Sometimes, when I am a passenger on a commercial flight, I hear a strange sound. I mean, I go in the plane....

Question

....., I take my seat, and some minutes before push-back, it seems to me that the whole plane gets no electricity-----Just everything is calm, but a few seconds later, all seems normal......I am thinking why?

My suggestion would be that the plane is changing from external power to APU, and therefore there are these few seconds without any power..... A "blank" of power if you want. Could this be it?

(By the way, I am a business class passenger, therefore I don't have any big knowledge)..

Answer 1

change-over from external power?  Starting a jet takes hi power, and the use external sources to save the battery stress and money.

Answer 2

depends on what kind of airplane you're on.  Before pushback they crew would be changing from ground power to APU power, and the relays need a second or two to make the switchover.  After pushback, the high draw electrical and pneumatic systems (ie fans and air conditioning) are suspended while the power is directed to engine start.

Answer 3

You are correct. They are swapping over to power from the apu. they reason it seems it takes so long . They are swapping to 2 sources of power which is called left bus and right right bus.  It does it in a sequence one before the other and that is the reason for the lag. If you take notice one set of interior lights come on before the other, due to the fact they are on different power source or buses. If you take notice it happens again after they start the engine due to the fact ,they are swapping to engine electrical power

Answer 4

I work for US Airlines doing ground work.... Many planes usually connect to ground power while they sit at the gate to conserve fuel with rising gas prices. The pilot usually start up the APU when they are ready to get going, after the APU starts up the pilot signals to the ground crew to disconnect the ground power, when we disconnect it the power is shifted to the APU. During this transfer the lights dim or shut off for a split second. There is no problem when this happens and is completely normal. Another time the lights will dim is when the plane is starting the main engines... To start the engines a large amount of power is needed so the lights dim a bit because most of the power is needed to start them.

If you didn't know what APU stands for it stands for Auxiliary power unit. An aircraft APU is a relatively small gas turbine used to produce shaft power to start the jet engines, usually with compressed air, and to provide electricity, hydraulic pressure and air conditioning while the aircraft is on the ground. Hopefully this is helpful, now when your traveling you can tell the business guy guy next to you whats going on!!

Answer 5

I think you are correct. They disconnect the external power cord AKA "shore power" first before backing out. For a while batteries provide power, then the APU is fired up. This takes a few minutes, as the APU is powered by a small jet engine which makes a lot of heat and noise.

Answer 6

As you say some airports or the trend nowadays is the host airport gives you electrical and pneumatic (for air-con) to your air plane for a fee and to reduce noise from APU.

Now before push back you can start the APU or unless the tow tug is equipped with GPU, APU staring is done during push back.

The lull you say is during elect. changeover but I guarantee you there is still power in the aircraft as power tripping cause by the changeover is eliminated now. It is called the "No Power Break Transfer" it is a relay that guarantees there is no power loss or trips during this electrical power transfer whether from Ground to APU, APU to Engine.

this is the beauty of the new air crafts.