Why do the Reading Lights Go Off During Pushback?
 

OK Delta Electrical Buffs. Why do the reading lights go off when being pushed back from the gate? All at once all the reading lights go off ... I hit the button (along with about 60% of the passengers) and nothing happens ... then all of the sudden the lights work again. The rest of the electrical stays on ... why can't the reading lights stay on?

Thanks in advance for the answer!

Happydad

I'm going to take a WAG on this. During pushback the engine(s) are being started and I'm thinking that all nonessential electrical connections are disabled until the engine(s) are up to speed.
But what do I know.....:confused:.
This might be an idea for a fun thread on the DL Forum. A fellow poster, DeltaMechanic who, I've got to assume is a Delta Mechanic by his/her knowledgeable posts, could pose a question like the above.
We could all take stupid guesses and after a period of time, Delta Mechanic could give us "the rest of the story".
Might be fun.....

The aircraft has to switch from shore power to onboard power and then start it's engines from power supplied by the APU or batteries. I believe that all unnecessary electrical loads are disconnected during engine start-up to allow more current for that task. Reading lights, IMO, would be considered non essential. If you will notice, the air conditioning stops for a moment also. Typically, one engine is started at or right after push back. The second(and third) are not started until just before take-off.

-J

That's my opinion as an electrical engineer, not fact.

Happydad you have asked an interesting question. It is a question that, IMHO will find a better audience on Travelbuzz. Therefore, I am going to move this to the Tråvelbuzz forum. Great question, and I would bet that we will have great replies. Apparently, there is a technical problem and I am unable to move this thread. So, just continue to post and when all is well again I'll move it
Obscure2k
DL Moderator

Ijaybird's got it right. All nonessential systems are switched off for a split second during engine start to make sure there is enough power for the engines to get turning.

Its always fun to watch this process when flying in a Beech 1900 - the whole console goes dark and a "whoop-whoop-whoop" sound goes off. Caught me off guard the first time but now I just wait for it and smile now that I know whats goin on.

-JC

There was a guy sitting next to me on a 757 from ATL to DAY in..June I think it was that became very irate and started making profane complaints about his reading light not working. I think he was trying to read a Maxim or something like it.. He yelled for an FA to come and one came and he asked what was wrong and she tried to turn on his light and it wouldn't come on, she seemed clueless as to why his light went off. Several other lights in the cabin went off, but half or better of them remained on. This occured when we were beginning pushback and when an engine kicked on.. I figured it happened for the same reasons as a few of you already posted.

Back when the 57's and 67's first came out, I happened to be in Seattle for a period of time. I met somebody who knew their way around Boeing, and he gave me the direct dial number of one of the desgners of these planes. I called him and got straight through. He was rather astounded to be contacted directly by a Frequent Flyer (this was before SkyMiles), but was nice enough to talk to me for a few minutes. I explained how inconvenient it was to be without these lights, paricularly this time of the year. His explanation was pretty much what has already been posted, namely that on these planes, the APU (alternate power unit) does not have enough power to start an engine and maintain cabin lighting at the same time. This had not been an issue on other Boeing planes up until then, apparently because the APU's on the 727 etc. were relatively larger in comparison to engine size.

If you notice, something similar happens in many cars. When you're starting the engine, the lights, radio, etc go off.

There's an equivalent effect when lining up for takeoff that the cabin air conditioning goes off (if it's a noisy one you hear it stop). This is to ensure all engine power is available for takeoff, particularly if engine failure procedures are suddenly required. Cabin air conditioning draws a surprising amount of the engine power. The AC comes back on during the climbout. The same is done on final approach in case the crew have to make a sudden full power go-around.

You guys are pretty close. The reading lights (along with the video system) are connected to the aircrafts' left and right "Utility Busses" (read: non-essential), which are shed during engine start. The reason the video does not go dark during the safety video is that there is a power switching relay that gets power from a different bus during engine start.

:rolleyes: Of course it's because, when the Pilot turns his head to look down to the rear when he's backing up, he's not blinded by the interior lights! ;)

Don't you turn off the overhead lights in your car when you're looking behind you whilst backing up?

I think this must depend on the aircraft. I've seen video installations fall over during power switchovers on startup.

Hi y'all

Lights and other flickering electical stuff (emergency videos, reding lites, etc.) is normal during various departure phases. Many variations on the theme but basically....

While at the gate, the airplane is literally "plugged into" an electrical outlet (soemone previously said this and referred to it as "shore power". If your a boater, it's virtually the same thing) and a big yellow hose is pumping air conditioning into the plane.

As departure time approaches, the electric plug and a/c hose have to (obviously) be disconnected. If your flight will "powerback"from the gate then all engines will be running before departure, but one or more may be shutdown after powerback (save fuel until the next time all engines are needed - at takeoff). If you will "push" from the gate, then preferably an APU (aux power unit) will be started before pushout to supply electrical power (and engine start capability). But (except for ETOPS operations ... another topic altogether) APUs are not a "no go" item to fly the airplane. If the APU is broken, the flight may still depart (after meeting some specific safety checks), but will need a "blow to go" (sorry ladies .. a "start cart") before leaving the gate to get at least one engine running.

Once the engines are running, either one of their associated generators can supply the electical power to the plane. The flight crew may switch back-and-forth between generators (APU or engine #1 or engine #2 or engine#x). The process will vary depending on the aircraft type and wether ot not the APU is working. Bottom line is that as the crew switches to/from shore power/APU/engine #x versus engine#y, there may be some flickering going on. And keep in mind that airplane engines are designed to run at quite a high power setting. While on the ground, they don't run at anywhere near their designed speed, so naturally, their generators just are not putting out all their potential volts. Thus the afore-mentioned electrical busses. (I ain't running at full power yet so supply power to bus #1 first and if there's still some amps left over then you can feed them to bus #2, #3, etc).

My favorite (here's some trivia question stuff) is watching all the passengers reach up to their air conditioning vents at departure time. They think that despite the summer time temperatures outside, Captain decided to turn off the air conditioning ... cuz all the air stopped flowing from the vents. Actually, the Flight Crew is using the air conditioning to start the engines. Your car uses an electric motor to "spin" it's engine to get it started. Jet engines spin much too fast for an electric motor to start them. Jet engines basically have to get their insides spinning extemely fast to run. During normal ops, the expulsion of gasses that give the engine its power to not only, "fly the plane" but also spin a turbine blade. This turbine also spins the compressor blades that pushes air into the engine to mix with fuel, ignite, and blow out passed the turbine and exhaust pipe. It's a catch 22. The engine won't run until the compressor is spinning fast enough to push ignitable air into the engine. The compressor won't spin that fast until the turbine is spinning that fast. The turbine won't spin that fast until it has enough compressed air blowing over it. Well where do we get the air to blow over the turbines to spin the compressor to drive the turbines when we'rejust sitting at the gate??? We take all the air blowing out the passenger vents and blow it over the turbines of one engine instead. Once that one engine is running, we can blow the air conditioning back to the passenger vents and "bleed" air off the running engine to get the turbines of the other engines spinning. And until at least one of those engines are running, we're at a loss to how to supply the electricity to your reading light.

Deeper triva. So if (while at the gate, after the electric cord and yellow a/c hose have been pulled from the plane), no engines are running so whats powering the A/C that's going to get those engines running ?? There's that APU again. It's actually a really small jet engine that can get started from the spinning of an electric motor. So the crew starts it a few minutes before departure. When the shore power and yellow a/c hose get pulled, the APU takes over; there may may a flicker of electric power and/or a "whoose" from your air vent. When ready to start the 1st engine, the air from your vent is borrowed to get a turbine spinning. If the APU is broken, then before you leave the gate, an air start cart will be brought up to the plane to provide the air to get that 1st turbine spinning ... thus the "blow to go" comment I made earlier.

And finally, back to the original post (and kinda mentioned already), your reading lights are really insignificant to the safety of the flight. To fly from A to B, your plane is not connected to your Electric company like your house is. It must at some point become "self dependent". The transition from one power source to another and yet another (as in the case of an airliner: Shore power->APU->engine) will cause some interruptions. During departure/Taxi out, the engines (their generators being the only source of electricity at this point) are not not running at optimal power thus the electrical power being produced must be priortized; would we rather provide our highest revenue passenger the power for his/her reading light, or (IE) the Captain the power to program the navigation computers ???

Great info from steve64 and the other knowledgeable folks in this thread! Thanks for sharing!

There is so much misinformation in this thread that I don't know where to start.

The specific answer to the original question will be different for each type aircraft as each has a different electrical system.

The engines are passenger jets are not started by an electric motor, they are started by high pressure air. The only electrical demands during start are for at least one fuel pump and a small solenoid that holds open the start valve. Most regional aircraft use electric starters so they do shed most non-essential electrical loads during engine start. So, in a regional airplane you'll shed electrics during engine start but in the larger airplane you'll shed air-loads which would be the airconditioning system.

When power sources are switched, in a system that doesn't parallel sources, the source currently powering the busses must be disconnected before the new source can be applied. The process is automatic but everything does flash off/on in the process. Some less essential systems may have a short time delay before coming back on.

All passenger jets normally takeoff with the airconditioning on. Some smaller turboprops are required to turn them off but not the jets. In fact, some jets actually pressurize slightly before takeoff to prevent the pressurization bump right after takeoff. If you feel the A/C decrease just before takeoff it is probably because a recirc fan was being used on the ground to provide more airflown but won't be needed with the engines at higher power settings. Similarly, you don't turn off the A/C in preparation for landing in anything other than the smallest turboprops. The airplanes have plenty of power to go-around with the A/C on and will typically have some system which automatically shuts down the A/C if an engine fails during takeoff or go-around.

You don't start an engine for power-back only to shut it down again for taxi unless you're planning for a long wait for takeoff and there was no push-back tug available.

A generator must be providing rated voltage or it can not power anything on the airplane. The generators provide full rated votage and cycles at idle (usually 115v 400Hz). They may be limited in amperage on the ground but that limit is not particularly significant. A generator at idle produces plenty of power.

The air conditioning does not start the engines, the bleed air, which also powers the air conditioning, provides the power to start the engines.

I'm not aware of any passenger jets which do a cross-bleed start as their normal procedure. What normally happens is that the APU air starts both engines but once the first engine is started that engine's bleed air can power half of the A/C system.