787 Ground Test Enable.
 

I have noticed on one or two occasions of pilots using this switch to enable operation of the air con during turn rounds when only two GPU stingers are attached.
After about 20 minutes or so the GPU supply tends to give up and the a/c is plunged in to darkness. With all the inconvenience and delay that goes with clearing spurious status messages.

Normally three GPUs or APU is required to run the CACs. Safely.

Is this practice becoming common?

Please desist, it's getting to be annoying.

We normally have two GPU's for our turnarounds, but I generally just leave the APU running, incase they fall over and it all goes dark.

Not seen the pilots touching the "Engineer is King" switch.

Was told by an engineer there are potetial hazards with moving that switch unless you are fully aware of the aircraft status. I always get an engineer up to move it.

As a matter of interest do you know which is more demanding of external power, running one pack or starting an engine?

Running one pack isn't in the same league as starting an engine...

Engine start is a 'transient' phase, with a truck load of power at the start which usually dies away as the engine spins up.
Engine start from ground power involves load shedding loads of other less critical electrical services.
Pack operation (I'm guessing 1 pack from 1 CAC? Never done this myself!) is a constant high electrical load - Just look at the decrease in fuel flow when you have a CAC fail...

I don't think that would be an approved Boeing procedure. I also don't think your Flight Ops department would be too impressed playing with their expensive Jet.

Sounds like a really stupid thing to do. Behind the scenes the B787 is a very, very complex computer system. If Mr. Boeing designed load shed into it, it was probably for a very good reason. It will bite you on the arse if you go playing with circuit breakers or using any undocumented procedures.

We do it all the time in europe because of Apu running restrictions at certain airports. As far as i'm aware, no problems running one pack with maintenance enable, however, if you try to start the APU with the switch in enable and run the pack then all hell breaks loose!:)

The airplane will still load shed with the switch in maintenance enable, the load shed groups will just have different priorities.

You wont do it any harm.

GPU and APU
 

We never run 3 GPU's. The book says you can it will help during engine start if you start the right engine but I imagine that would not happen.

When were running APU we usually run both packs until people are off. Then we shut the packs down as well as the lower recirc fan. This is if ground personnel later hook up air. Our company has had to replace a couple lower recirc fans getting worn our because of the air pushing against it.

We also shut down the battery (unlike other Boeings the APU won't shut down). We do this to save battery if the APU were to shut down or External power is lost at the gate. Otherwise you end up replacing a battery that runs somewhere between 70K and 100K.

I have never run just one CAC. If were on ground and people are coming aboard when I crank the APU I turn both pack on and lower recirc and call them and tell them to unhook ground air. Ground air is usually not so good on bigger jets. It will cool and empty airplane fine and keep it reasonable but as soon as you start filling it up doesn't do the job.

Ken M
787

Ken,

Small point...Are you aware that when you switch off the battery your not actually switching off the battery. The battery switch has no direct link to the battery at all. It just changes the logic of battery power should there be reconfiguring needed later....like no external power. You can press the switch off in flight with no effect if you really want. Ground test enable switch is for engineers only !! and don't get me started on the idiots who play around with the MFD curser sensitivity settings on the EFB.

Maybe I'm not quite understanding you, however 3 GPUs are a must for Engine start (Right only, system logic wont let you start the left) when APU is inop. Tried it with just two and its not pretty.


You will when the ground power trips. The CACs are not supposed to be run off just two GPUs, if you want to run the air con without APU then get a 3rd GPU. Simple. :ok:

Well I guess it's down to the quality of the ground power. Having operated without an APU, with ground power supplied to both forward external AC connections the RH engine was started without too much drama. The EFB took a few minutes to become fully operational but apart from that there was no delay in getting going. Starting the LH engine from the RH engine generators appeared to be slightly quicker than using the APU generators but then again when using the APU we tend to start both engines together. At one point it did look as though we would have to use a single source of ground power for engine start. The engineers opinion was that this would work although it would be extremely slow and with the load-shedding it would take longer for all the systems to come back online.
Using the aft AC connector is a bit of a non-starter. A panel has to be removed to gain access and then re-fitted so a qualified engineer is required. Also probably not a nice job with the engine running.
The main problem with no APU is keeping the cabin cool especially with no ground conditioned air. Therefore, using the Ground Test Enable to run one pack from external power would be most useful. Obviously this does place extra load on the ground power source and it would be a disaster if it tripped off line. However, if it is going to be used to start an engine perhaps it would be better to know earlier rather than later if it was not up to the job.

The 3rd GPU works just fine and is no problem to disconnect after engine start.

If you don't have an APU then you get an external Ground conditioning supply like every other aircraft.

If Boeing wanted you to use The GND test enable switch for air conditioning it would be in the NOP or Boeing Supp procedure. Its not. They designed and fitted an external air connector for a reason.

We got to ask the Boeing design team about this GND test/pack issue a few months back. The Boeing chap pulled a face and said "don't do it".

8che.
Yes and no.
There are places in the world that do not have external Air con supplies.

Double Oscar,
It is a standard procedure to supply a 3rd GPU if APU is inop. Yes it means you have to have a qualified mech/tech/eng on site but if it works don't fix it. Closing the aft ext power panel with right eng running is no big deal with appropriately trained personnel.

Using just the forward ext power means only one starter motor engages during engine start. The 3rd aft GPU means both starter motors will work.

You CANNOT start the engines with just one GPU attached. 90KvA is not enough.