Piper PA-34-200 N56484

Summary

On 11 January 2003 at about 18:23 local time, a Piper PA-34-200 registered N56484, operated by The Aero Club, was involved in an accident near Westlake Villag, California, United States. 2 people were on board and 2 were seriously injured. The aircraft was destroyed. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

11 January 2003 at 18:23

Classification

Accident

Location

Westlake Villag, California, United States

Coordinates

34.1333, -118.7167

Aircraft

Piper PA-34-200

Registration

N56484

Category

Airplane

Engines

2

Operator

The Aero Club

Operating rule

Part 91: General Aviation

Purpose of flight

Instructional

Phase of flight

Not recorded

Route

Van Nuys → Unknown

Aircraft damage

Destroyed

Weather

VMC

Light

Day

NTSB number

LAX03FA065

2 people were seriously injured.

a fatigue crack in the propeller hub due to the inadequate design location of the grease fitting and the chamfering process.

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

Read the full NTSB narrative

The airplane collided with ground obstacles during a forced landing when the pilots could not maintain altitude after one propeller blade and part of the propeller hub from the right engine separated in flight. The CFI and the student practiced two simulated engine-out maneuvers. The CFI was reaching for the mixture control for a third simulation when he heard a loud bang. The airplane yawed violently to the right and began to lose altitude. The CFI turned toward his home field, but was having difficulty maintaining altitude. He decided that he would not be able to make it to the airport, and would have to make an off field landing. He landed in the only clear spot that he could see, which was a cemetery. The airplane collided with monuments and a wood chip pile before coming to rest next to a mausoleum. The right propeller hub fractured and separated. Investigators did not recover one blade and the corresponding part of the hub. The hub fractured as a result of fatigue cracking emanating from one of the grease-fitting holes. The fatigue cracking originated in the area of the grease-fitting threads closest to the interior surface of the hub, at the intersection between the grease-fitting hole and the chamfer. No material or manufacturing defect was found that could be identified as the source of the fatigue cracking. The fatigue cracking originated just outside the thread remnants of the grease fitting hole that were plastically deformed during a chamfering process. The grease fitting was located in a region where the wall of the hub was approximately at its thinnest, and where there was a slight change in the profile along the axis of the propeller. These geometrical factors would have led to high stresses at the edges of the hole. Chamfers were retrofitted to the grease-fitting holes by the manufacturer at both the interior and exterior surfaces of the hub in response to fatigue fractures that occurred in three-bladed propellers. This accident was the first occurrence of fatigue emanating from a grease-fitting hole for a two-bladed propeller. The chamfering was apparently introduced in an effort to smooth the stress distribution around the grease-fitting holes. The chamfer at the interior surface should have removed the stress concentration associated with the change in wall profile, but would also have decreased the amount of material in the wall, leading to higher stresses overall. The stress concentration associated with the grease-fitting hole and the chamfer, coupled with the location in a position where the hub wall was very thin, were likely sufficient to initiate fatigue cracking in this case. In more recently manufactured hubs, the grease-fitting holes have been relocated to positions where the hub wall is much thicker.

Quoted verbatim from the NTSB record.