why does mel ops procedure require to cross supply?

[gearpins]

B 737-800 36-05 Engine Bleed Air Shutoff Valves(PRSOV)

OPERATIONS (O)

For left engine bleed unusable:

A. At altitudes 17,000 feet and below, operate the leftpack using the APU bleed air, the right pack using the right engine bleed andthe isolation valve CLOSE.
B. At altitudes above 17,000 feet, operate with the APUbleed air OFF.
1) Limit altitude to FL 250.
2) Operate the leftpack using the right engine bleed, the right pack OFF and isolation valve OPEN.

For right engine bleed unusable:

A. Limit altitude to FL 250.
B. Operate the left pack using the left engine bleed, rightpack OFF, and isolation valve CLOSE.
NOTE: At altitudes 17,000 feet and below, increased airflow will occur when flaps are extended (takeoff and landing) and the APU isused instead of engine bleed to supply bleed air to
the operating pack. Refer to Boeing FCOM, NO ENGINE BLEEDTAKEOFF AND LANDING SUPPLEMENTARY PROCEDURE.
greetings to all.
trying to get my head around the highlighted part :ugh:
obviously I am missing something.....:confused:
why cant we simply use right bleed to supply right pack and keep it simple? :{
appreciate all help thanks

[galaxy flyer]

Not a B737 guy, but I'll bet a whole lot that Boeing requires the left pack airflow to give priority to the cockpit and avionics cooling.

GF

[gearpins]

greetings galaxy flyer.
thanks for the post. That makes sense however if I were to loose left bleed in flight my checklist is happy to allow the right bleed to supply the right pack!!
:ugh: and hence the head banging !
appreciate the input.:)

[galaxy flyer]

Don't confuse abnormal procedures with MEL procedures.

GF

[InSoMnIaC]

Not a 737 guy either. but I am guessing the same. ie that left pack gives priority to the cockpit. I will take another stab in the dark and say that by having the left pack operating in a single pack ops also ensures rearward flow of air in the event of smoke in the cabin. 3rd stab in the dark.. boeing figures that in the event of a bleed fault in flight, which could be associated with smoke. better leave the isol valve closed to prevent cross contamination of the bleed system.

[InSoMnIaC]

moral of the story though is, just do what it says to do as those procedures have most probably been written in blood

[His dudeness]

I think its a good sign if someone wants to know WHY to do things the checklist or MEL... and as he did not say he would not do it as "it says", there is no moral....

[Goldenrivett]

Hi gearpins,
however if I were to loose left bleed in flight my check is happy to allow the right bleed to supply the right pack!!
The QRH is written as a simple practical solution for a single sector when work load might be high.

The MEL OPS is written considering the chance of an another independent failure to the air supply system occurring during a permitted series of flights. There is no time pressure when the aircraft is on the ground (except commercial) so a more complicated working solution is often offered for consideration.

[InSoMnIaC]

I think its a good sign if someone wants to know WHY to do things the checklist or MEL... and as he did not say he would not do it as "it says", there is no moral....

That why I offered an explaination as to 'WHY'. Your comment however has no moral and no substance whatsoever.

[Skyjob]

The reason for it is simple based on design features of the NG.
The cabin is controlled by 2 packs, both feeding into the mix manifold.
The flight deck compartment is directly taken from the Left pack only, no air from mix manifold. Thus Left pack must always be used in this case.
Note that this defect and use is not the case for a Pack being inop, we are talking PRSOV here.

Furthermore, the details are in the rest of omitted text. Namely that:

"Airplane is not operated into known or forecast icing conditions"
"Valve is secured closed before eying start"
From a maintenance point of view, the engine bleed valve is locked CLOSED.
"Operating two air conditioning packs with one engine bleed source is not recommended"

http://www.aircrafttrainingaids.com/images/bleedair.png

[gearpins]

appreciate all the posts.
Understand the limitations of checklist and am able to differentiate MEL objective vs Inflight failure.
I am not alluding to ignore set procedures.
still ......New to the type and am
Hoping to use this forum to gain some wisdom from B737 pilots.


His dudeness, Skyjob,InSoMnIaC,Goldenrivett
thank you all for the response.


lets wait and watch if we can glean more from the pilot fraternity out there
standing by......

[aaa0808]

Before dispatch with one PRSOV inop, that valve is secured closed by the maintenance engineers. On the other hand, if it fails in flight, there is no sure way of knowing that it will stay closed throughout the flight since it is inop and might disagree with the switch position. If it opens midflight, then both the bleeds will be supplying air to the left pack which is not allowed and might damage the system. So, unless the inop PRSOV is secured closed, we keep the isolation valve closed.

I hope this helps.

[Nil further]

So if you have a pack fault or INOP do you turn one of the bleeds off ?

[TURIN]

My failing memory seems to think that the layout of the bleed system changed somewhere along the line. Possibly the transition from 100/200 series to the CFM powered. 3/4/500 series. But to maintain operational commonality the procedures were kept the same.

I could be barking up the wrong tree though.

[aaa0808]

Nil further
No. Because we still need bleed for anti-ice and other operations on that side, even if the pack on that side is not working. So we keep the bleed ON. But just ensure that the isolation valve is closed so both the bleeds are not supplying to a single pack.

[gearpins]

The question really is why the left pack is preferred?
In other words when dispatched with left bleed inoperative why not use right bleed to supply right pack all the way through?
:ugh:

[aaa0808]

It is as explained above that the left pack provides air to the electronics bay where temperature control is crucial. So it is always prioritised whenever the conditions allow.

[gearpins]

Nil further, Turin and aaa0808,
thank you for the response.


aaa0808,
I do see the logic where maintenance procedure locks out the PRSOV. however I find for MEL 36-08 bleed trip light there is no maintenance procedure requirement to lock out the PRSOV yet the Ops procedure still requires you to cross supply and hence my question.:ugh:

[gearpins]

B 737-800 36-05 Engine Bleed Air Shutoff Valves(PRSOV)

OPERATIONS (O)

For left engine bleed unusable:

A. At altitudes 17,000 feet and below, operate the leftpack using the APU bleed air, the right pack using the right engine bleed andthe isolation valve CLOSE.
B. At altitudes above 17,000 feet, operate with the APUbleed air OFF.
1) Limit altitude to FL 250.
2) Operate the leftpack using the right engine bleed, the right pack OFF and isolation valve OPEN.

For right engine bleed unusable:

A. Limit altitude to FL 250.
B. Operate the left pack using the left engine bleed, rightpack OFF, and isolation valve CLOSE.
NOTE: At altitudes 17,000 feet and below, increased airflow will occur when flaps are extended (takeoff and landing) and the APU isused instead of engine bleed to supply bleed air to
the operating pack. Refer to Boeing FCOM, NO ENGINE BLEEDTAKEOFF AND LANDING SUPPLEMENTARY PROCEDURE.
greetings to all.
trying to get my head around the highlighted part :ugh:
obviously I am missing something.....:confused:
why cant we simply use right bleed to supply right pack and keep it simple? :{
appreciate all help thanks
reposting to generate renewed interest.....
My best guess so far is zip.....
and that's worrying me....calling for B 737 veterans / experts....

[aaa0808]

Yeah. Well that's because in MEL 36-08, the inop component is the light and not the valve. So we know that if we put the bleed switch to OFF, then the valve will be closed since this time there are no chances of valve disagreeing with the switch position. So no need to close it secure by the engineers.

[gearpins]

aaa0808,
It still does not answer the primary question:
consider the case where the aircraft is dispatched with left bleed u/s.
as per maintenance procedure the valve is locked in closed position.
Now the aircraft departs with APU bleed supplying left pack and engine 2 bleed supplying right bleed after passing FL170 why cant we just turn of the left pack? :confused:

[Skyjob]

See the previous post on the matter:
The flight deck compartment is directly taken from the Left pack only (http://www.pprune.org/tech-log/576088-why-does-mel-ops-procedure-require-cross-supply.html#post9310131)

after passing FL170 why cant we just turn of the left pack?

Because the flight deck compartment would not get ANY air if we were to take the LEFT PACK - OFF.

By ensuring the available (Engine) Bleed supply is feeding the LEFT PACK, we ensure pilots get some air to operate the aircraft. Feeding the Left Pack from the Right Bleed will ensure air is available in the Mix Manifold AND the Flight Deck Compartment.
If we only feed the RIGHT PACK as you are asking, the Mix Manifold will distribute air only into the passenger cabin but NOT into the Flight Deck Compartment (by design).

Thus to ensure the two people in control of the aircraft get some air as well, in (almost) every scenario we operate the LEFT PACK and turn the RIGH PACK - OFF.

[gearpins]

Hi Skyjob,
that makes sense then the question that comes to mind is when you have a left bleed failure in flight why is it, that the QRH procedure does not require this cross supply? :confused: