B738 ONE PACK INOP dispatch - why engine bleeds off?

[wycjol]

Hi all,

Recently I had to dispatch with the right PACK inop. The D.D.P.G. in my company states the following:
1) Limit altitude to FL250
2) ER operations not allowed
3) Position associated PACK switch to OFF. To maintain isolation bleed sources, position the isolation valve switch to CLOSED
4) For increased air flow when flaps are extended (takeoff and landing) use APU to supply bleed air to the operating pack;
A: Right PACK inop:
Use the Boeing "No engine bleed takeoff and landing procedure"
B: ....

My question is: Why does a pack inop dispatch require one to depart with engine bleeds off??? I thought this only added complexity to a relatively simple issue - in particular when we did not require the extra performance that the engine bleeds off procedure provides...

Thank you for clarifying!

[A and C]

It is so that both engines make the same power, if you had one engine supplying a pack it would produce slightly less power than the engine that was "bleed off".

[ggofpac]

hmm i don't fly the 73
It is so that both engines make the same power, if you had one engine supplying a pack it would produce slightly less power than the engine that was "bleed off".

But i thought that was taken care by step no:4 For increased air flow when flaps are extended (takeoff and landing) use APU to supply bleed air to the operating pack

I don't have access to mel on the A320, does anyone know if taking off with 1 inop pack requires bleeds off too?

Many thanks.

[rudderrudderrat]

Hi wycjol,

I don't fly the 737 any more (-200) - but I seem to remember that we couldn't mix APU & Engine Bleed air. So if we used APU bleed - we had to configure Eng Bleeds Off.

Hi ggofpac,

A320 series has no restriction for TO & Landing - just select Hi Flow and limit to FL 370.
The Engine Bleed is signalled to close when the APU Bleed is signalled open.

FCOM 1.36.10 Engine Bleed:

The bleed valve is fully closed :

........

the Bleed air Monitoring Computer (BMC) in the following cases :

overtemperature; overpressure; leak; open starter valve;

APU bleed being ON.

[ggofpac]

thanks rudderrudderrat.

So (on A320 )assuming the right pack is inop and switch off, and we taking off with the left pack only ( plz note, its pack not bleed) WITHOUT using the APU, which engine bleed is powering the 1 and only pack? do they both share the load?
Otherwise, wouldn't there be some asymm thrust?

Edit:sorry forgot to specify a/c type. Its A320.:)
2nd Edit : Forgot to include without using APU.:O

[rudderrudderrat]

Hi,

The Cross Bleed remains closed - unless you select it open or you introduce APU Bleed Air.

If your engine power is set with EPR, then the Left engine works harder to produce the same EPR and supply the bleed air to the Left Pack.

If your engine power is controlled with N1 - then yes the power will be slightly up on the RH side (or down on the left if you've calculated Packs Off) - but not so much as you would notice on a normal day.

[iflytb20]

We had the same MEL about 2 months back - our DDPG made no mention of a Bleeds off Take-off. Other than an appx 1.2% difference in N1, everything else was normal.

[Denti]

Weirdly enough the procedure posted by the OP is not applicable to -700IGW and 800 with PATS Auxiliary Fuel Tanks as that is not a boeing configuration.

Anyway, just checked and our DDPG says:

For -300/-400/-500/-600/-700/-800:
1. When dispatching with one operating pack supplied by engine bleed air on
takeoff:
A. Determine V1(MCG) based on engine bleed for packs OFF.
B. Determine takeoff performance based on packs AUTO.
2. Limit altitude to FL 250.
3. ER operations are not allowed.
4. For galley 4B food cart chiller installed, use only one chiller inflight.
5. Position the associated PACK switch to OFF.
6. Position the ISOLATION VALVE switch to CLOSE.
7. For increased air flow when the flaps are extended (takeoff and landing), use the APU to supply bleed air to the operating pack.

A. Right pack inoperative
1) Do the Supplementary Procedure - Air Systems “No Engine Bleed Takeoff and Landing” (Refer to the Flight Crew Operations Manual).
NOTE: Keep the PACK switch to OFF and the VALVE switch to CLOSE.

B. Left pack inoperative - PRIOR to takeoff or landing
1) Engine No. 1 BLEED air switch to OFF
2) R PACK switch to AUTO
3) L PACK switch to OFF
4) ISOLATION VALVE switch to OPEN
5) Engine No. 2 BLEED air switch to OFF
6) APU BLEED air switch to ON.

C. Left pack inoperative - AFTER takeoff or landing
1) APU BLEED air switch to OFF
2) Engine No. 2 BLEED air switch to ON
3) ISOLATION VALVE switch to CLOSE
4) Engine No. 1 BLEED air switch to ON.

The MEL simply states that only one pack is required with a restriction to max FL250 and no ER operation allowed, without any direction to consult the DDPG as is usual if there is a procedure to be followed.

[747dieseldude]

With one pack inop, the other one will go to high flow, which is prohibited using engine bleed.

[MaxHelixAngle]

With one Pack Inop the remaining Pack provides 'High Flow' with the switch in the AUTO position except when the airplane is on the ground or inflight with flaps extended.

In my company's DDG the final step is notated: "If desired, increased airflow can be obtained when the flaps are extended (take-off and landing) by ...... The final step after this condition line (ie optional) then contains the No-engine bleed take-off with the Right pack OFF if inop, and an adjusted procedure if it is the left pack that is inop for obvious reasons.

ie. In my company a no-engine bleed take-off when dispatching with One pack INOP is at the flight crews discretion if they wish to obtain high flow from the remaining Pack.

Interested to hear from any flight crew who have flown with this MEL if there is any noticeable cabin environment change with one pack operating in normal flow while the flaps are extended.

Cheers,
MHA

[Spendid Cruiser]

Interesting, I have experienced it myself and I see it many times that the 737 MEL is often misread/misunderstood.

[LAHSO 06/03]

To answer the first question directly:
Pack inop dispatch does not require one to depart with engine bleeds off, using APU bleed does.

Like MaxHelixAngle I was also tempted to read that using AUP bleed for increased airflow is optional, but there is nothing in my Boeing DDG that indicates this (the usual Boeing language for options is put down as "If .... is desired, ...." like in you company DDG). The exception that I read is mentioned earlier (without PATS Auxiliary Fuel Tanks). Maybe this is the variation your company operates?

And yes, the AUTO position only gives high airflow in flight with the flaps up,
BUT with operating one pack with APU bleed and both engine bleeds OFF, regulates to high airflow (which is not a problem for that senario).

We have come back landing with only one pack operating in high arflow once due to a wing-body overheat
and have not noticed any difference in the cockpit.

My two pence, nice flights everyone!

[KristianNorway]

Hey guys

I don't have any documentation here in front of me, but one question:

Why select isolation valve to "close" at all? Departing with applicable pack switch off and isolation valve auto gives you an open isolation valve and normal pack flow until flaps are up (supplied by both engines). When flaps are up the remaining pack operates in high flow for the duration of the flight.
The thrust would not be asymmetrical and the bleed air demand would not be greater than normal.

[rudderrudderrat]

Hi KristianNorway,

Why select isolation valve to "close" at all?

It's to ensure the power available on the surviving engine, after a severe failure of the other engine with damage to the pneumatic ducting causing a bleed leak.

Look at the performance penalty of packs on, engine anti ice on, wing anti ice on. If you take off with the X Bleed open, suffer an engine failure and there's a rupture in the ducting - then the surviving engine will be pouring bleed air through the rupture. Big penalty.