APU shutdown during turnaround

[TO MEMO]

Hi guys

The airline where I work has per SOP to turn off the APU during turn around.
One question... What`s worst for the APU. Lots of hours or cycles? Or is APU like the engines nowadays... On wing until they have problems...?

Now, disregarding the environmental reasons and regs for some airports... if you had none of these... Is this really a good procedure? What are your toughts?

[kuwait340]

Hello...

well...in our company we keep the APU on during the turn-around...and we don't use EXT-PWR at all. in fact, we use the APU all the time unless the aircraft is day-stopping or night-stopping (aircraft on ground for 6 hours or more) where the engineer shut's down the APU once he's done with his checks.

we are doing this policy since a very long time , but that doesn't mean we didn't have any problem with the APU's ... sure we had.

i think it is better to use the APU since it has more stable current than the EXT PWR ( in terms of supplying the aircraft systems ) ...and i stand to be corrected .

regards

[electricdeathjet]

obviously the price of fuel is cheaper in Q8!!!!

[kuwait340]

spot on :ok:

[BOAC]

I guess it depends on too many variables, including maintenance 'deals' for APUs. E.G. In DanAir 737s, we were switching the APU off after start and lighting it on landing (as per SOPs) on LGW-CDG and LGW-BRU sectors (of which we flew many daily) and it was determined that the 'hot cycling' of the APU had a negative effect on service scheduling. We left it running for the 28 mins or so and APU serviceability apparently went up. It is the same deliberation as to whether brakes or reverse are cheaper on landing.

[hunterboy]

Many airports have local by-laws that force operators to turn APU's off during turnarounds and night-stopping aircraft too. Quite often, these are the limiting factors.

[Swedish Steve]

Well glad I don't work in Q8.
Here in ARN is a strict no APU policy.FEP is plugged in on arrival, and the APU goes off. Restart 5 mins before departure. In fact the MH B744 crews have now been trained to not start the APU on landing. We plug in two FEP, and he shuts down nbr 4. APU started when pax door closes. We have ground air supply on most stands, and of course a nice climate. It makes life on the ramp so much easier when you don't have to shout to each other.
By the way, aircraft take a long time to cool down in the cold. Leave an empty aircraft with the power on, and even at M10degC it keeps warm (until catering opens the doors!)

[cessna24]

the technical side is that, the more times you start the apu, the quicker the starter motor will wear. Which is indicated by a wear pin which is part of our weekly check {A319}. We have had a letter from the airline we work for saying that they lost x amount of money through having the apu running for no reason. {I think it was more directed to us engineers}. Another thing to remember is that for some reason during the turn-around, you go to start the apu and it does not start and you have no technical personal available, delays creep in.

[N1 Vibes]

My observation is it is the hours that kill an APU, not the thermal start/stop cycles. Modern units on A330/B777 etc should be good normally for 10,000hrs running time. As for the PBH contracts, not sure how this affects things. But if I was the OEM, I'd keep it on-wing as long as possible.

Regards,

N1 Vibes

[Capt. Inop]

I guess that depends of what you fly and where.
Well flying Dutch NG's that's green'n white i run the APU whenever i feel the need to do so.

What`s worst for the APU. Lots of hours or cycles?

The way i understand it it's hours that costs most money by running the apu.

[Engineer]

Articles in a boeing topic on fuel conservation showing the different in operating costs of running APU or GPU Table reproduced below

COSTS PER KWH (33 KW DEMAND LOAD)
TYPE POWER GALS $/GAL. S/KWH
GPU (Diesel) 0.085 0.36 0.03
GPU (Gasoline) 0.197 0.50 0.10
APU. 1.63 0.40 0.65

The APU cost is 6 times more expensive. Note that the operating cost were derived when the fuel price was $0.50/US gal. Today the average price is $2.50/gal:{ Draw your own conclusions but here the APU is only run when the aircraft is ready for departure and 5 minutes before arrival at the gate

[parabellum]

The direct operating cost of APU versus ground power may show the ground power is cheaper to operate but that may not be the actual cost if you use a ground handling agent, it is important to know what they charge for the hire and use of the ground equipment. Zürich were very noise sensitive about APUs running but their ground equipment was usually pretty reliable.
Nothing worse than arriving on stand, dark wet night in the tropics, ground power and ground air attached only for it all to drop out when the APU goes off! From an engineering point of view it probably shouldn't happen but it certainly does. Very dark aircraft warming up nicely with 400 pax still on board. There are stations where it makes sense to keep the APU running at least until the pax have disembarked.

[BOAC]

AH! ARN - yes - I ALWAYS lit and kept the APU running until the GP was plugged in (and working). Handling there was a bit 'flaky' to say the least. On a 25 min T/R, waiting a long 3 mins for GP before shutdown took a big chunk of time away. So much for a 'noise sensitive airfield':ugh:

I cannot see much relevance in this discussion of APU policy to those places where the use of APU is restricted?? It really does not matter which is 'cheaper', there, does it?

On the subject of 'cheaper', I repeat my previous:I guess it depends on too many variables, including maintenance 'deals' for APUs.

[Tako Yaki]

The most likely reason that a company sets a SOP to shutdown the APU after Ext Pwr is connected and restart just before STD will be due to a power by the hour agreement for the APU's.

Therefore less operating hours equals less $$$$$.

The bean counters, as always, rule the roost!

[World of Tweed]

In the World of Tweed we are told to shutdown the APU after engine shutdown and EXT PWR is connected. We are then permitted to start it again when before Pax board.....

However, on the day it up to the operating crew(i.e. CPT) as to the best course of action. Frying your Cabin crew and cleaners in 35 degree heat on a summer turnaround in greece whilst still expecting a fast turn is just stupid. As is chilling your fare paying pax at 4:00am on the first rotation as we wait for boarding to finish.

Common sense prevails on the line - but the company, in good faith, is trying to reduce fuel consumption and latterly reduce carbon emissions so therefore asks us to assist by either shutting the APU down or when EXTPWR unavailable by running the APU with the bleed valve closed resulting in a lower APU EGT (757/767) and lower burn.

As they say in the movies lately with the fuel price as it was "all other concerns are secondary" Maint. cycles etc... have been ignored and fuel price has been the driver.

[sdh2903]

More APU start cycles = more apu starter wear, ignitor and ignition system wear, higher egt on start, apu bleed valve cycles, thermal wear etc. If on a short turnround, from an engineering perspective I would prefer the APU to be left running.

[aveng]

My observation is it is the hours that kill an APU, not the thermal start/stop cycles

Wrong! It is definately thermal shock during start cycles that kills apu's. I know of a pacific island a/c that left its apu running from when the a/c was first started in the morning till when it was shut down at the end of the flying day (mostly because of the remoteness and likely hood of problems if it didn't start) and when it was removed due to hours and disected in the workshop it was in absolutely pristine condiction. With regards to apu starter wear - starter motor brushes are easily replaced. New gen 737's actually have a starter/generator these days.:ok:

[Callsign Kilo]

In my opinion it's a beancounter issue. If a GPU contract is in place, APU off after GP source is verified by aircraft. My airline has recently reviewed GPU contracts at the majority of airports where it is available. Costings now seem to dictate that GPU is the way to go....until it trips out and it all goes dark on you!

[TO MEMO]

I just read the "Getting to the grips with A320 performance retention and fuel sabing" where it states that reducing average APU usage by 5 minutes a day will save $10,000 a year per aircraft... only 5 minutes...
calculations made for an aircraft flying 3000 hrs a year and an average sector of 2hrs.

Now let`s suppose you reduce APU usage by 30 minutes a day... that`s $60,000 a year, by switching APU on only some 20, 15 or 10 miuntes before departure

Now will that pay off those extra cycles a year?

I think so, APUs are designed for a life full of cycles...

[Capot]

Just for information, the last time I set a price for GPU hire, several years ago now as a handling agent, it was justifiably at least 20 X the actual direct fuel and maintenance cost, which may be the figure used in the cost comparisons GPU vs APU above.

Equipment depreciation and maintenance, labour costs, training, airside insurance, airport fees, fuel, administration overhead etc etc etc usw.

And there's the cost of keeping the back-up units round the corner in case of failure.

I'm sure someone can advise the probable contract charge for a GPU for a 30 minute turnround...my data are out-of-date. Try that against 30 minutes APU time.

But as has been said the over-riding factor is usually airport regulations, often driven by local planning agreements.