northernlightIII

As a light aircraft pilot I hate things that sound like they should not be there so when I heard a loud sawing noise coming from under my seat on a commercial flight the other day I wondered what it could be. Hydraulic pump low on fluid ? electric pump ? I would love to know as it kept going for ages and it just did not sound right.
Can anyone enlighten me ? I was sitting behind the wings and on final approach this loud sawing noise could be heard but it kept going even after the aircraft had landed and had been cleaned up.

vs69

Hydraulic power transfer unit - It allows the yellow hydraulic system to power the green hydraulic system and vice versa depending on which has dropped below the pressure that triggers it - cant remember the figure.
If the engines arent running there is an electric pump supplying the yellow system hydraulics and the green system is powered via the PTU.
Its a strange noise for those not used to it and many think its a dog in the hold, barking. Very steadily.....

ampclamp

vs 69 is spot on.
It sounds like hell and scares the crap outa lots of folks.Sounds like someone hacksawing their way out of the cargo hold at various speeds.