NYC95LA157

The pilot was conducting aerobatic maneuvers. The pilot stated, that during a maneuver, '...As I pulled out of the maneuver (a 4 to 5 G pull) I heard a very loud 'shot gun' type noise and simultaneously experienced a hard jolt and a release of pressure on the stick as the plane's nose pitched downward....The throttle lever jammed at approximately the half closed position. The plane was descending; in order to stabilize the airspeed for a forced landing, I had to hold full back stick pressure.' Examination of the aircraft revealed the upper left and right longeron tubes from the fuselage were fractured. The fracture of the longerons restricted movement of the cables for the throttle and mixture control. Metallurgical examination of the longeron fractures revealed evidence of fatigue cracking.

On July 11, 1995, about 1630 eastern daylight time, a Pitts S-2B, N260BF, sustained substantial damage when the upper longeron tubes fractured while performing an acrobatic maneuver near Queenstown, Maryland. The commercial pilot was not injured. Visual meteorological conditions prevailed for the personal flight operated by Lynn Aviation, Inc. The flight had originated from Edgewood, Maryland at 1605. No flight plan was filed for the flight conducted under 14 CFR Part 91.

In the NTSB form 6120.1/2, the pilot stated,

...I was practicing aerobatic maneuvers....I initiated a one and one quarter left turn spin....As I pulled out of the maneuver (a 4 to 5 G pull) I heard a very loud "shot gun" type noise and simultaneously experienced a hard jolt and a release of pressure on the stick as the plane's nose pitched downward. I immediately pulled the throttle back to reduce power; the throttle lever jammed at approximately the half closed position (would not move forward or aft). The plane was descending; in order to stabilize the airspeed for a forced landing, I had to hold full back stick pressure....I initiated my approach to a grass field on a private farm and landed the aircraft.

Postaccident examination by the FAA Inspector revealed that the upper left and right longeron tubes from the fuselage structure had fractured. The fuselage was separated approximately two inches. The separation of the left longeron affected the throttle cable.

The upper left and right longeron tubes were examined by the National Transportation Safety Board, Office of Research and Engineering, Material Laboratory Division, Washington, DC. The metallurgist's factual report stated:

...The fracture surface in the right tube was on a flat transverse plane, typical of fatigue cracking. However, most of this portion of the fracture also contained rubbing damage from recontact with the mating face of the fracture. Isolated areas containing crack arrest positions, also indicative of fatigue cracking....The separation was on a 45 degree slant plane, indicative of an overstress separation in this portion of the fracture....The fracture surface on the right tube piece was examined with a scanning electron microscope (SEM). This examination confirmed that the fracture surface details within the fatigue area had been obliterated by rubbing between the mating fracture faces. Because of this damage, the SEM examination did not reveal the exact location of the fatigue origin area....

...Detailed visual examination of the separation in the upper left tube revealed a relatively small area with fatigue cracking characteristics. The features within this fatigue area emanated from the outside diameter surface of the tube at a position corresponding to the toe of the weld....The fatigue cracking propagated almost completely through the tube wall thickness and over a circumferential width of about 0.12 inch, based on the extension of the fatigue features into a damaged area on one side of the fatigue region. Initiation was from multiple sites on the outside diameter of the tube....Necking down deformation was found adjacent to all portions of the tube away from this fatigue area. Examination of the left tube with the SEM revealed that undamaged portions of the fracture surface outside of the fatigue region contained ductile dimples, typical of overstress separation areas. The SEM examination revealed no evidence of defects that may have contributed to the initiation of the fatigue cracking.

The metallurgical report further stated:

Necking down deformation is a reduction in thickness that usually accompanies overstress separation areas in ductile material.