Analog From United States of America, joined Jul 2006, 1900 posts, RR: 1 Posted (7 years 10 months 3 weeks 4 days 7 hours ago) and read 6303 times:
Last week I was on a CO 735 that had the most awful, seizure-inducing flickering cabin light. One of the bulbs was going bad (I assume). The flicker stopped when we pulled away from the gate. Why is this?
I assume that the flicker stopped when the aircraft stopped using ground power. Is this because ground power is at 60Hz and power generated on board is 400Hz? Or is it related to voltage (ground power or APU vs. generators on the engines)?
This wasn't the first CO 73x that had this problem. The previous one (733) flickered at the front of the F cabin where the FAs work. I can't believe that the FAs can live with this. If I were an FA I'd make damn sure to bitch and moan until these things were fixed. Sorry, I would bring this to the attention of the appropriate persons in an effort to resolve this maintenance issue.
Tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 80
Reply 1, posted (7 years 10 months 3 weeks 4 days 7 hours ago) and read 6295 times:
Quoting Analog (Thread starter): Last week I was on a CO 735 that had the most awful, seizure-inducing flickering cabin light. One of the bulbs was going bad (I assume). The flicker stopped when we pulled away from the gate. Why is this?
It sounds like a bad ballast in lamp (I'm assuming it was a fluorescent). Could be that ground power was just flaky enough to throw it off but aircraft power was OK.
Quoting Analog (Thread starter): Is this because ground power is at 60Hz and power generated on board is 400Hz?
Ground power is 400 Hz too (plus or minus some tolerance).
Bartonsayswhat From Canada, joined Oct 2007, 444 posts, RR: 0
Reply 2, posted (7 years 10 months 3 weeks 4 days 6 hours ago) and read 6279 times:
yeah ground power constantly fluctuates, be it from bridge power or a diesel GPU. APU power is much more consistant, which is why most of the fmc updating is done on the APU, as a power surge can wipe out the whole thing, or so i've been informed. also does anyone know, just as a plane arrives or leaves the gate, when it is switched from GEN/APU to GPU the lights flick once?
DALMD88 From United States of America, joined Jul 2000, 2847 posts, RR: 13
Reply 5, posted (7 years 10 months 3 weeks 3 days 21 hours ago) and read 6170 times:
Even relatively new ground power units can cause all sorts of strange happenings. Here in BOS our new terminal has new units at every gate. We frequently get calls in the morning about systems not testing correctly. The standard answer is switch to apu power and run the test again. If it is still a problem call back and will send someone out. I think the 737NG tends to have the most issues with flakey ground power.
ZaphodHarkonnen From New Zealand, joined Jan 2015, 189 posts, RR: 0
Reply 8, posted (3 weeks 5 days 2 hours 5 minutes ago) and read 2130 times:
I would have thought the power would have been transformed for the lights and other internal systems, something like 120V or 240V AC. Or do all the lights and such run off the standard power bus? I'm not including flight management stuff in that as I'm sure that has its own power requirements.
Shamrock137 From United States of America, joined Jul 2005, 206 posts, RR: 0
Reply 9, posted (3 weeks 22 minutes ago) and read 1515 times:
Quoting Bartonsayswhat (Reply 2): also does anyone know, just as a plane arrives or leaves the gate, when it is switched from GEN/APU to GPU the lights flick once?
Usually the transition from aircraft to ground power will cause a flicker or two.
Quoting Airbus_A340 (Reply 4): In English, that means u don't get the lights flickering or horrible 'clunk' sounds when you change from GPU to APU or APU to engine Gens, or vice versa that you would find on other older aircraft.
I noticed the opposite, the A320 had a very audible clunk in the cockpit and front galley when transiting from aircraft to ground power.
fr8mech From United States of America, joined Sep 2005, 6497 posts, RR: 16
Reply 12, posted (2 weeks 6 days 17 hours 54 minutes ago) and read 1386 times:
Quoting ZaphodHarkonnen (Reply 8): I would have thought the power would have been transformed for the lights and other internal systems, something like 120V or 240V AC. Or do all the lights and such run off the standard power bus? I'm not including flight management stuff in that as I'm sure that has its own power requirements.
Really depends on the electrical architecture of the aircraft. Been a long time since I worked pax aircraft but, I seem to recall that the PSU lighting was 28VAC...and, I suspect that would vary by aircraft type.
The typical aircraft (not B787 or A350, as I understand, they are a little different electrically), there are a few different voltages:
It all runs at 400hz, except the DC stuff, of course. Anything requiring a different frequency, would change the frequency after the electricity makes it to the unit.
As for the issue the OP asked about, I suspect a marginal ballast. A 737-500 should be able to handle some frequency/voltage fluctuations without too much trouble. Newer technology aircraft have a harder time dealing with these fluctuations. Regardless of aircraft age, we will always put an aircraft on APU power if the electronics are exhibiting issues on pre-flight.
Let me add the standard disclaimer: while aircraft design tends to be consistent, within the regulations, there is variation based on manufacturer philosophy and customer preference.
CanadianNorth From Canada, joined Aug 2002, 3417 posts, RR: 7
Reply 13, posted (2 weeks 3 days 14 hours 42 minutes ago) and read 883 times:
Was the cabin cold (first flight of the day after sitting outside on a cool night type deal)? Around here it's pretty normal when you power up a cold 737 the cabin lights will usually misbehave in a variety of ways, but once the heat has been on for a while and things warm up it seems 99% of the time the lights smarten up and work fine for the rest of the day.
Could also be as mentioned before a bad light or ballast, or a getting ready to be bad light or ballast combined with a GPU that's on the upper or lower end of tolerances. GPU power usually works fine enough but I have noticed over the years that APU power generally does seem more stable and consistent. Should all in theory be an even 120v 400 hz regardless of APU generator, engine generator or GPU, but as with most things there is a +/- tolerance.