What is with that noise on these planes. When the plane has come to a complete stop at the jetway, there’s some kind of rhythmical sound as they shut something off. For the life of me it always sounds like someone on the ground is taking a very big hacksaw to a major portion of the plane.
I’m sure it has something to do with hydraulics, but what specifically are they doing to make this noise?..other than hacksawing off a used tire or wing or something.
Um… yeah. Did I say something otherwise? As the plane is clearing the active runway, the pilot will run through his afterlanding checklist, and one of the steps will be to raise the flaps. He’ll move the flap handle. That will actuate the hydraulic pump, which will raise the flaps.
Verifying (flew A320 series several times last 6 months): the noise being referred to happens after jetway stop, all surfaces retracted, engines spooled down, aircraft running on APU. Sounds indeed like an unskilled worker using a saw, kind of a broken rhythm. Or else like a very large dog, trapped inside a metal can, barking.
It is most likely the Power Transfer Unit (PTU) which transfers hydraulic pressure from one system to another when the second system loses pressure. It seems that it can be heard both just after engine start and after parking.
The engines supply power to the hydraulic systems. Sometimes an aircraft will taxi on one engine to save fuel, in which case Airbus aircraft use a device called a power transfer unit to cross-pressurise one hydraulic system from the other engine. That’s what makes the alarming noise.
The same issue exists on Boeing aircraft, but they use a different technology.