Grob Aircraft Ag G120TP-A N196TP accident near Abbeville, Alabama

Probable cause

The flame-out of the engine due to a mis-set fuel control unit at the time of manufacture. Contributing to the accident was the pilots’ delayed recognition that the engine had flamed out, partially due to the operator’s requirement to disable the crew awareness system.

NTSB analysis

Shortly after takeoff on a local instructional flight, the turboprop engine’s continuous ignition switch was turned off, where it likely remained for the remainder of the flight. The pilot, who was receiving instruction, performed flight maneuvers, after which the instructor took control of the airplane to demonstrate a simulated loss of engine power procedure. The procedure was initiated with the crew alerting system (CAS) circuit breaker pulled, per the operator’s checklist, and the continuous ignition switch remained off, which was contrary to the recently amended procedure specified in the airplane’s flight manual. As part of the procedure, while flying with the power lever at flight idle at 3,350 ft pressure altitude and about 1.8 nautical miles from the intended runway, the flight instructor slowly moved the condition/propeller rpm lever aft to the “low” gate detent. One second later, the engine indications began to decrease, consistent with an engine flameout. Although visual annunciations of an engine flameout would have been available to differing degrees on the flight displays, no aural annunciations occurred because CAS circuit breaker had been pulled.

The instructor continued the maneuver, unaware of the loss of power, and extended the landing gear on the downwind leg of the airport traffic pattern. While on the base leg of the traffic pattern, about 2.5 minutes after the engine flamed out, the instructor recognized the situation, turned toward the runway, and immediately called for an engine restart; at this time, the airplane was less than 100 ft above the ground. The pilot attempted to restart the engine, and the engine began to restart, but about 3 seconds later, the airplane impacted trees and the ground.

The engine was placed in the manufacturer’s test cell and found to operate with minor discrepancies noted. The fuel control unit (FCU) was operationally tested at the manufacturer’s facility and was found to provide a flow of 20.4 pounds-per-hour (PPH); the specification was 28.0 to 30.0 PPH. The low setting occurred at manufacturing and was attributed to a flow bench indication error.

The manufacturer had experienced engine flame-out issues during production acceptance flight testing. These instances were initially attributed to either rapid movement of the condition lever or the placement of the continuous ignition switch in the off position. To address the issue, the flight manual procedures for simulated forced landing events were updated to address these scenarios. Eight subsequent engine flame-out events were eventually attributed to be from a mis-set minimum flow test point of the FCU at manufacturing, the same condition as the accident FCU.

The FCU manufacturer identified 124 units that were possibly mis-set, and service bulletins were published by the FCU and engine manufacturers to address the low fuel flow of the suspect units.

Text from the NTSB. Open the NTSB docket for ERA17LA125 for the full record.