1、Lessons Learned Entry: 1187Lesson Info:a71 Lesson Number: 1187a71 Lesson Date: 1992-11-12a71 Submitting Organization: KSCa71 Submitted by: Jackie E. Smith/Eric RaynorSubject: Vehicle Assembly Building 250 Ton Crane No. 1 Close Call Mishap Investigation Board Report of November 12, 1992 Description o
2、f Driving Event: At the Kennedy Space Center on August 11, 1992, in Vehicle Assembly Building (VAB) High Bay #1, the 250 Ton crane #1 was stacking the left forward Solid Rocket Motor (SRM) segment for STS52 at 0009 hours EDT. While moving the segment north, crane #1 trolley suddenly and unexpectedly
3、 accelerated. The crane operator reacted and brought the segment to rest without contacting any structure or work platforms. During the incident the segment was estimated to have attained a maximum velocity of 50 feet per minute and moved 7 feet to the north over a period of 17 seconds. Had the segm
4、ent moved an additional 6.5 feet, it would have contacted the work platform. This type of mishap is defined as a close call because no personnel injuries or damage to hardware was sustained. The crane operation was being performed by a contractor in support of SRM mating operations which were being
5、performed by another contractor. Both contractors are members of the Shuttle Processing Contractor (SPC) Team at the Kennedy Space Center.Lesson(s) Learned: An SPC Investigation Team and an independent NASA Investigation Board were unable to determine the exact cause of the close call. The closest d
6、uplication of the event was obtained from the Malfunction Laboratory tests of the crane metadyne and the crane 4NCR relay contacts. A metadyne is an externally driven D.C. generator, which amplifies an operator initiated control signal to regulate trolley speed. When closed, the 4NCR relay contacts
7、provide the control signal to the metadyne.Other crane components that could have caused an increase in trolley speed were removed from crane #1 and tested in the Malfunction Laboratory. None exhibited abnormal results. The Board recognized that operator actions could have produced the observed segm
8、ent motion. However, based Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-on testimony of the two operators, the investigation board concluded that the operators most likely did not induce the rapid unexpected movement of the segment.Metadyne output
9、 voltage changes with no change to the input voltage were observed on numerous occasions during crane tests as well as during Malfunction Laboratory testing. These output changes could be induced by misaligning brushes, lightly tapping the brushes, injecting cleaning stone silica or cleaning cloth f
10、ibers into the commutator/brush area, or elevating humidity and temperature. Further the crane metadynes are operated at 2 percent of output capacity, which exacerbates these output instabilities. Review of text books and technical papers, and conversations with technical experts indicate that metad
11、yne systems are susceptible to erratic outputs. The voltage output changes observed in the lab were significant in magnitude and, based on analysis, were large enough to produce the maximum calculated trolley velocity.On September 14, 1992, in VAB High Bay #2, crane #2 bridge (same design as crane #
12、1) experienced erratic acceleration, other unexpected responses, and eventually stopped completely. Troubleshooting determined that the control voltage input to the metadyne system was zero. Relay contacts that provide this control voltage were cleaned and crane operations returned to normal. Additi
13、onal Malfunction Laboratory tests were performed on the power supply relay contacts and potentiometers. The relay contacts should have a resistance in the milliohm range but laboratory measurements were unstable and in the range of 100 ohms to 3 gigaohms. High resistance of the power supply contacts
14、 on the night of the close call could have occurred and, after a few minutes, the high resistance could have returned to normal. This would result in a sudden increase of trolley speed from a reduced to a full FINE speed condition of approximately 25 feet per minute. This very closely duplicates the
15、 timing and speeds that occurred during the close call.In summary, the exact cause of this crane close call is not known. The most likely causes were determined to be an erratic operation of the metadyne caused by cleaning material contamination of the brush/commutator area or an excessive resistanc
16、e increase of the control system relay contacts that suddenly returned to its proper value.The contributing cause to the close call was determined to be that metadyne output varies with constant input.Because metadynes are susceptible to unexpected changes in output, careful consideration of design
17、and operating controls is required. Proper relay contact selection is especially important in a low current control system.Recommendation(s): To minimize the likelihood of personnel injury and flight hardware damage, some crane hardware replacements and procedural changes have been implemented. The
18、metadyne in use on the night of the close call and other selected crane components have been replaced. A new commutator cleaning Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-procedure relating to the brushes and debris removal has been implemented
19、. Operator awareness and engineering involvement in any off-nominal crane performance have been increased and operating rules regarding speed selection have been enhanced. The new procedures reduce the likelihood of a recurrence, but some residual risk remains.The crane operator did not stop operati
20、ons and report the unexpected trolley speed increase. All anomalies should be reported. Shift turnover briefings are especially important to communicate anomalous or unusual conditions.Continued use of the VAB cranes as presently equipped should be with extreme caution. The Board recommends that the
21、 new solid state control system being evaluated to replace the metadyne be expedited for immediate implementation on the 250 and 175 ton cranes with special attention to proper control system relay contact selection.Evidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMis
22、sion Directorate(s): a71 Space Operationsa71 Exploration SystemsAdditional Key Phrase(s): a71 Facilitiesa71 Ground Equipmenta71 Ground Operationsa71 Hardwarea71 Human Factorsa71 Industrial Operationsa71 Launch Processa71 Lifting Devicesa71 Mishap Reportinga71 Packaging Handling StorageMishap Report
23、References: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Vehicle Assembly Building 250 Ton Crane No. 1 Close Call Mishap Investigation Board Report of November 12, 1992Additional Info: Approval Info: a71 Approval Date: 2002-05-06a71 Approval Name: Gena Bakera71 Approval Organization: KSCa71 Approval Phone Number: 321-867-4261Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
