I did a search and couldn't really find anything that answered (I apologize if this is something beaten to death or anything, just couldn't find anything that answered this) so my question is this:

I notice airlines often leave APUs on even when they are hooked up to ground power and ACs and such (or so it seems) Why do they do this and not shut everything down? Wouldn't it reduce noise/wear and tear etc. by doing so?

Yes, ideally while at the gate the aircraft should be hooked up to ground power and air. Normally the APU is started sometime before departure, it depends on the airlines procedures or the aircraft type, but generally within 15 min. There are also situations where the ground air is not strong enough to keep the cabin cool. Either the unit is not functioning properly, its a very warm day, or the hose is kinked. This seems to be especially true on longer narrow bodies like the 757-300, 737-900 and A321, where unless everything is functioning perfectly, the single air hose isn't enough. Similar things can happen with ground power, if the plug isn't connected tightly, or the unit trips offline, the crew might just start the APU to keep the power from switching off repeatedly.

In addition to what shamrock137 mentions, in some places ground power is so expensive that running the APU is cheaper.

Another thing to consider is APU start cycles - sometimes it’s cheaper to keep it running, rather than shutting it down after taxi in and re-starting it before departure.

However, it’s a bit of black magic, depending on airline maintenance contracts etc, company policy, price of ground power at a particular airport etc. - airlines themselves often get it wrong

Thanks for your replies everyone!

One thing is airline policy, but in this neck of the woods the pressing issue is actually airport restrictions. At my home airport, CPH, the restrictions are 5 minutes after on-block and 5 minutes before expected off-block. Exceptions are allowed for meteorological reasons or unavailability of GSE, extending the limits to 10/45 minutes for wide-bodies and 5/15 minutes for all other aircraft. Most, if not all, major (and not so major) European airports will have similar rules.

While most airlines readily accept and abide by those rules (the odd ad-hoc operator may need to be educated), some private jets do have a problem wrapping their heads around the fact that Mr. VIP will not be welcomed in a sufficiently cooled or heated cabin. Crews often argue that Mr. VIP is paying top-Dollar to hire a private jet, and has certain expectations. In all those cases, however, the airport employees dispatched to deal with the issue has stood their ground, and the crews have subsequently - and frequently under loud protests - shut the APU down.

I do appreciate the conundrum this presents to the crews, as they will be at the receiving end of the passengers dissatisfaction. But there is a ready-made solution to that problem: Order a heater/cooler and GPU from your ground handler and the job's jobbed. Sure it costs money, but compared to the cost of hiring or operating a private jet we're talking peanuts.

https://www.cph.dk/48da29/globalassets/ ... mpagne.pdf

B777LRF wrote:

One thing is airline policy, but in this neck of the woods the pressing issue is actually airport restrictions. At my home airport, CPH, the restrictions are 5 minutes after on-block and 5 minutes before expected off-block. Exceptions are allowed for meteorological reasons or unavailability of GSE, extending the limits to 10/45 minutes for wide-bodies and 5/15 minutes for all other aircraft. Most, if not all, major (and not so major) European airports will have similar rules.

While most airlines readily accept and abide by those rules (the odd ad-hoc operator may need to be educated), some private jets do have a problem wrapping their heads around the fact that Mr. VIP will not be welcomed in a sufficiently cooled or heated cabin. Crews often argue that Mr. VIP is paying top-Dollar to hire a private jet, and has certain expectations. In all those cases, however, the airport employees dispatched to deal with the issue has stood their ground, and the crews have subsequently - and frequently under loud protests - shut the APU down.

I do appreciate the conundrum this presents to the crews, as they will be at the receiving end of the passengers dissatisfaction. But there is a ready-made solution to that problem: Order a heater/cooler and GPU from your ground handler and the job's jobbed. Sure it costs money, but compared to the cost of hiring or operating a private jet we're talking peanuts.

https://www.cph.dk/48da29/globalassets/ ... mpagne.pdf

Obviously you haven’t actually operated a large bizjet in those locations. GPU is hard to get when there’s 5 or 10 getting ready at once on a ramp, not a gate. No mobile A/C units, very rare. Oh, it takes, at least 40 minutes for dark cockpit start oincluding outside and inside preflights.

SaschaYHZ wrote:

I did a search and couldn't really find anything that answered (I apologize if this is something beaten to death or anything, just couldn't find anything that answered this) so my question is this:

I notice airlines often leave APUs on even when they are hooked up to ground power and ACs and such (or so it seems) Why do they do this and not shut everything down? Wouldn't it reduce noise/wear and tear etc. by doing so?

On larger aircraft the ground air is not enough to cope normally when the cargo doors are open, it just lets too much hit air into the aircraft. The external air hoses are smaller than the size of a passenger window, they can handle a cabin door being open but not the much larger cargo doors.

When ground power connect and the APU running we still take advantage of the ground power, the APU runs at a lower setting.

Some maintenance procedures also need the APU, eg the cold soak procedures need the APU running for a couple of hours prior to the flight, often towing needs the APU as many tugs do not have suitable electrical sources.

APU’s usually prioritize electrics, if your parking place doesn’t have external cooling air available it’s more effective to leave the APU running but offload it by using external power then it can provide its maximum bleed air output and in turn, maximum cooling

GalaxyFlyer wrote:

Obviously you haven’t actually operated a large bizjet in those locations. GPU is hard to get when there’s 5 or 10 getting ready at once on a ramp, not a gate. No mobile A/C units, very rare. Oh, it takes, at least 40 minutes for dark cockpit start oincluding outside and inside preflights.

You mean apart from the 3 years I flew a Falcon 900?

I can't recall ever seeing "5 or 10" business jets on the ground at the same time in CPH, let alone 5 or 10 of them seeking to depart at the same time. Even so, every single stand is equipped with a GPU so that's hardly an issue, and there's plenty of mobile heaters/coolers as well.

We went from cold and dark to ready in less than 30 minutes on the Falcon, and was perfectly able to do that being connected to a GPU. Granted, we did it on APU power with great regularity as well at airports where a GPU was not readily available and APU usage restrictions were more lax, and running off the APU certainly had benefits making the temperature in the aircraft comfortable. But when we operated into airports with APU restrictions, we just had to accept those are the rules. Luckily the guy we flew for was a sensible chap, who hadn't been caught in delusions of his own grandeur. But I've certainly encountered more than a few who thought that because they were rich and owned a 50 million Dollar jet, rules didn't apply to them.

B777LRF wrote:

One thing is airline policy, but in this neck of the woods the pressing issue is actually airport restrictions. At my home airport, CPH, the restrictions are 5 minutes after on-block and 5 minutes before expected off-block. .....

https://www.cph.dk/48da29/globalassets/ ... mpagne.pdf

Interesting! Very uncommon in the US. I know some airlines have "APU Police", teams that will go around the ramp to speak with flight crews and reduce APU use and identify gates that do not have sufficient ground services, but its usually company driven, not airport. I've heard the opposite as well, that when contract negations are stalled between pilots and the company, usually over pay, APU usage will spike.

B777LRF wrote:

I can't recall ever seeing "5 or 10" business jets on the ground at the same time in CPH, let alone 5 or 10 of them seeking to depart at the same time. Even so, every single stand is equipped with a GPU so that's hardly an issue, and there's plenty of mobile heaters/coolers as well.

Everyday thing at MIA & OPF. I see the departure end on 9L (@OPF) all day long, several times a day it looks they are queued at least five deep.

I spent many years working on the ramp at Stockholm ARN.
When I arrived in 1987 there was no APU policy. Most aircraft had them running when the crew were on board.
then one day around 1991 a British Airways BAC111 was parked on a gate nearest to the terminal, and it stayed there for four hours from 1130 until it returned to Manchester at 1530. The crew stayed on board, and even though ground power was connected and free, the APU stayed on. In the terminal about 50 meters away the airport director was holding a board meeting with the constant drone of the BA APU.
A month later an APU policy was introduced which was very strict. Taxy in with the APU shut down and wait for Ground power before shutting down the engines. Start the APU at STD -5mins on departure. And it worked, with very efficient FEP on every gate, and Heating available (we are at 59N, not much AC required!)
Later when the B737-600 arrived, it was allowed to taxy in with the APU running as the ground power socket was inside the engine safety zone, and now this is standard due to Health and Safety rules that do not permit the ground crew to approach the aircraft on arrival when the engines are running.
It makes working on the ramp much nicer. Some times there is silence outside the aircraft and you can talk to each other without shouting!
Having worked on a ramp previously where APUs run all the time, I know which I prefer.

SaschaYHZ wrote:

I notice airlines often leave APUs on even when they are hooked up to ground power and ACs and such (or so it seems) Why do they do this and not shut everything down? Wouldn't it reduce noise/wear and tear etc. by doing so?

SaschaYHZ, what Shamrock137 stated in Reply # 2 exactly.

Your question speaks very directly to airport infrastructure issues, at least in the United States. At my operator, the way it is SUPPOSE to work is to start the APU after landing so it reaches full operational status just prior to gate arrival. Further, the ground crew is suppose to hook up external power and air within 3 minutes after block in at which time we shut down the APU. We start it again approximately 3 minutes prior to departure.

But, it rarely works that way because at many of our stations, during warm weather, the air supplied by the ground air source (unit connected to the jetway or terminal building, or ground air source) is inadequate to maintain a comfortable cabin environment. Even when it is not especially warm--27 C / 81 F, the cabin can get warm very quickly. And, yes, sometimes there are kinks or twists in the hose due to inattention by the ground crew. Their focus is generally on getting luggage unloaded. As a result, we generally end up starting the APU prior to passenger boarding, which can be anywhere from 30 to 45 minutes prior to departure (narrow body).

While company policy is to use external power and air as much as possible to reduce fuel consumption, passenger comfort takes a higher priority.

With regard to airport infrastructure, I notice passenger facilities are generally updated and modernized; however, ground equipment--environmental units, belt loaders, baggage carts, tugs, etc., don't always get the same amount of attention. If using a company gate, our facility maintenance personnel do their best to keep the air conditioning equipment working satisfactorily, but if we are assigned a common use gate, we are dependent on the local airport authority to maintain satisfactory environmental controls.

The normal result: yes, we often run the APU while hooked up to ground power to maintain a comfortable cabin temperature.

e38

Can I ask a dumb question? I've never seen an air conditioning hose that didn't have a fold or kink in it. Are they not actually *designed* to be that way?

Florianopolis wrote:

Can I ask a dumb question? I've never seen an air conditioning hose that didn't have a fold or kink in it. Are they not actually *designed* to be that way?

Actually they do. You just cant notice it when it is supplying High Volume Air as it becomes inflated. However old hoses do leak from areas where they are usually folded up and stowed.

Our air hoses used to have a fixed head, so any kink would “run” along the length of the hose as it inflated and kink right where it hooks into the aircraft. It wasn’t obvious on glance as the rest of the hose was fully inflated. Kind of a hidden twist.

Over the past 5 years most of those have been replaced with a rotating head. So when the hose inflated, any kinks will “run” towards the head, and then the pressure of the twist will actually rotate the head and unkink the hose. It’s very nice and works quite well most of the time. It used to be such a chore unrolling a hose that was lazily stowed with a million twists in it - with the fixed head you’d have to twist the hose around to try and get it to connect right. Then a big gust of wind would come and twist the hose around again before you could get it straightened out.

Now, just plug and play. Doesn’t matter if it’s twisted it will straighten itself out. We have also moved to a lighter fabric that isn’t as rigid as the old ones.

But, only so much can be done. Even if they blow cold air, many times it doesn’t push the same amount of volume as the aircraft’s own AC packs. Also, the long run of these hoses with no insulating value means the air may increase in temperature substantially from the time it enters the hose to when it enters the aircraft.

Edit: Also, from a ground handling perspective we can get frustrated by conflicting messaging. Often times this results in us not even bothering to hook it up. Our supervisor will come out and tell us to hook the air up, as soon as we hook it up the pilot comes down and says they don’t want it they want to run the packs. So we unhook it. The next flight crew comes in, wants air hooked up but it’s a crew change and they leave. The next crew boards and tells us to take it off. Or the inbound crew doesn’t want it, but the outbound crew wants it all the way up until departure time. This back and forth usually results in us saying “screw it”. There is not much consistency.

One other thing that comes into play these days, is that the APU cooled/heated air has passed through a HEPA filter. External A/C normally isn't, so it might be preferable to use APU air whenever there is pax on board. Most airports in Europe have relaxed the APU rules specifically for this reason, but I remember flying into CPH last year and it was made very clear to me, that they are not relaxing anything, so the APU was off ASAP.

Certain older A320 aircraft can't run the potable water system off of GPU power, so lots of operators during the pandemic wanted those aircraft to run the APU so that lav washing water was available.