View Full Version : B737 Single Pack FL250

18th Mar 2008, 04:39
hi all, this one has really got me wondering...
case 1... a/c arrives with a pack problem, engg. releases a/c with single pack and mel complied with FL 250 Max Alt.
Case 2... same a/c takes off with no pack problem and just at ..say 5000 feet has a pack light and qrh says nothing about FL250, you conviniently climb your filed altiutude.

why did boeing penalise the airline for dispatching with 1 pack and no change for airborne non normal?
also is one pack is sufficient to pressurise the aircraft for almost all stages of flight, then why the restriction?
and if its really that important to have 2 packs all the time, then who NOT the restriction for inflight non normal?

would love to know how you feel about this...

18th Mar 2008, 05:41
From a dispatch point of view you need redundancy.

The same restriction does not apply ones airborne since your not restricted by the MEL.

However I'd think think twice before climbing to any higher cruise altitudes after a pack failure.

I guess good airmanship will have to come in to play.

18th Mar 2008, 06:44
.. in support of mrjet ..

(a) launch with a KNOWN defect requires that the MEL considers the intent of the certification standard and come up with appropriate restrictions to achieve a similar capability with respect to risk.

(b) in-flight failure is a crew-assessed situation .. if you don't have any option for a pack (fuel/diversion) then the risk of normal level is accepted .. but, if you have enough fuel, then it would be difficult to argue the decision to go above 250 at the enquiry ...

(c) in general, an in-flight failure warrants review of the MEL to get the guidance therein .. and, if you reasonably can do so .. it is a good idea to observe the MEL restrictions. Keep in mind that there often are requirements which the pilot may not be aware of ...

(d) it is not the OEM imposing the restriction .. the MEL is based on the MMEL which is approved (and issued) by the FAA.

18th Mar 2008, 08:51
The difference between MEL penalties and ECL restrictions for the same failure, can be clarified as

MEL: The requirements in the MEL are, amongst others, based on a (next critical) en-route failure, after dispatching the aircraft with the respective component unserviceable.

ECL: In flight, procedures are based on the assumption that the next critical failure, during that particular flight, will not occur. The manufacturer,s Operations Manual, AOM and ECL procedures
are therefore based on different criteria than the DDG.

When consulting the MEL in flight, it should be realized that restrictions in the MEL are only applicable for (the next) departure. Safety of the flight is fully covered by the applicable ECL procedure directives.

Copied this directly from one of our handbooks, hope it helps

Walker Texas Ranger
18th Mar 2008, 09:36
I do not fly the 737. But other than the MEL, is there a specific single pack FL limitation? I would assume in most cases that specific aircraft limitations are utilized in MEL's regardles of aircraft type. Like i said before, I dont fly the 737...

18th Mar 2008, 09:42
There is no problem with climbing or cruising at filed altitude if one pack fails after departure. Descent planning is a must. You may need increased thrust on the associated side operating pack's engine to ensure that there will be enough bleed air supplied, especially during anti-ice operations, so you don't lose the cabin and are able to re-pressurize the aircraft during descent.

Thus you may not be able to maintain a specified rate of descent when requested.

18th Mar 2008, 14:00
So 1 pack is capable of pressuring the aircraft above FL250? Lets say 1 pack fails when above this, should we then descend?

18th Mar 2008, 19:20
It's a very good idea to do what the manuals say.

Continue the flight as planned, but keeping a close eye on the pressurisation, and in particular - and far too few people know this - watching for correct functioning of the 5th/9th stage valve when thrust is reduced for descent.

From a practical point of view, prior to despatch you're able to uplift low level cruise fuel to address the FL250 cruise restriuction, but once airborne a decision to descend (unnecessarily) might well put you into a low fuel state to add to your other woes...

18th Mar 2008, 20:38
Can you elaborate on how exactly you can determine correct functioning of the 5th/9th stage valve for descent?

18th Mar 2008, 20:42
If on two engines/bleeds, monitor the duct pressure to see there is no significant split at idle thrust.

If on one bleed (possibly, one engine), monitor the duct pressure as the thrust lever is closed to see that it falls in synch with the thrust descrease, then increases again as the 9th stage valve opens. It's very obvious.

18th Mar 2008, 23:13
Consider the scenario with 1 pack available.

To operate this pack at FL250 where the ambient air is considerably 'warmer' than FL400, this pack would be working hard.
If you operated at FL400 with 1 pack, the same pack would not be working as hard, thus less prone to falling over.
Regulatory authorities place the restriction of FL250 for depressurisation considerations (I guess).
I would certainly continue to a higher than FL250 altitude in a single pack operation.

Flight Detent
19th Mar 2008, 03:22
Re the FL250 restriction with only one a/c pack operating...

I always understood that the altitude restriction was to allow the airplane to descend to 10,000 feet in the case of the other a/c pack failing. (in 2 a/c pack airplanes)

At 'normal' cruise altitudes, the time required to descend verses the cabin altitude climb rate is the concern.

Of course, this is still a concern if the initial a/c pack failed just after takeoff, but then complicated by the fuel usage rate at the lower level.


Old Fella
19th Mar 2008, 03:28
Some interesting comments on this subject from a pilot's point of view. Dubya says the single pack would not be working as hard at FL400 as it would at FL250 due to the "warmer" ambient temperature at the lower level.

ISA temp of -35C at FL250 does not seem warm at all to me. I have had nought to do with B737 aircraft, however I have had considerable B707/B747 time as a F/E and I don't believe "Working less hard and less prone to falling over" would even come into the thinking as far as the pack is concerned. I would guess that the only part of the pack doing any "work", even at FL250, would be the heat exchangers. The ACM would likely be being by-passed at those sorts of levels once the initial cooling of the cabin was accomplished.

Purely from a pressurisation loss consideration I would not like to be in an aircraft operating on one pack at FL400. Precisely the reason the MEL does not allow for dispatch with a single operative pack for flight at levels above FL250 is that in the event of a further failure the time to descend, initially to below FL140, is minimised and later to 10000', thus probably avoiding the "rubber jungle". If fuel is a consideration most routes flown by the B737 would likely have suitable diversion airfields available for additional fuel uplift if required.

19th Mar 2008, 08:22
Commercial air transport safety revolves around statistical probability of various kinds of event.

The reason why it's OK to continue flight at normal cruise levels on one pack is that following a pack failure, a failure of the other pack is considered highly improbable, and therefore no special mitigation is required.

Put another way, it is considered that the probability of a poor outcome from a single pack cruise at normal cruise levels is as low as other acceptable risks.

If you genuinely would not like to be in an aircraft operating on one pack at FL400, then you need to start worrying about lots of other hazards, too, all at about the same level of probability.

As I said earlier, it's best simply to do what the books say, rather than attempting to second-guess them.

19th Mar 2008, 08:30
Can you elaborate on how exactly you can determine correct functioning of the 5th/9th stage valve for descent?
http://img149.imageshack.us/img149/3508/b737bleedpressure1nx1.th.jpg (http://img149.imageshack.us/my.php?image=b737bleedpressure1nx1.jpg)
http://img149.imageshack.us/img149/9024/b737bleedpressure2cj2.th.jpg (http://img149.imageshack.us/my.php?image=b737bleedpressure2cj2.jpg)
http://img149.imageshack.us/img149/7163/b737bleedpressure3lh9.th.jpg (http://img149.imageshack.us/my.php?image=b737bleedpressure3lh9.jpg)


19th Mar 2008, 09:02
Thanks Ingo,

There is variation amongst a fleet of aircraft in the way that they achieve (or more often, don't achieve) the pressures charted.

But the crucial thing is to watch the guage as you close the thrust lever and observe the pressure reduction and then increase shown in Ingo's second chart.

19th Mar 2008, 09:15
One of many MEL requirements on the A320 with operating on one pack ( At any level ) is the speedbrakes must be operative in such a case an emeregency descent is required.

On descent a higher idle setting on the engine with the operative pack may not maintain profile accurately ( speaking from experience-after a long day it can get interesting when ATC ask you to slow down on descent with constraints ahead):ugh:

19th Mar 2008, 23:27
one of our guys had a pack trip climbing thru F150. QRH done and went up to F380. Passing F330 on clb - second pack failed and emerg descent conducted (with the rubber jungle) and diverted to nearest suitable airport..

it turned out to be a ram door and outflow valve problem that we incorrectly identified by the LAME on a quick 30 minute turn around..

I would think twice about going high if you have had a trip..

20th Mar 2008, 01:28
And what hazards do FL250 pose given you have the fuel to continue?

.. only that you get to the hotel that little bit quicker .. and that presents its own hazards ...

20th Mar 2008, 01:49
Not a 737 or an A320, but my ship has one big difference in single pack ops when comparing QRH to MEL. Applying the MEL requires an engineer to go to the tail compartment and manually secure the pack valve to CLOSED.

The system is therefore in a different state under MEL than in QRH.

The time to descend is a red herring. The higher N2 in single-pack ops is NOT going to make an appreciable difference to RoD in an Mmo/Vmo, airbrake out, emergency descent.

20th Mar 2008, 09:57
It is within AMM Limit when you lost up to approximately 1PSI within 60 seconds without any air source and all valves closed. :ok:
With one perfect working pack it might be possible to maintain the ∆p at FL400, but is your structure in perfect condition?:E

20th Mar 2008, 21:51
I dispatched with a 1 pack operation restricted to FL250 one day and on descent with the flight attendant asking what we wanted to drink was awarded with a take off warning horn. I looked at the copilot because he had been pulling tricks all day wondering how he did it. He looked confused so I thought, WAIT! that sound is for loss of pressurization too but I had never heard it. Looked up and sure enough, the cabin was above 10,000 and climbing. Putting on the bad pack fixed it so we didn't have to add power to the working side. Some cooling equipment outflow valve had stuck open letting the air leak out. No big deal but it sure gets your attention. I am happy we didn't get the problem enroute and continue to a higher altitude as is allowed.

20th Mar 2008, 22:10
Putting on the bad pack fixed it so we didn't have to add power to the working side

...and people ask me which operator I fly with...

21st Mar 2008, 23:16
Sometimes MEL requires not using a pack for reasons that only occur on take off at high power settings and works fine at cruise or descent. You know why they MELed it so sometimes you know it is fine in the regime of flight you are in so you can consider using it if another failure happens. Most pilots use common sense to decide if they should use the option of using a system that was not MEL available but was operationally usable.

22nd Mar 2008, 10:44
Let me add some of my experiences of aircraft condition and put IFixPlanes last comments into pilot speak.

One of the airtest checks is to select each pack off one at a time at max alt to verify that either pack will maintain cabin pressure. Then we switch both off we measure the “leak rate” on the cabin VSI. The acceptable limit is 2000fpm, or 2500fpm on short body a/c.

With new build a/c the test is a non-event but with in-service a/c the results can be alarming. I have seen some aircraft that gave a leak rate of almost 2000fpm when a single pack was switched off – this “single failure” would give you about 3min before the PSUs drop. I have also seen a/c that have a leak rate of over 4000fpm when both packs are switched off.

The above figures were of course out of limits and the aircraft were rectified, but they and many others (yours!) could be flying in a similar condition and you won’t know it. The law allows continued flight above FL250 if a pack fails in-flight, but you should be aware that your aircraft may not perform as well as a new aircraft. From max altitude, if the cabin descends at 4000fpm you will only have 90secs before the rubber jungle deploys. How long would it take you from recognising the problem to reach FL100 from FL400?