Ayres S2R-G6 N447AT

Summary

On 11 May 1996 at about 15:00 local time, a Ayres S2R-G6 registered N447AT was involved in an accident near Williams, California, United States. One person was on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

11 May 1996 at 15:00

Classification

Accident

Location

Williams, California, United States

Nearest airport

Private Ag Airstrip

Coordinates

39.1101, -122.2706

Aircraft

Ayres S2R-G6

Registration

N447AT

Category

Airplane

Engines

1

Operator

Not recorded

Operating rule

Part 137: Agricultural

Purpose of flight

Aerial Application

Phase of flight

Not recorded

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

LAX96LA196

Nobody was injured.

The pilot's failure to maintain directional control during a rejected takeoff, and the catastrophic failure of the engine's turbine section due to an improper overhaul procedure. A factor in the accident was the close proximity of the drainage ditch to the edge of the private airstrip's runway.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

The pilot reported that as the aircraft reached the takeoff rotation point, the engine's turbine section disintegrated, throwing large chunks of turbine wheels and other debris through the top and side of the engine cowling. The pilot lost directional control while attempting to stop the aircraft on the 2,700 foot-long dirt runway and it veered off the right side of the airstrip, crossed a drainage canal, and collided with the opposite embankment. Engine disassembly revealed that the second stage turbine wheel rim separated over almost its entire circumference. Detailed examination of the hub's fracture found machining type scoring to a depth of about 0.1 inches on the forward face of the wheel, which reduced the strength in the wheel web until overload failure occurred. Inspection of the second stage turbine stator assembly found that the sheet metal support for the turbine wheel to stator air seal was separated from the stator and had shifted forward to contact the failed wheel. Twenty-two out of 31 cooling passages which pass through the stator vanes from the outside diameter of the stator to the seal support area were completely plugged by braze repair material. The braze material had flowed from the outside diameter toward the inside during an attempt to braze repair the sheet metal on the castings outside diameter. The seal support showed evidence of operation at a temperature a few hundred degrees hotter than normal. Allied Signal reported that this overtemp condition will cause the seal support to crack after a few hundred hours of operation. Review of the Allied Signal overhaul documents for the second stage stator assembly disclosed that a braze repair is only authorized for the rear air seal attachment to the casting, and not in the area of the cooling passages. The historical engine maintenance records disclosed that the second stage stator was overhauled on August 23, 1995, and installed in the engine on October 23, 1995, 207 hours and 230 cycles since overhaul.

Quoted verbatim from the NTSB record.