Odd things happening upon reaching parking stand

[R04stb33f]

Hi all,

I have a question relating to a LH flight that I was on from MUC to LYS last thursday night.

Aircraft was 737 300/500 not sure which one, but was an event free flight... until we reached the stand at LYS.

The normal crap began when the aircraft reached a stop before the seatbelt signs went off and everyone stood up to grab their luggage etc. Engines still running.

Then, all the lights went off, and the PA went "BING!", the emergency lights came on... and the engine thrust increased as if the aircraft was about to move forward / take off. (incidentally, we were being connected to a tunnel facing the terminal...)

After around 1 to 2 seconds the emergency lights went off, main cabin lights came back on, "BING!" from the PA and the engines reduced back to idle speed.

This happened 3 or 4 times before finally the engines were cut off.

Does anyone have any idea what on earth was going on there?

I am particularly interested as to why the thrust would be increased when the cabin lights failed/were switched off and emergency lights switching on... (possible fault?)

Thanks all

Rich

[marlowe]

Sounds like a dodgy GPU being plugged in and failing!!

[PAXboy]

Non pilot speaking.
When the Ground Power Unit is connected, there will be a transfer of power that that causes a deliberate break in the lighting circuit. If this failed, the crew might have boosted the Auxiliary Power Unit to compensate. The APU is a small jet engine mounted at the base of tail fin and supplies electrical power whilst on the ground, unless mains power is available.

Certainly sounds like a problem in connecting, and then transferring the a/c services, to the ground power. The increase in thrust may be a perceived, rather than actual acceleration. Of course, if the a/c did move forward when the noise increased, they may have been nudging closer to the air bridge. Although the GPU will not be connected until the a/c has come to a halt, several things are going on.
.

[R04stb33f]

the crew might have boosted the Auxiliary Power Unit to compensate.

I'm not an expert, but I don't believe that the APU was running upon reaching the stand because once the engines were finally shut down, there was a good 20 - 30 seconds of quiet before I heard the APU start up.

Also, when engine noise increased there was no movement at all - brakes were clearly firmly on.

What was very strange was that, although I don't think this is possible, the apparent engine thrust increase was linked to the cabin lights going out...

If the GPU was dodgy and was failing like Marlow and Paxboy say, would the main engines be required to compensate by increasing thrust? (I wouldn't have thought so, because at idle, the aircraft is running off power from the engines anyway, right? Unless there was a short?)


Also flight was FRA to LYS LH4178 9th Oct 2008, A/C was a 733. Sorry - I am getting confused with all the travelling I'm doing at the moment!

Cheers
Rich

[TopBunk]

A plausible explanantion is:

1. Aircraft arrives on stand and transfers to ground power (GPU)
2. Main engines shut down
3. GPU then fails causing
4. emergency lights to come on
5. Flight crew attempt to re-establish gound power which may come on for a second or so at a time before dropping off (hence the 'flashing' of lights)
6. Flight crew decide that it is best to start APU (which is the noise you hear sounding like increasing thrust on the main engines)
7. APU at operational speed and electrical power tranferred to it.
8. Normal service resumes

.... end of story

[Rainboe]

I think you are confusing the sound of aircon packs (which give noisy air) being turned on after engine shutdown with engine noise and APU noise. Fact is unless the cabin is absolutely silent, you will not hear the APU operate or start up unless you are right in the tail.

[R04stb33f]

Thanks for your responses

Heh... I was in fact in the very last seat right in the tail.

The APU most definitely started up after the shutdown of both main engines, which was after the lights flashing / thrust increases etc. I can tell the difference between the sounds of the two main engines, the APU and aircon packs...

As an experienced SLF I notice that the most "usual" thing to happen is to arrive at the stand, aircraft stops moving, small pause... engines shut down and then seatbelt lights off. (probably a whole lot of other work going on in the front office that won't get noticed in the back)

On this flight I noticed a different sequence of events and I was sufficiently curious to ask in this forum (hopefully in the right place)

Thanks again
Rich

[tablelover]

I watch Holby City, doesnt make me a doctor or knowledgable in their arena, and years of watching LA Law Rumpole of the Bailey unfortunately dont make me an expert in legal subjects. Indeed although a somewhat 'experienced' drinker doesnt make me an expect in brewing techniques!

All meant tongue in cheek, of course!

Think Rainboe & Topbunk have given you the answer, takes more than 20 secs for an APU to wind up! Have flown a few types and can assure you it is highly unlikely you'll hear the APU start up in a busy cabin with lots going on, you'll be hearing the air bleed being turned on!

[Bob_Harris_721]

When there is a power transfer, sometimes these engines sense the momentary break in Essential AC power and react by going to High Idle. A few seconds later they sense that Essential AC has returned and go back to Low Idle.

[R04stb33f]

When there is a power transfer, sometimes these engines sense the momentary break in Essential AC power and react by going to High Idle. A few seconds later they sense that Essential AC has returned and go back to Low Idle.

Thank you very much for that explanation.

[Maude Charlee]

Boosting the APU????

Really? Is the 737 so advanced?

On most types, it's a simple matter of on or off. Guess I'll have to go back to the manuals and have a really good look for my APU thrust levers.

As for the question, no idea. Happy to have been of service.