APU start technique A320
 

Hello everyone,

my previous airline used this lenghty procedure for apu start on A32X family to avoid air/pack contamination events.
start apu, after avail wait one minute, then bleed on with both packs off for another minute and finally packs off. Is anyone else using this technique? My new airline has no limitation in apu bleed use immediately after apu start, just a note on fctm suggesting a 3 minutes warm up before selecting the bleed.

My company recommends a 3 minute warm-up time after APU start before selecting bleed on. Utilising this delay gives the seals time to warm-up and supposedly reduces cabin fumes incidents.

Same at my company.

Also;

The APU is a gas turbine engine running on high internal temperatures, so the longer you can leave it to thermally stabilise before making big power demands, the better for its health and longevity.

You wouldn't select TOGA straight after starting a main engine from cold, or if its oil was cold, and the same general principle applies to the APU.

Starting the APU then immediately selecting APU bleed and one or both packs puts a hell of a load and strain on the internal components. When I was PF setting up the cockpit I would start the APU, then do some other tasks, then a few minutes later, switch on the APU bleed, then continue setting up the FMGS and another few minutes after that, select a pack on - if circumstances allowed.

You can use the APU straight away in an emergency, but if you don't need to, it's better not to. You might need it in a real airborne emergency one day, so best to treat it gently as much as you can.

I see your point, but what’s the logic behind selecting the bleed without the packs on? There’s no demand on the system, right?

Here we used to do in order to vent any possible contamination in the bleed system…

I found a very long procedure as it required more time to start everything up, so delaying APU start to just before closing doors was not feasible.

To be able to get coffee asap and running water in the lavatory.:p

Muy importante.

But the A32X APU is the Honeywell 131-9(a), and the 737 Max APU is the Honeywell 131-9(b). (Someone really smart allocated the letters…).
Now assuming these APUs are not significantly different, why the complicated mucking about on the Airbus?

@ Check Airman: Yes there is some demand : APU bleed by itself pressurises the water systems and the hydraulic reservoirs, and of course the bleed pipe from the APU forwards, and seats the seals, so it uses some airflow and pressure but not a huge amount.

Doing it this way, also gives the APU hot parts a little longer to stabilise, before hitting it with the very big load of the pack(s).

Ah thanks. Hadn’t considered all those “smaller” user systems. My excuse will be the all-night leg I just finished.

I'd let the metallurgists take care of that. Or better yet, delay the APU start by those 3 minutes if you don't need the bleed yet and save the planet a little. 1 min = how much, 3 USD worth? People starting APU early to 'give more time to stabilize' fleetwide sounds like not overly bright SOHabit.

The EGT increase by introducing bleed demand is marked but whether or not it is real-life damaging becomes debatable. Keep the EXT PWR connected to mitigate that, why not.

'Over the lifetime' is academical truth because the supposed savings only arrive with the overhauls.

No dispute with the other reasons, your honour.

My simple rule on which hairs to split, when disussing small issues like this one:

Does this improve anyting for the given day/sector?
such as
- fuel burn
- noise
- workload
- complexity
- customer experience
- ontime performance

in my opinion having used the first procedure in my first post (which is even more complicated than airbus 3 mins wait rule) and the new airline one with no delay rule.

3 mins procedure/ avail 1 min, bleed on packs off 1 min, then packs on
fuel burn: more as inevitable apu is started earlier compared than before seconds before closing doors. Even when required on hot days is started even earlier than just seconds before 1st customer on board
noise: probably same as above
workload: higher for most compex procedure
complexity: same as above
otp: needs time so you need to anticipate apu start

Of course, the cabin air quality and duct contamination issues might be over-ruling.

We have the 3min limitation.

Is it certain company procedure or is it airbus procedure to wait 3 min before selecting apu bleed? Thank you guys

The procedure to wait 1 minute idle, then 1 minute with bleed on and packs off and then all (or 1 pack) on is a company procedure. The 3 minutes idle instead is an airbus policy.

I’ve flown for three airlines and never heard of this before. Legacy, low-cost and in between. And fleets of 30 to 300.

The OP's original operator is incorporated in a country where air cabin contamination and possible toxic fumes at workplace is a very big and important agenda.

Can't blame the flight ops team for playing their part to mitigate the risk. Fair job, at least that 1+1+1 procedure is easy to understand, work-flow wise.

3 mins seems reasonable to me - especially if Airbus state that and they obviously know much more about it.

I always winced in the bad old days, when folk would bang on the APU bleed and packs immediately, (and the APU GEN would also be demanding power), the second the APU ready light illuminated.

All I was saying was to err on the gentle side. Metallurgists have no control over the abuse of equipment, but we do, and we might need the APU one dark night halfway across the Atlantic......

Some operators lease all their APUs, so "don't care" about them, but as I say, we pilots might care if it doesn't work when we need it.

Has anyone heard of the APU starting before the inlet door opens?