Usage of EXT PWR and APU

[Walter Kranl]

Hi all,

If EXT PWR is available, are there reasons to use the APU instead?
From my (limited) observations, most planes have the APU running before start, even on big airports.
At first glance it seems that EXT PWR has only advantages (no fuel, wear, tear).

Thanks Walter

[Jeroen Hoppenbrouwers]

Power Quality. Pilots and dispatchers hate it when the power transfer from a GPU to the APU screws up their carefully programmed flight deck and they have to start all over again while the pushback truck already accelerates.

Jeroen

[Walter Kranl]

Thanks Jeroen.

Does it mean that the power delivered by the GPU has too much fluctuations in voltage? And if so, aren't there protections to deal with voltage peaks?

Walter

[Jeroen Hoppenbrouwers]

I will pass the exact reply to the experts, but generally the APU is an excellent generator and the GPUs are of a different order. Frequency, stability, voltage, reliability... it all makes a difference.

The power transfer from GPU to APU may introduce a brief power interrupt. There are circuits to make this seamless by adjusting the APU to match exactly the GPU and then briefly bridging the two generators, but when the GPU is far out of specs, it may fail.


Jeroen

[John H Watson]

If EXT PWR is available, are there reasons to use the APU instead?

The APU also offers pneumatics for airconditioning in hot climates and in some cases, APU bleed air is the only source of power for (ADP-driven) #1 hydraulics (for maintenance purposes).

Stability is an issue, especially with older style external power units, but, apparently, the beancounters are willing to take this risk as the fuel savings are great.

Use of external power, especially non-deisel types, makes the tarmac a lot less noisy and polluted.

The APU is, of course, required for pushback and, especially, engine start.

Application of external air and electrics is very labour-intensive. Labour costs are another negative for not-running the APU. Also, stopping and starting the APU can induce thermal shock, which shortens the life of the APU.

Regards
JHW

[Walter Kranl]

Thanks John & Jeroen.

Walter

[OKD]

Hi Guys

Just go on a bit further from Walter left off.

I am just wondering for example if a foreign airlines makes an early arrival at SYD, but schedules for departure 6 hours later, naturally the aircraft would have had all electrics shut off until the such time the gruond staff are ready to restart the APU / GPU for flt preparation.  

My question is that what is the time frame for a normal a/c 744 preparation for departure, and if my rough guess is 1.5 hours to 2 hours (presuming the crew report duty just after this time frame), will the ground staff choose the APU instead of the GPU, even though if the ground temperature soars to 45 -50 degree celcius?

[John H Watson]

Like most airports, Sydney has unique facilities and restrictions.  
It now has aerobridges equipped with aircraft air-conditioning units/hoses which are hooked up as soon as is practicable after aircraft arrival. However, due to airport congestion (the owner of the airport makes more money from constructing Duty Free stores, than it does from creating extra aircraft gates), the aircraft has to be repositioned if ground time is more than 3~4 hours. This means that within an hour or so of arrival, a long transit aircraft will have to have the aircon/power disconnected and the APU restarted to move the aircraft to either a remote bay or to a maintenance area (often requiring the aircraft to cross active runways and generally compete with aircraft taxying under engine power). This can take half an hour or more each way, burning fuel in the process.

Heat in the cabin is a big issue (not only in summer). Any semi-sealed/insulated  metal tube filled with lots of lights and electronics is going to get hot inside very quickly, almost irrespective of outside temperature. If the cabin gets above 28C,  aircon must be supplied for both cleaning staff and to keep things like galley refrigeration units( chillers) from overheating.

It's a very inefficient process, however you look at it.

Anyway, assuming no maintenance work to be done, your estimate of preparation time is pretty much spot on. Fuelling for a SYD ~ LAX flight is going to take at least an hour, even longer if there is only one refueling truck. It is possible to leave the aircraft hooked up to external air/power supplies until the pilots arrive (25~35 mins before the flight), but manpower requirements and stress levels are pretty high if you leave it this late. On hot days, the external aircon supply will never be as efficient as 3 packs running at full cool. Pax boarding will be delayed if the cabin temps are too high.

Regards
JHW

[Jeroen Hoppenbrouwers]

Electrics in an isolated tube: add 300 people times, what, 100 W = 30kW of constant heating.

Jeroen

[OKD]

Thanks guys....very informative indeed....

Let's see what I can on my PS 1.3 later...!! :mrgreen:

[Will]

Speaking of fueling time... Any chance, Hardy, for fulfilling my dream request: real-time refueling and defueling?  I picture something like requesting the desired quantity somehow, then a random delay until the fuel hoses are attached, then a long time goes by watching the fuel quantity indicators slowly rise, then the quantity is set +/- 1% of your original request...  It would be fun to do the cockpit prep while the ground crew is busy working outside!

Thanks for at least considering it.

[Hardy Heinlin]

If the development of this feature won't take more than an hour, then there is a chance. My todo list is 5 miles long.

[Hardy Heinlin]

Actually, you could refuel and defuel PSX also via Internet. Send the fuel through the network into the tanks of your choice.

If an addon developper could do this, I would have more time for other things :-)

[Tor]

Like so many other things in aviation the use of APU or GPU is about money.
IF
GPU cost   <   APU fuel use pr min.   X   min. at stand
THEN use GPU

Where I work we have a list with recommendations about what to use, since we obviously have no idea what it costs. Someone makes this calculation based on our turnaround times. Other factors are outside temperature (air condition!) and noise. A lot of airports have guidance to what extent the APU may be used.

The most restrictive destinations that I fly to, have a requirement that the APU must be shut off 5 min after arriving on stand (provided that GPU is available of course = that it works). And that it must not be started until 5 min prior to expected off block time.

We happily prepare FMC on GPU, this is actually quite common, probably more that 50% of the time. I once experienced loss of FMC when switching from APU to ENG driven generators. But this was a whole difference affair and a return to stand anyway due to another malfunction. Never experienced any problems with GPU to APU or vice versa. And during pushback you would already be running on APU anyway.

[John Golin]

My todo list is 5 miles long.

Statute or Nautical?

[Hardy Heinlin]

Galactical.

[Jeroen Hoppenbrouwers]

C'me on.

It compiles.

Ship it!









 :twisted:

[Zinger]

Ben Gurion International Airport limits APU use to a maximum of 5 minutes prior to pushback, due to environmental contamination. Terminal gates are equipped to supply electricity and cooling.

[Zinger]

Just to add something about GPU to APU power transfer. The B747 family requires a special type of pushback truck. Unit cost around 1970 was $1,000,000, suggesting its specification, including quality electric power supply was high. Smaller aircraft are catered by a vast array of less standardized ground equipment, and could therefore be more susceptible to power transfer probelms as explained above.