Abstract:
During a recalibration test of the Galileo NIMS flight instrument in a subsystem vacuum chamber, a facility failure subjected it to subjected to a temperature below its qualification limits. Subsystem facilities must be critically examined for the probability of any failure modes which may subject flight hardware to out-of-specification environments. Specification limits should be alarmed, and there should be straightforward safing procedures available to the operator.
Description of Driving Event:
During a recalibration test of the Galileo NIMS flight instrument in a subsystem vacuum chamber, a facility failure occurred. A liquid nitrogen supply line fitting separated, causing liquid nitrogen to spray on the outside of the chamber, and filling the room with fog. After the break was repaired and in preparation to restart the test, four additional failures occurred, one of which caused the flight instrument to be subjected to a temperature below its qualification limits. These failures can be attributed to substandard liquid nitrogen plumbing, a known idiosyncrasy in a control circuit and human error. Before the flight instrument was permitted to be tested in the subsystem facility, the required safety certification process had been accomplished and a flight projects office waiver had been requested and approved. The reason for requesting to test in a facility other than the JPL environmental test lab was so as not to introduce unknowns into the recalibration process. Additional Keyword(s): Science Instruments
Lesson(s) Learned:
Environmental test chambers are subject to several hazards not familiar to most practicing engineers and technicians. Cryogenics, vacuum systems, and thermal control systems all have serious potential safety risks if wrong materials, parts, or assembly techniques are used. Once incorporated, these improper elements may perform well for indefinite periods before bursting, leaking, or otherwise failing. They look ok, they work ok, but they aren't ok.
Recommendation(s):
- Subsystem facilities must be critically examined for the probability of any failure modes which may subject flight hardware to out-of-specification environments. Specification limits should be alarmed separately from control system limits. Operator response to alarms should be covered by straightforward safing procedures.
- The proposed use of nonstandard facilities for flight hardware should be very carefully weighed during the planning phase of a project or task and the following should be considered:
- Safety certification isn't a foolproof process.
- Availability of qualified operators is a greater problem for small subsystem teams than for the officially prescribed environmental test lab, particularly if long-term tests are planned.
- Upgrading facilities to meet requirements that are imposed because of the testing of flight hardware is likely to be expensive in schedule and dollars.
- Damage and subsequent repair to flight instruments or subsystems is costly in both schedule and dollars and could have severe project and mission impact.
Evidence of Recurrence Control Effectiveness:
N/A
Documents Related to Lesson:
N/A
Mission Directorate(s):
N/A
Additional Key Phrase(s):
- Cryogenic Systems
- Environment
- Facilities
- Test Facility
Additional Info:
|
