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 and, Minimum Thrust Required does not equal Max Range Cruise ......what was min drag speed? *edited |
Do not forget Econ with cost index 0 still corrects for wind. (+, not -).
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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 :) |
Originally Posted by Cosmo Kramer
So, which speed would give you the max range then?
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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). 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 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. 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. 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. 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. 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. |
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 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. As you quoted Normal flight (faster than L/D max) is inherently stable with respect to speed. any disturbance causing a decrease in airspeed will result in a further decrease in airspeed 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. 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). |
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... So far you have now admitted, that everything I have posted so far is correct. 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. |
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 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. 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. At the end of the day I think you are simply comfortable with lower margins than me. |
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. 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. 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 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) I wish you thirty years of safe cruising Cosmo. Thanks for the conversation. I make a mental note of what Min drag is and 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. |
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? 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. 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. 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. |
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) 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 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 :E ). 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?” 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 :) 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? :hmm: |
What is "suddenly"? If you mean instantly - then you would with 99% have had forewarning. 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. 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 ). To whom are they talking? Who is this "you"? Clearly, it is someone who didn't do his ATPL: On what do you base this assumption? What is "BEST SPEED"? I have never heard of "BEST SPEED" before, "best speed" for what? BEST SPEED Displays computed best holding speed based on present altitude and conditions. Note: May exceed maximum speed permitted by regulatory agency. ...but the again, who holds at FL380? Righto, off for a two sector eleven hour day. Have fun. |
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. 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. 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. |
framer,
It's quite well done really. It is aimed at pilots who have an ATPL. |
Pictures for De Facto:
Beautiful day for flying: http://s23.postimg.org/ebmv7gqcb/IMAG0146.jpg And nice to pass over those white fluffy things... (not the dangerous kind, but still more comfortable) http://s23.postimg.org/nuwk0xduj/IMAG0148.jpg ...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). http://s9.postimg.org/soqbuyuj3/IMAG0144.jpg ...and a whooping 18 knots margin... :eek: http://s23.postimg.org/p3kfmk5sr/IMAG0143.jpg ...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): http://s23.postimg.org/wyzkf44t7/IMAG0149.jpg Questions? :ok: |
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). 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. |
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. |
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: http://s24.postimg.org/btdca8jpx/IMAG0150.jpg 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: http://s17.postimg.org/4p7xpyg9r/IMAG0151.jpg 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? |
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. |
Well with the thrust you are wrong. And the proof lies in the holding speed. Of course there would be thrust enough to maintain 240 knots, otherwise the FMC wouldn't display it as a holding speed. If I would fly that speed or not is another story, the point is that if flying e.g. .785, like in the pictures, I have plenty of time to react and increase thrust if the speed should decay for one reason or the other. Even if I had bad luck and it decayed to 240, I would still have enough thrust to pull me back.
This goes for this particular instance, other instances may be thrust limited (like maybe with a standard + 15 degs temperature, in the case of the pics it was pretty close to standard). I don't fly with 40 degs bank at high altitude, max 15 degs as pr. Boeing recommendation. I am sure you have a copy of the FCTM too, so there is really no point for me to quote what the flaps up amber band means, when you can have a look yourself. |
Pictures for De Facto: |
Very demanding! ... I'll keep the camera ready next time, it's not like it's every day there is an operational benefit in climbing as soon as the max hit the next level. ;)
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Your operation must be much better about your actual weight than ours. If we were showing a 300' difference between cruise FL and max FL, there would only be a few knots between the amber bands.
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Im sure ull make an operation benefit exception for me:p
Thanks. |
Of course nothing would happen. And if it would scare some people, change your cruise CG to 25%.
For the ones that never get scared, change it to all the way fwd, now you are flying in the yellow band, one way or the other. But apart from that, no, nothing happens..If you can get there in the first place, you can stay there.. Again, how exactly is your weight today? How correct is your load sheet? And now you have your engine degradation of a few percent. Your flight plan takes it into consideration for fuel calculation, but your FMC has still the same old database. This database determines your so called Cruise Trust Limit. All this leads me to select MCT as a limit, when I deem appropriate, as Pilot In Command. And that was this topic all about. |
I think that my statement of
you wouldn't have had the thrust available to maintain the speed and commence even a 5 degree bank angle turn Cosmo, you said I don't fly with 40 degs bank at high altitude, max 15 degs as pr. Boeing recommendation. If we look at what Boeing use to determine the max altitude in any given situation and then apply it to your photo it might help strengthen my argument. 1/ The altitude for 1.3 g margin to initial buffet. In the picture this speed is 17 or 18 kts slower than the speed you are cruising at and is not limiting you, it is an aerodynamic consideration and not a thrust consideration. 2/The service ceiling ( altitude for 100 fpm climb rate at max climb thrust) This is a thrust consideration and may be limiting you but maybe not. 3/ Max cruise thrust limit ( altitude for 0 fpm climb capability at cruise thrust) This is a thrust consideration and is sometimes lower than the service ceiling limit and may have been limiting you. 4/ normal accel to initial buffet at optimum altitude + 2000 feet. An aerodynamic consideration and was not limiting you, if it was the selected speed would have had you sitting on the bottom of the upper amber band. 5/bank angle to max cruise thrust at opt alt + 2000 feet, ISA+15 Another thrust consideration and what I think was most likely the limiting consideration. Your opt was 359, so why wasn't the max 379? Because when a bank angle limit of 15 degrees was applied to 379 it failed to meet the requirement ( maybe the thrust limit was reached at 10 degrees) so the calculation s were done to where the aircraft could comply and your max was calculated at 373 where a 15 degree bank could be sustained with the thrust available. If you slowed to a speed where drag was less, the bank angle capability would have increased, but as you slow beyond that the ability to sustain the minimum required bank angle would have diminished. I maintain that at 240kts it would have been less than the 15 degrees required. What we really need is someone with sim access to go and do it and report back to us. Another way of looking at it: Yesterday in the cruise I determined the max for three different speeds .76 .77 .79 ( LRC) The max altitudes were FL385 FL 386 FL382 So if I had a great desire to fly at FL 386 I would have to cruise at 0.77, if I then changed my speed to 0.76 I would be 100ft higher than the max altitude calculated for the new speed. Which limit am I not complying with? Who knows but at my new speed I no longer have the margin of one of the 5 limits written above. If I used 0.77 to climb up there and then accelerated to 0.79 I would be 400ft above where one of those limits kicked in. |
If I used 0.77 to climb up there and then accelerated to 0.79 I would be 400ft above where one of those limits kicked in. Which limit am I not complying with? Who knows I know: you would be maneuver margin limited, but by the high speed band. I actually did exactly what you did to comfort my FO that we had plenty of margin (some are worried, because other captains tells them they alway need 600 feet or more to climb, I try to tell them exactly what we have been discussing now for 109 posts). I selected cost index back to 4 and the FMC showed 376 as max. With 55 it was the 373, that is show in the pictures (actually 372 before the climb). The highest max you get with ECON cost index 0. Had I put in .81 as speed, it would probably have shown max as 350ish. So now: Do you still think I was thrust limited? Am I cheating? I quote myself from post 54: 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". So how do I know if I have 10 knot or not? Simple, use the FMC as a tool instead of being tooled by the FMC: Hit ALT HOLD, put in desired level in the FMC and execute. Put in 200 knots. FMC now tells you e.g. MIN SPEED 242 KNOTS. Put in 300 knots. FMC now tells you e.g. MAX SPEED 256 KNOTS. So in this example I know the range between low and high speed buffet margin (with 40 degs bank) is 14 knots, completely independent from what I will actually select as cruise speed when I get to that level. If the margin is around 10 knots or more and it is desirable to climb, I climb :ok: If you were to fly an old machine with conventional instruments and no speed tape, would you be comfortable climbing if I told you your stall speed was 200 knots, your MMO was 270 and you would have enough thrust to keep you flying? Probably... I maintain that at 240kts it would have been less than the 15 degrees required. This is again what I mean, there are so many things you can do, and so many margins on margins that adding your own ones on top makes no sense, when it is not justified. I quote myself from post 68: 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. And like latetonite, I select CON when I need to. And I do so routinely due to the way I operate the aircraft (de facto's objection against routinely, was what got me involved in this thread, although it is completely in line with the recommendations from Boeing). In fact, I always select CON with a <15 knots window. And that was this topic all about. |
C-K, ref you comment on my post 56,
Hobo, are you referring to 737 classic? If so, then I agree. It is quite uncomfortable if going into the high speed buffet band, |
I think that you are correct Cosmo.
I had another look at your photos and a read of the books and I now concede that you were most likely limited by the high speed buffet margin. My mistake was in thinking that if you were, your selected speed would have been coincident with the bottom of the upper amber band which it probably would have been if you had climbed the extra 300ft to max. My apologies. A question for you for my own learning; If you were at max altitude and the selected speed was below the upper amber band, how would you know which of the three thrust based limitations were ' active' so to speak? The holding speed is calculated that you are actually able to do turns in the holding. And like latetonite, I select CON when I need to. And I do so routinely due to the way I operate the aircraft (de facto's objection against routinely, was what got me involved in this thread, although it is completely in line with the recommendations from Boeing). |
How can you have min fuel flow at a speed below min drag?
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thats the md80.
cant fly that high in imc where youll be using the eai. performance penalty is significant. drop 2000' or even 4000' until clear. |
Derfred,
because TSFC increases with speed (explained in Boeing JTPM). |
If you were at max altitude and the selected speed was below the upper amber band, how would you know which of the three thrust based limitations were ' active' so to speak? can you provide a reference for that because the JTPM written by Boeing has lead me to believe that BEST SPEED is nothing more than the speed at which the engines would have the minimum rate of fuel flow and can be a speed well below min drag. Seeing as Max alt is sometimes limited by the ability of the engines to power you through a 15 degree banked turn I can't see how that speed would allow 15 degrees with the extra drag of being below min drag. I have no argument with this practice but can see why pilots coming off other Boeings might, in some Max Con is only to be used in emergencies. If the Climb N1 and the Max Con N1 are the same there is no restriction and that applies to the NG. MCT = Max Continuous Thrust. You can fly continuously all day with that setting if you like. Max takeoff thrust is more of an "emergency" thrust setting, as you are limited to 5 or 10 mins. |
How can you have min fuel flow at a speed below min drag? For that reason the lowest fuel flow does not occur at the min drag/thrust. It occurs at the lowest power required. Power = force x (distance/time) = force x speed = Drag x TAS. At min drag speed (Max Range), your True Airspeed is higher than at min power required (Max Endurance). Hence: Lower TAS x higher drag < Higher TAS x lower drag You are fighting a higher force, but you are doing it at a slower rate. Rate of fuel flow is therefore lower. You are not getting very far (lower speed = lower distance covered over time), great for holding, but not so great for cruise. Hope this helps refreshing what is basic (European) ATPL knowledge. It has nothing to do with TSFC - that only modifies the curves a bit, but the basic physics remain the foundation. I give again the link I gave before, which show the basic curves for power and thrust required. And yes basic physics applies to all aircrafts, no matter if they have propellers, jet engines, straight or swept wings, obviously the real curves may not be so nice and linear because we do not live in a perfect world, anyway: Maximum Endurance, Maximum Range, and Optimum Cruise Speeds |
max endurance speed for jets
derfred & cosmo cramer
Have a look at the lower graph in an earlier post. I give again the link I gave before, which show the basic curves for power and thrust required. |
It doesn't matter if the thrust is created by a propeller or a jet or a turbofan (with the fan being a propeller too). Those graphs, to which I provided a link twice, shows the aerodynamical most efficient speeds.
Why TSFC is relevant is because a jet doesn't run efficiently at the minimum power required. So it's fuel consumption is actually higher at this lower power setting. Hence, the actual speed for max endurance is slightly higher than the aerodynamical one... A small technicality, that doesn't render the basic principles of physics invalid. It has nothing to do with TSFC - that only modifies the curves a bit, but the basic physics remain the foundation. |
cosmo cramer,
Quoting from your source: Maximum Endurance (...) Since the fuel flow is proportional to the power required, the fuel flow will be minimized at the point where the power required is a minimum. Both are simplifying assumptions, as shown for the PW jet engine in the graphs I referred to. You are right about two things though: the drag characteristics of a given airframe are not affected by the means of propulsion, and the fuel efficiency of a fan jet engine is somewhere between a straight jet and a turboprop. |
[Kramer - thread drift, but why don't you think a PAN allows you to deviate from a clearance? PAN = declaration of emergency = do whatever you like]
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Pan pan is a prefix for an urgent message, nothing more. It was just discussed in this thread too:
http://www.pprune.org/questions/5199...tuation-3.html |
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