Observation:

1

• Observation
• During the acceptance testing of the Rudder/Speed
  Brake Hydraulic Valve Module at Moog, the Rudder
  channel D bypass solenoid valve failed to
  energize
• Concern
• Inability to bypass a failed servo valve in
  flight
• Discussion
• RSB HVM S/N006 was sent to Moog for repair due to
  power spool stop being out of its end cap
  restricting movement of the power valve
• Unit was repaired by replacing the old stops with
  vented stops with tightest fit stop/end cap
• S/N006 HVM Rudder channel D bypass solenoid valve
  failed the pull-in test requirement of the ATP

2

• Actions Taken
• Failed solenoid valve was removed from the power
  valve assembly
• Good solenoid valve from S/N010 HVM (10 year old)
  was tested and validated the test equipment
• S/N006 HVM solenoid valve was then tested, and
  failed the pull-in requirement
• Current to the solenoid coil was increased to
  maximum allowable value
• Solenoid valve exhibited a small amount of
  leakage indicating small armature movement
• Disassembly of the failed solenoid valve revealed
  that the ID of the pole piece o-ring was damaged
• O-ring particles were found between the pole
  piece and the armature, restricting armature
  movement
• Removed o-ring particles and changed all o-rings
  with new ones and re-tested the solenoid valve
• Valve passed all ATP requirements

3
DISASSEMBLY OF THE FAILED SOLENOID VALVE FROM RSB
HVM S/N006
O-RING PARTICLES FOUND BETWEEN POLE PIECE AND
ARMATURE
DAMAGED POLE PIECE O-RING
4

• Actions Taken (Cont)
• Tear down and inspection of three other solenoid
  valves from the S/N006 HVM (21 year old) revealed
  only debris on 2 of the solenoids, the third
  solenoid had similar damage as the failed unit
  but no particles
• Review of the solenoid valve drawings revealed
  that there was a drawing change which modified
  the pole piece in June of 1979 by adding a
  groove/relief for the o-ring
• Design change was because of the problems with
  meeting the drop out voltage requirements of the
  ATP
• Review of the build papers revealed that all
  OV-103 actuators incorporate the new design
  except for engine 1 Yaw actuator
• All o-rings recovered from the disassembled
  solenoid valves were examined by Boeing Materials
  and Processes
• All o-rings show compression set
• O-ring hardness ranges from 80 to 88 (requirement
  for new o-rings is 60-85)
• 12 out of 16 O-rings inspected meet all
  specification requirements
• No degradation due to age was noticed on any of
  the O-rings

5
O-ring Most likely pinched
O-ring particles
Original Design Without O ring groove
Modified Design with recessed groove is less
likely to pinch O ring
Servo Bypass Solenoid
6

• Actions Taken (Cont)
• Dimensional analysis of the o-ring groove
  revealed
• Pole piece o-rings have formed a square shape
  cross section after being in the groove for 12 to
  21 years
• Original o-ring width was .070 in diameter and
  round
• O-ring recovered from the solenoid valves look
  almost square in cross section and measure
  .06X.08
• Original groove dimensions are .045X.085
• 1979 design change groove dimensions are
  .045X.095
• Parker O-ring recommends .050X.095 groove
• Pole piece and sleeve dimensional checks (at
  Moog) are complete and meet drawing requirements

7
21 year old valve HVM006
12 year old valve HVM010
8

• Risk Assessment
• Solenoid valve failure to energize is criticality
  1R3
• Bypass function is required to isolate any
  channel failure in an aerosurface or TVC actuator
  subsystem
• Unbypassed channel failure results in A 3-on-1
  force fight which is an acceptable operational
  mode
• Actuator system is vulnerable to next (third)
  failure on another channel of the same actuator
• Flight rule 8.1.3-8 requires FCS channels be
  placed in override for the three remaining good
  channels
• Third failure must be identified by ground
  monitoring and isolated manually

9

• Risk Assessment(continued)
• Worse case flight condition (three failure case)
• One channel fails hardover
• Channel Remains Unbypassed due to solenoid
  failure
• Second Channel fails hardover in same polarity
• Condition Results in 2-on-2 force fight
• Following ground call crew must place FCS channel
  switch back to auto to bypass second channel
  failure and restore actuator control
• Bypass function is verified each flight during
  ground turnaround and during Ops 8 FCS Checkout

10

• Acceptable for STS-92 Flight
• Servo valve bypass solenoid valve criticality is
  1R3
• All OV-103 actuators incorporate the new design
  with the o-ring relief/groove, except for engine
  1 Yaw actuator
• OV-103 has successfully passed all OMRSD testing
  which include the servo bypass solenoid check
• This O-ring installation is unique to the bypass
  solenoid valve and is not found anywhere else in
  the actuators