Masonaries From United States of America, joined Jun 2000, 93 posts, RR: 0 Posted (11 years 1 month 3 weeks 6 days ago) and read 7383 times:
Pardon my ignorance but what is the noise these aircraft make as they are about to pull away from the gate? It sounds like a drill is tightening something below deck. Are they locking down the luggage compartment? Just wondering.
NWB757300 From , joined Dec 1969, posts, RR:
Reply 3, posted (11 years 1 month 3 weeks 6 days ago) and read 7354 times:
As was explained to me in the Tech/Ops forum, the sound is the Power transfer unit (PTU), where the (i believe) green and blue hydraulic systems power each other if they have a large difference in psi.
Hope this helps but tech/ops may be more informative...
LongHauler From Canada, joined Mar 2004, 5863 posts, RR: 43
Reply 5, posted (11 years 1 month 3 weeks 5 days 16 hours ago) and read 7240 times:
Just as the second engine (engine 1) is starting, but before it reaches idle, the hydraulic system goes through a test. The PTU is tested by using the Yellow system power (powered by the engine 2 hydraulic pump) to power the Green system before engine 1 reaches idle.
When engine 1 reaches idle, then the Green system is powered by the engine driven pump, and the PTU shuts down.
The sound you hear when the cargo doors are closing, is the auxiliary Yellow pump. The cargo doors are Yellow hydraulic powered, and this auxiliary yellow pump, (like the PTU) is located under the cabin, mid fuselage. When a cargo door is opened or closed, you will hear this pump start/stop.
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!