GE/MRAS Comments to NPRM FR Doc. E8-3463

## Refer to 'Summary'

Comment 1: End of second sentence add, 'or show signs of corrosion'.

Rationale: OEM type design part is manufactured from a Stainless Steel
alloy; there should be no corrosion.

Comment 2: After the fourth sentence, before the fifth sentence, add
'The source of this unapproved part can not be determined, therefore we
are proposing this AD.'

Rationale: The airline could not provide records of where the
un-approved part came from or when it was installed.

 

**[Refer to \"Discussion\"]{.underline}** 

 

Comment 1:  The NPRM indicates that the \"airplane landed with one
actuator inoperative\" and \"When a single actuator is inoperative\". 
MRAS believes that clearer language would be \"disconnected from the
translating sleeve structure\" rather than \"inoperative\".

*Rationale:  The end actuator flexshafts were still intact and connected
to the actuator so technically it was still \"operating\".  However
since the actuator attachment point was disconnected from the
translating sleeve structure, no load was being carried which helped
contribute to the eventual structural failure.*

 

Comment 2: The NPRM indicates that \"Upon landing the thrust reverser
deployed and two of the clevis pins failed\...." MRAS believes that this
should state \"Upon landing the thrust reverser deployed and the clevis
pin failed at the CDU location with one actuator disconnected\....\" 

*Rationale:  MRAS is aware that one clevis pin failed at deployment at
the mid or CDU location.  The pin did not fail at the upper actuator
location.*

 

Comment 3: The NPRM indicates, \"Of the three thrust reverser central
drive unit pins affected" MRAS believes that this should read, \"Of the
three thrust reverser clevis pins affected\"  

*Rationale: Each TR halve only has (1) clevis pin at the CDU and (1) at
each of the (2) end actuators.*

 

Comment 4: third bullet. After the word 'pin' add 'based on its visual
lack of corrosion and hardness test value'

*Rationale: Clarify criteria for statement.*

**[Refer to 'FAA's Determination and Requirements of the Proposed
AD']{.underline}**

Comment 1: Add 'The source of the unapproved part that contributed to
the incident could not be identified'.

*Rationale: Clarify that the unapproved part was not installed by
GE/MRAS.*

[**Refer to \"Unsafe Condition\"**]{.underline}

 

Comment 1:  Refer to section (d).  NPRM indicates, \"The failure was due
to lack of clevis pin hardness\".  MRAS believes that a better root
cause statement would be \"The failure was caused by a clevis pin
manufactured from material with strength and corrosion inhibiting
characteristics inferior to the original or OEM part\".  

*Rationale:    The material selection, of choosing low-strength carbon
steel that also allowed attack by corrosion was root cause of
the event. In addition to lower hardness, the failed clevis pin sheared
due to insufficient strength. The fracture surface exhibited significant
corrosion attack over greater than 50% of the final fracture surface
indicating that lack of corrosion resistance was a significant
contributing factor to the clevis pin failure. Low strength carbon steel
or other materials can be heat treated to RC 31-38; however, the
finished part still may not have the necessary material properties for
all design conditions.  *

 

**[Refer to \"Compliance\"]{.underline}**

 

Comment 1:  Refer to step (1).  NPRM indicates to \"remove any corrosion
from the head of the pin.\"   MRAS comment would be that if any
corrosion is present, that maintenance should immediately suspect the
part and replace the pin.    

*Rationale:  There are no established corrosion limits for this part.
Corrosion would indicate a pin manufactured from sub-standard material.*
 

Comment 2:  Refer to step (4).  NPRM indicates to \"if the hardness
measured is within the range of 31 to 38 RC\...and the pin has no
visible defects, the pin can remain in service.\"   MRAS comment is
that a hardness test and visual inspection alone may not be sufficient
to ensure that the installed clevis pin has the required material
properties for the operating environment.    

*Rationale:   Various materials can be heat treated to RC 31-38;
however, the finished part still may not have the necessary material
properties for all design conditions. *

 

**[Refer to \"Install Approved Part Clevis Pins\"]{.underline}**

 

Comment 1:  Paragraph (h) relies on a hardness test and visual
appearance to inspect for proper clevis pins.  MRAS comment is that
these inspections may not fully capture all sub-standard clevis pins. 

*Rationale: Refer to comment 2 under Refer to "Compliance".*

 

Comment 2:  It may be helpful to add some terminating action or end date
for stock inspections.  Likewise MRAS suggests clarification that
indicates new OEM parts will not require any inspection before
installing.

*Rationale: MRAS has no intention of performing a hardness test on each
OEM pin we install in new product or sell as a spare, and therefore the
language as written would make the OEM part non-compliant.  Hardness
is checked on each heat treat lot using standard manufacturing
techniques, but not to each small part.  This section needs to be
reworded such that OEM Spares are shown to be compliant to all
regulatory requirements.*