Narrative:

I needed to crank up the APU so I could build head pressure back up on my hydraulic reservoirs. Once the APU was up and running; (I) put the APU bleed switch to the 'on' position. I immediately noticed a high pitch squeal noise coming from outside the aircraft that I was not used to hearing. I assumed it was a turbofan running; because we have seen the turbofan control valve fail open and if this is the case; the turbofan will immediately start running as soon as APU bleed is placed on. Anyway; when I got outside; it was abundantly obvious that the noise wasn't coming from the turbofan; but rather the #1 engine. Popped the outboard cowl open; got a ladder and found the source of the noise without difficulty; as the large amount of air blowing out of the empty hole in the starter duct was hard to miss. The hole was a union fitting that was supposed to be connected to the downstream sense line for the high pressure regulator. The sense line was laying there; with a cap on it; and nothing on the union fitting which was the source of the high squeal noise. I can only assume this was capped off for some reason; in the 'C'-check; back in january; because their is no reason line mechanics would disconnect and cap this sense line for trouble shooting; or to change the high stage regulator. Additionally; the sense line is on the opposite side of the engine vs. The high stage regulator; which is on the inboard side. I believe this is why our guys didn't catch it; when they changed the high stage regulator reference the recent log page write-up. Anyway; I reconnected the downstream sense line to the starter duct and accomplished the bleed air control system operational check on #1 engine; with no defects noted; reference MM 36-11-05-715. This downstream sense line is the reference pressure for the (reverse flow protection at .3 psi; as sensed from the starter duct.)

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Original NASA ASRS Text

Title: A Mechanic reports hearing a loud squeal noise coming from #1 engine after he opened the APU bleed switch. Air was blowing out from the starter pneumatic duct; where a union fitting was attached; that was supposed to have the downstream sense line connected to it for the high pressure regulator.

Narrative: I needed to crank up the APU so I could build head pressure back up on my hydraulic reservoirs. Once the APU was up and running; (I) put the APU bleed switch to the 'ON' position. I immediately noticed a high pitch squeal noise coming from outside the aircraft that I was not used to hearing. I assumed it was a turbofan running; because we have seen the turbofan control valve fail open and if this is the case; the turbofan will immediately start running as soon as APU bleed is placed on. Anyway; when I got outside; it was abundantly obvious that the noise wasn't coming from the turbofan; but rather the #1 engine. Popped the outboard cowl open; got a ladder and found the source of the noise without difficulty; as the large amount of air blowing out of the empty hole in the Starter duct was hard to miss. The hole was a union fitting that was supposed to be connected to the downstream sense line for the high pressure regulator. The sense line was laying there; with a cap on it; and nothing on the union fitting which was the source of the high squeal noise. I can only assume this was capped off for some reason; in the 'C'-Check; back in January; because their is no reason Line Mechanics would disconnect and cap this sense line for trouble shooting; or to change the high stage regulator. Additionally; the sense line is on the opposite side of the engine vs. the high stage regulator; which is on the inboard side. I believe this is why our guys didn't catch it; when they changed the high stage regulator reference the recent log page write-up. Anyway; I reconnected the downstream sense line to the starter duct and accomplished the bleed air control system operational check on #1 engine; with no defects noted; reference MM 36-11-05-715. This downstream sense line is the reference pressure for the (Reverse Flow Protection at .3 psi; as sensed from the Starter duct.)

Data retrieved from NASA's ASRS site as of April 2012 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.