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Why would B738 be limited to FL250?

Posted: Fri Apr 24, 2020 7:17 pm
by Pomayer
Hello Fellow Aviation Enthusiasts: I ask my first-ever question to a group that is impressive in its knowledge and experience; I apologize if it's a rookie question: Several months ago, I flew SMF to MSP on N3743H, a Delta B738. It cruised at FL250, rather climbing to a higher (e.g. more typical) FLs. The weather or winds had no bearing assigned altitude, because all the other traffic was up in the normal range. Moreover, I noted that N3743H had observed the same FL250 maximum for a number of other flights before and after. What was the reason? Is there a point when the economics of flying at a lower altitude outweigh whatever benefits there might be in doing so? Thank you for your assistance.

Re: Why would B738 be limited to FL250?

Posted: Fri Apr 24, 2020 7:28 pm
by JoseSalazar
If one pack is deferred, it affects the max altitude per the MEL. The 737 is limited to FL250 when single pack.

Re: Why would B738 be limited to FL250?

Posted: Fri Apr 24, 2020 7:32 pm
by atcsundevil
Pomayer wrote:
Hello Fellow Aviation Enthusiasts: I ask my first-ever question to a group that is impressive in its knowledge and experience; I apologize if it's a rookie question: Several months ago, I flew SMF to MSP on N3743H, a Delta B738. It cruised at FL250, rather climbing to a higher (e.g. more typical) FLs. The weather or winds had no bearing assigned altitude, because all the other traffic was up in the normal range. Moreover, I noted that N3743H had observed the same FL250 maximum for a number of other flights before and after. What was the reason? Is there a point when the economics of flying at a lower altitude outweigh whatever benefits there might be in doing so? Thank you for your assistance.

I'll start off by saying that I'm not a pilot. However, there are a lot of possible explanations. One could be that the aircraft may have been unable to fly in RVSM altitudes (FL290-410), which occasionally happens due to equipment failure. Another could be an MEL'ed item. Again, not a pilot, but I believe an inop pack will limit them to around FL250, just as an example.

Also, just because other aircraft were at higher altitudes doesn't mean they weren't getting a bad ride up higher. Delta is uhh — how do I put this? — notoriously particular about their rides. All major carriers have their own apps that correlate PIREPS submitted by ATC and by company pilots through the app, so pilots can see actual maps and altitude stratums of ride reports on their iPads. Sometimes they'll opt for the penalty of a lower cruising altitude than accept a climb where the ride might be rougher. Other crews may have simply chosen to ride it out with the benefit of fuel savings being at a more optimum altitude.

However, given the length of the flight, I'd put my money on something broke.

Re: Why would B738 be limited to FL250?

Posted: Fri Apr 24, 2020 7:38 pm
by ERAUMBA
Why did you not ask the crew while you were deplaning?

Re: Why would B738 be limited to FL250?

Posted: Fri Apr 24, 2020 7:53 pm
by American 767
Back in 2004 I flew on an MD-80 with American from DFW to LGA. We cruised at FL 250, yes only 25000ft. I think that the reason for that must have been high head winds at higher flight levels. Well this was 16 years ago, I barely remember.

Re: Why would B738 be limited to FL250?

Posted: Fri Apr 24, 2020 7:54 pm
by bigb
My best guest would be a deferred pack.

Re: Why would B738 be limited to FL250?

Posted: Fri Apr 24, 2020 7:56 pm
by bigb
American 767 wrote:
Back in 2004 I flew on an MD-80 with American from DFW to LGA. We cruised at FL 250, yes only 25000ft. I think that the reason for that must have been high head winds at higher flight levels. Well this was 16 years ago, I barely remember.


This was probably due to having deferred pack as well.

Re: Why would B738 be limited to FL250?

Posted: Fri Apr 24, 2020 8:05 pm
by JoseSalazar

Re: Why would B738 be limited to FL250?

Posted: Fri Apr 24, 2020 9:12 pm
by strfyr51
bigb wrote:
My best guest would be a deferred pack.

A deferred Pack would limit you to FL250 Or even an outflow valve inop in auto mode. limited to standby mode. I've seen them both and have had to use them both to get an airplane beck to a maintenance station before I retired, Doesn't happen but once in a blue moon? But it has happened.

Re: Why would B738 be limited to FL250?

Posted: Sat Apr 25, 2020 12:39 pm
by bigb
strfyr51 wrote:
bigb wrote:
My best guest would be a deferred pack.

A deferred Pack would limit you to FL250 Or even an outflow valve inop in auto mode. limited to standby mode. I've seen them both and have had to use them both to get an airplane beck to a maintenance station before I retired, Doesn't happen but once in a blue moon? But it has happened.


I agree with you, I fly the CRJ, we are limited to 250 when when we have a deferred pack, even though the 700/900 are certified to operate at FL310 on a single pack.

Re: Why would B738 be limited to FL250?

Posted: Sun Apr 26, 2020 11:43 am
by T18
bigb wrote:
strfyr51 wrote:
bigb wrote:
My best guest would be a deferred pack.

A deferred Pack would limit you to FL250 Or even an outflow valve inop in auto mode. limited to standby mode. I've seen them both and have had to use them both to get an airplane beck to a maintenance station before I retired, Doesn't happen but once in a blue moon? But it has happened.


I agree with you, I fly the CRJ, we are limited to 250 when when we have a deferred pack, even though the 700/900 are certified to operate at FL310 on a single pack.

As stated above, answer to op is likely pack deferral, I find it interesting that your outfit limits to FL250 on the CRJ while mine we are permitted to FL310 with a pack inop, granted on most sectors our flights are short enough we stay below FL250 anyway.

Re: Why would B738 be limited to FL250?

Posted: Thu Apr 30, 2020 3:25 am
by Sancho99504
bigb wrote:
strfyr51 wrote:
bigb wrote:
My best guest would be a deferred pack.

A deferred Pack would limit you to FL250 Or even an outflow valve inop in auto mode. limited to standby mode. I've seen them both and have had to use them both to get an airplane beck to a maintenance station before I retired, Doesn't happen but once in a blue moon? But it has happened.


I agree with you, I fly the CRJ, we are limited to 250 when when we have a deferred pack, even though the 700/900 are certified to operate at FL310 on a single pack.

About 15 years ago, I was onboard an A320 PHX-ORD. Captain advised we'd be cruising at FL310 due to an inop Pack. Thought it was weird since I flew CO ANC-SEA-IAH on the same 757 with an inop Pack and were restricted to FL250 iirc.

Re: Why would B738 be limited to FL250?

Posted: Thu Apr 30, 2020 4:52 am
by teachpdx
I was on a UA 735 back in the college days, prolly 2008, OKC-DEN. Had an inop pack and couldn’t go above 250. A huge squall line of thunderstorms stretched from the Texas panhandle north to Nebraska. We had to fly around it, nearly over the border with Mexico, before turning back north. A scheduled 1:30 flight turned out to be nearly 4:00, and of course ended with a missed onward connection.

Re: Why would B738 be limited to FL250?

Posted: Thu Apr 30, 2020 3:09 pm
by Tristarsteve
The pack inop MEL on the A320 has changed over the years. Old aircraft were FL250, and the latest have no limitation. In between it was FL310.
Don't know if it can be changed, but you must check the MSN against the MEL.

Re: Why would B738 be limited to FL250?

Posted: Sat May 02, 2020 9:48 am
by Avgeek21
Could also be a wing body overheat detector system that is not working. Limited to FL250. Or it could be Left or Right Engine Bleed system not working. Also limited to FL250. However most typically it's a Pack issue. Not a big deal. I had a pack inflight failure a few months back. Even though the QRH is applicable inflight and it doesn't tell you to descent it is good airmanship imho to descent (if possible) to FL250/240. Winds could also play a part if there are no technical issues. However going that low is a bit unusual. But it happens.

Re: Why would B738 be limited to FL250?

Posted: Fri May 08, 2020 2:46 am
by FriscoHeavy
American 767 wrote:
Back in 2004 I flew on an MD-80 with American from DFW to LGA. We cruised at FL 250, yes only 25000ft. I think that the reason for that must have been high head winds at higher flight levels. Well this was 16 years ago, I barely remember.


DFW-LGA is eastbound and therefore, you almost certainly had a tailwind, not a headwind.

With that said, a strong tailwind could have been a possibility for the altitude.