HAP experienced occasional un-commanded windshield wiper sweeps when transferring power from ground to APU. For info: the wiper switch position is in Park-Position.

As a subsequent damage un-commanded movement of wipers on dry windshields can cause scratches.

An un-commanded wiper sweep during power transfers do not affect the airworthiness of the airplane and there are no restrictions regarding further flight.

For trouble-shooting Panel P5-4 with integrated wiper switch (PN233A3204-1) had been Cross-changed with a new one. Result: No help.

Question: Did other operator had seen this phanomenum and what was the final action

DAL has encountered this condition on one aircraft. The Captains and/or First Officers windshield wipers moved momentarily when the wiper switches were in the PARK position and electrical power was transferred from external to APU.

Extensive troubleshooting found that the DC Bus Tie relay R9 would cycle from close to open and back to close during a 50 ms period. The opening and closing of this relay caused voltage transients to occur on the DC busses and influenced the voltage levels within the wiper control switches resulting in uncommanded movements.Boeing was contacted about this phenomenon and instructed Delta of similar occurrences in Seattle.

Boeing advised that this fault was a nuisance which would not affect the airworthiness of the aircraft.DAL has implemented a procedure to test the windshield wiper system to determine if the uncommanded wiper movement during external to APU power transfer is a nuisance fault or not.

The procedure uses the Bus Transfer switch in the cockpit to open relay R9. In this way, the DC 1 bus and DC 2 bus are completely isolated from each other and the voltage transient that may occur because of relay R9 is no longer possible.

By eliminating the voltage transient produced by relay R9, the TRUs are able to provide cleaner voltage outputs to the wiper switch controller resulting in no wiper anomalies. In addition, if the test identifies that the wipers do continue to malfunction even with R9 removed from the circuit, then another problem with the system exists and must be addressed prior to flight in accordance with proper maintenance procedures.Ref. DAL-DAL-10-2763-08B.