While on initial approach; a Q400 Captain noticed the Power Transfer Unit (PTU) light did not come 'On' with selection of Flaps Five; or when the PTU switch was selected 'On'. A typographical error in Step # 10 of the Maintenance Action portion of the MEL led to an unsuccessful test outcome; when attempting to satisfy the MEL Maintenance Action requirement.

Narrative

On approach to ZZZ; Q400 aircraft Power Transfer Unit (PTU) light did not come 'On' with selection of Flaps Five or when PTU switch was selected 'On'. Upon landing and shutdown; PTU advisory bulb tested good. Maintenance Control was consulted; and as per their request and the 'PTU Light INOP' MEL provisions; engine # 1 was started to attempt to verify PTU operation. Following engine shutdown; I realized there was a probable typographical error in the Step # 10 of the Maintenance Action portion of the MEL; leading to an unsuccessful test outcome (referring to System # 3 Hydraulic pressure vs.the appropriate System # 2 Hydraulic pressure). Maintenance Control was consulted on this issue; whereupon they sent out Contract Maintenance to redo the MEL Maintenance Action; which resulted in a satisfactory PTU System Test. Upon consultation with and regulation interpretation from the Duty Officer; the PTU MEL was accepted by the pilots and Maintenance Control. The unsuccessful PTU Operation's Test conducted In Accordance With (IAW) PTU Light Inop MEL; was resultant from a typographical error in Step # 10 of the Pilot or Maintenance procedure. The Step tells the performer to verify System # 3 Hydraulic pressure vs. the correct System # 2 pressure.