Auto cooldown-cycle. 737-6/7/8/900

As you will know the NG APU has a cooldown cycle: After the APU switch is set to the “OFF” position the APU does not stop directly but it continues to keep running without being used. This ensures that there is less internal wear and that the life span is extended. A cooldown-cycle can only be interrupted by switching the main battery – switch to “OFF” while the APU is still running. The number of interrupted cooldown-cycles are saved in the data memory module (DMM) and can be read out on the CDU (maintenance page). It appears that this number is considerably high, resulting in a shortened life span of the APU. Since a complete shopvisit of an APU costs between 100,000 and 150,000 euro, the life span must be stretched as long as possible. Before the battery switch is set to OFF, it is worth the trip to the tail to verify if the APU is still running (and not to look to the position of the APU switch only).

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