Maintenance Personnel’s Improper Installation Of An Engine-Driven Vacuum Pump

Location: El Cajon, California Accident Number: WPR24LA135
Date & Time: April 28, 2024, 18:15 Local Registration: N8800V
Aircraft: Bellanca 17-31ATC Aircraft Damage: Substantial
Defining Event: Loss of engine power (total) Injuries: 1 Minor
Flight Conducted Under: Part 91: General aviation - Personal

Analysis: The pilot was conducting a test flight following the completion of an annual inspection. During the takeoff climb, the pilot observed an abnormal oil pressure indication and elected to return to the airport. While returning to the airport, the engine sustained a complete loss of oil pressure followed by a loss of all power. Unable to maintain altitude, the pilot elected to make an off-airport landing to a nearby road. During the landing, the airplane struck a power line and a utility pole before it impacted the ground and came to rest upright.

A serviceable engine-driven vacuum pump had been installed on the airplane about 25 days before the accident by a mechanic trainee. After the installation, an engine test ground run was completed and the airplane was returned to service. A mechanic with airframe, powerplant, and inspection authorization (A&P IA) was present but did not verify that the engine-driven vacuum pump had been properly installed or set to any specific torque value. Postaccident examination of the engine revealed a crack in the engine case between cylinder Nos. 5 and 6, as well as no presence of oil in the oil sump. Internal thermal discoloration and damage identified during the engine teardown were consistent with lubrication deprivation.

Additionally, the engine-driven vacuum pump remained attached to but not secured to its mounting pad. All four of the mounting studs were loose, which allowed for forward and aft movement, or “play,” in the vacuum pump of about 1/8 inch with minimal resistance. The vacuum pump accessory mounting pad has a drilled passage that can supply pressurized lubricating oil. In the accident installation, this oil passage was blocked off by mechanical clamping force and a gasket. Absent sufficient clamping force, the oil would leak out until the oil sump is depleted of oil.

Probable Cause and Findings: The National Transportation Safety Board determines the probable cause(s) of this accident to be -- Maintenance personnel’s improper installation of an engine-driven vacuum pump, which resulted in oil starvation to the engine and a subsequent total loss of engine power. 

Contributing to the accident was the A&P IA’s lack of supervision.

FMI: www.ntsb.gov