Narrative:

The first leg began with a successful hydraulic built in test (bit) for the flight controls and no adverse issues noted throughout the first leg. The leg back also began with a successful hydraulic bit. During climb out; through FL200; the crew responded to an EICAS bleed 1 fail that cleared after following the QRH and procedures. Upon landing (in ZZZ) the ca subconsciously noted slight variations in the performance of certain flight controls but; with lack of any prior knowledge or any indications form the aircraft's warning/advisory systems; just assumed these variations were normal fluctuations or brought on by external forces due to light turbulence. However; the ca did remark to the first officer that the conditions felt uncharacteristically 'bubbly.' the flight terminated successfully and once at the gate a logbook entry was made with the cooperation of maintenance control. Maintenance personnel arrived at the aircraft to test the bleed 1 system and subsequently return the aircraft to service. Our crew remained onboard the aircraft for the impending 3 hour sit prior to a turn.after approximately 2 hours of down time local maintenance personnel returned to the aircraft with rushed and concerned demeanors. They announced to myself and the crew that this aircraft wasn't going anywhere for a long while. The ca queried the mechanic. The mechanic then proceeded to share the story of how earlier that morning and before the first flight of the day (which was my flight) a mechanic was to service all hydraulic systems by draining fully and the refilling them. However; instead of refilling them with skydrol hydraulic fluid; the mechanic refilled the systems with the improper 5606. He explained how this type of oil was to be used in our aircraft's landing gear shocks and had no application as a hydraulic fluid for our system and that continued use predicated certain catastrophic failure. What's most troubling is that the mechanic speaking to me also shed light on the actions taken by the mechanic after he improperly serviced the aircraft. The mechanic who attempted to service the hydraulic systems discovered his mistake before leaving the aircraft on the line and then attempted to hide his mistake. He recognized the improper oil was used and decided to cover his tracks by not entering his actions in the logbook. Later in the morning the crew arrived to the aircraft and noticed all hydraulic pumps were in the off position (which is not a standard selection by pilots leaving an aircraft to remain overnight) but without a logbook entry they figured no service had been attempted nor accomplished. So; the crew took the plane for face value; a serviceable aircraft and this aircraft flew two legs with improper fluid in all its hydraulic reservoirs. This had the potential to lead to a full hydraulic loss. The actions by the mechanic reflected gross negligence that were initially recognized only by the mechanic himself and instead of admitting his/her mistake; he/she executed a successful cover-up only to be foiled by his/her own conscious. Nevertheless; the flight had already operated 2 legs and was about to operate two more. Luckily; no lives were lost. I have personally had many mechanics express their own concern due to the lack of training they received as new hire mechanics. An evaluation of the hiring; training and oversight of the airlines maintenance department may help ensure our operation is held to our continued standard of safety.

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Original NASA ASRS Text

Title: EMB-175 Captain reported the hydraulic system was improperly serviced with the wrong fluid which had no uses as hydraulic fluid.

Narrative: The first leg began with a successful hydraulic Built in Test (BIT) for the flight controls and no adverse issues noted throughout the first leg. The leg back also began with a successful hydraulic BIT. During climb out; through FL200; the crew responded to an EICAS Bleed 1 Fail that cleared after following the QRH and procedures. Upon landing (in ZZZ) the CA subconsciously noted slight variations in the performance of certain flight controls but; with lack of any prior knowledge or any indications form the aircraft's warning/advisory systems; just assumed these variations were normal fluctuations or brought on by external forces due to light turbulence. However; the CA did remark to the FO that the conditions felt uncharacteristically 'bubbly.' The flight terminated successfully and once at the gate a logbook entry was made with the cooperation of Maintenance Control. Maintenance personnel arrived at the aircraft to test the Bleed 1 system and subsequently return the aircraft to service. Our crew remained onboard the aircraft for the impending 3 hour sit prior to a turn.After approximately 2 hours of down time local Maintenance personnel returned to the aircraft with rushed and concerned demeanors. They announced to myself and the crew that this aircraft wasn't going anywhere for a long while. The CA queried the mechanic. The mechanic then proceeded to share the story of how earlier that morning and before the first flight of the day (which was my flight) a mechanic was to service all hydraulic systems by draining fully and the refilling them. However; instead of refilling them with Skydrol hydraulic fluid; the mechanic refilled the systems with the improper 5606. He explained how this type of oil was to be used in our aircraft's landing gear shocks and had no application as a hydraulic fluid for our system and that continued use predicated certain catastrophic failure. What's most troubling is that the mechanic speaking to me also shed light on the actions taken by the mechanic after he improperly serviced the aircraft. The mechanic who attempted to service the hydraulic systems discovered his mistake before leaving the aircraft on the line and then attempted to hide his mistake. He recognized the improper oil was used and decided to cover his tracks by not entering his actions in the logbook. Later in the morning the crew arrived to the aircraft and noticed all hydraulic pumps were in the off position (which is not a standard selection by pilots leaving an aircraft to remain overnight) but without a logbook entry they figured no service had been attempted nor accomplished. So; the crew took the plane for face value; a serviceable aircraft and this aircraft flew two legs with improper fluid in ALL its hydraulic reservoirs. This had the potential to lead to a full hydraulic loss. The actions by the mechanic reflected gross negligence that were initially recognized only by the mechanic himself and instead of admitting his/her mistake; he/she executed a successful cover-up only to be foiled by his/her own conscious. Nevertheless; the flight had already operated 2 legs and was about to operate two more. Luckily; no lives were lost. I have personally had many mechanics express their own concern due to the lack of training they received as new hire mechanics. An evaluation of the hiring; training and oversight of the airlines maintenance department may help ensure our operation is held to our continued standard of safety.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.