Single Pack operation B737 NG
 

Dear All
i’d like to know why with 1 Pack Inop we’re limited to FL250 if on ground whereas if the Pack gets Inop while at cruise level we can maintain current Flight Level?

Is this an MMEL question?

No it’s not i was just wondering why?

I don't fly the B737, but this MEL requirement applies to other jet types I have flown.

I believe the rationale is as follows:

Should you lose a pack enroute, the remaining pack will provide adequate flow for pressurisation (at FL350 etc). You may not have the additional fuel for cruise at FL250.

After arriving at your destination and applying the MEL, from then on your sectors will be planned, and fuel carried for, cruise at or below FL250. This limitation is in place for safety should your aircraft lose the remaining pack.

I hope that helps,

Duke.

As I understand it....

QRH assumes no further failures...

MEL assumes there will be further failures and mitigates the risk.

A consideration is time to descend to 10,000’ in the event of a pack trip when on one pack . Even if you can get there you don’t want to be at 39’0000 with a trip on the remaining pack. I have had a pack trip at top of descent as the thrust was brought back. Heat exchanger in the pack was blocked by fod it was later found. Mel dispatch requirements only ever apply after an MEL has been applied on the ground. QRH takes precedent airborne. QRH lets you stay at planned level because it assumes descending to 25,000 might cause endurance issues. But you wouldn’t plan to be in that position.

:ok: It is an MMEL question. The dispatch release using its provisions must be conservative, taking into account the possibility of a single independent failure on the same system (don't quote me on this verbatim).

You can depart with 1 brake (of 2+2) inop, because (after applying the penalties) a failure of another one (1+1 or 2+0) still works.
You cannot depart with 2 brakes inop (2+0 or 1+1) because a subsequent loss creates an unsafe state (1+0).

Sometimes the required 'survivability' after MMEL release is achieved by not pushing all the corners of the envelope. That is your case.

One failure in flight is expected.
- if you are departing with -1 already, need to make provisions for -2
- if you depart fully operational and encounter -1 inflight, another failure is not expected but will be dealt with using the appropriate EMERG C/L - should it actually occur.

"QRH lets you stay at planned level because it assumes descending to 25,000 might cause endurance issues."

Politely tend to disagree, QNH doesn't give a **** whether YOUR decisions see you land safely or end up in a smoking hole regards fuel awareness/management.

My thought was that in flight the pressurisation system had pumped the cabin up to required diffrentials and losing only one pack/bleed would be able to maintain that pressure.
A further problem in the remaining pack/bleed - well firstly you're having a really, really bad day, if a slow leak follow depressurisation, if an explosive depressurisation then all applicable checklists.

Was always a discussion point when training - in fairness no absolute right or wrong - most responses tended to be:
- stay where I am (reasons above)
- stay where I am and review procedures in case further problems
- descend to a more mid point cruise, more "comfortable"....whatever that meant.

Finally lead into a discussion about what happens when we arrive, MEL applied for the return trip, the biggest difference/consideration on the flight plan??

Cheers.

I am quite sure the designers of the Boeing QRH considered the issue. That is exactly why the Checklist is not proscriptive.

Cough nailed it. The probability of independently loosing both packs on a single flight is quite low, so once one fails the assumption is the other won't fail before landing.
When doing an MEL dispatch, you must assume a failure will occur during that next flight, since the probability of a single failure on the next flight is not that low.

And despite what the statistics would seemingly indicate, a disturbing number of inflight incidents occur after departing with a legal MEL on the related system.

I’m aware of 3 depressurisation events in my own airline following legal MEL bleed dispatches, and one famous 747 nose-gear-up landing which was dispatched with a legal hydraulic pump MEL.

Beware the MEL, because while the mathematicians say a further failure is unlikely, they don’t look at the additional load on the remaining systems.

The remaining systems will have been tested to handle the additional load during certification, but when was the last time they were stress-tested to that load prior to your impending departure?

In the case of a B737 pack, it will be running at high flow, not normal flow. When was that last stress-tested?

Agree. I fly the 320, but same thing, one pack MEL'd, other pack in high flow for days. Always thought I would prefer to baby my one remaining option.

Isn't that's what having a 25,000ft ceiling (737) for single pack/bleed dispatch doing?

Should the remaining system fail descent to 10,000ft should be achievable without oxygen/rubber jungle coming into play.
An unlikely explosive depressurisation? a) what are the chances b) you're having a really, really bad day c) deal with it, earn some of those big bucks some (completely incorrectly of course :) ) allude to. :ok:

Cheers