TB 1-1520-240-20-93
(1) Wholesale Stock. Upon receipt of this TB, depot commanders shall ensure inspection of all
items listed in paragraph 7 below located in all wholesale depots including all satellite wholesale depots and
war reserve depots. Upon receipt of this TB, commanders and others maintaining wholesale stock shall
ensure that serviceable materiel (condition code "A","B" and "C") is placed in condition code "J" and tagged
with a suspended tag/label - DD Form 1576/1576-1. The authority block must reference inspection required
per this TB (TB 1-1520-240-20-93). Any nuts found discrepant per the inspection procedures cited in para-
graph 8 below shall be disposed of by mutilation to prevent resale, or reuse.
(2) Retail Stock. Upon receipt of this TB, commanders and others maintaining retail stock of
items listed in paragraph 7 below shall contact their support aviation organization to perform the inspection
required by paragraph 8 below and to perform the correction procedures of paragraph 9 below on discrepant
materiel. All items listed in paragraph 7 below shall be immediately placed in condition code "D" and retagged
with a test/modification tag/label - DD Form 1576/1576-1 (blue color). The authority block must reference
inspection required per this TB (TB 1-1520-240-20-93). Items identified as serviceable in accordance with
(IAW) paragraph 8 below shall be retagged with a serviceable tag/label - DD Form 1574/1574-1 (yellow
color). Indicate compliance with this TB (TB 1-1520-240-20-93) in the remarks block. Any items found
discrepant per the inspection procedures cited in paragraph 8 below shall be disposed of by mutilation to
prevent resale, or reuse. Report compliance with this TB in accordance with paragraph 14d(2) below.
g. Components/Parts in Work. N/A.
2. Task/Inspection Suspense Date. Within 10 hours/14 days.
Reporting Compliance Suspense Date. No later than 03 April 1997 per paragraph 14a of this TB.
3.
Summary of the Problem.
4.
a. There have been three reported instances of AN320-12 castellated nuts found cracked. One during
pre-flight inspection of a CH-47D helicopter where a lag damper attachment nut at the rotor head connection
was found cracked. The second during post flight inspection of a MH-47E helicopter. Both nuts were
forwarded to Corpus Christi Army Depot (CCAD) for failure analysis which revealed the nuts were cracked
at the manufacturer's designation (logo) impression stamp. The defective nuts were both double impression
stamp with the capital letter G, which is the registered logo of Greer Stop Nut Inc (CAGE 27687). A third
instance involving a MH-47E helicopter was very recently reported and is being investigated.
b. The CCAD failure analyses concluded that there were three contributing causes of failure. The
double stamp impression on the cracked nuts and the excessive depth of the impressions provided an area
of stress concentration that assisted in the cracking. The torque value for the nut in the TMs were to high in
the drive arm attachment installation and needed to be lowered to not to exceed 3,000 inch-pounds. Also,
the grain structure contained islands of pearlite and ferrite, both of which are characteristic of soft phases of
steel. The nuts should have been quenched and tempered steel.
c. For manpower/downtime and funding impacts, see paragraph 12 below.
d. The purpose of this TB is to inspect the forward and aft rotor system and controls installations, as
delineated in paragraph 8 below, and to replace AN320-12 castellated nuts that exhibit a capital letter G
vendor identification impression stamp.
End Items to be inspected. All CH-47D, MH-47D, and MH-47E aircraft.
5.
Assembly Components to be Inspected.
6.
PART NUMBER
NOMENCLATURE
NATIONAL STOCK NUMBER
145R0002-Series
N/A (XC)
Rotor System lnstl, Forward and Aft
145R3550-Series
Control lnstl, Forward and Aft
N/A (XC)
7.
Parts to be Inspected.
PART NUMBER
NATIONAL STOCK NUMBER
NOMENCLATURE
AN320-12
Nut, Thin, Plain, Castellated, Shear
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