MCT at cruise
Hi Cosmo, I like the way you play the ball and not the man, I generally stick to that as well.
I don't consider myself overly experienced, but not green either. I think I fall into the 'journeyman' category but everyone will have different ideas on what experience qualifies someone to comment on this topic. I have 19 type ratings if you include turbo props, (737-300/400 and 800 counting as one). I'm current on the 737-400 and 800.
Yes. We were below optimum at the time and managed to maintain our FL. the Met guys were consulted and queried about the forecasts and the result was as I commented earlier. There are quite a few other examples of this happening around the world hidden away in incident reports on the net.
My opinion is that there is a reason that you are in the minority with your Max altitude cruising so I'll try and covey my reasoning:
In an earlier post you talked about adding margins to already existing margins. I put it to you that there is no margin when at Max altitude as you are already at a limitation of 100fpm capability and any degradation of performance puts you outside of that requirement.
You have often referred to the lower min manoeuvre band, I don't think that the lower min manoeuvre band is well understood when the aircraft is operating at high altitude and I think it is potentially giving you a false sense of security, that is why I pointed out that it won't move as the temperature increases, (I would appreciate a response to that part of the conversation). Also on that topic, what is it giving you? Nothing relevant in my mind. It is telling you the speed at which the airframe/wings can handle 40 degrees of bank prior to stick shaker if you had unlimited power......you don't, so it's not very useful. The reality is you probably would struggle to maintain straight and level flight at the top of the bar ( at altitude) as it is lower than min drag and the drag is too high to be overcome by the engines.
Can you answer this for me please?
Cheers.
I remember a thread about a year ago, where we discussed correct flare/thrust reduction technique for the 737. It seemed to me at the time, that you were quite green?
Did you encounter an instant un-forcasted +20 deg temp. increase in clear weather?
My opinion is that there is a reason that you are in the minority with your Max altitude cruising so I'll try and covey my reasoning:
In an earlier post you talked about adding margins to already existing margins. I put it to you that there is no margin when at Max altitude as you are already at a limitation of 100fpm capability and any degradation of performance puts you outside of that requirement.
You have often referred to the lower min manoeuvre band, I don't think that the lower min manoeuvre band is well understood when the aircraft is operating at high altitude and I think it is potentially giving you a false sense of security, that is why I pointed out that it won't move as the temperature increases, (I would appreciate a response to that part of the conversation). Also on that topic, what is it giving you? Nothing relevant in my mind. It is telling you the speed at which the airframe/wings can handle 40 degrees of bank prior to stick shaker if you had unlimited power......you don't, so it's not very useful. The reality is you probably would struggle to maintain straight and level flight at the top of the bar ( at altitude) as it is lower than min drag and the drag is too high to be overcome by the engines.
Can you answer this for me please?
At -56 degrees the aircraft was capable of a 100fpm climb rate, what is it capable of now that the temp is -46 ?
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I thought you were done here?
Max 381 with a 9 knots margin between low and high speed buffet (240 to 249, with 249 equal to approx .79). We climbed at it was smooth at FL380. We kept the speed at just below .79 (ECON cost index 55, can't remember the exact speed) and with the variations it was momentary slightly in the high speed buffet band at times (autothrottle acting more fast to correct so not a problem for me)
Does you airline monitor your excesses?(QAR)..
Concerning temp increase,and i only flew this type for about 9 years on the left seat,,is that a 1-2 increase in temp can reduce your max by 100 ft...5c is plenty enough to get you in the lower band and be in an underthrust condition.
Again,i have never got into a high speed or low speed on the 737,call me conservative but i rather have my coffee without going into the clacker,save fuel and deal with what you describe as light turbulence.
Maybe your cabin crew tend to overfill your coffee,since your 'light turbulence' at Fl360 wa a potential tie ruining scenario
You seem a perfect fit for a cost 55 airline...
Last edited by de facto; 2nd Oct 2013 at 05:44.
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If you are cruising at 380(max 381), 4 kts upper margin sounds correct but to MMO ..
If your speed stayed within the upper margin,how close did it come to an overspeed,if you do it again please take a picture and post..im interested.
In any case flying at max will get you in the overspeed one day or the other.
You must be flying in easa land,the FAA
use 1.2G(33 deg bank to clacker).
If your speed stayed within the upper margin,how close did it come to an overspeed,if you do it again please take a picture and post..im interested.
In any case flying at max will get you in the overspeed one day or the other.
. Also on that topic, what is it giving you? Nothing relevant in my mind. It is telling you the speed at which the airframe/wings can handle 40 degrees o
use 1.2G(33 deg bank to clacker).
Last edited by de facto; 2nd Oct 2013 at 06:59.
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This happened to me 2 weeks ago. A321 c.83tons. Open CLB
Climbing through a long band of Cb around SE Europe at night, passing about fl280 encountered sev turb., a/c stopped climbing and periodically entered descent. Speed decayed to Green Dot -10 in level flight. MCT selected, which unsurprisingly did nothing to improve the situation at this altitude. V/S -300 selected, a/c regained normal speed range in level turbulent flight.On leaving Cb area, Op CLB reselected and back to normal.
MCT or TOGA do not really help at high altitudes, if you enter this situation keeping the a/c under control inside the flight envelope is the key and accepting whatever flightpath that entails, with obvious regard to terrain and other traffic.
Climbing through a long band of Cb around SE Europe at night, passing about fl280 encountered sev turb., a/c stopped climbing and periodically entered descent. Speed decayed to Green Dot -10 in level flight. MCT selected, which unsurprisingly did nothing to improve the situation at this altitude. V/S -300 selected, a/c regained normal speed range in level turbulent flight.On leaving Cb area, Op CLB reselected and back to normal.
MCT or TOGA do not really help at high altitudes, if you enter this situation keeping the a/c under control inside the flight envelope is the key and accepting whatever flightpath that entails, with obvious regard to terrain and other traffic.
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Framer,
I would consider a +20 degs temperature change an event comparable to encountering severe clear air turbulence, and would start an emergency descent. I don't plan my flight on the odd occasion that might happen every 50.000 flight. Though I know that you have a more extreme environment down under. I mean when a jellyfish can kill you...
You wrote the you were even below the optimum and only just managed to maintain altitude. I assume that is was more a coincidence that you were below optimum... hence had you been higher you would have had to descent too?
Therefore, it would be very inefficient always to fly with the worst possible scenario in mind. As an extreme example, then we could stay below 10000 feet too, to be on the safe side in case of a rapid decompression.
That is exactly what I mean, we can't continue to add personal margins on established margins... where will it end? ...that we stay on the ground.
Boeing say it is safe to operate the aircraft at max altitude, so that is what I am doing (with the use of common sense as pointed out previously).
I have no problem adding margins to margins, when it is justified. I happily bring 2 tons extra fuel if there is a reason to do so. But I also fly with min required if there is no reason to bring extra. What I do not agree with is people saying "I don't fly with less than 800 kg extra", "I always want to see +800 feet on the max altitude before I climb" etc. I have only a big WHY? to say to that.
Aha, there we have the misunderstanding!! You are confusing FMC max altitude with "service ceiling". This is absolutely incorrect. Flying at max FMC altitude is not flying at service ceiling. Because Boeing already build in a margin for you:
So I guess I don't have to answer your question, as it is irrelevant.
As for the flaps up amber band, it doesn't say anything about thrust available vs. thrust required. Hence, with a decreasing speed approaching the top of the amber band it would be a good idea to select CON to have the extra thrust available (if you didn't already do so, routinely, as pr. Boeing recommendation). If the speed continues to show a decreasing trend and getting close, a slight (to ATC unnoticeable) descent may help you out, as pointed out earlier.
Just to point out, despite my "dangerous" operating of the aircraft I was never in the low speed band. Except briefly one time, flying well below max altitude and flying through the edge of a CB with moderate turbulence. Lost about 300 feet altitude in the process with autopilot in CWS.
De facto,
Sorry, with you I really can't keep my eye on the ball anymore. You really have no clue, do you? Clacker for exceeding maximum maneuver speed?
Maybe Boeing should have added:
Been a captain for 9 years? Complacency setting in already. I can almost hear it: "...I have always done so...."
Of curiosity what is a cost index 55 airline? Actually the cost index was 4 on the OFP, what does that then say about my airline?
As we were late and I wanted to bring the aircraft back on time for the next crew to have the min required turn-around time available I had changed the cost index 55, which is equal to LRC. The benefit is that the climb and descent speeds are changed too. For me it is more efficient to change one entry, than having to modify climb and descent pages manually too.
I would consider a +20 degs temperature change an event comparable to encountering severe clear air turbulence, and would start an emergency descent. I don't plan my flight on the odd occasion that might happen every 50.000 flight. Though I know that you have a more extreme environment down under. I mean when a jellyfish can kill you...
You wrote the you were even below the optimum and only just managed to maintain altitude. I assume that is was more a coincidence that you were below optimum... hence had you been higher you would have had to descent too?
Therefore, it would be very inefficient always to fly with the worst possible scenario in mind. As an extreme example, then we could stay below 10000 feet too, to be on the safe side in case of a rapid decompression.
That is exactly what I mean, we can't continue to add personal margins on established margins... where will it end? ...that we stay on the ground.
Boeing say it is safe to operate the aircraft at max altitude, so that is what I am doing (with the use of common sense as pointed out previously).
I have no problem adding margins to margins, when it is justified. I happily bring 2 tons extra fuel if there is a reason to do so. But I also fly with min required if there is no reason to bring extra. What I do not agree with is people saying "I don't fly with less than 800 kg extra", "I always want to see +800 feet on the max altitude before I climb" etc. I have only a big WHY? to say to that.
I put it to you that there is no margin when at Max altitude as you are already at a limitation of 100fpm capability and any degradation of performance puts you outside of that requirement.
Optimum/Maximum Altitude (OPT/MAX)
Also displays the maximum possible altitude based on the selected target speed and the specified maneuver margin.
Values are advisory only. They are provided for crew reference.
Also displays the maximum possible altitude based on the selected target speed and the specified maneuver margin.
Values are advisory only. They are provided for crew reference.
As for the flaps up amber band, it doesn't say anything about thrust available vs. thrust required. Hence, with a decreasing speed approaching the top of the amber band it would be a good idea to select CON to have the extra thrust available (if you didn't already do so, routinely, as pr. Boeing recommendation). If the speed continues to show a decreasing trend and getting close, a slight (to ATC unnoticeable) descent may help you out, as pointed out earlier.
Just to point out, despite my "dangerous" operating of the aircraft I was never in the low speed band. Except briefly one time, flying well below max altitude and flying through the edge of a CB with moderate turbulence. Lost about 300 feet altitude in the process with autopilot in CWS.
De facto,
Sorry, with you I really can't keep my eye on the ball anymore. You really have no clue, do you? Clacker for exceeding maximum maneuver speed?
Maybe Boeing should have added:
Flight crews intending to operate at or near the maximum operating altitude should be familiar with the performance characteristics, instrumentation, limitation and basic priciples of how airplane works in these and any other conditions. Flight crews should be certified and have been tested in their knowledge prior to operating the aircraft
Of curiosity what is a cost index 55 airline? Actually the cost index was 4 on the OFP, what does that then say about my airline?
As we were late and I wanted to bring the aircraft back on time for the next crew to have the min required turn-around time available I had changed the cost index 55, which is equal to LRC. The benefit is that the climb and descent speeds are changed too. For me it is more efficient to change one entry, than having to modify climb and descent pages manually too.
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Just going to quote it before you edit it away. I am not done with you. 
...and yes, light turbulence is a potential tie ruiner. If there is moderate turbulence (unsecured objects flying around in the cockpit and cabin) I ask for descent.
And you are quite fine with that,high speed clacker? I am quite familiar with high altitude flight issues and i really dont understand how you could come here and tell us that having intermittent clacker is operationally wise or professionnaly acceptable...Did you note this excursion into the technical log?
Does you airline monitor your excesses?(QAR)..
Concerning temp increase,and i only flew this type for about 9 years on the left seat,,is that a 1-2 increase in temp can reduce your max by 100 ft...5c is plenty enough to get you in the lower band and be in an underthrust condition.
Again,i have never got into a high speed or low speed on the 737,call me conservative but i rather have my coffee without going into the clacker,save fuel and deal with what you describe as light turbulence.
Maybe your cabin crew tend to overfill your coffee,since your 'light turbulence' at Fl360 wa a potential tie ruining scenario
You seem a perfect fit for a cost 55 airline...
Does you airline monitor your excesses?(QAR)..
Concerning temp increase,and i only flew this type for about 9 years on the left seat,,is that a 1-2 increase in temp can reduce your max by 100 ft...5c is plenty enough to get you in the lower band and be in an underthrust condition.
Again,i have never got into a high speed or low speed on the 737,call me conservative but i rather have my coffee without going into the clacker,save fuel and deal with what you describe as light turbulence.
Maybe your cabin crew tend to overfill your coffee,since your 'light turbulence' at Fl360 wa a potential tie ruining scenario
You seem a perfect fit for a cost 55 airline...
Last edited by cosmo kramer; 2nd Oct 2013 at 12:33.
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Quote:
You must be flying in easa land,the FAA use 1.2G(33 deg bank to clacker).
I'm in FAA land and my book has 1.3 (40deg).
You must be flying in easa land,the FAA use 1.2G(33 deg bank to clacker).
I'm in FAA land and my book has 1.3 (40deg).
Maximum Altitude
Maximum altitude is the highest altitude at which the airplane can be operated. It is determined by three basic characteristics, which are unique to each airplane model. The FMC predicted maximum altitude is the lowest of:
• maximum certified altitude (structural) – determined during certification and is usually set by the pressurization load limits on the fuselage
• thrust limited altitude - the altitude at which sufficient thrust is available to provide a specific minimum rate of climb. (Reference the Long Range Cruise Maximum Operating Altitude table in the PI chapter of the QRH). Depending on the thrust rating of the engines, the thrust limited altitude may be above or below the maneuver altitude capability
• buffet or maneuver limited altitude - the altitude at which a specific maneuver margin exists prior to buffet onset. This altitude provides at least a 0.2g margin (33° bank) for FAA operations or a 0.3g margin (40° bank) for CAA/JAA operations prior to buffet.
Last edited by de facto; 2nd Oct 2013 at 15:21.
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If there is moderate turbulence (unsecured objects flying around in the cockpit and cabin) I ask for descent.
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Back to our friend
JET TRANSPORT PERFORMANCE METHODS
Walt Blake and the Performance Training Group Flight Operations Engineering
Boeing Commercial Airplanes
It seems my airline has gone the more conservative 1.3 route, rather than the 1.2. It's interesting that, according to Blake, there is no regulatory requirement, yet FAA and EASA differ.
JET TRANSPORT PERFORMANCE METHODS
Walt Blake and the Performance Training Group Flight Operations Engineering
Boeing Commercial Airplanes
regulatory requirements for maneuver capability
At the time of this writing, there are no known regulatory requirements for minimum maneuver capability at cruise altitude.
Many airlines have a policy of requiring a maneuver capability of 1.3 gees or better at the selected cruise altitude, and more if turbulence is expected. Operators are free to require more conservative margins if they wish – we understand that there are some operators who do require maneuver capabilities greater than 1.3.
Flight at maneuver capabilities less than 1.3 should not be thought of as inherently dangerous, but rather that the speed margins to buffet are less. Flight in light buffet isn’t dangerous, but it is a natural aerodynamic reminder that the airplane is approaching its operating limits.
At the time of this writing, there are no known regulatory requirements for minimum maneuver capability at cruise altitude.
Many airlines have a policy of requiring a maneuver capability of 1.3 gees or better at the selected cruise altitude, and more if turbulence is expected. Operators are free to require more conservative margins if they wish – we understand that there are some operators who do require maneuver capabilities greater than 1.3.
Flight at maneuver capabilities less than 1.3 should not be thought of as inherently dangerous, but rather that the speed margins to buffet are less. Flight in light buffet isn’t dangerous, but it is a natural aerodynamic reminder that the airplane is approaching its operating limits.
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De facto: the FMC predicted max altitude you refer to is dependent on the stuff you feed it.
Again, your wing does not care about the FMC.
If you let the FAA airplanes believe their 25% Cruise CG default in the FMC, you might be in for a surprise. The JAA 8% will safely underestimate the values.
Cosmo Cramer, you take the words out of my mouth.
Again, your wing does not care about the FMC.
If you let the FAA airplanes believe their 25% Cruise CG default in the FMC, you might be in for a surprise. The JAA 8% will safely underestimate the values.
Cosmo Cramer, you take the words out of my mouth.
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Seems the 737 club took over. As I recall it was a320 question. At high altitude, max CLB, MCT and TO are the same limit. However selecting MCT or TOGA will react much faster then the autothrust on a speed-decay.
On a 320 you are also lucky that the high speed limits are more generous and in general the margin between low and highspeed is much bigger then on a 737.
I also loved that statement that take-off thrust can never be selected in any other phase then take-off.
You guys need to get out more instead of watching the SAT. There is also a difference between optimum, max recommended and max cruise altitude.
Max recommended really is not that dramatic, and you can only plan for what you know. Our company downloads winds and temperature for the entire route into the fmgs, so yes the fmgs can foresee what will happen in the future a long as it is forecast.
On a 320 you are also lucky that the high speed limits are more generous and in general the margin between low and highspeed is much bigger then on a 737.
I also loved that statement that take-off thrust can never be selected in any other phase then take-off.
You guys need to get out more instead of watching the SAT. There is also a difference between optimum, max recommended and max cruise altitude.
Max recommended really is not that dramatic, and you can only plan for what you know. Our company downloads winds and temperature for the entire route into the fmgs, so yes the fmgs can foresee what will happen in the future a long as it is forecast.
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You might want to check your definition of moderate, that actually would be considered severe.
(Ok, I exaggerated a bit to prove a point). "Dislodged" - happy? The point being, the amazing number of time I hear colleagues reporting moderate turbulence along the exact same route I am flying in light chops. Hence, if I say that I would climb for light turbulence (like the examples previously), it's because it is uncomfortable for the passengers and cabin crew. However, I refuse to use the word moderate, despite other seemingly doing so casually for light turbulence.
latetonite, thanks for the support.
(Framer, I am not alone you see. )
You wrote the you were even below the optimum and only just managed to maintain altitude. I assume that is was more a coincidence that you were below optimum... hence had you been higher you would have had to descent too?
Therefore, it would be very inefficient always to fly with the worst possible scenario in mind. As an extreme example, then we could stay below 10000 feet too, to be on the safe side in case of a rapid decompression.
Therefore, it would be very inefficient always to fly with the worst possible scenario in mind. As an extreme example, then we could stay below 10000 feet too, to be on the safe side in case of a rapid decompression.
Aha, there we have the misunderstanding!! You are confusing FMC max altitude with "service ceiling". This is absolutely incorrect. Flying at max FMC altitude is not flying at service ceiling. Because Boeing already build in a margin for you:
The FMC compares three limits in finding the Max altitude
1/Maximum Certified Altitude (Structural).
2/ Thrust Limited Altitude
3/Manoeuvre Limited Altitude
We all know the Structural limit. The thrust limited altitude is based on a residual rate of climb of 100fpm. You say Boeing have built in a margin on this but I have never read that anywhere. Can you provide a reference?
The Manoeuvre Limited Altitude is an Aerodynamic consideration and isn't related to the other two.
Most of the time in an NG you will be operating restricted by number 2.
When limited by the thrust the lower amber band provides no guidance as to whether or not straight and level flight can be maintained at that speed, and it does not change with temperature. If you are at ISA + it is even more misleading. So again, if you are at your maximum altitude and the temperature increases, can you maintain your required 100fpm? Unless you can describe the margin that Boeing have built in then I say no.
The ability to maintain the 100fpm is not the big issue here. It just illustrates how limited your options are when operating at Max. But either way we should determine which of us has the misunderstanding so that we can carry on with increased knowledge
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You say Boeing have built in a margin on this but I have never read that anywhere. Can you provide a reference?
It's in the note in the brackets below:The FMC predicted maximum altitude is the lowest of:
• thrust limited altitude - the altitude at which sufficient thrust is available to provide a specific minimum rate of climb. (Reference the Long Range Cruise Maximum Operating Altitude table in the PI chapter of the QRH).
Service ceiling is determined at the speed for minimum drag = minimum thrust required. Minimum thrust required = Max Range Cruise. MRC in the FMC is ECON cost index 0 (no wind). Reference:
Cruise Speed Determination
Entry of zero for cost index results in maximum range cruise.
Entry of zero for cost index results in maximum range cruise.
But it is certainly enough to maintain level flight, even with the odd speed variations due to changing wind and temp. Even more so when you increase your thrust available by approx. 5% N1 by selecting CON on the N1 page, as recommended by Boeing.
I ask again, where is the problem?
Boeing also provides this note in the FCTM, by the way:
On airplanes with higher thrust engines, the altitude selection is most likely limited by maneuver margin to initial buffet.
Since the upper band is not a problem for your thrust available and really doesn't affect the type of maneuvering you are doing, it would be safe to fly in the band temporarily. Hence your total margin would be from top of flaps up maneuvering band, practically to MMO. This means that at max FMC predicted altitude, you have maybe 8-9 knots on each side of your bug speed (when flying just below the upper amber band) of to absorb wind variations. Is this a problem on normal day for you? Or let me ask you another way, how many knots margin do you need on a normal smooth day?
Yes efficiency is another matter. I reiterate, I do not blindly climb to max altitude. But there can be a number of operational reason why it can be a benefit - "Maintain level 340, sorry level 360 is occupied same destination as you, can you accept level 380? I have an aircraft 80 nm ahead of you level 380, he reported it is smooth", "able level 380"
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Ohhh, I missed this pearl:
Can you please tell me what the bank angle has to do with the clacker (MMO)?
De facto:
You must be flying in easa land,the FAA
use 1.2G(33 deg bank to clacker).
You must be flying in easa land,the FAA
use 1.2G(33 deg bank to clacker).
Last edited by cosmo kramer; 3rd Oct 2013 at 00:33.
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De facto: the FMC predicted max altitude you refer to is dependent on the stuff you feed it.
Again, your wing does not care about the FMC.
Again, your wing does not care about the FMC.
No i tend to write fast and kramer loves to catch on it so from now on i wont jump steps;-)
Kramer,the reason the clacker comes to my mind each time we speak about buffet is that i believe the vmax band is quite close to the max maneuvring one at your max altitude.
The MMO is a fixed speed(yes the clacker is dependent on this fixed value) but going into high speed buffet isnt acceptable to me at least routinely as you seemed to be ok zipping away your coffee...
Now we agree to disagree...
(I must admit i would love you requesting fl 380(max alt) from fl 340 and as you climb it isnt smooth as reported,again it seems like a subjective definition ,and the temp going up by a few degrees and now your fmc,passing through fl360,shows you max is 378.....,what would you do ? Ask for descent with the tail between your legs or continue up,(i bet u do that),and try to convince your Fo about all the margins built in....and squeeze your sorry
?How often have you exceeded MMO Kramer? Be honest.
I know even if you exceed mmo,theres extra margin,it aint the end of the world..but we are paid to keep the aircraft within given margins,few seem to believe that to be at exactly those margins is acceptable,,i dont.
Hence my earlier analogy to opearting the flaps at 19900 ft or selecting flaps at 249kts...whats the difference Kramer?
You are within limits?and yet if you overspeed the flaps,the aircraft wont fall off the sky...but you respect that dont you?so why not the same with your alitude?
Last edited by de facto; 3rd Oct 2013 at 07:25.
Ok Cosmo, we are starting to make some headway into the 'misunderstanding ' that you pointed out. I agree that there is a level of misunderstanding on this topic but am not convinced that it lies with me.
You are quick to point out that adding an extra margin is a wives tale but it's genesis may lie in truth. Your conscious, considered decisions, based on facts at hand will be drawing on your knowledge base and the learnings you have made over the years in order to make the decision. It is neccassary to have self confidence in that knowledge base, but also to be prepared to re-jig it if it is found wanting so that your decision making skills evolve as you go. I want to question that knowledge base without it seeming like a personal attack .With that said, I am more than prepared to eat humble pie and rethink my alt selection if you can direct me to where I have misunderstood. Your last shot at directing me there fell short. I'll show you where so that there is no confusion.
Only two of those three assertions are correct.
The speed for minimum drag does not equal the speed for minimum thrust required in a swept wing Boeing, and, Minimum Thrust Required does not equal Max Range Cruise in a swept wing Boeing. I agree that nil wind CI zero equals MRC.
So shall we sort out those basics before we move on to determining why you and Latetonite seem to be on the fringe of what B737ng pilots think is an acceptable way to operate the aircraft?
I'm glad you mentioned min drag actually because that speed is probably the only relevant speed to you when you are operating at Max altitude, that and Vmo of course. I have one question for you, in your flight the other day when you said
......what was min drag speed?
Every decision I make in the cockpit is a conscious, considered decision, based on the facts at hand... Not an automated decision based on old wife's tales or "that I always do/need so".
Service ceiling is determined at the speed for minimum drag = minimum thrust required. Minimum thrust required = Max Range Cruise. MRC in the FMC is ECON cost index 0 (no wind)
The speed for minimum drag does not equal the speed for minimum thrust required in a swept wing Boeing, and, Minimum Thrust Required does not equal Max Range Cruise in a swept wing Boeing. I agree that nil wind CI zero equals MRC.
So shall we sort out those basics before we move on to determining why you and Latetonite seem to be on the fringe of what B737ng pilots think is an acceptable way to operate the aircraft?
I'm glad you mentioned min drag actually because that speed is probably the only relevant speed to you when you are operating at Max altitude, that and Vmo of course. I have one question for you, in your flight the other day when you said
Coincidentally I had exactly this scenario last night, so I can back up with some numbers too. Cruising at 360 top of clouds with continuous light turbulence (what other usually report as moderate, for some reason). Max 381 with a 9 knots margin between low and high speed buffet (240 to 249, with 249 equal to approx .79)