ahmetdouas
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Why does the 77W cruise so low

Fri Oct 21, 2016 6:20 am

Guys is it me or is the 777-300ER a real struggler for high altitude cruising? Isn't it's service ceiling 43,000 ft? Yet on long haul trips I barely see it go above 35,000 feet even on low fuel! I mean even the 747-400 cruises higher and it's 30 years old!
 
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 6:25 am

43000 feet is the maximum ceiling.
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 6:32 am

ahmetdouas wrote:
Guys is it me or is the 777-300ER a real struggler for high altitude cruising? Isn't it's service ceiling 43,000 ft? Yet on long haul trips I barely see it go above 35,000 feet even on low fuel! I mean even the 747-400 cruises higher and it's 30 years old!

A B744 usually cruises at what, FL360, FL370? I can't ever recall being on a B747 of any type cruising at higher than that. I'm sure that there are people who've been on them that cruised at FL380 or FL390 and the odd person here that hit the FL400 mark on a B747 but FL350 seems pretty normal to me.
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EK2
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 6:38 am

I've flown on quote a lot of 77W's and have only had a handful of flights at FL350 or 360 , most have got up to FL390 or 400.
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 6:59 am

It really all depends on the route, the fuel load, the passenger load and the winds. The FMS is going to compute the most efficient cruising altitude for any given situation. I've flown both 744's and 77W's on the same route many times (JFK-NRT and back) and haven't really noticed a difference in cruising altitude. Both start out in the low 30's and end up in the high 30's. Once or twice I've hit 40 in both types.
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 7:00 am

It doesn't. /thread
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 7:51 am

ahmetdouas wrote:
Guys is it me or is the 777-300ER a real struggler for high altitude cruising? Isn't it's service ceiling 43,000 ft? Yet on long haul trips I barely see it go above 35,000 feet even on low fuel! I mean even the 747-400 cruises higher and it's 30 years old!


The fact that the 744 is 30-years-old has absolutely nothing to do with its cruising altitude and a newer design does not necessarily cruise at higher altitudes.
 
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 8:23 am

There was a thread a few months ago similar to this. I remember reading that the 77W has relatively high wing loading which correlates to lower initial cruise altitudes than other wide body frames.
 
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 8:38 am

jpetekyxmd80 wrote:
It doesn't. /thread


Actually, it does.

The original wing has essentially been used on all models. Except there is over 100 Tonnes of MTOW difference between a -200A and a -300ER...

Now, the 300ER and 200LR do sport wingtip extensions, but they can't compensate for the extra weight being carried, especially in the case of the 77W.

As a result, a -200A will gladly cruise in the high 30s or low 40s all day long, but a heavy -300ER will barely be able to make 30,000ft as its initial cruise level.

Of course, what they lack in wing, they make up for in the engine department. A heavy 77W will not cruise very high, but it will climb there like a homesick angel, thanks to its massive power. On the other hand, A models (not too familiar with the 200ER) will struggle more to reach their higher cruise level.

It's a tale of an engine-limited aircraft vs. its wing-limited brother.
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 9:39 am

At the end of the day I think ATC has the final say. :)
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ahmetdouas
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 10:17 am

Yeah because the 77W always seems to cruise between 3-5000 feet below the 787 and A380 on the LHR-LAX route throughout the flight! Same route, so its a matter of aircraft feeling comfrotable. Highest I've seen a 77W on that route is 36,000 FT!!! As said before, even the 777-200ER goes up to 39 or so.
 
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 10:56 am

The 77W at MTOW will generally start out at FL290-300, and step climb up to FL360-370 if it's close to MZFW, of course the faster you fly, the lower the optimum altitude and the lower the cruising level.
 
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 12:38 pm

ACDC8 wrote:
ahmetdouas wrote:
Guys is it me or is the 777-300ER a real struggler for high altitude cruising? Isn't it's service ceiling 43,000 ft? Yet on long haul trips I barely see it go above 35,000 feet even on low fuel! I mean even the 747-400 cruises higher and it's 30 years old!

A B744 usually cruises at what, FL360, FL370? I can't ever recall being on a B747 of any type cruising at higher than that. I'm sure that there are people who've been on them that cruised at FL380 or FL390 and the odd person here that hit the FL400 mark on a B747 but FL350 seems pretty normal to me.


I remember in the late nineties being on an SAA 747-200 that happily cruised JNB-LHR at FL41 at well over 1000km/h ground speed (it was a very very empty plane)... Thrilling!
 
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 1:30 pm

ACDC8 wrote:
ahmetdouas wrote:
Guys is it me or is the 777-300ER a real struggler for high altitude cruising? Isn't it's service ceiling 43,000 ft? Yet on long haul trips I barely see it go above 35,000 feet even on low fuel! I mean even the 747-400 cruises higher and it's 30 years old!

A B744 usually cruises at what, FL360, FL370? I can't ever recall being on a B747 of any type cruising at higher than that. I'm sure that there are people who've been on them that cruised at FL380 or FL390 and the odd person here that hit the FL400 mark on a B747 but FL350 seems pretty normal to me.


I flew UA from LAX-ORD on a 744 and we for sure were at FL410 for most of that flight
 
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 3:37 pm

Not enough wing. You're observation is correct. Very capable aircraft obviously but it is stuck quite low until quite late in long sectors. The current 2nd gen 777 could have used that extra 12 feet that the X will have.
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zeke
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 4:59 pm

LY777 wrote:
43000 feet is the maximum ceiling.


Doesn't the oxygen mask in the cockpit states not for use above FL400 ?
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 6:26 pm

zeke wrote:
LY777 wrote:
43000 feet is the maximum ceiling.


Doesn't the oxygen mask in the cockpit states not for use above FL400 ?


There is no restriction on using the mask up to the aircrafts service ceiling, otherwise what's the point in having it? A few regulatory authorities (including FAA) state that when above FL410, an oxygen mask shall be used at all times by one crew member. I can vouch from personal experience you really don't want to wear that mask any longer than absolutely necessary. For this reason, it will be a cold day in hell before you see a commercial airliner from the states above 410 for any length of time.

As to the original question, in the two years I flew the 777 I never saw it get above FL360. On 12 hour sectors it would more often than not recommend an initial cruise level of FL290. One of the reasons has already been explained but also as far as I am aware some of it was down to the fan diameter and it performed better lower down.
 
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Re: Why does the 77W cruise so low

Fri Oct 21, 2016 9:15 pm

zeke wrote:
LY777 wrote:
43000 feet is the maximum ceiling.


Doesn't the oxygen mask in the cockpit states not for use above FL400 ?


Something around that - 400 or 410, I can't remember exactly but I know it's not 430!. The ceiling is 43,100 though - presumably the assumption is that even with an very rapid decompression the cabin altitude won't get above 40,000 for very long if at all!

In my experience, a heavy 77W normally starts out cruising around 280-310, ending up around 350-390 depending how close to MZFW we are. I've never been up to 400 in a 77W.
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njxc500
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Re: Why does the 77W cruise so low

Sat Oct 22, 2016 12:58 am

I saw 450 aboard a qf 744 doing LAX-SYD
 
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zeke
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Re: Why does the 77W cruise so low

Sat Oct 22, 2016 4:00 am

skyhawkmatthew wrote:
Something around that - 400 or 410, I can't remember exactly but I know it's not 430!.


I have read TSO to 40,000 ft on the 737, 777, and 787 mask, very similar to this

Image
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Re: Why does the 77W cruise so low

Sat Oct 22, 2016 4:31 am

sixtyseven wrote:
Not enough wing.


It has as much wing as it needs.

As Francoflier says:
Francoflier wrote:
The original wing has essentially been used on all models. Except there is over 100 Tonnes of MTOW difference between a -200A and a -300ER...


Boeing could have relofted the entire wing for the 77W. But that would have raised the cost per frame to an unacceptable amount. So instead they put big, honking engines on the airplane and made a trade-off: in exchange for a smaller wing and a lower cruise, they also got a lighter airframe (remember, wings have weight) with good payload and longer range than the then gold-standard 744.

A higher cruise altitude is not necessarily "better." It is less draggy, but it also constrains the airspeed and requires a bigger wing. The 77W is one of the most wildly successful widebody models Boeing has ever built. So obviously its many operators don't seem to think that the lower cruise altitude is a significant detractor.
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Re: Why does the 77W cruise so low

Sat Oct 22, 2016 3:40 pm

DocLightning wrote:
sixtyseven wrote:
Not enough wing.


It has as much wing as it needs.

As Francoflier says:
Francoflier wrote:
The original wing has essentially been used on all models. Except there is over 100 Tonnes of MTOW difference between a -200A and a -300ER...


Boeing could have relofted the entire wing for the 77W. But that would have raised the cost per frame to an unacceptable amount. So instead they put big, honking engines on the airplane and made a trade-off: in exchange for a smaller wing and a lower cruise, they also got a lighter airframe (remember, wings have weight) with good payload and longer range than the then gold-standard 744.

A higher cruise altitude is not necessarily "better." It is less draggy, but it also constrains the airspeed and requires a bigger wing. The 77W is one of the most wildly successful widebody models Boeing has ever built. So obviously its many operators don't seem to think that the lower cruise altitude is a significant detractor.


You seem to prove my statement with yours. Boeing realized the wing wasn't big enough so they slammed the biggest stoves they could find to get it into the air. I'm not sure if you took my statement as an attack on your favourite airplane or what have you, but I think it's a phenomenal airplane. Just needs more wing.

If higher cruise altitudes aren't always "better", just "less draggy" as you say, why then does the 787 cruise around the upper 30's and low 40's all the time and sports TREMENDOUS fuel economy? If the 77W could get up to those altitudes would you think it's customers would spend 13 hour legs in the low 30's for the most part? I think you know that answer.

What happened was Boeing saw what they needed, what people would pay for and this is what you have. Thats all. But don't for a second think it's optimal. It has the wing people paid for. But it would be even better if it had MORE. I don't think the X is going to have 12 more feet of wing just because it'd be nice to have.
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Re: Why does the 77W cruise so low

Sat Oct 22, 2016 5:02 pm

Operating higher doesn't automatically translate to more efficient. One of the many factors is the height of the Troposphere. Mach number is always related to temperature. If the temperature at altitude gets too low, the net result is that true airspeed and goundspeed can be greatly reduced as well. It can become more efficient to operate at a lower altitude for range efficiency. Another factor that sometimes limits altitude on long range operations is Ozone and radiation. These factors are a consideration in flight planning as well.
 
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Re: Why does the 77W cruise so low

Sat Oct 22, 2016 5:42 pm

sixtyseven wrote:
I'm not sure if you took my statement as an attack on your favourite airplane or what have you, but I think it's a phenomenal airplane. Just needs more wing.


The 744 is my favorite airplane. But with 697 units delivered (more than 744s) and a few more to go the operators disagree with you. The 77W has enough wing to do its job. Any more wing would have made a more expensive airplane.

If higher cruise altitudes aren't always "better", just "less draggy" as you say, why then does the 787 cruise around the upper 30's and low 40's all the time and sports TREMENDOUS fuel economy? If the 77W could get up to those altitudes would you think it's customers would spend 13 hour legs in the low 30's for the most part? I think you know that answer.


That isn't how it works. The optimal cruise altitude is the optimum. Not the highest. For the 77W the cost of increasing the wing size did not make up for the overall difference in fuel burn per ASM spread out over the expected frames for delivery.

And the 77W offered superb fuel economy for 2003. What it gave up in lower cruise altitude it gained in decreased unit cost and in improved CASM by using a -200 wingbox and wing for a -300 fuselage. In summary, if the 77W had a wing that was "too small" it would not have been the slam-dunk success that it was. Boeing simply chose a solution with a different set of benefits and trade-offs than their typical big-wing design. And Boeing isn't the only company to have done something like this. Airbus has chosen high wing loading for the A330 and A321

Now that Boeing is forced to re-loft the wing for the 77X, they have chosen a larger wing that will allow for a higher cruise altitude and also for a bigger plane with lower-thrust engines. But this was not a low-cost decision. The 777-9 lists for almost as much as an A380. There are always trade-offs and the plane that cruises higher is not always automatically better.
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Re: Why does the 77W cruise so low

Sat Oct 22, 2016 6:36 pm

DocLightning wrote:
The 777-9 lists for almost as much as an A380.


And given the new Boeing financial focus, other than for launch customers, will sell for more than the A380.
 
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Re: Why does the 77W cruise so low

Sat Oct 22, 2016 7:01 pm

Optimal for itself yes. If it had a bigger wing, it's optimal cruise altitudes would be higher. Fuel savings more. I'm not saying it's a bad airplane. But it could use more wing for the weight it hauls.

Clearly what Boeing saw was what it could provide compared to the competition at the time and what it had to work with. It's a tremendous airplane but sitting at 31,000 for five hours costs money. More than it would if it could go higher, earlier in the stage.
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Re: Why does the 77W cruise so low

Sat Oct 22, 2016 7:33 pm

[quote="sixtyseven"
Clearly what Boeing saw was what it could provide compared to the competition at the time and what it had to work with. It's a tremendous airplane but sitting at 31,000 for five hours costs money. More than it would if it could go higher, earlier in the stage.[/quote]

And less than fuel saved plus cost of a relofted wing.
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Re: Why does the 77W cruise so low

Sat Oct 22, 2016 11:24 pm

I've recently flown quite a few long-haul sectors on the 744, 772ER, and 332. Each flight leveled off fairly low for the first few hours (FL29 on the 744, low-30s on the others). On all of them, we eventually ended up in the high-30s, even a multi-hour segment at FL41 on an HA332.
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Re: Why does the 77W cruise so low

Sun Oct 23, 2016 12:26 am

A 777 Captain at EK that I know very well told me that FL370 is the optimal altitude for the 77W. He also said that 77W's flying Polar routes won't fly as high as they are capable of in order to keep the fuel in the tanks slightly warmer. They also fly at Mach .01 faster than the company's preferred Mach number (I forgot the number) for that type because that little bit of extra speed, again, combined with the lower altitude, helps keep the fuel temperature warm.
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Re: Why does the 77W cruise so low

Sun Oct 23, 2016 12:29 am

As a slightly nervous flyer, my main issue is that it can be stuck ploughing through weather that other planes on the same route can fly over, I remember leaving London for Singpaore on a nasty European winter's day and us being stuck at FL290 in some mucky stuff that an A380/B787 would have been nicely over. It's always frustrating being stuck in the layer when you know a few thousand fee above is nice clear air...
 
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Re: Why does the 77W cruise so low

Sun Oct 23, 2016 5:46 pm

DocLightning wrote:
[quote="sixtyseven"

And less than fuel saved plus cost of a relofted wing.


What on earth is this term "relofted" anyways lol? I've been in the business for 30 years and have never heard the term before.
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Re: Why does the 77W cruise so low

Sun Oct 23, 2016 8:24 pm

sixtyseven wrote:
DocLightning wrote:
[quote="sixtyseven"

And less than fuel saved plus cost of a relofted wing.


What on earth is this term "relofted" anyways lol? I've been in the business for 30 years and have never heard the term before.


Fairly common term in the design end of the business -- not so much if you're at the pointy end.

Lofting definition:

"Lofting is a drafting technique (sometimes using mathematical tables) whereby curved lines are generated, to be used in plans for streamlined objects such as aircraft and boats. The lines may be drawn on wood and the wood then cut for advanced woodworking."
 
sixtyseven
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Re: Why does the 77W cruise so low

Sun Oct 23, 2016 9:04 pm

Gotcha cheers. I figured it had to be an engineering term.
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Re: Why does the 77W cruise so low

Mon Oct 24, 2016 10:45 am

So what if it cruises lower than some other planes. The 77W is under winged. The A380 is over winged. Or maybe they both have the correct wing... Get over it people... the designers generally know more about this stuff than any of us ever will. Commercial jets cruise anywhere from 20000ft to 40000ft depending on their mission. All Boeing and Airbus commercial jets fall into this regime.
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Re: Why does the 77W cruise so low

Mon Oct 24, 2016 5:44 pm

ACDC8 wrote:
ahmetdouas wrote:
Guys is it me or is the 777-300ER a real struggler for high altitude cruising? Isn't it's service ceiling 43,000 ft? Yet on long haul trips I barely see it go above 35,000 feet even on low fuel! I mean even the 747-400 cruises higher and it's 30 years old!

A B744 usually cruises at what, FL360, FL370? I can't ever recall being on a B747 of any type cruising at higher than that. I'm sure that there are people who've been on them that cruised at FL380 or FL390 and the odd person here that hit the FL400 mark on a B747 but FL350 seems pretty normal to me.

I was on a 744 at FL390 just this morning from SEA-FRA.
 
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Re: Why does the 77W cruise so low

Fri Oct 28, 2016 1:57 pm

Everything in aircraft design is a compromise. The 777 started out as a much lighter, smaller plane than the 77W, and I do not know if the original designers foresaw what it would become (I suspect not, because in 1990 the thought of a 115,000 lb thrust engine was pretty much in the science fiction category.) So I suspect the designers designed the wing for the plane they were building then (with the weight bump for the 77E and 773 probably figured in) but when it came time for the 77W they were outside of the original parameters. Seeing as how the most expensive part of an existing plane to change is the wing, the 77W designers worked around the existing wing to achieve the desired result. Would a bigger wing have gained more? Highly likely, but apparently not enough to make the extra cost worthwhile. What they did come up with was such a smashing success that for over a decade it has been THE airliner for long-haul routes, its only competitor being the A380.

Here's the interesting part. While the 77W may have suffered because the original designers did not put a big enough wing on it for what it eventually became, the A380 has suffered from the fact that the original designers envisioned a stretch that never came. From all I have read the A380 really has too big a wing, and has suffered an efficiency hit because of it. The original wing was really designed for the A389/388F, and hence is bigger than the A388 needs. It suffers because of excess weight and too low aspect ratio, and hence the new planes coming out (787, A350, 778/779) are able to equal or exceed its CASM while being smaller. So the moral of the story is that while hindsight is 20/20, foresight rarely is.
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Re: Why does the 77W cruise so low

Fri Oct 28, 2016 10:16 pm

SEPilot wrote:
Here's the interesting part. While the 77W may have suffered because the original designers did not put a big enough wing on it for what it eventually became, the A380 has suffered from the fact that the original designers envisioned a stretch that never came. From all I have read the A380 really has too big a wing, and has suffered an efficiency hit because of it. The original wing was really designed for the A389/388F, and hence is bigger than the A388 needs. It suffers because of excess weight and too low aspect ratio, and hence the new planes coming out (787, A350, 778/779) are able to equal or exceed its CASM while being smaller. So the moral of the story is that while hindsight is 20/20, foresight rarely is.


Indeed. I would bet you that a lightly-loaded A380 from a purely aerodynamic standpoint would be able to exceed FL 450. Of course, the engines might not work and it would violate so many regulations I can't even count them and there might be issues with cabin pressurization even if the engines and APU didn't quit. But I'd bet that the wing is big enough to allow it.
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Re: Why does the 77W cruise so low

Sat Oct 29, 2016 2:05 am

DocLightning wrote:

Indeed. I would bet you that a lightly-loaded A380 from a purely aerodynamic standpoint would be able to exceed FL 450. Of course, the engines might not work and it would violate so many regulations I can't even count them and there might be issues with cabin pressurization even if the engines and APU didn't quit. But I'd bet that the wing is big enough to allow it.


The engines would work and so would the pressurization (although the cabin would be higher than normal). The APU might work there but might have to be started at a lower altitude.
 
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Re: Why does the 77W cruise so low

Sat Oct 29, 2016 9:55 am

DocLightning wrote:
Indeed. I would bet you that a lightly-loaded A380 from a purely aerodynamic standpoint would be able to exceed FL 450.


I haven't read this thread in detail but a quite search for "lift coefficient" comes up blank. That's really the simple explanation of cruise FL for a plane that isn't engine-limited. Most airliners cruise most efficiently at Cl~.50-.55. Go above that and you have problems with buffet and/or wave drag rise. This is why the A380 typically cruises higher than other airliners.

A higher cruise FL generally means a higher L/D. As Ferpe pointed out in one of his old threads, however, the effect on L/D is somewhat small because induced drag actually increases linearly with the inverse of air pressure, whereas every other form of drag decreases (with some caveats) linearly air pressure. Per Ferpe, the bigger altitude effect is on the engines, which have greater thermal efficiency at higher, colder altitudes. This effect drops off after reaching the stratosphere at ~FL 37 in standard atmospheric conditions.

The 77W surely would have benefited from a longer, bigger wing that allowed it to fly higher and mitigated induced drag - that's the whole idea behind the 777-9. Simply adding wing area, however, probably would not have helped with anything but takeoff performance. Even there, the 77W is constrained more by V2 OEI than by its ability to reach rotation speed in a given length. As an equally-powered quad it would have amazing shortfield performance. So I don't think there's much point in critiquing its cruise FL.
 
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Re: Why does the 77W cruise so low

Sat Oct 29, 2016 2:58 pm

zeke wrote:
skyhawkmatthew wrote:
Something around that - 400 or 410, I can't remember exactly but I know it's not 430!.


I have read TSO to 40,000 ft on the 737, 777, and 787 mask, very similar to this

Image


I've also noticed that while I was cruising up at FL430 on a lightly loaded plane. My take on it is that the Diluter Demand function is only good up to FL400, but the 100% setting is not limited. Thus my company compensates for that by requiring us to always set the mask to the 100% setting rather than the "Norm" setting, so we don't use the diluter functionality.

In any event everything in aviation essentially boils down to balancing risk vs. rewards, and not about eliminating risk altogether. Will we have a rapid depress on this flight? During the limited time up at that high altitude? If so how long to get down below FL400? Statistically it's staggeringly improbable that it will happen at that unfortunate moment vs. all the much longer times spent at lower altitudes, so I suppose having such a limit on the mask is an "acceptable risk" given the limited time up at those extreme altitudes.
 
ahmetdouas
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Re: Why does the 77W cruise so low

Mon Nov 07, 2016 5:29 pm

I flew a 77W twice over the last week LHR-LAX-LHR. Outbound started at FL 300, then FL320, then FL340, then FL 360. Return FL 310, 330, 350, 370. Both the A388 and B787 do better than this! I remember on an A388 starting altitude was 360 before going up to 400 later on!
 
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DocLightning
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Re: Why does the 77W cruise so low

Mon Nov 07, 2016 8:19 pm

ahmetdouas wrote:
I flew a 77W twice over the last week LHR-LAX-LHR. Outbound started at FL 300, then FL320, then FL340, then FL 360. Return FL 310, 330, 350, 370. Both the A388 and B787 do better than this! I remember on an A388 starting altitude was 360 before going up to 400 later on!


The 788 and 789 have a decent wing for their fuselages as the 787 wing area was optimized around the 789. The 78J will probably have similar cruise profiles to the 77W. The A388 has too much wing as it was designed with the A389 and A38J in mind. In fact, I think the A388 could climb so high that you'd lose the engines before the wing stopped being able to support the frame.

The 777 wing was optimized around the -200, so the 77W has a relatively small wing, leading to a lower cruising altitude. In spite of that, the 77W was wildly successful and offered unparalleled efficiency and performance. It replaced the 744 at many major international carriers and offered a cargo UPGRADE over the 744. It had no valid competitor until the A359 just this last year.

The 77X wing will be optimized for the aircraft's size, so expect a higher initial cruise. But with that I expect more disappointing sales because the aircraft will be a bit overcapable for most airlines' needs.
-Doc Lightning-

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Revelation
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Re: Why does the 77W cruise so low

Tue Nov 08, 2016 12:54 am

DocLightning wrote:
The 788 and 789 have a decent wing for their fuselages as the 787 wing area was optimized around the 789.


The 787-9 originally was supposed to have a larger wing than the 787-8 but it was found that the benefits of having a common wing size and less weight than the larger wing meant that they went with the same sized wing for both.

https://www.flightglobal.com/news/artic ... ng-336055/ tells the story.
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Viscount724
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Re: Why does the 77W cruise so low

Tue Nov 08, 2016 4:49 am

DocLightning wrote:
SEPilot wrote:
Here's the interesting part. While the 77W may have suffered because the original designers did not put a big enough wing on it for what it eventually became, the A380 has suffered from the fact that the original designers envisioned a stretch that never came. From all I have read the A380 really has too big a wing, and has suffered an efficiency hit because of it. The original wing was really designed for the A389/388F, and hence is bigger than the A388 needs. It suffers because of excess weight and too low aspect ratio, and hence the new planes coming out (787, A350, 778/779) are able to equal or exceed its CASM while being smaller. So the moral of the story is that while hindsight is 20/20, foresight rarely is.


Indeed. I would bet you that a lightly-loaded A380 from a purely aerodynamic standpoint would be able to exceed FL 450. Of course, the engines might not work and it would violate so many regulations I can't even count them and there might be issues with cabin pressurization even if the engines and APU didn't quit. But I'd bet that the wing is big enough to allow it.


The Canadian Pacific DC-8-43 (Rolls-Royce Conway engines) that went slightly supersonic on a pre-delivery test flight in 1961, did it on a dive from 52,000 ft., although the DC-8's maximum certified operating altitude is 42,000 ft. (40,000 ft. if fewer than 3 turbocompressors are operating). I flew on dozens of DC-8's of almost all models (including that "supersonic DC-8" several times when I worked for CP) and never recall reaching 40,000 ft.
http://www.airspacemag.com/history-of-f ... -27846699/
 
OldAeroGuy
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Re: Why does the 77W cruise so low

Tue Nov 08, 2016 4:34 pm

[quote="Matt6461"]

I haven't read this thread in detail but a quite search for "lift coefficient" comes up blank. That's really the simple explanation of cruise FL for a plane that isn't engine-limited. Most airliners cruise most efficiently at Cl~.50-.55. Go above that and you have problems with buffet and/or wave drag rise. This is why the A380 typically cruises higher than other airliners.

A higher cruise FL generally means a higher L/D. As Ferpe pointed out in one of his old threads, however, the effect on L/D is somewhat small because induced drag actually increases linearly with the inverse of air pressure, whereas every other form of drag decreases (with some caveats) linearly air pressure. Per Ferpe, the bigger altitude effect is on the engines, which have greater thermal efficiency at higher, colder altitudes. This effect drops off after reaching the stratosphere at ~FL 37 in standard atmospheric conditions.

The 77W surely would have benefited from a longer, bigger wing that allowed it to fly higher and mitigated induced drag - that's the whole idea behind the 777-9. Simply adding wing area, however, probably would not have helped with anything but takeoff performance. [/quote

Correct on all counts Matt except for the takeoff comments. More 77W wing area would mainly help takeoff by lowering Vlof, relieving the tire speed limits that sometimes reduce TOW at high, hot airports.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
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Matt6461
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Re: Why does the 77W cruise so low

Fri Nov 11, 2016 10:54 am

OldAeroGuy wrote:
More 77W wing area would mainly help takeoff by lowering Vlof, relieving the tire speed limits that sometimes reduce TOW at high, hot airports.


Interesting, hadn't thought of that constraint. I guess that makes sense that the wheels must have some maximum speed. Probably this would be very relevant for hot/high airports that attempt to compensate for conditions by supplying a super-long runway. Denver comes to mind. A highly-loaded wing just wouldn't be able to use all that space due to the wheel constraint, not an aero/engine constraint. Is that about right? Thanks.
 
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Re: Why does the 77W cruise so low

Fri Nov 11, 2016 2:50 pm

Matt6461 wrote:
OldAeroGuy wrote:
More 77W wing area would mainly help takeoff by lowering Vlof, relieving the tire speed limits that sometimes reduce TOW at high, hot airports.


Interesting, hadn't thought of that constraint. I guess that makes sense that the wheels must have some maximum speed. Probably this would be very relevant for hot/high airports that attempt to compensate for conditions by supplying a super-long runway. Denver comes to mind. A highly-loaded wing just wouldn't be able to use all that space due to the wheel constraint, not an aero/engine constraint. Is that about right? Thanks.


That's right! Tyre speed is one of the major limiting factors on 77W operations out of JNB.
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OldAeroGuy
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Re: Why does the 77W cruise so low

Fri Nov 11, 2016 6:13 pm

Matt6461 wrote:
OldAeroGuy wrote:
More 77W wing area would mainly help takeoff by lowering Vlof, relieving the tire speed limits that sometimes reduce TOW at high, hot airports.


Interesting, hadn't thought of that constraint. I guess that makes sense that the wheels must have some maximum speed. Probably this would be very relevant for hot/high airports that attempt to compensate for conditions by supplying a super-long runway. Denver comes to mind. A highly-loaded wing just wouldn't be able to use all that space due to the wheel constraint, not an aero/engine constraint. Is that about right? Thanks.


Yes, that's correct. Tires typically rated to 235mph. Wing loading and flap setting are the ruling parameters impacting Vlof.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
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Re: Why does the 77W cruise so low

Sat Nov 12, 2016 6:32 pm

Viscount724 wrote:
The Canadian Pacific DC-8-43 (Rolls-Royce Conway engines) that went slightly supersonic on a pre-delivery test flight in 1961, did it on a dive from 52,000 ft., although the DC-8's maximum certified operating altitude is 42,000 ft. (40,000 ft. if fewer than 3 turbocompressors are operating). I flew on dozens of DC-8's of almost all models (including that "supersonic DC-8" several times when I worked for CP) and never recall reaching 40,000 ft.
http://www.airspacemag.com/history-of-f ... -27846699/


Oh, now that's interesting. If the old DC-8 could do it, I'd bet an A380 could do it. Airbus would have to override every electronic safety limit in the airplane, but I'd bet they could do it if they really wanted to for some strange reason.
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Re: Why does the 77W cruise so low

Sun Nov 13, 2016 2:20 am

DocLightning wrote:
Viscount724 wrote:
The Canadian Pacific DC-8-43 (Rolls-Royce Conway engines) that went slightly supersonic on a pre-delivery test flight in 1961, did it on a dive from 52,000 ft., although the DC-8's maximum certified operating altitude is 42,000 ft. (40,000 ft. if fewer than 3 turbocompressors are operating). I flew on dozens of DC-8's of almost all models (including that "supersonic DC-8" several times when I worked for CP) and never recall reaching 40,000 ft.
http://www.airspacemag.com/history-of-f ... -27846699/


Oh, now that's interesting. If the old DC-8 could do it, I'd bet an A380 could do it. Airbus would have to override every electronic safety limit in the airplane, but I'd bet they could do it if they really wanted to for some strange reason.


I dunno. You'd have to be in Direct Law, which is actually not that hard to get to "activate". All you need to do is switch off all three PRIM computers. Done it in the 330 sim (direct law, not supersonic). It makes for "finicky" flying to put it mildly. I'm assuming a 60s jet is much more naturally stable since it was designed for flight without electronic augmentation. My feeling is that modern super-optimized aerodynamic design would make a modern Airbus trickier to fly in very unusual regimes like very high transonic and supersonic compared to a DC-8. I can't think that it would be easily controllable, and the relevant computed data for such regimes probably doesn't exist.

I wouldn't recommend turning the PRIMs off in the real plane unless you're an Airbus test pilot and have planned your flight profile really carefully... ;)
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