B737NG Bleed Trip off
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B737NG Bleed Trip off
Hi,
With Ref, to B737NG, if you had a No. 2 Bleed Trip off on ground as per MEL, you are restricted to below FL250, however, if it happened in flight, there are no altitude/FL restrictions....any idea why this is so??
Regards
With Ref, to B737NG, if you had a No. 2 Bleed Trip off on ground as per MEL, you are restricted to below FL250, however, if it happened in flight, there are no altitude/FL restrictions....any idea why this is so??
Regards
Last edited by B777Heavy; 16th Apr 2010 at 19:06.
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to my understanding the reasoning is:
it's unlikely to have TWO packs fail on one flight.
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don't we just love the boeing non-normal checklists where it says:
push button - yellow light will extinguish.
-> but it doesn't says what to do if the darn light stays ON ?!
it's unlikely to have TWO packs fail on one flight.
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don't we just love the boeing non-normal checklists where it says:
push button - yellow light will extinguish.
-> but it doesn't says what to do if the darn light stays ON ?!
Also once you are up and about at altitude one side can maintain cabin pressurization up to service ceiling but too tough an ask to do it from the ground up to FL410.
The MEL is about planning to operate with a know system deficency, whereas the QRH is about operating with a systems failure during flight.
The MEL infer a "risk management" for planned single pack operation V the QRH and the likelyhood of a second pack failure in the case of one pack failing in flight.
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A single pack will regulate in high flow with a pack trip off. It will maintain pressurisation up to Max service ceiling. There is probably a question of airmanship with this kind of scenario though.
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All valid reasons so far. Aviation is all about mitigating risk and trying to keep the chance of avoiding the worst happening as high as humanly poss. Lots of things are like ETOPS, defined by mathematical equations that someone smarter than me has come up with. With this in mind leaving with one pack is perfectly possibly but it degrades your protection. Time of useful consciousness calculations may well come into the equations as FL250 is the cut off for aircraft operating without drop down o2. Just i thought.
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(a) intentional operation requires that the Type Certificate risk levels be maintained ie the MMEL addresses a mini recertification if you like. Hence the various restrictions in the MMEL
(b) FL250 matches older style aircraft limitations in the event of the remaining single pack failing in flight
(c) in the event of an abnormal/emergency in flight, ie not tied up with MMEL considerations, the OEM prescribes appropriate actions to recover the situation but the increased risk levels are part of the gameplan.
(d) it is, however, still a good idea to review the MEL after the QRH items are completed to facilitate consideration of those matters which pertain to certification as the subsequent flight management decision processes may well be influenced by those considerations.
(b) FL250 matches older style aircraft limitations in the event of the remaining single pack failing in flight
(c) in the event of an abnormal/emergency in flight, ie not tied up with MMEL considerations, the OEM prescribes appropriate actions to recover the situation but the increased risk levels are part of the gameplan.
(d) it is, however, still a good idea to review the MEL after the QRH items are completed to facilitate consideration of those matters which pertain to certification as the subsequent flight management decision processes may well be influenced by those considerations.
