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787 AC Ramp Operations

Posted: Sat Nov 10, 2018 6:55 pm
by lifecomm
It appears that 787 operates similar to a 767/777 when it comes to AC ramp operations. But I wondered, was there any consideration to increasing the electrical/power consumption capabilities so that on-board AC could be utilized and therefore, conditioned air connections could be gradually and generally eliminated? This way only an electrical connection would be required at the gate and require less effort.

Re: 787 AC Ramp Operations

Posted: Sun Nov 11, 2018 1:44 am
by Starlionblue
Aircraft already has enough power generation on board. Using the APU to power the packs for air conditioning will almost always give you better cooling than ground air.

However, many airports impose restrictions on APU use for noise reasons. And burning the fuel it is typically more expensive than ground air.

Re: 787 AC Ramp Operations

Posted: Sun Nov 11, 2018 1:59 am
by zuckie13
I'd guess the airlines still would want to save the wear on the onboard systems by giving them that break.

Re: 787 AC Ramp Operations

Posted: Sat Nov 17, 2018 4:27 am
by shamrock137
lifecomm wrote:
It appears that 787 operates similar to a 767/777 when it comes to AC ramp operations. But I wondered, was there any consideration to increasing the electrical/power consumption capabilities so that on-board AC could be utilized and therefore, conditioned air connections could be gradually and generally eliminated? This way only an electrical connection would be required at the gate and require less effort.


Believe the 787 has 3 GPU ports just for this. Problem is though, most airports aren't setup for it, and the 3rd port is down by the wing root so bit of an awkward setup. Its got 2 low pressure connections like most wide bodies for PC air, but those units are less reliable and are better at maintaining whatever temp the cabin comes in with vs drastically heating or cooling it. If its hot out, and the aircraft has been sitting with no air connected, ground air will take a long time to cool the cabin, if at all.

Re: 787 AC Ramp Operations

Posted: Sat Nov 17, 2018 6:27 am
by Francoflier
I don't know the 787, but I'd think they would have engineered plenty of electricity generation on the APU for just that purpose.
The airplane needs to be self-sufficient in places where it can't be plugged in and ventilated.

Re: 787 AC Ramp Operations

Posted: Sat Nov 17, 2018 12:29 pm
by stratclub
The 787 APU has 2 VFSG's (Generators) for a total of 450 KVA. Plenty of electrical power. Your assumption is correct, the 2 packs on a 787 are electrically powered not powered by bleed air as on earlier model aircraft.

Re: 787 AC Ramp Operations

Posted: Sat Nov 17, 2018 7:16 pm
by Lpbri
Not trying to nitpick, but the APU starter generators on a 787 are not referred to as VFSGs even though their output frequency is variable.

Re: 787 AC Ramp Operations

Posted: Sat Nov 17, 2018 8:51 pm
by stratclub
Pretty sure you were trying to nitpick. If you look at the pilots overhead electrical panel, none of the 6 VFSG's are labeled as VFSG's.

Image

Re: 787 AC Ramp Operations

Posted: Sun Nov 18, 2018 9:05 pm
by Okcflyer
What is the AFT EXT PWR connected too if not the AC bus?? That graphic doesn’t show lines from it

Re: 787 AC Ramp Operations

Posted: Sun Nov 18, 2018 10:25 pm
by unimproved
Okcflyer wrote:
What is the AFT EXT PWR connected too if not the AC bus?? That graphic doesn’t show lines from it

It's connected to the AC bus, but IIRC only used for speeding up APU-less engine start and maintenance tests.

Re: 787 AC Ramp Operations

Posted: Mon Nov 19, 2018 4:52 pm
by 744lover
unimproved wrote:
Okcflyer wrote:
What is the AFT EXT PWR connected too if not the AC bus?? That graphic doesn’t show lines from it

It's connected to the AC bus, but IIRC only used for speeding up APU-less engine start and maintenance tests.


Since the engines sit around, what, 100-150 feet from the nose of the aircraft where the main GPU connectors are, there is a wiring resistance increase which will limit the current you can provide to the engine start motor. For this reason, Boeing created the AFT GPU connector (around the wingbox). This shorter wiring from the wingbox to the engines allow greater currents, therefore allowing for a faster engine start.

I have yet to see an airport with such a long GPU cable that reaches from the nose section (where the GPU normally sits) to the wingbox.



BR,
744lover

Re: 787 AC Ramp Operations

Posted: Mon Nov 19, 2018 5:57 pm
by stratclub
The Aft External Power receptacle is used for maintenance so that you can have full aircraft systems functionality without having to run the APU. With no operating APU, the aircraft will start just fine on the front 2 external power receptacles although you would not be able to do a simultaneous 2 engine start, would have longer crank times on the first engine started and an additional amount of the aircraft electrical loads would be automatically shed during start.

Re: 787 AC Ramp Operations

Posted: Tue Nov 20, 2018 10:18 am
by unimproved
744lover wrote:
unimproved wrote:
Okcflyer wrote:
What is the AFT EXT PWR connected too if not the AC bus?? That graphic doesn’t show lines from it

It's connected to the AC bus, but IIRC only used for speeding up APU-less engine start and maintenance tests.


Since the engines sit around, what, 100-150 feet from the nose of the aircraft where the main GPU connectors are, there is a wiring resistance increase which will limit the current you can provide to the engine start motor. For this reason, Boeing created the AFT GPU connector (around the wingbox). This shorter wiring from the wingbox to the engines allow greater currents, therefore allowing for a faster engine start.

I have yet to see an airport with such a long GPU cable that reaches from the nose section (where the GPU normally sits) to the wingbox.



BR,
744lover

A normal engine start would use the APU generator, which is on the other side of the plane as well. The wire resistance is just a case of using thicker conductors.

The problem lays with keeping ramp operations simple, and the normal ground power connection is 90 kVA (x2 if available). Compare that to the APU generator which is 2x 225 kVA.

Re: 787 AC Ramp Operations

Posted: Fri Nov 23, 2018 5:05 am
by Lpbri
stratclub wrote:
The Aft External Power receptacle is used for maintenance so that you can have full aircraft systems functionality without having to run the APU. With no operating APU, the aircraft will start just fine on the front 2 external power receptacles although you would not be able to do a simultaneous 2 engine start, would have longer crank times on the first engine started and an additional amount of the aircraft electrical loads would be automatically shed during start.

By systems functionality you mean full hydraulic power. Yes. For ground use the rule of thumb is 1 hydraulic pump per stinger. Meaning if you want o run all three hydraulic systems you need to three ground power sources