Certification Service Washington, DC U.S. Department of Transportation Federal Aviation Administration
June 29, 2011
This is information only. Recommendations
This Special Airworthiness Information Bulletin (SAIB) informs
registered owners/operators of all Air Tractor, Inc. AT-300, AT-301,
AT-302, AT-400, AT-400A, AT-401, AT-401A, AT-401B, AT-402, AT-402A,
AT-402B, AT-501, AT-502, AT-502A, AT-502B, AT-503, AT-503A, and AT-504
model series aircraft (under Type Certificate Data Sheets A9SW
and A17SW) of an airworthiness concern. Specifically, this SAIB
provides general and detailed inspection for fatigue cracking of the
fuselage frame wing attach tube.
At this time, this airworthiness concern has not been determined to be
an unsafe condition that would warrant airworthiness directive (AD)
action under Title 14 of the Code of Federal Regulations (14 CFR) part
The Federal Aviation Administration (FAA) received reports of a fatigue
crack at the upper bushing location in the fuselage frame wing attach
tube on two Air Tractor, Inc. AT-502B model airplanes. Both airplanes
had factory installed gussets around the upper bushing area and earlier
production thinner walled tubing. Originally, reports of fatigue cracks
at the upper bushing on AT-300/400 series airplanes were addressed by
Snow Engineering Co. Service Letter #69. Cracks were also reported on
early AT-501/502 series airplanes at the two middle bushing locations.
Inspection and repair of early production AT-501/502 model series
airplanes was addressed by Snow Engineering Co. Service Letter #92.
These early airplanes lacked the reinforcing gussets at these
locations. Later AT-502 series airplanes received gussets at all four
bushing locations starting at serial number -0129, and were produced
with a thicker walled tube starting at serial number -615.
Left unaddressed, cracks can continue around the perimeter of the wing
attach tube and result in loss of connection of the fuselage wing
attach tube with the remainder of the fuselage frame. The wing attach
angles that bolt to the fuselage wing attach tube are also known to
crack in the radius of the attach angle. Inspection for cracks in the
wing attach angles was originally addressed by Snow Engineering Co.
Service Letters #89 and #89A.
In response to these concerns, Air Tractor, Inc. recently issued Snow
Engineering Co. Service Letter #296 to conduct directed visual
inspections of the right and left hand fuselage wing attach tubes and
wing attach angles. The service letter specifies recurring directed
visual inspections every 100 hours using a borescope with a more
thorough inspection at each annual inspection with the attach angles
removed. If cracks are indicated, confirm by removing the paint and
performing a dye penetrant inspection and contact the Air Tractor
Service Department if any cracks are found to obtain a repair method.
The FAA recommends you inspect your Air Tractor, Inc. airplane fuselage
frame wing attach tubes and wing attach angles at the time intervals
and using the inspection procedures directed in Snow Engineering Co.
Service Letter #296. Repair or replace any cracked structure found
during any inspection in accordance with Air Tractor Service Department