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fiscal
Topic Author
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Does Longer Lower Approaches Save Fuel?

Tue May 25, 2010 9:13 am

A local newspaper's article on aircraft noise at Perth International Airport, claims that in an attempt to save fuel, airlines (and with the consent of air traffic control regulators) are being allowed to make longer low level approaches.

I had always believed that lower altitude flying equated to a higher fuel consumption.

Who is right?
 
Glom
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RE: Does Longer Lower Approaches Save Fuel?

Tue May 25, 2010 10:11 am

We would need to know more details. If they're talking about glide path angle, then maybe they're being allowed to use the optimum for the aircraft. Sometimes at difficult airports, aircraft have to descend more steeply.
 
B737200
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RE: Does Longer Lower Approaches Save Fuel?

Tue May 25, 2010 10:20 am

Maybe usually the aircraft are kept above a certain altitude until they reach a certain distance to the airport. To keep level they will need to use a certain amount of thrust.

To make it more efficient they can perform a constant descent at idle (hence less fuel usage). This would mean a shallower, constant descent without level stops. As such the approach will be lower over certain areas and may be longer (you need space to descend so the descent from cruise may start before). So as such, the lower and longer is a by product of the constant idle descent. I suppose in certain cases you may be higher using the idle approach.

Maybe someone who knows the subject better can help you better.
Lady Guinness is ready to fly...
 
fiscal
Topic Author
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RE: Does Longer Lower Approaches Save Fuel?

Tue May 25, 2010 10:34 am

Quoting b737200 (Reply 2):
To make it more efficient they can perform a constant descent at idle (hence less fuel usage). This would mean a shallower, constant descent without level stops.

Perth is basically flat, so a constant descent is probably quite OK and has probably always happened , but what the protesters are saying is that there is more noise from longer lower flight approaches (asserting at the same time that airlines are doing it to save fuel).

So, it seems to me that they do not really know what they are talking about, as it seems obvious to me that long, low and level approaches do not benefit the airlines, as there would be no fuel savings to be had on this type of approach.

I suppose it is also fair to assume that a decent using idle is going to be a lot quieter than using thrust to keep level.
 
B737200
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RE: Does Longer Lower Approaches Save Fuel?

Tue May 25, 2010 10:58 am

Well I concur, they must be messing something up becasue the way I know it, is that the lower the fly, the least efficient you are.
Lady Guinness is ready to fly...
 
oly720man
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RE: Does Longer Lower Approaches Save Fuel?

Tue May 25, 2010 11:33 am

I get the impression that the problem is more to do with approach routes to the airport having changed without too much consultation with the locals rather than giving long low approaches, as such.

Route changes since Nov 2008

http://www.airservicesaustralia.com/...ts/waroutereview/communityinfo.asp
http://www.airservicesaustralia.com/...flight_routes_before_and_after.pdf
wheat and dairy can screw up your brain
 
KELPkid
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RE: Does Longer Lower Approaches Save Fuel?

Tue May 25, 2010 8:10 pm

A traditional instrument approach is far less than an optimal descent profile for saving fuel...there are many points at which power must be added at low altitude to level off, or power must be carried to achieve a specific descent gradient (like on an ILS approach).

Does Australia follow the US example, where airliners on IFR flight plans are allowed to make a visual descent in visual meteorological conditions, or do they follow Europe's example, where you are shooting the ILS even on a severe clear day?

An optimal descent profile for an aircraft should, in theory, reduce noise, as the power settings would be pretty low...
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
mrskyguy
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RE: Does Longer Lower Approaches Save Fuel?

Tue May 25, 2010 8:15 pm

I've heard over at WN that the pilots prefer a parabolic approach for short length trips, and later releases than others for descent on longer legs all for the same reason.. it's believed that the higher usage of fuel on climbout is negated by 20 minutes of closed throttle descending. Seems to make sense in my head, but I have no numbers to prove it.
"The strength of the turbulence is directly proportional to the temperature of your coffee." -- Gunter's 2nd Law of Air
 
cobra27
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RE: Does Longer Lower Approaches Save Fuel?

Tue May 25, 2010 8:34 pm

Ok this doesn0t make any sense to me now. how can lower approach save fuel. As it is ils approaches are at about 55% N1, (that is a lot of power, because the put LDG and flap miles before). The only way to reduce consumption would be to fly steeper and wait with configuration
 
jetmatt777
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RE: Does Longer Lower Approaches Save Fuel?

Tue May 25, 2010 10:46 pm

Quoting MrSkyGuy (Reply 7):
I've heard over at WN that the pilots prefer a parabolic approach for short length trips, and later releases than others for descent on longer legs all for the same reason.. it's believed that the higher usage of fuel on climbout is negated by 20 minutes of closed throttle descending. Seems to make sense in my head, but I have no numbers to prove it.

It is my understanding that WN is now using RNP (RNAV) approaches at fields that have them to use idle thrust, from the T/D all the way to the FAF. (when possible due for traffic and spacing)
 
pilotpip
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RE: Does Longer Lower Approaches Save Fuel?

Wed May 26, 2010 1:08 am

Quoting MrSkyGuy (Reply 7):
I've heard over at WN that the pilots prefer a parabolic approach for short length trips, and later releases than others for descent on longer legs all for the same reason.. it's believed that the higher usage of fuel on climbout is negated by 20 minutes of closed throttle descending. Seems to make sense in my head, but I have no numbers to prove it.

What is a parabolic approach?

Quoting Jetmatt777 (Reply 9):

It is my understanding that WN is now using RNP (RNAV) approaches at fields that have them to use idle thrust, from the T/D all the way to the FAF. (when possible due for traffic and spacing)

They are qualified for it, just like many other airlines (mine included) however most RNAV approaches are base on the same descent angle (~3 degrees) that an ILS is predicated on. They're also confined to the same restrictions we are including other traffic in the area. Many other airlines have tested idle descents from TOD to the runway however these are extremely impractical in all but the least congested airspace.
DMI
 
jetmatt777
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RE: Does Longer Lower Approaches Save Fuel?

Wed May 26, 2010 1:22 am

Quoting pilotpip (Reply 10):
They are qualified for it, just like many other airlines (mine included) however most RNAV approaches are base on the same descent angle (~3 degrees) that an ILS is predicated on. They're also confined to the same restrictions we are including other traffic in the area. Many other airlines have tested idle descents from TOD to the runway however these are extremely impractical in all but the least congested airspace.

I have talked to a WN pilot who tells me they use the idle to FAF procedure very often when available.

Quoting pilotpip (Reply 10):
What is a parabolic approach?

Non-standard terminology, but I think that is, for a short stage length, climbing to a higher altitude without much of a level cruise (certainly there but not for a long period of time) and then starting the descent. Like a giant rainbow in the sky from Point A to Point B.
 
pilotpip
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RE: Does Longer Lower Approaches Save Fuel?

Wed May 26, 2010 3:34 am

Quoting Jetmatt777 (Reply 11):

I have talked to a WN pilot who tells me they use the idle to FAF procedure very often when available.

I jumpseat with WN on average twice per week and sit in the cockpit about 80% of the time. Not once have I seen them do what you describe here and two of the airports I go to are pretty dead. One I often get cleared for visuals at 10,000 and 20 miles from the runway. I'm not trying to discount what you're saying, but I'm telling you that this is something that is rarely accomodated for anybody unless you're going into somewhere like CID.
DMI
 
mrskyguy
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RE: Does Longer Lower Approaches Save Fuel?

Wed May 26, 2010 3:49 am

Quoting Jetmatt777 (Reply 11):
Non-standard terminology, but I think that is, for a short stage length, climbing to a higher altitude without much of a level cruise (certainly there but not for a long period of time) and then starting the descent. Like a giant rainbow in the sky from Point A to Point B.

Right. The typical flight has three main phases: climb, cruise, and descent. A parabolic profile is one that does not have a long cruise segment, which looks like an arc if plotted in 2 dimensions. Pilots would be carrying a lighter load of fuel, allowing a "rocket" climb, and then closing the throttles for the entire descent to maximize inertia and use as little fuel as possible.
"The strength of the turbulence is directly proportional to the temperature of your coffee." -- Gunter's 2nd Law of Air
 
cobra27
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RE: Does Longer Lower Approaches Save Fuel?

Wed May 26, 2010 3:57 am

One more thing, there is other interesting thing.
A lot of people think that reduced thrust reduces consumption, it actually increases it by few percent bu extends engine life and safety remarkably
 
jetmatt777
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RE: Does Longer Lower Approaches Save Fuel?

Wed May 26, 2010 4:26 am

Quoting cobra27 (Reply 14):
A lot of people think that reduced thrust reduces consumption, it actually increases it by few percent bu extends engine life and safety remarkably

Not questioning you, but explain?
 
tdscanuck
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RE: Does Longer Lower Approaches Save Fuel?

Wed May 26, 2010 5:37 am

Quoting cobra27 (Reply 14):
A lot of people think that reduced thrust reduces consumption

It does reduce fuel flow (consumption per unit time). It may not reduce mileage (consumption per mile) because of the SFC and speed increase at higher thrusts.

Tom.
 
AirframeAS
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RE: Does Longer Lower Approaches Save Fuel?

Wed May 26, 2010 4:23 pm

Quoting tdscanuck (Reply 16):
It does reduce fuel flow (consumption per unit time). It may not reduce mileage (consumption per mile) because of the SFC and speed increase at higher thrusts.

The question I have is: The air is so much heavier at 7,000' or below which makes the engine work harder. How exactly does that save fuel? Once you get that low, you are thrusting more on that engine vs. at 15,000'.
A Safe Flight Begins With Quality Maintenance On The Ground.
 
miller22
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RE: Does Longer Lower Approaches Save Fuel?

Wed May 26, 2010 6:26 pm

Steeper approaches burn less fuel. This applies to descent as well. The whole idea is to move forward up high where you can save fuel, and then pull the thrust levers to idle and glide down from the top of descent. Note that if you extend the flight spoilers, you went too far  

But an approach is hardly a low fuel burn scenario, even though you are descending. The longer you're on the approach, the more fuel you'll burn. It's best to stay clean and maintain ~250 knots to the outer marker, then dirty everything up and slow down in between.
 
swiftski
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RE: Does Longer Lower Approaches Save Fuel?

Thu May 27, 2010 7:36 am

Quoting KELPkid (Reply 6):
Does Australia follow the US example, where airliners on IFR flight plans are allowed to make a visual descent in visual meteorological conditions, or do they follow Europe's example, where you are shooting the ILS even on a severe clear day?

The first one.
Normally "Report Visual" followed by a visual approach, or visual approach on glidepath.
 
mrskyguy
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RE: Does Longer Lower Approaches Save Fuel?

Sun May 30, 2010 3:25 am

Quoting miller22 (Reply 18):
Note that if you extend the flight spoilers, you went too far  

Not really.. because there's no fuel penalty for using the spoilers in such a descent profile. In fact, doing so almost ensures a convenient low-fuel descent, vs. having to use the engines to maintain speed.
"The strength of the turbulence is directly proportional to the temperature of your coffee." -- Gunter's 2nd Law of Air
 
mandala499
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RE: Does Longer Lower Approaches Save Fuel?

Sun May 30, 2010 4:15 am

Assuming no terrain concerns... there is a way where you can end up at the final altitude before the approach for longer than usual...   

We have here, where I am, an airport where "lower for longer" can save time and fuel... hence lower the noise exposure to the neighbours...

Constant descent down to say, 1500 or 2000... with no speed restriction below 10,000... one can go in and reach 1500 or 2000 at 280 - 300KIAS... and take 5-10NM or so to slow down to approach config for the glideslope intercept.
This way, you use the potential energy as kinetic momentum unimpeded(except for drag) until the final approach, on that level portion prior to the glideslope. Hence, lower for longer... but this is a very narrow window where you can save by lower for longer... The problem with this method is, you need to maintain high speed below 10,000 (which goes against several companies' SOP)

Quoting miller22 (Reply 18):
But an approach is hardly a low fuel burn scenario, even though you are descending. The longer you're on the approach, the more fuel you'll burn. It's best to stay clean and maintain ~250 knots to the outer marker, then dirty everything up and slow down in between.

Or do the above... slow down clean to green dot, and with careful planning... the throttles won't go above idle until you're in landing config... or just as you intercept the glideslope... But this would be difficult if not dangerous in a high traffic volume situation... For that, the continuous descent method is better... which means, no longer "lower for longer". In most situations, miller22's stay clean for as long as possible would have some savings and less noise already.

Mandala499
When losing situational awareness, pray Cumulus Granitus isn't nearby !
 
flybaurlax
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RE: Does Longer Lower Approaches Save Fuel?

Sun May 30, 2010 3:06 pm

Quoting Jetmatt777 (Reply 9):
It is my understanding that WN is now using RNP (RNAV) approaches at fields that have them to use idle thrust, from the T/D all the way to the FAF. (when possible due for traffic and spacing)

AS has been doing this for a few years now. They were one of the first to do so. As mentioned, it's only realistic to do in non congested airports, but in Alaska they are abundant. The RNP procedures not only save fuel due to continuous descent, but they often cut down on track miles. This also applies to RNP departures.
Boilerup! Go Purdue!
 
pilotpip
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RE: Does Longer Lower Approaches Save Fuel?

Mon May 31, 2010 4:12 am

Quoting flybaurlax (Reply 22):

AS has been doing this for a few years now. They were one of the first to do so. As mentioned, it's only realistic to do in non congested airports, but in Alaska they are abundant. The RNP procedures not only save fuel due to continuous descent, but they often cut down on track miles. This also applies to RNP departures.

The biggest advantage is that they can be configured in ways that traditional approaches using ground-based systems are left unable to do due to terrain or other obsticals to line of sight.

Alaska (the airline and the state) are often the first to adopt new technology on the navigation front because the state is so reliant upon aviation.
DMI
 
mrskyguy
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RE: Does Longer Lower Approaches Save Fuel?

Mon May 31, 2010 5:37 am

Quoting pilotpip (Reply 23):
Alaska (the airline and the state) are often the first to adopt new technology on the navigation front because the state is so reliant upon aviation.

..that and the nature of Alaska's flying up north often results in Alaksan being the first to truly benefit from leaps in such technologies.
"The strength of the turbulence is directly proportional to the temperature of your coffee." -- Gunter's 2nd Law of Air

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