Cre From United States of America, joined Jun 2000, 187 posts, RR: 0 Posted (11 years 3 months 1 week 3 days 14 hours ago) and read 7028 times:
I was on a UA A320 last week for an evening flight from CLE-ORD. I was sitting over the wing in an aisle seat. As we left the gate and started taxing, a very sound vibrating sound was coming seemingly under the floor. My feet were vibrating with the sounds. It would be a 1-2 second burst, then silent for a few seconds, then go it again.
It almost sounded like a dog barking, some people were laughing at this sound, which again was very loud. The sounds lasted for about 2 minutes, then just stopped.
RobinJH From Australia, joined Nov 2000, 14 posts, RR: 0 Reply 3, posted (11 years 3 months 1 week 3 days 11 hours ago) and read 6892 times:
It's been explained to me as the self test that the hydraulics system does at the time of start up (if you observe the outboard ailerons, they 'droop' until the hydraulics come on) - the hydraulics reservoir(s) for A320/1/19 are located between the main undercarriage bays - I noticed this sound the 1st time I flew on an A320 (AN97 CBR>MEL circa May 1995) & thought it sounded like someone below the floor working with a hacksaw - I suggested this to some nervous flyer neighbours in the cabin - probably not such a funny joke post 11/9/01...
Ifix747s From United States of America, joined Nov 2001, 18 posts, RR: 0 Reply 4, posted (11 years 3 months 1 week 3 days 10 hours ago) and read 6900 times:
This sound is normal. When the yellow or green hyd system has a higher pressure than the other .. And the ptu switch is in the normal position the higher press system will drive the ptu (Power transfer unit) A (reversible pump) to pressure up the lower system. So it will "bark" one way then the other until they are the same. The ptu transfers pressure only no fluid the systems stay separate. Hope this helps
Erasmus From Italy, joined Jun 2007, 0 posts, RR: 0 Reply 5, posted (11 years 3 months 1 week 3 days 4 hours ago) and read 7175 times:
The sound you're talking about is indeed the PTU (Power Transfer Unit).
This is a bidirectional power transfer unit which enables the yellow system to pressurize the green system and vice versa.
The PTU comes into action automatically when the differential pressure between the green and the yellow systems is greater than 500 PSI.
The PTU is inhibited during the first engine start.
In "my" company (as in most companies) engine 2 is started first. Once eng 2 is started the PTU detects a pressure difference between the yellow system and the green hydr. system and the PTU starts automatically. (The yellow system is normally supplied by an engine driven pump on engine 2 and the green system is supplied by a pump driven by the engine1)
So you will hear this strange noise, and indeed it IS a very strange noise, every time one engine is running and the other is not (yet).
(I liked the way you described the sound )
You will hear this sound for quite a long time during a power push. (When the pushback is done with a tug connected to the main landing gear. (see pictures below) In this case the pilot has to steer the airplane during the pushback via the nose wheel steering.)
In this case eng 2 is started at the gate BEFORE the pushback starts. The reason for this is that hydraulic power is needed for the nose wheel steering. In A319/320/321 the nose wheel steering (NSW) uses the green hydraulic power. After the eng 2 is started, hydraulic pressure is provided to the NSW via the PTU and the green hydraulic system. Eng 1 is only started AFTER the pushback is completed because these tugs have limited power and would have a hard time pushing the aircraft back with both engines running at idle.
The PTU is situated near the wing roots.
Remember: you will hear this sound whenever only 1 engine is running.
The fact that you continued to hear this sound after the taxi started, indicates to me that UA is one of the companies that allows taxi-out on a single engine. The second engine is started during the taxi or at the holding point. This procedure is used for fuel saving.
I hope this answer is clear enough.
The F/A's in "my" company are used to questions from the passengers about this sound. They get this question on nearly every flight.
Delta-flyer From United States of America, joined Jul 2001, 2676 posts, RR: 7 Reply 7, posted (11 years 3 months 1 week 1 day 17 hours ago) and read 6748 times:
Erasmus is absolutely right.
I just want to add that the PTU that UA uses is built by Garrett (I think it's now part of Frisby Corp.) and it starts very rapidly due to its low inertia -- that causes the pressure in the green system to rise back to 3000 psi very quickly, at which time it the PTU stops. This makes it sound like a dog barking. As hydraulic fluid is used from the green system, the pressure drops to a certain level (maybe 2400 psi or so) and the PTU starts up again and repeats the cycle.
I worked on a competing PTU that has been incorporated in some of the A320 family which has increased damping in the hydromechanical controls, thereby starting more gradually, and more quietly.