TB 1-2835-205-20-4
2.
Task/Inspection Suspense Date. Prior to next flight or APU start.
NOTE
Aircraft may complete a one time flight, with intermediate stops, to a suit-
able maintenance facility prior to inspection. The one time flight will not
exceed 20 APU starts.
this TB.
4.
Summary of the Problem.
a. Background.
(1) In 1995 there was an incident where an APU compressor wheel burst during start--up (see
reference 13.a.). The failure was attributed to fatigue which originated from bolt holes on the back of the wheel
(see reference 13.b.). The damaging fatigue cycles are incurred by starting the APU.
(2) Previously, APU starts have not been recorded and have not been maintained and trans-
ferred with the APU. Currently, start cycles for a particular compressor wheel are estimated based on the
hours logged on the aircraft records. The estimates use four APU starts for one flight hour.
(3) APU's overhauled since November 1996 were subject to magnetic particle inspection of the
compressor wheel for fatigue cracks and are given a one time life extension of 2100 starts since inspection
at depot (see "NOTE" in para 9). Only APU's specifically listed in paragraph 9.b. below are authorized to
operate up to 525 hours or 2100 starts since inspection (SSI). Future APU's from depot shall have forms
and records reflecting this new retirement life.
(4) The data from these inspections indicate that with no life limit, which is the current situation,
the T--62T--2B APU compressor wheel has a high probability of catastrophic failure. In the event of an impeller
burst, up to three pieces of the impeller may exit the APU radially. These pieces will weigh up to 1.1 pounds
each and will be moving at extremely high velocity.
(5) A new design has been approved for the compressor wheel which will have a greater fatigue
life. The new wheels will be incorporated by OLR team using MWO or by depot.
b. For manpower/downtime and funding impacts see para 12.
c. The purpose of this message is to impose a retirement life on APU compressor wheels installed on
APU P/N 160150--10 or 160150--100 and starts since inspection (SSI) limits from time of depot inspection.
Additionally, this message adds historical record requirements that will allow tracking of APU usage by starts.
It also imposes restrictions on APU's with non--inspected wheels.
5.
End Items to be inspected. All CH--47D and MH--47D and MH--47E aircraft.
NOTE
APUs received from depot accounts direct from Cherry Point will Not
require inspection. APUs in ASL/PLLS will require compliance with this
technical bulletin.
Assembly Components to be Inspected.
NOMENCLATURE
PART NUMBER
NATIONAL STOCK NUMBER
Auxiliary Power Unit
160150--10
160150--100
2835--01--092--2037
7.
Parts to be Inspected. N/A.
Inspection Procedures.
a. Inspect aircraft condition change DA Form 2408--16 to determine time since new (TSN) of the APU
(based upon aircraft flight hours).
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