# Why does load shedding still occur in jetliners (e.g., 787) when there is so much power available?

I had a few questions about how electrical generator works on an aircraft, the 787 to be specific although I know it is similar for all aircraft.

So the 787 engines have 2 VFSG each which are rated at 250 KW each which is 1000 KW -> 1 MW which is said to power a 2000 population town. Since the power is generated using the rotation of the engine, im assuming it does not always produce 1 MW , only at peak rotations?

What on earth in the plane sucks that much energy, I know the 787 is a more electric aircraft, but still how much power does it really take? If the total power output of the plane is 1.45 MW, I'm assuming the APU Gens are able to provide 0.45 MW in total to the airplane.

If there is so much power available, why does load shedding still occur in aircraft?

• I don't have a good breakdown of where all the 787 power goes, but the first couple slides here give you an idea how much power is required to replace what was done by the bleed air system on other aircraft and some of the typical loads. Commented Sep 23, 2018 at 5:55
• Hi @ZachLipton - I think the presentation you found can be used to formulate a good answer regardless of the aircraft type.
– user14897
Commented Sep 23, 2018 at 14:12
• @ZachLipton, actually, the slide that provides most concrete answer is in the middle. Commented Sep 24, 2018 at 12:34

Load shedding always occurs during engine start, according to the FCOM of the B789. Both engines are allowed to start simultaneously. The B789 engines are started using both VFSGs of each engine, which are mechanically connected to the N2 shaft via the accessory gearbox. So, the VFSGs are using power instead of delivering during engine start. The APU cannot provide enough power to prevent load shedding.

If the engines are started using external power, at least two 90 kVA external power sources are required. Optimal start performance is achieved using 3 external power sources (2 on the left forward fuselage and 1 behind the left wing on the fuselage). If only 2 external power sources are used, significant load shedding can occur (e.g. First Officers displays blanks and even the audio control panel receive and trasmit selections may be lost).

The APU is normally switched off during flight, unless e.g. one VFSG is U/S.

Load shedding should normally not occur when both engines are running and all VFSGs are operational (I haven’t experienced load shedding during flight).

First of all, I am not familiar with the electrical system on the 787.

In a modern twinjet airliners, if one engine-driven generator fails, load shedding is activate to protect the electrical system. What will normally be shedded is IFE and galley equipment (ovens, coffee makers, etc.). Should this happen, a checklist will direct flight crew to start the APU in order to establish normal power supply for all the equipment of the aircraft. If APU starts, the flight can continue normally. If APU is unable to start for some reason (flight could be dispatched with APU INOP, or APU generator INOP), then the crew will likely have to land at the nearest suitable airport, as the electrical reduncancy is lost.

So, in case 1 when generator fails and crew successfully starts APU, the IFE and galley equipment will not be powered for perhaps 5 minutes or so. No big deal to be honest. In case 2, where the APU is not available, the flight will have to divert to an airport, so no real need for ovens or IFE anyway.

Now, of course the manufacturers could design electrical systems that would handle this kind of failure, but the cost/benefit just doesn't support it. The generators would have to be more powerful, which would in turn lead to increased weight, wires would have to be thicker, other electrical elements would have to be designed to take the increased current/load, etc. All this, just so passengers could enjoy IFE and a hot meal during their short diversion to a nearby airfield.

Technically the VFSG are 250kVA (kilo VoltAmps). VoltAmps and Watts are only equal assuming a perfect power factor. The various systems are largely motors and controllers and do not have a particularly good power factor, so there is some losses there.

As you noted, the VFSGs are direct coupled to the engines, so they vary in rotational speed based on engine speed, so there are some engine speeds where peak power may not be available.

I don't have wattage numbers for the various systems, but on the 787 everything is electrical, so there are a lot of loads you might not think of.