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- Thread starter aquajet
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aquajet said:
I think this rule applies mainly to piston engines, which seriously degrade in power output with higher altitude. I'm not entirely certain, but I believe that turbine engines don't experience such a big drop-off in performance in thinner air.
Jets also suffer from this problem, because they tend to be much larger and heavier. For example, the Airbus A320 is most fuel efficient at 37,000AGL despite having a service ceiling of 39,000 feet. It's a similar story for many other turbofan aircraft, but I don't know any other numbers off the top of my head. Stacking will also affect assigned flying altitudes, meaning that some planes will occasionally be asked to fly higher or lower to avoid any chance of collision.IJ Reilly said:
I don't get why it is so f*****g hard for people to turn their phones and iPods off. I fly almost every other week and I can't tell you how many times my flight has been delayed or stuck at the gate because of electronic problems. I was on a flight and some ass decided he was going to talk on his phone for the first half of the flight (Phoenix to Burbank). Every time the flight attendant walked by he would hide the phone. About 50 miles outside of Burbank, CA the pilot informs us we are going back to Phoenix because the navigation system stopped working. I am convinced his phone screwed up the navigation system. Why, I have been on the same flight several other times where someone decided they were going to talk on their phone and we had electronic problems.
Oh when returned to Phoenix I finally turned around and took the guys phone away from him.
Oh when returned to Phoenix I finally turned around and took the guys phone away from him.
matticus008 said:
I'm speaking strictly of engine power output. Piston engines suffer greatly at increased altitude due to the thinner air and the need to lean the fuel mixture to compensate. The result is sharply lowered horsepower. I believe that turbine engines don't have as much of an altitude penalty because they produce power in a completely different way.
Right, but the original point posted by aquajet is that maximum fuel efficiency is not always at the top of the service ceiling, and in fact rarely is this the case. Thinner air results in less thrust potential in a turbofan engine because of the lower oxygen content--combustion is combustion. The effect is more pronounced on piston engines because of their much lower power output, but the gist remains the same: higher is only better to a certain point, and there's a sweet spot for every weight and engine power combination.IJ Reilly said:
I think maybe we may be talk about different points.
Generally this is true.skoker said:
However, there are factors to consider such as the additional fuel used to climb to altitude, winds aloft, aircraft type, aircraft loading (GW), duration of the flight, etc. Bottom line, it can be more economical to fly at a lower altitude than the AC is capable of flying.
So while your comment is generally true, it depends on the situation.
sushi said:
It's a two sided story. If you've got the jet stream flowing in your favor, you could take 'er up to 29,000, set engines to idle, work the trim a bit and fly on little to no fuel at all.
Of course, the rule is different for different AC types. A Dash-8 or a EMB145 will be way different than a 737 or A319 or a 777 for that matter.
IJ Reilly said:
Not at all. Several airlines (such as B6, who's A320 fleet can't do transcons without a fuel stop) as their pilots to cruise like that and 'float' for like a half hour or so per flight if conditions are favorable. It saves on fuel and saves time too if you're not having to stop for fuel halfway cross-country.
Wait a sec while I stuff my eyeballs back into my head. Okay, now... I think someone is confusing ground speed with airspeed. Ground speed = airspeed ± the wind vector. The only aircraft that can float are airships and helicopters. All the others need a minimum amount of airspeed, or they will stall, aka, plummet. Some aircraft stall more gracefully than others, but in all cases a stall involves a sharp pitching downwards of the nose, a condition which persists until minimum airspeed is obtained, and the nose can be raised, at which point, if thrust is not applied, another stall will ensue. And so on, until....
Have you guys ever thought of this?
What happens when an ETOPS plane has a double engine-out at 39,000 ft over the atlantic? Does it just fall into the water? NO! Those planes (assuming that one of the engines can be re-turned over and the APU is still operable) can fly for three hours at just a little less than cruise altitude with a proper cockpit config (pitch, trim, flaps in limited cases)
The same idea holds true for what I'm saying, you can lower your engines to near idle if you've got a proper jetstream config and ride it out at altitude for half-hour chunks.
What happens when an ETOPS plane has a double engine-out at 39,000 ft over the atlantic? Does it just fall into the water? NO! Those planes (assuming that one of the engines can be re-turned over and the APU is still operable) can fly for three hours at just a little less than cruise altitude with a proper cockpit config (pitch, trim, flaps in limited cases)
The same idea holds true for what I'm saying, you can lower your engines to near idle if you've got a proper jetstream config and ride it out at altitude for half-hour chunks.
skoker said:
If a twin has a double flame-out over the Atlantic, and the pilots aren't able to relight, then you've got probably about 30 mins of gliding time provided the aircraft is at cruise altitude. If the pilots aren't able to reach an airport within that time, then a water landing will be the result.
A jet airliner can fly without engines, but the only way it can maintain airspeed is by pointing the nose towards the ground. The laws of physics are at work here.
skoker said:
You're confusing airspeed with ground speed.
You must have propulsion in order to maintain airspeed. Without it, you must point the nose towards the ground in order to maintain airspeed. Your situation above is the equivalent of a free lunch. That's not possible.
skoker said:
Let's think of a hypothetical situation for a moment. Let's pretend a 737 is trying to take off in a hurricane. There's a continuous wind speed of 130 knots in the exact direction of the runway. Point the 737 down the runway, and we're looking at a 130 knot tailwind. Let's also pretend that Vr (rotation speed) for our 737 is 130 knots. Can this tailwind push our 737 down the runway and make it take off? No, it can't, and your situation described above is basically the equivalent of saying a 130 knot tailwind can push a 737 down the runway and make it take off. The reason why this can't happen is because we're using the ground as our reference point. Ground speed means nothing to the flying machine.
Now, let's say we throttle the 737 up to take off thrust. We begin our takeoff roll, and at 130 knots ground speed, the indicated airspeed in our cockpit will be zero. An airplane is a flying machine; the only speed it cares about is the speed of the wind across its wings. So, while the airplane is traveling at 130 knots ground speed, we still need an additional 130 knots ground speed in order for our indicated airspeed to equal 130 knots, which is required for rotation. By the time the airplane is traveling 260 knots down the runway, our indicated airspeed will equal 130 knots, and we can rotate the aircraft and be on our way.
Of course, I wouldn't want to actually try to take off in a hurricane, but you get the picture, right? The situation I presented above isn't possible. It would most likely cause our 737 to tumble over on the ground, but it illustrates the difference between airspeed and ground speed. A 737 can't actually take off in a hurricane. That would be suicide.
As a side note skoker: If you're familiar with X-Plane, you can simulate this exact scenario. Create a continuous wind in the direction of the runway; no gusts, deviations, etc. Just a continuous wind. Now try to take off from both directions and see what happens.
My Dad's a pilot, it's his job to ensure that 450 people get from where they are to where they are going by hurtling high above the ground in a pressurised aluminium tube.
When I am on a plane I do whatever the people who are there to ensure NOT that I have a comfortable and enjoyable flight but that I have a safe flight, say. No questions asked.
It irritates the life out of me when I hear people complaining about "oh I was only 5 minutes late" or "I don't need to turn my phone off" etc. You get to your gate at least half an hour before take off, check-in even earlier. NO EXCUSES. Get to the gate early. Turn off your phone, iPod, laptop, all electronic devices at least for take off and landing.
If you don't like it I offer you my most profound piece of advice:
Suck it up, princess.
When I am on a plane I do whatever the people who are there to ensure NOT that I have a comfortable and enjoyable flight but that I have a safe flight, say. No questions asked.
It irritates the life out of me when I hear people complaining about "oh I was only 5 minutes late" or "I don't need to turn my phone off" etc. You get to your gate at least half an hour before take off, check-in even earlier. NO EXCUSES. Get to the gate early. Turn off your phone, iPod, laptop, all electronic devices at least for take off and landing.
If you don't like it I offer you my most profound piece of advice:
Suck it up, princess.
aquajet said:
In theory, yes, but few if any modern airliners are tested for these conditions, so the best rate of glide number will be unknown to both the manufacturers of the airplane and the pilots flying it. Have you ever heard of the famous Gimli Glider incident? These guys were forced to improvise everything about how A B-767 would perform fully engine-out. It's probably one of the most amazing feats of piloting in recent history.
skoker said:
ETOPS is a standard for a twin engine aircraft flying with one engine out, not both.
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