An interesting moment on SQ947

[SQtraveller]

post deleted

[Desert Traveller]

I might get round to writing a full blown trip report, but I wanted to tell you about a slightly disturbing experience I had on my flight back from Bali a few weeks ago.

Forgive my description as I am only a frequent flyer and not technically minded when it comes to planes

We were in the process of boarding and as normal, the lights dim briefly and the engines start up - I believe this is the switch from external to internal power.
However, in this case, the lights dimmed, the engines started up and then it all went dark and the engines went quiet.
Then they appear to switch back to external power as the cabin lights came back on.

[This has happened before to me on other flights but usually only once. I've always thought that it was because some sort of timing issue - i.e. switching to internal power before the engines had a chance to start generating power]

Anyway, what was most disconcerting about this, was that this happened at least 10 times - I lost count at 8. We were sat on the tarmac for at least 30mins as they cycled through this again and again, with no communication from the cockpit.

Fortunately once they got it all working, the captain came on and said there was a short in the external power and that the reason they hadn't talked to us, was because there wasn't any power to the address system.

As a frequent flyer I'm not normally one to get nervous, but this did get my nerves jangling a bit. I was wondering if anyone had had such an experience?

Haven't had an experience like this before, but it sounds like they did not have enough ground power to start the engines and/or APU. But I'm no expert, would love hear more views.

[flying.monkeyz]

It has happened before. I remember there was a similar case on a SQ777 to CGK which the APU failed to transfer power to the engine generators. Nothing to be worried about as each engine has a generator. Aircraft obviously cannot depart if there's no power.

[StarG]

Aircraft was still on the ground, they tried and finally got things going. Then gave explanation. I don't get the 'disturbing' part.

[jhm]

I can understand why it might have been disturbing for an already nervous flyer if s/he feels that the plane is flaky even before it takes off.

If it really was 30+ mins of on/off for at least 10 times without any news, that's also a surprising lapse. It wouldn't have taken much for a member of the crew to walk through the cabin and tell people what's going on.

[boing]

I might get round to writing a full blown trip report, but I wanted to tell you about a slightly disturbing experience I had on my flight back from Bali a few weeks ago.

Forgive my description as I am only a frequent flyer and not technically minded when it comes to planes

We were in the process of boarding and as normal, the lights dim briefly and the engines start up - I believe this is the switch from external to internal power.
However, in this case, the lights dimmed, the engines started up and then it all went dark and the engines went quiet.
Then they appear to switch back to external power as the cabin lights came back on.

[This has happened before to me on other flights but usually only once. I've always thought that it was because some sort of timing issue - i.e. switching to internal power before the engines had a chance to start generating power]

Anyway, what was most disconcerting about this, was that this happened at least 10 times - I lost count at 8. We were sat on the tarmac for at least 30mins as they cycled through this again and again, with no communication from the cockpit.

Fortunately once they got it all working, the captain came on and said there was a short in the external power and that the reason they hadn't talked to us, was because there wasn't any power to the address system.

As a frequent flyer I'm not normally one to get nervous, but this did get my nerves jangling a bit. I was wondering if anyone had had such an experience?

Was this at the gate itself or after pushback ?

During a normal start, the APU supplies electricity and air for the cabin prior to engine start. When engine start is initiated and the engine starts to supply electrical power, the pilot would usually transfer electrical load of the aircraft from APU to the engines and that's when you would see a flicker of the cabin lights for a second. If it happened when the APU was serviceable, there could be a problem with the electrical system.

When the APU is inop, electrical power is supplied by a ground power unit and is only disconnected when the Captain says so. There is also an air con supply and pneumatic supply for engine start. For most airlines, they would disconnect the air con prior to engine start and when they give the disconnect command, some ground staff tend to mistakenly switch off the ground power as well leading to a blackout in the cabin. Although I find it impossible they will repeat the same mistake 10 times over.

Did anyone enter your plane throughout the entire engine start and after ?

[sunnyday]

Engine start is only initiated after doors are closed and during or end of push back.

The only time engine start is initiated at the gate if APU is in-op. Even then, only one engine is started using ground air cart and only after doors are closed. The second engine is started after push back using bleed air from the engine that has started.

"However, in this case, the lights dimmed, the engines started up and then it all went dark and the engines went quiet.
Then they appear to switch back to external power as the cabin lights came back on."

I could be wrong but I believe this is the time when the pilots started the hydraulic right demand pump. This usually happens 15 mins before STD.

I can't remember if they provide external power and ground air con in DPS. Even if they do, the pilots will start the APU and ensure it's running first before asking the engineers to disconnect the external power and air con. There shouldn't be any issue once the APU is started as the APU is capable of supplying the electrical power and air con on it's own. The APU is left on in most stations after the engines are shut down.

[SQtraveller]

Was this at the gate itself or after pushback ?
Did anyone enter your plane throughout the entire engine start and after ?

Engine start is only initiated after doors are closed and during or end of push back.
The only time engine start is initiated at the gate if APU is in-op. Even then, only one engine is started using ground air cart and only after doors are closed. The second engine is started after push back using bleed air from the engine that has started.

post deleted

Aircraft was still on the ground, they tried and finally got things going. Then gave explanation. I don't get the 'disturbing' part.

post deleted

[boing]

What aircraft type was it ?
As Sunnyday pointed out, engine start is only allowed when all doors are closed and all ground equipments like aerobridge and cargo loader are removed. Forgot to add that in earlier.

But going by your records that this happened before the doors closed, then engine start couldn't even have been initiated at all. They could have either shut down all electrical power knowingly to reset some spurious messages in the cockpit. But I believe the Capt would have at least warned his pax through the intercom about it. Or the GPU could have shorted leading the aircraft to blackout intermittently. Or there could have been a problem with the cabin lighting controller.

The reason why I asked if anyone entered the aircraft throughout the process was if an engineer came on board to do troubleshooting. Anyway, if the aircraft departed, that means the problem was a minor one.

I could be wrong but I believe this is the time when the pilots started the hydraulic right demand pump. This usually happens 15 mins before STD.

When the hydraulic pumps are switched online, lights wouldn't dim. The only indicator it's running is a humming noise, depending on the aircraft, from the centre of the aircraft. But you got the timing correct.