View Full Version : Limitation of 25,000ft when one bleed air inoperative for 737-300?

10th May 2003, 12:09
hi, everybody
I am a new guy in this forum. This forum is so professional and i wanna find some answers at here.
I am a 737-300 pilot from China. I had met a "bleed trip off" few days ago. The left "bleed trip off" light illuminated when we did take-off rolling, we continued takeoff, we asked ATC for 1500m to maintaining(in China, about 5000'), we executed the NNC "bleed trip off", but the light was still on. According to the QRH&OM, we have no altitude limit, so we continued to climb, after 3-5min, we pushed the reset switch again, light was off, we flied to the destination......
When i back to home and check with some documents, problem is coming:
1. in QRH NNC.2.4"BLEED TRIP OFF" , it says:

¡° Condition: The BLEED TRIP OFF light illuminated indicates the
related engine bleed air temperature or pressure is
TRIP RESET SWITCH. . . . . . . . . . . . . . . . . . . . . . . . . . . . PUSH
[The BLEED TRIP OFF light extinguishes if bleed air temperature has
cooled below limits.]
If the BLEED TRIP OFF light remains illuminated:
PACK SWITCH (affected side) . . . . . . . . . . . . . . . . . . . .OFF¡±

No altitude limit.

2. in Operation Manual page 2.30.1, it says:
¡° A single pack is capable of maintaining pressurization and acceptable temperatures throughout the airplane up to the maximum certified ceiling.¡±

No altitude limit.

3. but in MEL item 36-5¡± Engine Bleed Air Valves¡±, it says:
1. avoid icing conditions
2. left engine bleed air off:
a. when blow 17000ft, left pack use APU bleed air and right pack use right engine bleed air with isolation valve switch closed
b. turn off the APU bleed air when above 17000ft
1) limit maximum flight altitude to 25000ft,
2) isolation valve switch on and use left pack(use right engine bleed air), turn off right pack switch
HERE, we have a maximum flight altitude limitation of 25000ft!!!

So, my question is how is the difference come. Is there any limitation of 25000ft when one bleed air inoperative for 737-300? If there is, what is 25000ft for? For structure limit? I don¡¯t think so. For one bleed air work load limit or sth. else?
Although the MEL is to be used before engine start, the instructions in it still have the value as a reference in flight. At least Boeing should add a notification¡°avoid icing conditions¡± in the QRH.

10th May 2003, 13:52
The subtle difference in your quoted case between the QRH and MEL is that the QRH lets you carry on at any chosen altitude while of course avoiding icing etc. The assumption is that you started out with the whole system serviceable and suffered a failure or fault after dispatch.
The further assumption is probably that you would be very unlucky to have a further fault or failure in the system, thus they don't really expect that you are going to end up depressurized as the result of a pack or bleed trip. Same reasoning lets twin engine aeroplanes fly - i.e. they only ever expect one engine to fail on any one flight!
The MEL however, is intended to be used to get you away legally, that is dispatched, with one system already failed (obviously nothing as radical as an engine!). To satisfy the certifying authorities the MEL is therefore more conservative in that it has to allow for the possibility that the other system will fail or fault along the way. Also because MEL/DDG references usually form part of the planning process, it gives you the heavy hint to replan at the lower flight levels for fuel, time etc. How your company views 'dispatch' in this situation should be spelled out in the preamble to the MEL and/or ops manuals. It could be from commencement of pushback, commencement of taxy or commencement of takeoff roll, or it might be left entirely to commonsense in that there is no point in dispatching from a maintenance base under the MEL if it then grounds the aeroplane at an outstation.

10th May 2003, 14:08
I was trying to figure out a way to say what fruitbatflyer
just said.
So here it is:

What he said!!!:p

10th May 2003, 14:58
What a good answer from FBF. Well done - saves a lot of typing and misdirection.

10th May 2003, 19:00
Truly exceptional, FruitBatFlyer. Just like to add one more thing; If you do have a problem in the aircraft while flying, opening up the MEL is always an option for information gathering, but if you decide to follow the limitations, you can end up in an uncomfortable situation.

Suppose you have this problem while flying at 35000 ft, 1/4 of the way on a 4 hour flight. If you decide to descend to 25000 ft, you may have a range problem. So, be carefull on what you choose to do. In this case, I will only descend to 25000 ft if the cabin altitude cannot hold. This problem is quite pervasive in our airline a few months back. Engineering seem to have solve it now but its just a matter of time before it comes back...

10th May 2003, 23:34
The B707 had this 25,000/one pack limitation as well.
Recall years ago when enroute at FL350, one pack just quit. No reset possible. Decided to land enroute (9hr flight) due to suspected problems with the remaining pack (there was). Marketing was very upset, but the fleet manager told 'em to back off, pilots had the final authority...period.
Airline involved...SQ, many years ago.
How things have changed over the years.:rolleyes:

11th May 2003, 09:43
Hi folks,
I´m not pretty sure you all (mainly FBF) gave our chinese folk an important statement: there is no disagreement between QRH and Ops Manual.
Let me try to be short: if you´re, for instance, taking off, and have such a problem, one bleed could maintain cabin pressure within limits until 25000 ft.
On the other hand, let´s suppose you´re flying at FL330. If you loose one pack, the other one should maintain cabin pressure for the remaining of the flight (that is, above 25000ft).
I hope this clears a little bit more.
Best regards,


12th May 2003, 13:16
thank you! i appreciate your replys above so much.
and there is still a qz:
what's the 25000ft limit for? is it for a shorter time from 25000' to 14000' when we in a emergency descent? or for the one system working reliability is better in 25000' than in higher level?
and when i review the hole thing, i think it's a better choice to choose 25000' as the crz level in that case(we had enough fuel),especially when we had the bleed failure at the early phase.

13th May 2003, 22:17
As stated above, the reason the QRH permits you to maintain your cruise altitude is because: As the cabin pressure differential is maintained by the outflow valve, not the pack, the system is capable of operating at maximum differential on one air conditioning system, albeit with reduced airflow. You have planned your flight at that cruise altitude, and even though you will have allowed fuel for depressurisation, it is probably safer to keep to the original plan than to complicate the rest of the flight with a forced decent, and The statistical chance of suffering a separate, independant failure during the remainder of the flight is sufficiently remote.The reason the MEL requires a higher safety margin is because you no longer require two independant failures of the airconditioning system for a depressurisation, but only a single failure. Under those circumstances a higher margin of safety is required to allow a flight to depart.

Why 25,000? 25,000 is simply a "line in the sand", like the 10,000' limit (14,000 in the USA) for pilots to use oxygen. At 10,000 most people will be able to sustain consciousness indefinitely, with blood oxygen saturation areound 90%. However, while oxygen pressure decreases a bit less than linearly with altitude, the ability of the hemoglobin to hold oxygen follows a much different curve, called the oxygen dissociation curve.
There is a big change for the worse in the hemoglobin's ability to combine with oxygen that occurs in the low twenties.
At 25,000 feet the oxygen saturation is only 55%. The partial pressure of oxygen in alveolar air at 25,000 feet is 14% of 281.8 mm Hg or 39.5 mm Hg -- slightly less than that normally found in venous blood returning from the tissues. Above 25,000 feet oxygen will diffuse from your blood into the air. Any time over three minutes or so at this altitude, and brain cells are dying!

Altitudes above 25,000 feet are know by mountain climbers as "The Death Zone,". Given that climbers who work in these altitudes spend weeks at high levels getting acclimatised, allowing their bodies to increase the amount of heamoglobin in their blood, and still experience deaths, this isn't just a "cute term".

Keeping the aircraft below 25,000 feet is simply an added safety measure for your passengers, given the increased likelyhood of suffering a depressurisation while flying on one pack.

Image referred to from the article:
When Humans Fly High: What Pilots Should Know About High-Altitude Physiology, Hypoxia, and Rapid Decompression (http://www.avweb.com/news/aeromed/181893-1.html) at Avweb. (http://www.avweb.com/)

14th May 2003, 13:19
so professional u are!
and thank u so much!

14th May 2003, 16:41
Pif Paf, are you saying that if you loose one pack after takeoff you should not climb above FL250?

maybe it's useful to highlight that the problem is not much in being relatively safe below FL250, while above that level most of the passengers would die.
Nobody would remain at those altitudes after a decompression.
What the authorities want is to limit the TIME it takes, assuming max descent rate of your type of aircraft, to reach 14000 ft.
I'm probably wrong, but maybe an airplane capable of descending at an amazing rate would be cleared to a higher level on a single pack.

Standing to be corrected!

16th May 2003, 10:52

I am pretty sure this was said during my gnd school (a long time ago), as I've found it written in my handbook. But I have to admit that I've not found any text about this in the manuals.
I'll keep researching and will notify if I discover anything new.


17th May 2003, 12:37
i mean will you guys check the MEL as reference when you have something fail/wrong in the air,not only the QRH?
in case of the failure take place at the early phase inflght like t/o rolling or initial climb, i don't think there is a essential difference compared with the failure taking place before dispatch.

17th May 2003, 14:39

Seems to me that you have done quite a lot of thought about this problem...and suspect you are right on target.

If I was operating a pax jet transport aircraft with only two a/c packs (have not done this for awhile, generally three) and one pack failed/no reset possible, certainly would not continue to climb to the expected final flight-planned cruise altitude, unless of course that altitude was lower, and the flight was of short duration.

And yes, to consult the MEL in addition to the QRH would seem to me to be a very good idea. Others of course might disagree.

16th Jun 2003, 03:06

Just checking on a few posts regarding MELs when I came across this.

The discussion about why there is a difference between QRH/OM and MEL has been right on the money. However, I thought that it remained a little unclear that consulting the MEL in flight following a system failure is generally conducted only to determine the consequences after landing of that failure if there is limited or no engineering support.

The MEL has no in-flight relevance other than reminding you of the conditions under which you agreed to operate the aircraft.

Stay Alive

17th Jun 2003, 02:25
Good point made by 4dogs.

We use the MEL/DDG in the same way. Once moving under own power, we use the QRH only and the MEL serves as a guideline to determine whether it is more advantageous to come back and use another acft or to continue and have it sorted out at or destination.

Good thread.

17th Jun 2003, 03:46
Nobody has stated so far if, having experienced a bleed or pack failure after takeoff, he would climb to only 25000ft or to 35000 as planned.

I understand it's a difficult question. Let's keep in mind that if the other pack fails at fl250, we'll have to make an emergency descent anyway!


17th Jun 2003, 05:47
I will climb FL 350 because 25000 it's only if you have had the failure on the ground.

the MEL anticipate the failure of the second bleed or pack
it's just in case of

in flight also you read the MEL it gives you information and helps you to take your decision.

and this it's also true for an other aircraft like MD80.
We had the same limitations.

in flight you have just to follow the QRH instruction.

28th Jul 2003, 17:31
I said earlier that maybe an airplane capable of descending at an amazing rate would be cleared to a higher level on a single pack.

An old Captain told me a few days ago about the Caravelle:
that outstanding airplane did not have a fixed passengers oxygen system because it was capable of a 12000 ft/min descent (!) thanks to speedbrakes which extended symmetrically above and below the wings (the ones with holes).

So in case of decompression it was capable of reaching safe altitudes in a couple of minutes! :ooh: