I was on this flight.

https://www.dropbox.com/scl/fi/00om0g02o820a314bxhpi/Screenshot_20230822-160815.png?rlkey=wjs9dzd4dtphwdj1xhtcxs0et&dl=0

ATC needed to delay its arrival in Sydney, but an orbit at 41 thousand
feet is not something I'd heard of before.

Sylvia.

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On Tue, 22 Aug 2023 22:08:02 +1000, Sylvia Else <sylvia@email.invalid>
wrote:

I was on this flight.

https://www.dropbox.com/scl/fi/00om0g02o820a314bxhpi/Screenshot_20230822-160815.png?rlkey=wjs9dzd4dtphwdj1xhtcxs0et&dl=0

ATC needed to delay its arrival in Sydney, but an orbit at 41 thousand
feet is not something I'd heard of before.

Sylvia.

10 days ago (14th August) all flights were cancelled to Sydney
creating a backlog.
Maybe still clearing backlog?
--
Petzl
Good lawyers know the law
Great lawyers know the judge

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On 24-Aug-23 12:47 pm, Petzl wrote:

On Tue, 22 Aug 2023 22:08:02 +1000, Sylvia Else <sylvia@email.invalid>
wrote:

I was on this flight.

https://www.dropbox.com/scl/fi/00om0g02o820a314bxhpi/Screenshot_20230822-160815.png?rlkey=wjs9dzd4dtphwdj1xhtcxs0et&dl=0

ATC needed to delay its arrival in Sydney, but an orbit at 41 thousand
feet is not something I'd heard of before.

Sylvia.

10 days ago (14th August) all flights were cancelled to Sydney
creating a backlog.
Maybe still clearing backlog?

Unlikely.

There were thunder storms around at the time. One doesn't fly through
thunder storms if one likes life, so they will cause some disruption. I
expect that was the problem.

Sylvia.

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On 26-Aug-23 5:17 am, Rod Speed wrote:

Sylvia Else <sylvia@email.invalid> wrote

I was on this flight.

Bugger, the death squad were too busy setting up
for the execution of Prigozhin to shoot it down.

You just can't get any reliable people anymore.

https://www.dropbox.com/scl/fi/00om0g02o820a314bxhpi/Screenshot_20230822-160815.png?rlkey=wjs9dzd4dtphwdj1xhtcxs0et&dl=0

ATC needed to delay its arrival in Sydney, but an orbit at 41 thousand
feet is not something I'd heard of before.

Makes, sense if it can be organised, saves fuel.

I doubt that. Aircraft fly high because a particular indicated airspeed,
and thus drag requiring thrust, gives a higher true air speed. Doing an
orbit at high altitude just means a greater distance has to be flown for
a given required delay.

Sylvia.

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On 01-Sept-23 5:37 am, Rod Speed wrote:

Sylvia Else <sylvia@email.invalid> wrote

Rod Speed wrote

Sylvia Else <sylvia@email.invalid> wrote

I was on this flight.

ÿBugger, the death squad were too busy setting up
for the execution of Prigozhin to shoot it down.
ÿYou just can't get any reliable people anymore.

https://www.dropbox.com/scl/fi/00om0g02o820a314bxhpi/Screenshot_20230822-160815.png?rlkey=wjs9dzd4dtphwdj1xhtcxs0et&dl=0

ATC needed to delay its arrival in Sydney, but an orbit at 41
thousand feet is not something I'd heard of before.

ÿMakes, sense if it can be organised, saves fuel.

I doubt that.

More fool you.

Aircraft fly high because a particular indicated airspeed, and thus
drag requiring thrust, gives a higher true air speed.

You are just plain wrong about that.

Doing an orbit at high altitude just means a greater distance has to
be flown for a given required delay.

And that too.

Any chance[*] that you'll explain your reasoning?

Sylvia

[*] Actually, there's no chance, because you never do. I'm just
indulging in hope over experience.

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On 02-Sept-23 5:57 am, Rod Speed wrote:

That's just saying what I said in different words, lower drag and so
greater fuel efficiency.

Your claim was that an orbit at high altitude saves fuel. Now, given
that the purpose of the orbit was to achieve a delay, how would flying
for the same period of time at a lower altitude, but the same indicated airspeed, save fuel?

Sylvia.


--- MBSE BBS v1.0.8.4 (Linux-x86_64)
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On 2/9/2023 10:02 am, Sylvia Else wrote:

On 02-Sept-23 5:57 am, Rod Speed wrote:

That's just saying what I said in different words, lower drag and so
greater fuel efficiency.

Your claim was that an orbit at high altitude saves fuel. Now, given
that the purpose of the orbit was to achieve a delay, how would flying
for the same period of time at a lower altitude, but the same indicated airspeed, save fuel?

My guess is that there a lot of other aircraft in the pattern, if the
aircraft you were on was already at 41,000 no reason to descend until
there was space for it at a lower altitude.
If it had descended on approach then climbed back to 41,000 then IMHO
that would be a very strange thing to do.

--
Daryl


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Sylvia Else wrote:
-----------------------------

On 02-Sept-23 5:57 am, Rod Speed wrote:

That's just saying what I said in different words, lower drag and so greater fuel efficiency.

Your claim was that an orbit at high altitude saves fuel. Now, given
that the purpose of the orbit was to achieve a delay, how would flying
for the same period of time at a lower altitude, but the same indicated airspeed, save fuel?

Sylvia.

** Consider the following:

At 41,000 feet, the *amount* of oxygen in a given volume of air drops by over 4 times, while the percentage remains constant.
Fuel flow is necessarily reduced to maintain the optimum combustion temperature ( and so thrust) at higher altitudes.
The low drag condition allows higher speeds despite this and so longer range.



.... Phil


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On 02-Sept-23 3:49 pm, Phil Allison wrote:

Sylvia Else wrote:
-----------------------------

On 02-Sept-23 5:57 am, Rod Speed wrote:

That's just saying what I said in different words, lower drag and so
greater fuel efficiency.

Your claim was that an orbit at high altitude saves fuel. Now, given
that the purpose of the orbit was to achieve a delay, how would flying
for the same period of time at a lower altitude, but the same indicated
airspeed, save fuel?

Sylvia.

** Consider the following:

At 41,000 feet, the *amount* of oxygen in a given volume of air drops by over 4 times, while the percentage remains constant.
Fuel flow is necessarily reduced to maintain the optimum combustion temperature ( and so thrust) at higher altitudes.
The low drag condition allows higher speeds despite this and so longer range.

... Phil

By the time the air is being used to burn fuel, its velocity relative to
the engine has been reduced, and it has been passed through multiple compression stages, so the volumetric oxygen density in the ambient air
is pretty much irrelevant.

The point at issue is whether fuel is saved by flying a greater distance
at higher speed in more rarified air, compared with flying a smaller
distance at lower speed in denser air, in order to achieve the same
delay. This last aspect is important, because the aircraft is not trying
to get somewhere more quickly, and use less fuel thereby. Regardless of
its altitude, the time spent doing the orbit is the same, because that
was the required delay.

Sylvia.





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On 02-Sept-23 8:42 pm, Rod Speed wrote:

Sylvia Else <sylvia@email.invalid> wrote

Phil Allison wrote

Sylvia Else wrote

Rod Speed wrote

That's just saying what I said in different words, lower drag and so >>>>> greater fuel efficiency.

Your claim was that an orbit at high altitude saves fuel. Now, given
that the purpose of the orbit was to achieve a delay, how would flying >>>> for the same period of time at a lower altitude, but the same indicated >>>> airspeed, save fuel?

ÿ Consider the following:
ÿAt 41,000 feet, the *amount* of oxygen in a given volume of air
drops by over 4 times, while the percentage remains constant.
Fuel flow is necessarily reduced to maintain the optimum combustion
temperature ( and so thrust) at higher altitudes.
The low drag condition allows higher speeds despite this and so
longer range.

By the time the air is being used to burn fuel, its velocity relative
to the engine has been reduced, and it has been passed through
multiple compression stages, so the volumetric oxygen density in the
ambient air is pretty much irrelevant.

Have fun explaining why jet aircraft cruise at those altitudes.

They can fly faster up there.

I know people think in terms of lower drag, and for a give true airspeed
it is lower. But so is the lift, and the same lift is required
regardless of altitude. So the aircraft has to fly faster to get the
same lift, and at that faster speed it has the same drag, because the lift-drag ratio of the wing doesn't magically improve due to thinner air.

Sylvia.


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On 03-Sept-23 7:41 am, Rod Speed wrote:

Sylvia Else <sylvia@email.invalid> wrote

Rod Speed wrote

Sylvia Else <sylvia@email.invalid> wrote

Phil Allison wrote

Sylvia Else wrote

Rod Speed wrote

That's just saying what I said in different words, lower drag and so >>>>>>> greater fuel efficiency.

Your claim was that an orbit at high altitude saves fuel. Now, given >>>>>> that the purpose of the orbit was to achieve a delay, how would
flying
for the same period of time at a lower altitude, but the same
indicated
airspeed, save fuel?

ÿ Consider the following:
ÿAt 41,000 feet, the *amount* of oxygen in a given volume of air
drops by over 4 times, while the percentage remains constant.
Fuel flow is necessarily reduced to maintain the optimum combustion >>>>> temperature ( and so thrust) at higher altitudes.
The low drag condition allows higher speeds despite this and so
longer range.

By the time the air is being used to burn fuel, its velocity
relative to the engine has been reduced, and it has been passed
through multiple compression stages, so the volumetric oxygen
density in the ambient air is pretty much irrelevant.

ÿHave fun explaining why jet aircraft cruise at those altitudes.

They can fly faster up there.

That's not the reason they fly there.

I know people think in terms of lower drag, and for a give true
airspeed it is lower. But so is the lift, and the same lift is
required regardless of altitude. So the aircraft has to fly faster to
get the same lift, and at that faster speed it has the same drag,
because the lift-drag ratio of the wing doesn't magically improve due
to thinner air.

Pity the real reason is that the lower drag means better fuel efficiency
and since that is the main controlable cost. that's why they fly there.

The drag isn't lower, for the reason I stated. I thought you understood
this stuff. Apparently, I was wrong.

Sylvia.

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