On a recent flight on a 738, one (out of the two) A/C unit were broken. Unable to fix it, they decided continue the flight as is and the captain mentioned that they would need to fly at 25k feet.
Can someone explain why it was necessary to fly lower? Does it have something to do with the pressurization of the aircraft?
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RE: Flying Lower With Broken A/C
MEL requirement. When dispatched with one AC Pack inop, max altitude for flight is FL250. The plane can physically fly at higher altitudes; however, for reliability/safety reasons the flight is dispatched with max altitude of FL250.
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RE: Flying Lower With Broken A/C
| Quoting rta (Thread starter): On a recent flight on a 738, one (out of the two) A/C unit were broken. Unable to fix it, they decided continue the flight as is and the captain mentioned that they would need to fly at 25k feet. Can someone explain why it was necessary to fly lower? Does it have something to do with the pressurization of the aircraft? |
To expand on what AAR90 said, the one pack can keep the plane pressurized, but if it fails you'll be left with no means of pressurizing the aircraft. It's a lot faster to get to safe altitude from FL250 compared to FL400.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
RE: Flying Lower With Broken A/C
| Quoting rta (Thread starter): Can someone explain why it was necessary to fly lower? Does it have something to do with the pressurization of the aircraft? |
Pressurisation of the aircraft goes hand in hand with air conditioning: it is the same system for both.
So if you goes U/S, you fly lower to reduce the time getting down from F250
CFI/Gr. III, L-382 Loadmaster, ex C-130B Navigator
RE: Flying Lower With Broken A/C
Should the failure happen in flight however, there is no requirement to descend to FL250.
RE: Flying Lower With Broken A/C
| Quoting AAR90 (Reply 1): MEL requirement. When dispatched with one AC Pack inop, max altitude for flight is FL250. The plane can physically fly at higher altitudes; however, for reliability/safety reasons the flight is dispatched with max altitude of FL250. |
How about for a long ETOPS flight? Can they still apply the MEL requirement and release the flight (say LAX-HNL) with a AC Pack inop if they stay at 25,000? I'm guessing no, because even though getting down to 10,000 is faster, it's still quite a long time before you're going to land.
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RE: Flying Lower With Broken A/C
| Quoting MrBuzzcut (Reply 6): Quoting AAR90 (Reply 1): MEL requirement. When dispatched with one AC Pack inop, max altitude for flight is FL250. The plane can physically fly at higher altitudes; however, for reliability/safety reasons the flight is dispatched with max altitude of FL250. How about for a long ETOPS flight? Can they still apply the MEL requirement and release the flight (say LAX-HNL) with a AC Pack inop if they stay at 25,000? I'm guessing no, because even though getting down to 10,000 is faster, it's still quite a long time before you're going to land. |
That depends if the MEL states something like "no ETOPS" with a pack inop. There may also be other restrictions, e.g. Pack 1 and 2 must be operative for polar routes or something.
Barring that, t becomes a fuel thing. The extra burn can be rather significant. Also the fact that you'll be slower so your maximum distance to an en route alternate will decrease, meaning your ETOPS route could change increasing your track miles.
On a side note, there may be another more critical point for fuel, such as engine inoperative or pressurization failure, that is more restrictive than the critical ETOPS point(s). It is not always ETOPS that restricts the fuel. In fact in many cases none of the critical points restrict the fuel at all and you simply need what you'd need for the trip disregarding any critical cases.
[Edited 2016-03-21 18:07:23]
[Edited 2016-03-21 18:12:17]
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
RE: Flying Lower With Broken A/C
| Quoting MrBuzzcut (Reply 6): How about for a long ETOPS flight? Can they still apply the MEL requirement and release the flight (say LAX-HNL) with a AC Pack inop if they stay at 25,000? |
AA B738s do not fly to Hawaii so it is not something addressed in our manuals; however, our MEL does place this restriction on B738 operations with only one AC Pack....
C. Do not operate flight out of the 48 contiguous states to South American destinations south of 5° N. If
outside the 48 contiguous states enroute operations and return are authorized.
*NO CARRIER* -- A Naval Aviator's worst nightmare!
RE: Flying Lower With Broken A/C
AS 737's are restricted from ETOPS with one pack inop per the MEL.
A couple of other scenarios that can require FL250 or less is having less than the required decompression portable oxygen bottles or inop passenger oxygen system.
[Edited 2016-03-22 09:23:23]
A couple of other scenarios that can require FL250 or less is having less than the required decompression portable oxygen bottles or inop passenger oxygen system.
[Edited 2016-03-22 09:23:23]