut down, autorotation practice, and emergencies. The handbook also states that the emergency procedure in response to an illuminated oil light is to "land immediately," which the pilot proceeded to do. However, the helicopter did not lose engine power; therefore, there was no need for the pilot to manipulate the throttle. It is possible that, without throttle manipulation, the pilot could have performed a normal landing to a suitable landing area and landed safely under power without damage to the helicopter.

A postaccident engine functionality check did not reveal any anomalies; the engine ran for 8 minutes without any apparent oil leaks. However, the next day, fresh oil was noted under the engine oil filter. Further examination revealed that the torque required to rotate the oil filter was significantly less than normal. In addition, the threaded fitting that the filter threads onto the adapter was about 2 threads in, and the filter cavity was empty of any oil. The loss of oil in flight was likely a result of the loose oil filter adapter.

Text from the NTSB. Open the NTSB docket for WPR18LA269 for the full record.