B777 difference between APU to Packs and Packs off T/O

[Kapitanleutnant]

All...

Can someone explain this difference to me.

It seems that in either instance, the bleed air will not be used from the engine to the packs and therefore would be the identical same performance adjustment... but obviously I'm missing something as there are the two separate checklists in the supplementals.

Thanks

Kap

[VNAV PATH]

When APU to Pack is used there is only one engine bleed going to one pack.

[Kapitanleutnant]

Thanks VNAV.... knew I was missing something there

K

[RAT 5]

Can't say for a B777, but I suspect the principle is same as B737. APU to packs = APU supplying air for pressurisation and air-conditioning. Some a/c might allow this to be via 1 pack or multiple. Packs Off takeoff could imply unpressurised. Remember, the APU can be u/s. The performance improvement depends on engine bleeds on/off not being pressurised or not.

I think VNAV PATH is confused.

[ACMS]

VNAV PTH wrong

[VNAV PATH]

APU to pack takeoff configuration (when selested on Take off FMC page):

• left engine bleed valve is closed : L ENG OFF light is illuminated

• APU provides air to ADP C1 and to left pack

• center bleed isolation valve is closed

• the right engine provides bleed air to ADP C2

• the right pack valve is closed, and the R PACK OFF light is illuminated


Should have written : When APU to Pack is used there is only one engine bleed AND one pack



Sad I cannot find a good schematics dispaly from my doc.

[BANANASBANANAS]

APU running = APU Air Inlet Door open = extra drag.

[ACMS]

APU to Pack on the ER and LR :-----


APU TO PACK TAKEOFF

Before engine start, selecting APU from the FMC scratch pad to the SEL-APU field on the THRUST LIMIT PAGE arms the system for APU to pack takeoff. Approximately one minute after starting the second engine, the bleed air supply system configures for APU to pack takeoff.

When APU is selected to the SEL-APU field after both engines are operating but before takeoff, the bleed air supply immediately configures for APU to pack takeoff.

In APU to pack takeoff configuration:

• the left engine bleed valve is closed,

• the L ENG OFF light is illuminated and remains illuminated,

• the APU provides air to air demand pump C1 and the left pack,

• the center bleed isolation valve is closed,

• the right engine provides air to air demand pump C2,

• the right pack valve is closed, and

• the R PACK OFF light is illuminated and remains illuminated.

The system reconfigures from APU to pack takeoff to normal operation when:

• thrust is reduced to climb thrust, or

• approximately 10 minutes after takeoff, or

• the airplane is above 11,400 feet altitude, or

• APU to pack configuration cannot be continued because of failures in other systems, or

• APU to pack mode is deleted manually

[ACMS]

Yes and your original post is still wrong VNAV PTH.

APU to PACK there is no Eng Bleed Air going to any packs.

L Pack is supplied from APU, R Pack is off.

So Basically:--

PACKS OFF::Bleeds both open, L and R supplying C1 and C2 demand pumps
APU to PACK:: L Bleed off, R Bleed supplying C2 Demand pump.

So if you were picking differences APU to PACK would give a little extra thrust over Packs off. Wouldn't be much though.....

[Wizofoz]

AND, if you run both through the OPT, you will find identical performance for both.

Yes, the APU inlet door is open, but apparently the difference is negligible.

The reason the use pack off instead of APU to Pack would be either the APU is U/S, or the APU to PACK function is not fitted (as is the case on earlier model 777s)

[JammedStab]

Correct Wiz.

Ran the numbers the other day in the OPT and it was the same performance gain for APU-to-pack as Packs off. One or two tons benefit. I can't remember for sure.

[Kapitanleutnant]

Wondering why Boeing would not simply utilise the APU to run BOTH packs and therefore air conditioning in front AND back cabins. APU Air is good up to what... FL220? Simply use APU to power both packs and maybe turn off the bleeds (Don't know if that affects any anti icing required... no FCOM handy)

I can't quite recall but in my early years on the 737-800, it seemed like we did something similar to that, but I could be mistaken.

Thanks guys.

K

[felixthecat]

Single pack operations are sufficient to pressurise the cabin, and since the packs feed the air conditioning manifold the front and the back of the cabin still receive adequate air. The flight deck still receiving air directly from the L pack for any problems. If both bleeds were off then the C1 and C2 demand pumps would need to be fed from the APU. The APU may not be able to supply both packs and the C1 plus C2 demand pump in a high demand state like after take off when both demand pumps are on for gear retraction..... That would be my guess anyway Kapitanleutnant.

[agg_karan]

Originally Posted by Kapitanleutnant

Wondering why Boeing would not simply utilise the APU to run BOTH packs and therefore air conditioning in front AND back cabins. APU Air is good up to what... FL220? Simply use APU to power both packs and maybe turn off the bleeds (Don't know if that affects any anti icing required... no FCOM handy)

K

As stated above -
PACKS OFF::Bleeds both open, L and R supplying C1 and C2 demand pumps
APU to PACK:: L Bleed off, R Bleed supplying C2 Demand pump.

Your question was pertaining to why APU to Pack doesn't supply BOTH packs and additionally why not turn off eng bleeds for extra desired thrust?

Apu to pack function (apart from other good reasons) considers a single pack takeoff by APU Bleed.
Kindly ASSUME an APU failure in such a takeoff roll, BOEING has ensured that your both packs DONT TAKE ANY bleed from any ENGINE. How? Right pack is off, left Eng bleed is OFF.

This ensures performance weights/calculations complied with using FULL ENG THR with no PACK Extraction.
If APU were to supply both packs as you questioned, an APU failure would somewhat invalidate/deviate from calculated PERF weights.

Why ENG bleeds can't be off WHILE APU supplies both packs and demand pumps as you questioned, Assume APU failure and there is no BLEED supply for HYD RESERVIORS pressurization /air for ADPs / 'maybe' Gear Retraction / 'maybe' RTO Braking

I wish I could back this explanation with a legal document, I was studying the same issue and this is what I concluded.

Any corrections to above is welcome

Regards

[ACMS]

because the APU bleed would have to run the C1 C2 pumps AND both Packs.
So they share the load between the right bleed and the APU bleed.