Amateur-built LEGACY FG N3UH accident near Hollywood, Florida

Probable cause

The pilot/owner’s failure to properly reinstall the number 1 cylinder spark plug, which resulted in a partial loss of engine power during the initial climb. Contributing was the pilot's exceedance of the airplane’s critical angle of attack during an attempted return to the airport following the partial loss of engine power, which resulted in an aerodynamic stall, loss of control, and impact with terrain.

NTSB analysis

The pilot was departing in the experimental, amateur-built airplane from his home airport. Data retrieved from an onboard electronic flight instrument indicated that, shortly after takeoff the engine lost partial power. While attempting to turn back to the airport, the pilot exceeded the airplane’s critical angle of attack and the airplane entered an aerodynamic stall and rolled to the right, impacting terrain in a nose- and right-wing-down attitude.

Further examination of the data indicated that, when the partial loss of power occurred, the exhaust gas temperature for cylinder No. 1 showed a marked divergence from the other cylinders and the engine speed dropped to 1,600 rpm with manifold pressure decreasing to about 12 inHg. A few seconds later, the manifold pressure had increased to about 22 inHg and engine speed had increased to about 2,300 rpm, consistent with the other three cylinders continuing to function, and also consistent with the engine responding to pilot inputs. The data also indicated that at this time, the pilot allowed the airplane to pitch up, and the airplane reached its critical angle of attack; a right roll of over 100° developed before the airplane impacted the ground.

No preimpact failures or malfunctions of the airplane or engine were discovered that would have precluded normal operation, with the exception of the No. 1 cylinder’s top sparkplug and adapter, which had blown out of the top sparkplug hole in the No. 1 cylinder. The sparkplug was still attached to its high-tension lead, and was still threaded into its adapter, but displayed thread damage on the two lower threads of the adapter, with no evidence of an anti-seize compound being applied to the threads. This was likely indicative of improper installation, either by application of inadequate torque or cross-threading during installation. The airplane’s maintenance logbooks were not located and it could not be determined whether the airplane had a current condition inspection at the time of the accident.

The mechanic who performed the airplane’s most recent condition inspection reported that he had observed the pilot maintain the accident airplane on a number of occasions, most recently within the month that the accident occurred. The mechanic reported that at that time, he saw the airplane “opened up,” with numerous sticky notes on various parts of the engine.

The pilot kept an air compressor and differential compression tester in his hangar, which suggested that the pilot was familiar with removing the sparkplugs and adapters and how to perform a compression test. Additionally, a notebook the pilot kept contained a partial date that was likely entered about 3 days before the accident, which appeared to be cylinder pressure readings for each cylinder. It is possible that the pilot may have performed a compression test on the engine in anticipation of it being accepted by a mechanic as part of an owner-assisted inspection and that he improperly reinstalled the No. 1 cylinder top sparkplug adapter following that compression test.

Based on the available information, it is likely that the pilot experienced a partial loss of engine power after takeoff when the No. 1 cylinder top sparkplug separated from the cylinder as a result of improper installation. During the pilot’s attempt to return to the airport, he exceeded the airplane’s critical angle of attack and the airplane entered an aerodynamic stall.

Text from the NTSB. Open the NTSB docket for ERA21LA052 for the full record.