MCT at cruise

[cosmo kramer]

De facto,

I don't understand why you think MMO would be more easily exceeded at max altitude?

If I fly .79 at FL360 or FL380, I still have .03 margin in both levels, with the difference of the .03 being the yellow band in FL380. Actually, I believe it to be less likely, because the auto throttle will "wake up" as soon as the speed goes into the yellow band. I am sure you are familiar with cruising at well below max, .79 bugged, but seeing the speed go to e.g. .815 and the auto throttle still asleep.

In the previous times, it was normal to fly 340/.820 if late (before the rising fuel prices), and have the clacker come on intermittently. Ask older colleagues, no aircrafts fell out of the sky for that reason. I don't fear the clacker, but I respect it and avoid it, I don't like to fill out unnecessary paperwork (inefficient use of time). I wasn't in the clacker for years, and when I was it was never at max altitude, because I don't go there if it's turbulent.

During my climb from 340 to 380, my max will most likely go up to 382 during the climb. I think the FMC doesn't* consider the fuel burned during the stepclimb, but I can't back that up with anything in writing.

I did have once that on the way up to FL360 the max altitude dropped down to FL359. And yes we continued up (it was perfectly smooth) and within minutes max and selected again agreed.
Like already been said the wing doesn't know what's written on the cruise page of the FMC. This has to be an individual evaluation what to do.

The difference to using flaps in 19999 or 249, is that there is no point to doing so. Give me one objective example of where it could possibly be an operational benefit of doing so? I can't see one. To make your example slightly less extreme, even if fast, I can't see any objective accomplished in selecting flaps 1 at e.g. 240 knots. I have a gear that is much more effective in slowing me down, and it needs to be extended to land anyway.

Framer:

The speed for minimum drag does not equal the speed for minimum thrust required in a swept wing Boeing

Aha, but it's the same curve. It's just two different names for the same point on the same curve. So how in would could they not be the same...

and, Minimum Thrust Required does not equal Max Range Cruise

Ok So, which speed would give you the max range then? (If you say long range cruise, explain to me why we always fly slower to save fuel).

......what was min drag speed?

I'll maintain ECON cost index = 0. Didn't check what is was at the day, probably around .76.

*edited

[latetonite]

Do not forget Econ with cost index 0 still corrects for wind. (+, not -).

[framer]

Cosmo, you were right about min drag and min thrust required. I confused min fuel flow with min thrust required. My apologies.
Off to work with me now. I'll have a look at some numbers in the cruise and see if I can bolster my argument with them tomorrow

[HazelNuts39]

Originally Posted by Cosmo Kramer

So, which speed would give you the max range then?

Well ... if you assume constant TSFC (thrust-specific fuel consumption), then max L/D speed will give you minimum fuel consumption per hour. Max range speed is minimum fuel per NM and requires a higher speed -1.32 times minimum drag speed for a 'parabolic' drag polar. The actual max range speed will be somewhat different due to Mach effects and variation of TSFC. Hope this helps.

[framer]

Righto, back to it.

Ok So, which speed would give you the max range then? (If you say long range cruise, explain to me why we always fly slower to save fuel).

Cost index zero. It will always have you arrive overhead with the most gas in the tanks.It will give you a speed faster than the minimum thrust required speed. In addition, as I'm sure you know, it will give you a faster speed again if you are into a headwind. LRC is pretty much irrelevant in today's CI environment, it doesn't take the wind into account and simply gives you an extra 5% speed for an extra 1% burn. Handy in the days when you didn't have Cost Index to do the maths for you.
I think I have identified where you and I are at crossed purposes Cosmo. You stated

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

You have identified a range of airspeed where you feel safe operating, but the lower limit of this range is incorrect in my mind. The lower limit you suggest is the top of the lower amber band, whereas the lower limit should be min drag speed. The top of the lower amber band is some way from min drag speed. I will give you some cut and paste examples of why I think this;
The Royal Aeronautical Society has this to say.

The lowest point on the total drag curve is known as L/D max (or Vmd-minimum drag speed). The speed range slower than L/D max is known as slow flight, or the “back side of the power-drag curve” or the “region of reverse command.” Speed faster than L/D max is considered normal flight, or the “front side of the power-drag curve”.
Normal flight (faster than L/D max) is inherently stable with respect to speed. When operating in level flight at a constant airspeed with constant thrust, speed-stability ensures that any airspeed disturbance (such as turbulence) is of short term duration and airspeed will eventually return to the original airspeed if the total thrust and attitude have not changed.

So it is obvious from this that we want to stay in the Normal flight regime or we will end up in the slow flight regime which is described here;

Slow flight (slower than L/D max) is inherently unstable with respect to speed and thrust settings. When operating at a constant airspeed, with constant thrust setting, any disturbance causing a decrease in airspeed will result in a further decrease in airspeed unless thrust is increased. The lower speed subjects the aeroplane to increased drag. This increase in drag causes a further decrease in airspeed, which may ultimately result in a stalled flight condition.

The paper then goes on to say this;

Flight slower than L/D max at high altitudes must be avoided, due to the inefficiency and inherent instability of the slow flight speed range.

And

Flight slower than L/D max must be avoided in the high altitude environment. Proper flight planning and adherence to published climb profiles and cruise speeds ensures that speeds slower than L/D max are avoided.

So from that, if we are to believe the Royal Aeronautical Society ( and it's American counterparts) we should be cruising above min drag speed. That isn't really up for debate unless you consider that you know better than the folk who collaboratively produced the guidance document. That is why I asked you what min drag was for you example flight. You took a guess at about Mach 0.76. With respect, I say that taking a guess after the flight is complete is not really good enough. The Captain needs to know where the lower limit of safe flight is while he or she is conducting the flight.
Min Drag speed is usually ten knots or more ( sometimes as much as 18kts in the NG) faster than the top of the lower amber band when at high altitude. My experience is that there is usually only a couple of knots lee-way when operating at Max, not the 8-9kts that you described.
Is there a chance that the reason you feel comfortable is that you have erroneously identified a larger safe operating band than many other pilots?
Most pilots that I fly with can't identify the min drag speed if asked, yet it is only two button pushes away. The fact that it is not easily displayed to the pilots is a failing on the manufacturers part in my mind because both the Royal Aeronautical Society and the FAA produced 'jet upset training aid ' warn of not going below it.
I hope that this doesn't come across as an attack on your professionalism, rather a robust discussion and a chance for both of us to learn something. I have learnt already so thanks for that.

[cosmo kramer]

Framer, it is time to admit you are wrong, instread of digging up more and more obscure arguments as I shoot down your previous ones...

So far you have now admitted, that everything I have posted so far is correct. Min drag = min thrust required = cost index 0 (no wind, as I wrote already in the first post mentioning cost index 0).

So I is really time to let it rest.

LRC is pretty much irrelevant in today's CI environment

Yet this is the speed Boeing uses to calculate the thrust limited altitude. Like I have now written several times, this gives you an extra margin.


About the last non-sense you posted, this is basic ATPL knowledge:

The lower limit you suggest is the top of the lower amber band, whereas the lower limit should be min drag speed.

No, this is the lowest speed that should be used as cruise speed (the selected cruise speed). Because a speed lower that this speed is ineffective.

As you quoted

Normal flight (faster than L/D max) is inherently stable with respect to speed.

(hope my formatting stays, because it dissappeared from my previos posts, the emphasis should be on NORMAL)....

any disturbance causing a decrease in airspeed will result in a further decrease in airspeed unless thrust is increased.

(emphasis "unless thrust is increased")

So it is quite obviuous that we should not plan to fly at a speed lower than L/D max (Max range cruise = cost index 0 (no wind). And we don't.. we always plan to fly faster that this speed, because...

due to the inefficiency and inherent instability of the slow flight speed range.

It is inefficient, because we have a higher drag, and fuel flow plus we will have a lot of throttle movement to maintain the speed.

That all together means that if you cruise at a faster speed, a temporary excursions into this regime is acceptable, but must be corrected by increasing the thrust (pretty obvious - the "unless thrust is increased" emphasis).

We will always cruise faster than MRC, because it makes no sense to cruise with a lower speed. And should the speed temporarily go below MRC, towards the top of the amber band, we will correct it (auto throttle too).

[framer]

Righto Cosmo, I see where you are coming from, correct me where I am wrong.
Cruising at speeds planned to be at or faster than min drag is acceptable because if you drop below min drag you will use thrust to overcome the extra drag and drive on out of it?
I can see how that works in theory but have you never experienced changes in wind and/or temperature that result in a situation where your only option is to descend as the thrust available can't pull you out of it? I know I have and I also know of several departures from flight level because of that very reason.

Framer, it is time to admit you are wrong, instread of digging up more and more obscure arguments as I shoot down your previous ones...

The minute I recognised that I was wrong about min drag equaling min thrust required I posted saying just that and apologising. I am yet to see a similar post from you admitting that min thrust required does not give you MRC. I have to admit that I'm not holding my breath.

So far you have now admitted, that everything I have posted so far is correct.

Not at all. There are three basics we were trying to sort out
1/ cost index zero = MRC ...... We agreed from the start
2/ min drag = min thrust required......we now agree after I admitted I was wrong.
3/ min thrust required = MRC......we still disagree, I say MRC is faster.

I can see your thought process though. At the end of the day I think you are simply comfortable with lower margins than me.

[cosmo kramer]

I can see how that works in theory but have you never experienced changes in wind and/or temperature that result in a situation where your only option is to descend as the thrust available can't pull you out of it?

Yes, well below max altitude due to moderate turbulence. At close to max altitude, no, because I don't go there if it to be expected or is forecasted.

I am yet to see a similar post from you admitting that min thrust required does not give you MRC. I have to admit that I'm not holding my breath.

The best source I can dig up on google to be able to provide you with a link is this:
Maximum Endurance, Maximum Range, and Optimum Cruise Speeds

Not the best source, but have a look af the lower curve on figure one anyway.

3/ min thrust required = MRC......we still disagree, I say MRC is faster.

Even if your were right, it only makes my case stronger. It would make the margin even bigger. Since Min Drag Speed would be even lower, than cost index 0 speed. But have a look at the link above.

At the end of the day I think you are simply comfortable with lower margins than me.

I am comfortable with lower margins when I don't need them. If I need higher margins I will take them (more fuel, lower altitude etc.). But there has to be a reason....

[framer]

The best source I can dig up on google to be able to provide you with a link is this:
Maximum Endurance, Maximum Range, and Optimum Cruise Speeds

Not the best source, but have a look af the lower curve on figure one anyway.

Hi Cosmo, thanks for the link. That link provides good information for straight wing propellor aircraft only.
In 2009 the 'Performance Training Group Flight Operations Engineering Boeing Commercial Airplanes ' updated their document 'Jet Transport Performance Methods' . It is a great document and explains the Boeing ethos behind all of their Performance Engineering. PAge 32-14 has a nice graph and explanation regarding the relationship between Min Thrust Required and MRC. It says this:

We've drawn in a line of MRC. Here you see clearly that the speed for maximum range cruise doesn't occur at the point for minimum thrust required, but rather at a slightly faster speed.

So here is where we stand, feel free to correct me:
Min drag = min thrust Required
MRC = cost Index Zero
MRC/ CI zero > Min Drag/min Thrust Required

Even if your were right, it only makes my case stronger. It would make the margin even bigger. Since Min Drag Speed would be even lower, than cost index 0 speed.

I see what you are saying and I agree that the margin to min drag is greater than you originally suggested. It is the same as I always imagined because every time I level out in the cruise I make a mental note of what Min drag is and don't accept slower speeds. I am fairly confident though that next time you climb up there and have a look at what your min drag speed is it will be within a knot or two from the speed you are cruising at. It is always ten or so knots greater than the lower amber band ( often 15-18kts) so when you have a gap of 8 or 9 knots like you suggested, you are very close to it. Obviously some are more comfortable with that than others. Anyway, apart from MRC= min thrust we roughly agree on the basics , the difference being that I am more conservative regarding deviations into the slow speed regime. Fair enough, I wish you thirty years of safe cruising Cosmo. Thanks for the conversation.

[cosmo kramer]

I am aware that the drag curves are greatly simplified, most likely they are too simple for a small prop a/c too. However, they are enough for the purpose of explaining something in an easy and understandable way. I guess that is why they are used for learning material for pilots. I don't pretend to be an engineer, I like the KISS principle. I will give you the following.

MRC/ CI zero ≈ Min Drag/min Thrust Required (for practical purposes)

If you say you have a document from Boeing, that states that MRC is slightly higher than Vmd, I believe you (compressibility, TSFC etc.). Still, it has no practical bearing on the discussion about cruise margins (and even strengthens my point of view). Furthermore we can probable put "≈" in every argument in every discussion, starting with why an aircraft flies at all (was it because of Newton or Bernoulli?).

I wish you thirty years of safe cruising Cosmo. Thanks for the conversation.

Thanks, you too. But before you go, I have one more question:

I make a mental note of what Min drag is and don't accept slower speeds.

Not sure what do you mean with "don't accept slower speeds"?

Do you mean that you won't accept a slower speed as selected speed (say an ATC restriction). And, if so, I assume you mean in high altitude cruise flight only? (in which case I from the beginning agreed, as it is ineffective).

or

You will not accept that the speed temporarily decays below Vmd with wind/temp variations during cruise, and for that reason you will fly with such great margins as to make it unlikely to happen (this is how I interpret your statement). If this is the case, I have to ask a big "WHY?" again. Vmd is not a dangerous speed, flying slower isn't dangerous either. The speed that is dangerous, is "V drag too great for the thrust at hand to pull you out". Clearly this speed is slower than Vmd.

[framer]

Do you mean that you won't accept a slower speed as selected speed (say an ATC restriction). And, if so, I assume you mean in high altitude cruise flight only?

Yeah that assumption is correct for how I operate and yes, only at high altitude.

You will not accept that the speed temporarily decays below Vmd with wind/temp variations during cruise, and for that reason you will fly with such great margins as to make it unlikely to happen (this is how I interpret your statement). If this is the case, I have to ask a big "WHY?" again. Vmd is not a dangerous speed, flying slower isn't dangerous either. The speed that is dangerous, is "V drag too great for the thrust at hand to pull you out". Clearly this speed is slower than Vmd.

If I am cruising at high altitude and the speed decays to Vmd and the A/T doesn't wake up I will open the speed window and select a higher speed to wake it up, when the speed recovers to the target speed I close the window.
I agree that Vmd in itself is not a dangerous speed and that the speed at which you no longer have the thrust required to recover to a 'speed stable ' situation is what is important. This is where you and I probably part ways in our assessment of the situation. I am guessing that we have different ideas about how much thrust we have available to us at max altitude to do just that.
My assessment is that we have juuust about enough thrust to pull the skin off a rice pudding ( 100fpm + the margin you have described earlier) and that if we suddenly gain twenty knots of tail, or lose twenty knots of head, or if the temp increases, or we experience wake or other turb, we may find ourselves having to descend when we are not cleared to. I don't expect these things to occur very often, maybe once or twice a decade, but considering that I want to do this for another twenty years I would rather avoid three or four pan calls in the remainder of my career.
Your assessment of the thrust available in those situations is obviously different and that is not surprising, it is a subjective scenario with many unknowns and the call is therefore judicious. Each pilot will assess it slightly differently.
It is probably worth mentioning that under the right conditions I would climb to max altitude. They would be exceptional conditions as the benefit of being up there would have to outweigh the fact that we would burn more fuel and have less margin.I have yet to encounter a situation where every level below optimum has severe turb but it is smooth above. If I am faced with the choice of being inefficient above optimum or being inefficient below optimum I choose the latter most of the time.
So there we have it, different pilots making different subjective decisions, at least we have both thought about it independantly and not just blindly followed our mentors habits.
If we want to throw another spanner in the works we could talk about the BEST SPEED on the holding page, it is often twenty knots below min drag when at altitude as it is literally the BEST SPEED for minimising RATE of fuel flow, it is normally 15-20kts below min drag and results in extra burn in the turns and thrust levers working hard in the turns. Boeing has given the pilot the best theoretical speed for staying in the air the longest time possible but practically it is rarely that
The Boeing performance Engineers say this:

In the holding pattern, however, while the emphasis is still on fuel efficiency in this case we want to minimize the rate of fuel flow. Minimizing the fuel flow rate means minimizing the amount of fuel consumed while holding.

“All right,” you say, “we'll just calculate the fuel flow at the speed for minimum drag. Minimum drag means minimum fuel flow, right?”

Sorry, no. It's a little more complicated than that. Actually, the minimum flow will occur at a speed slightly slower than the speed for minimum drag.

They then go on to give an example of a 757 and their slightly slower speed is this

It will occur at the point where a fuel flow line is tangent to the thrust required line. In the example you see here, the fuel flow at the tangent point is slightly more than 3500 pounds per hour per engine. And notice the speed at the point of tan-gency: about Mach 0.71. What's the speed for minimum drag in this graph? Approximately Mach 0.75, which is 0.04 Mach or 23 knots true airspeed faster than the speed for minimum fuel flow.

This prompted me to select holds in the cruise and compare the BEST SPEEDS for the NG, they too are significantly ( or slightly depending on what school you went to) slower than min drag.

[cosmo kramer]

My assessment is that we have juuust about enough thrust to pull the skin off a rice pudding ( 100fpm + the margin you have described earlier)

Edit: misread it like you meant I had described that we have a +100 fpm margin, I'll leave first part of the reply unchanged anyway:

No, I didn't - on the contrary, I said we have a lot higher margin because the +100 feet is calculated at LRC, where we have a higher drag than the Vmd. Hence, flying at Vmd the residual climb rate would be higher than as calculated for LRC. Plus, the +100 feet is calculated at CRZ thrust limit. With CON you have 4% extra N1.

Hence, my assessment is that you greatly underestimate the thrust available.
Again I quote the FCTM:

On airplanes with higher thrust engines, the altitude selection is most likely limited by maneuver margin to initial buffet.

---

if we suddenly gain twenty knots of tail, or lose twenty knots of head, or if the temp increases, or we experience wake or other turb, we may find ourselves having to descend when we are not cleared to. I don't expect these things to occur very often, maybe once or twice a decade

What is "suddenly"? If you mean instantly - then you would with 99% have had forewarning. The wind doesn't change 20 knots instantly in smooth conditions. Or like I said before, it is just as likely as meeting Godzilla at FL380.

I do maybe 250-300 flights pr. year or up to 3000 flights in a decade. Will I make all 3000 flights is FL100, because of the odd chance I might experience a pressurization problem? Of course not, to me this is similar in likelihood.

Even if I had to descent, a quick call to ATC, "unable to maintain speed, require descent", may often be in time to avoid having to make a mayday call (pan call - urgent message- will get you nowhere, and does not allow you to deviate from your clearance - but that's a different story ). Remember, you do not need to make an emergency descent with 4000 fpm sink rate, a shallow descent to the non standard level 1000 below will with all likelihood be enough to regain your speed. Or maybe 1-200 feet will even be enough and you can do that on your own, without even bothering the ATC controller.

---

And now for something completely different (as Monty Python would say):

The Boeing performance Engineers say this:
“All right,” you say, “we'll just calculate the fuel flow at the speed for minimum drag. Minimum drag means minimum fuel flow, right?”

To whom are they talking? Who is this "you"? Clearly, it is someone who didn't do his ATPL:

Min thrust required = Max Range Cruise
Min Power required = Max Endurance

This is really basic ATPL knowledge and I really don't know why you bring Max Endurance into the picture. But I will follow your lead...

Boeing has given the pilot the best theoretical speed for staying in the air the longest time possible but practically it is rarely that

On what do you base this assumption?

What is "BEST SPEED"? I have never heard of "BEST SPEED" before, "best speed" for what?

I have a target speed on the holdings page with is "Max Endurance" or "Min Power Required" (at least ≈ , to avoid making it too complicated). If indeed you mean target holding speed from the FMC holdings page, it just shows how much thrust you mostly have available, since it will be just a few knots above the amber band. I wouldn't personally want to fly at that speed at high altitude though, since a 1-2 knots margin to the amber band is on the low side for me (yes I know I am being conservative and that I still have 1.3G margin to stick shaker).

...but the again, who holds at FL380?

[framer]

What is "suddenly"? If you mean instantly - then you would with 99% have had forewarning.

Not really instantly, I'm thinking more over a time frame of three or four seconds.

I do maybe 250-300 flights pr. year or up to 3000 flights in a decade. Will I make all 3000 flights is FL100, because of the odd chance I might experience a pressurization problem? Of course not, to me this is similar in likelihood.

Everything we do is commercially driven at the end of the day. If it had no economic impact then yes, you would.
What does it cost you to cruise at 10,000 ft? Lots
What does it cost you to fly within cooee of optimum? Nothing.

Even if I had to descent, a quick call to ATC, "unable to maintain speed, require descent", may often be in time to avoid having to make a mayday call (pan call - urgent message- will get you nowhere, and does not allow you to deviate from your clearance - but that's a different story ).

I will deviate from my clearance as I need to to fulfill my obligations. If nothing on the TCAS Pan will suffice. It is an urgent situation but there is no immediate threat to life - but that's a different conversation

To whom are they talking? Who is this "you"? Clearly, it is someone who didn't do his ATPL:

Clearly. The document is written largely in a conversational tone. It's quite well done really. It is aimed at pilots who have an ATPL. You should read it, I reckon you'd dig it.

On what do you base this assumption?

Practical observation. I explained it in my last post. The thrust levers work hard.

What is "BEST SPEED"? I have never heard of "BEST SPEED" before, "best speed" for what?

Bottom right hand corner of the HOLD page. It says BEST SPEED and gives a number. The FCOM says this about it

BEST SPEED

Displays computed best holding speed based on present altitude and conditions.

Note: May exceed maximum speed permitted by regulatory agency.

Does your FMC not have this?

...but the again, who holds at FL380?

I do on a regular basis. Believe me it's not by choice.
Righto, off for a two sector eleven hour day. Have fun.

[cosmo kramer]

A change of windspeed of 20 knots over 4 seconds, is what I would call instant. It would be foreseeable by the presence of at least light turbulence in the area you are flying. And no, if you descent without clearance and you are only calling pan-pan, you are breaking the rules. To deviate from any clearance you have to call mayday.

Bottom right hand corner of the HOLD page. It says BEST SPEED and gives a number.

Thanks for the pointer. Didn't pay much attention to that speed, it's the same speed as LSK1R (predicted target speed), if no manual entries are made.

So as you said yourself:

This prompted me to select holds in the cruise and compare the BEST SPEEDS for the NG, they too are significantly ( or slightly depending on what school you went to) slower than min drag.

So there you have it with your own conclusion above: Your margin to your "V drag too great for the thrust at hand to pull you out" is significant.


I actually wrote a longer reply to your practical holding observations, but deleted that again as it really doesn't have anything to do with the topic. I'll just leave it at saying: be careful about unfounded assumptions.

[HazelNuts39]

framer,

It's quite well done really. It is aimed at pilots who have an ATPL.

Caveat: It's about Jet Transports, doesn't discuss propellers.

[cosmo kramer]

Pictures for De Facto:

Beautiful day for flying:



And nice to pass over those white fluffy things...
(not the dangerous kind, but still more comfortable)



...in FL370, efficiently 1100 feet close to optimum, 10 knots less headwind compared to 350, and with 300 feet to spare... Max was 372 as climb was initiated, pictures taken a few mins later (when this thread sprung to mind).



...and a whooping 18 knots margin...



...actual weight 72 tons at the time (forgot to take a picture), still NOT thrust limited (26k engines). As can be seen from the holds page, with a 240 knots recommended holding speed, which is actually 1 knot below the amber band (was changing back and forth between 240 and 241, assume it's a rounding issue and it really wanted to be just at the top at the amber band):



Questions?

[misd-agin]

cosmos Kramer wrote -
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). We climbed at it was smooth at FL380.
...
After 15-20 mins the margin increased to about 15 knots (still flying close to the upper band giving me 13-14 knots to low speed band).

In 15-20 minutes your margin increased by 6 kts?

Looked at 737NG buffet charts. 20,000 lbs change(9000 kg) changes buffet speed by 18 kts. That's a lot longer than 15-20 minutes.

[framer]

Thanks for the photo's Cosmo.
I will ask a question.
What do you think the result would have been had you opened the speed window and selected either 240kts or the equivalent Mach number?
Cheers
edit: PS the reason for my question is to challenge the idea that you had an 18kt margin to operate in.

[cosmo kramer]

misd-agin, yes as far as I recall. The pictures are from a different day but show pretty much the same.
On the very first picture, in the post above, in the distant horizon is a long flat cloud. As we reached that I did another few pictures, this is the same cloud:



Didn't take a lot of minutes to get there, from the first photos. And look how fast the margin increased by 3-4 knots:




Framer, I think nothing would have happened. The speed would have been 240 knots, and with the natural variations it would temporarily be above 240, and temporarily below 240 (which would then be in the amber band). What do you think would happen?

[framer]

I think that if you reduced to cruise at M0.77 your N1 would have been lower, then if you slowed to M0.76 it would have had to increase to counter the extra drag, more again at M0.75 and by the time you got to M0.74 most likely you wouldn't have had the thrust available to maintain the speed and commence even a 5 degree bank angle turn.
Do you think that the top of the amber band means you can cruise there and have 40 degrees of bank angle to the stick shaker or initial buffet? I'm assuming you know better than that but thought it worth asking seeing as the FCOM makes that statement.