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[i]Hydrogen Low Pressure Master Alarm The caution and warning system, upon receipt of a malfunction or out-of-tolerance signal, simultaneously identifies the abnormal condition and alerts the crew to its existence. Each signal (both oxygen and hydrogen pressure are on one indicator) will activate the system status indicator, light the master alarm light, and place an audio tone in the crew's headsets. The crew can turn off or reset the master alarm; however, the particular system status malfunction indicators remain lit, blocking further master alarms on this indication, until the malfunction is cleared. At lift-off, the quantity readings for the hydrogen tanks no. 1 and no. 2 were 91 percent and 93.4 percent, respectively. This was due to initial loading values (98.7 percent for tank no. 1 and 99.4 percent for tank no. 2) and the difference in usage during countdown. At approximately 32:00 g.e.t., a quantity unbalance of 2.38 percent existed between the hydrogen tanks, and a quantity balancing procedure was conducted to prevent tank no. 1 low-pressure master alarms during the sleep period. In the "auto" mode the tank heaters are turned on and off by pressure switches connected in series. When the pressure in either tank reaches about 260 psi, the heaters in both tanks are switched off. The heaters remain off until the opened pressure switch closes at approximately 225 psi. Since one tank pressure switch normally remains closed, the tank that controls the upper pressure will also control at the lower pressure. During the flight, tank no. 2 was controlling. Tank no. 1 pressure was almost reaching the caution and warning low pressure point (224.2 psia) prior to tank no. 2 reaching its pressure switch activation point of 233.6 psia to turn on the heaters. Since tank no. 2 had the greater quantity, at 32:00 the tank no. 1 heaters were manually turned off by the crew while tank no. 2 remained in auto. This condition would allow the fuel cells to obtain hydrogen from tank no. 2 because of its higher pressure and in turn reduce its quantity of hydrogen. Several master alarms occurred immediately after this change (33:10, 33:41, 34:01, and 34:32). At 36:48 the hydrogen tank no. 1 heater was placed back to auto for the sleep period. On the first "down" pressure cycle a master alarm occurred (38:00) due to hydrogen tank no. 1 pressure dropping lower than 224.24 psia, awaking the crew. The crew reset the alarm, and no master alarms occurred through the sleep period although the heaters cycled several times. To obtain a balanced condition for the next sleep period, the ground controllers devised the following plan for the next day's operation: [list=1][*]After crew wakeup, turn hydrogen tank no. 2 heater to off and leave hydrogen tank no. 1 in auto for two to three pressure cycles to determine if this will transfer heater control to tank no. 1 in anticipation of using this configuration for sleep. [*]If successful, tank no. 1 heaters will be turned off during the day and tank no. 2 heaters left in auto to create a quantity unbalance in favor of tank no. 1. [*]During the next sleep period, the tanks will be balanced by placing tank no. 1 heaters in auto and tank no. 2 heaters to off.[/list] This plan was executed when the crew awoke the next day. At the time of the accident, tank no. 1 was in off and tank no. 2 was in auto, and the caution and warning master alarm was reset with a low hydrogen pressure indication present at 55:52:30. This hydrogen low pressure indication locked out the master alarm during the time of the increasing pressure in oxygen tank no. 2.[/i]
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