Based on the preceding failure analysis, this problem was attributed to manufacturing error due to a faulty  production process. S/N's 1021, 1027 and 1036 were labeled "Limited Use (Non-Flight Part)" and were not  used for flight.  Except for S/N 1021, 1027 and 1036, all the other pots passed this screening process. Absence of any  detent feel means only that the conductive plastic disk/wiper/bearings were not damaged and that there  was some clearance between the rotating and stationary parts at room temperature. It was not a measure  of how much clearance existed, nor was it a guarantee that clearance existed over the entire operating  temperature range. However, it would have been impractical to check for detents at other than room  temperature since detecting them requires a very sensitive touch in a quiet room with no vibration. This  risk was accepted and the remaining pots were approved for flight use. Lessons learned include:  LESSON 9. Sometimes you have to take off the gloves. In a quiet, vibration-free room, extremely  subtle mechanical effects can be felt reliably only through bare skin contact.  LESSON 10. An X-ray inspection machine with real-time imaging capabilities is an extremely  valuable tool for performing non-destructive evaluation on assemblies that are sealed or for which  disassembly would affect internal clearances or invalidate qualification testing already conducted.  LESSON 11. Interferences within the MIL-PRF-39023-style pots are possible. The pot manufacturer  should:  a) individually specify and inspect the angle or direction that the flat ribbon wire points as it leaves  each of the three terminals. The angle should be chosen to provide a direct route to each flat  ribbon wire's respective hole in the conductive plastic disk.  b) specify and inspect that each flat ribbon wire must be bent towards the case immediately upon  exiting the hole in the conductive plastic disk. This would ensure that the flat ribbon wires will be  against the inner wall of the case and maximize the clearance between the rotating and stationary  parts. A cutaway cover or functional gage could be designed that would permit inspection and  verification of a minimum acceptable clearance.  c) investigate a shorter wiper bracket or an alternative "lower profile" method of insulating the flat  ribbon wire, to provide more clearance between the rotating and stationary parts.  Set Screwed  While concentrating all my tactile attention on the flight pot shafts rotating between my fingers, I noticed  that many were not smooth. Close examination revealed deep impressions with raised edges that made  each precision 3.1 70 +0.000/-0.005-mm-diameter shaft reminiscent of a bastard-cut round file. Some EM  pots showed this damage as well. During testing per the ATP, the manufacturer must have used a breaker  bar to repeatedly tighten R, 52 hardened steel cup-point set screws into the Rb 82 annealed 303 stainless  steel pot shafts. To correct this damage, the affected pots needed to have their shafts deburred with an  India oilstone before I could deliver them to their end users. Deburring generates metal chips and abrasive  fines, so to prevent contamination each pot was bagged and taped to expose just its shaft for deburring.  After deburring was complete, the shafts were cleaned and the bags removed. The following lesson was  learned:  LESSON 12. If for testing purposes EM pots are ordered and received long before the flight pots,  then immediately upon receipt the EM pots should be carefully inspected and the manufacturer  should be contacted about quality issues like precision shafts marred by set screws. Before  fabricating the flight pots the manufacturer should be instructed to tighten setscrews against the  shaft's machined flat and not its precision outer diameter, to properly control setscrew torques, to use  brass or nylon tipped setscrews, and/or to switch to non-setscrew style couplings on their test  equipment.  Observed Trends  Continuing the investigation, the total resistance measurements captured my attention next. A plot of the  total resistance of the MIL-PRF-39023-style pots showed it to be decreasing with either time and/or  thermal cycles. The plot in Figure 8 shows the history of these total resistance measurements with each  line representing a different serial number pot. The spec for these pots was 6250 Ohms f 20%, or 5000 to  7500 Ohms. There was minimal drift measured by the pot manufacturer before and after their thermal  31 1