Hi there,

Kind of a newbie question, but one I'm curious about - when a commercial airliner is inside a hangar, to what degree are electrical systems like lighting, etc made functional, or are they? Most pictures I've seen seem to have shop lights hung and powered by extension cord for example to provide light for maintenance crews inside. Is it possible to power certain systems like interior lights/inflight entertainment without needing an APU or ground power unit? Is that something you *can* do safely inside a hangar or does that not happen indoors?

I don't work in a hangar but I'm sure there are appropriate AC electrical hookups in serious, commercial hangars. Probably DC as well. You wouldn't need to run the APU or have a fuel-powered GPU. AC power is available at most jetways so having it in a hangar shouldn't be an issue.

falter wrote:

Hi there,
Is it possible to power certain systems like interior lights/inflight entertainment without needing an APU or ground power unit? Is that something you *can* do safely inside a hangar or does that not happen indoors?

Hello and welcome! For aircraft power, there are 2 main sources, onboard or external power. The generators onboard the aircraft in the engines or APU provide onboard power, while a ground power unit will provide external power. Ground power units, GPU's, are either diesel powered in the case of the portable carts you see pulled to the front of the plane after it parks, or building mounted. Building mounted ones are typically found on the jetbridge, the plane parks, and a cable is lowered from the bridge and plugged in to the aircraft. These types are also found inside hangars. Aircraft have batteries, but these are small and are used for things like emergency lights, or starting the APU if no GPU is available.

To answer the first question, no, you need either the APU, onboard power, or a GPU, external power to run things like lights or IFE. Most hangars have ground power so they can plug the aircraft in and run systems like the IFE or avionics. You can't run an APU indoors, even if the hangar doors are open. Typically when you see things like hanging work lights or extension cords to run tools its because the cabin lights may not be bright enough in that particular spot, or lots of power is needed to run a particular tool. The outlets onboard the aircraft are really only designed to run smaller appliances like vacuums for the cleaning crew, not high amperage power tools.

Thank you! You answered another question I had in my head but assumed was obvious - that they wouldn't run the APU or engines indoors.

falter wrote:

Thank you! You answered another question I had in my head but assumed was obvious - that they wouldn't run the APU or engines indoors.

You're welcome! Besides the noise and exhaust fumes, most hangars have a foam fire suppression system. These are usually activated by smoke beams, or infrared heat sensors. Even running an APU outside close to an open hangar door can set these systems off, and activating one of them by accident would ruin your day!

https://youtu.be/wYAOHYKBYas

I've seen people start APU's inside a hangar, usually it's a mistake, but it can happen. Lucky enough the hangar we worked in then had no such sophisticated fire detection systems.

As for powering up the aircraft in the hangar, some airplanes can be sufficiently powered by a DC cart (such as the Dash 8), although you can also use an AC cart, especially if you need to power the Hydraulic pumps, or Aux fuel pumps.

On CRJ's, I've only ever connected a DC cart to "jump start" the APU, and only outside. Everything else uses AC power on most jets. We also do have portable AC and DC power carts inside hangars, they plug into a 600V power receptacle in the hangar and provide Either 28VDC, or 115VAC, depending on the unit you are using

Usually the aircraft provides plenty of lighting for inside work, however if you need to shut off the aircraft power, then we use trouble lights. During a heavy maintenance check, these lights are used extensively as the power will be shut off for a week or more during the middle part of the check.

To answer the other part of your question, we can run practically any system on the airplane in the hangar using AC or DC power as required. The exceptions are leak checks on the engine and APU, or testing the air conditioning system, for these we have to tow the airplane outside and carry our APU runs, or engine runs as required. For brake or steering checks, we will need to taxi the airplane. We can check the steering in the hangar to see if it "swings" back and forth, but if there's a steering related complaint, then it'll usually end up with a taxi test.

We often power up most of the airplane's systems in the hangar using the ground power units (GPUs). They are the same units you'd find at the terminal, just mounted to a roll around cart instead of to a bridge. The only things we can't really power up on most aircraft are the engines and APU systems themselves, those we test outside for obvious reasons. With the right GPU we can do pretty much everything else on most airplanes. Cabin lights the avionics are easily powered by the GPUs.

As for the cords it's usually to run tools and such, and since the power is there now anyway some inspections it helps to have some additional lighting. Also sometimes we'll be working on things inside the aircraft while someone else is working on the electrical system so usually when you see lots of shop lights and extension cords going into the airplane it's because the airplane's electrical system is taken apart in some way.

Just a quick add-on question.
For aircraft to be powered up - do you need to switch battery on, or will external power completely do the trick? I could imagine some sort of protection on the battery circuit preventing that, but Bat/GPU running parallel with switch on each seems a simpler solution.

Also, are there any systems not powered up with GPU, that would with battery? Except for those that do not need to be powered at ground (eg flight systems, yaw damper etc.).

Thanks, Cheers
Adam

gloom wrote:

Just a quick add-on question.
For aircraft to be powered up - do you need to switch battery on, or will external power completely do the trick? I could imagine some sort of protection on the battery circuit preventing that, but Bat/GPU running parallel with switch on each seems a simpler solution.

Also, are there any systems not powered up with GPU, that would with battery? Except for those that do not need to be powered at ground (eg flight systems, yaw damper etc.).

Thanks, Cheers
Adam

Depends on the aircraft, some aircraft require the battery to be on to energise the relay/contractor to allow ground power to be connected, however after ground power is connected the batteries can be turned off without disconnecting the external power.

falter wrote:

Thank you! You answered another question I had in my head but assumed was obvious - that they wouldn't run the APU or engines indoors.

Oh it's been done, it's one of those where the manual doesn't always specifically say you can't. Mainly because the tech writers didn't think they needed to put that warning in the manual since in most places its against fire and or noise regulations to operate a turbine in an enclosed space.

gloom wrote:

Just a quick add-on question.
For aircraft to be powered up - do you need to switch battery on, or will external power completely do the trick? I could imagine some sort of protection on the battery circuit preventing that, but Bat/GPU running parallel with switch on each seems a simpler solution.

Also, are there any systems not powered up with GPU, that would with battery? Except for those that do not need to be powered at ground (eg flight systems, yaw damper etc.).

Thanks, Cheers
Adam

Unless there is some specific need (e.g. maintenance procedure) to select or deselect certain systems, it is our* procedure to power the aircraft in the hangar the exact same way it is powered on the ramp. That includes switching to ground service when powering the whole aircraft is not required.

*other operators may do thing differently.

On the Boeing WBs, there is the ability to power the ground service bus which is the cabin equipment, electrical plugs, lights, PSU lighting and galleys. No battery required for that. In most of the hangers I have the privilege to be in, there were built-in power packs to provide aircraft power when needed. No running GPU or any other external power generator.

gloom wrote:

Just a quick add-on question.

Depends on the airplane, but most airliners seem to follow a similar theme...

HS748: The battery power and ground power are selected on and off from the cockpit, and they can each be selected on and off independently. The only catch was the battery and ground power switches must both be on to charge the batteries from the GPU power.

ATR: Plugging in and turning on the GPU automatically turns on some lights for servicing the airplane (loading, cleaning, etc). Everything else requires going into the cockpit and turning battery switch on then pushing the ground power button.

737: With the GPU plugged in and turned on there is a ground service switch by the forward entry door (I think the combi had one at the aft entry door as well, can't remember for sure though), selecting this switch powers up the cabin lights, fuel panel and cargo pit lighting, but leaves the rest of the airplane dark. To power up everything else requires going in the cockpit and selecting battery switch on then ground power switch on.

Standard operating procedures is usually rampies, cleaners, etc. can all use the ground servicing power themselves, but to go in the cockpit and turn on the real power you're supposed to go and get a pilot or mechanic to do it.

Systems it's hard to really split between battery and GPU because normally sitting in the hangar there are some things coming straight from the battery, but while that is happening the battery is being charged by the GPU. In the hangar the HS748 you could do most systems off of either the GPU or batteries, but the batteries ran down pretty quick if you had a lot turned on. The ATR and 737 the battery only really gives you a few basic things and you'll need the GPU for everything else. The ATR and 737 I can't name any reason why you would have the GPU on and not the battery switch as well (I'm not even 100% sure the GPU power would engage without the battery) so I don't know what all would or wouldn't work.

At our hangar here in IND, there is an over-head power drop-down station, some hangars have floor recessed power stations. The station has ground power for the aircraft, two air hoses for maintenance purposes, and 120V power, again, used for maintenance purposes. Each aircraft bay has this. The aircraft ground power plug in is the same as you would see out on the flight line ramp. It will provide power to the aircraft for all its electrical needs. The reason you see maintenance personnel using portable drop-lights and light stands is that aircraft don't come equipped with 115/120V plug ins like you would see in your house. Thus, we string out power cords to run our maintenance electrical equipment, such as work lights, fans, and such. Here, we can use portable electrical equipment out on the ship if it is deemed safe. By this I mean DeWalt or Mikita power drills, etc. In the past, I have worked at places where this was not allowed. Pneumatic powered tools are still the dominant choice for us in the structures shop because so many of our specialized tools are air powered. Stringing out air hoses and electrical cords are a daily part of my work process out on the aircraft. Some times it can become tedious such as when you need to get power, electrical or air, to some locations such as the top of the vertical stabilizer or on top of the fuselage of a MD11. It's all in a day's work.

falter wrote:

Thank you! You answered another question I had in my head but assumed was obvious - that they wouldn't run the APU or engines indoors.

If the tail of the aircraft is kept outside of the hanger door, it is common to run the APU if you require hydraulics for moving flight controls on aircraft like the 747.