More questions for pilots - LAC
Thread Starter
Join Date: May 2003
Location: Sarf England
Posts: 138
Likes: 0
Received 0 Likes
on
0 Posts
More questions for pilots - LAC
Recently, various banners have appeared around Swanwick, apparently distributed by those fine employees of NATS who are behind the "Acting Responsibly" initiative of Vision 2011. Two examples caught my eye:
Naturally, we are all aware of the aviation industry going through tough times at the moment - NATS management rams this message down our throats on an almost daily basis to justify any or all of their recent unpopular actions. On a day when one of our most important customers has reported record half-year losses, my questions to anyone who wishes to answer them are:
LTP
The most efficient descent speed of most modern jets (even a B747!) is in the range 250-270kts.
A Boeing 737-800 descending at 310kts instead of its most efficient descent speed will use up to 100kgs more fuel.
LTP
Join Date: Mar 2002
Location: Euroland
Posts: 2,814
Likes: 0
Received 0 Likes
on
0 Posts
Simple no wind basic explanation.
Jet engines are most efficient at relatively high power settings.
Airframes have certain speeds which are most efficient.
to have the two happening at once usually requires the aircraft to be operating high - say FL300 to FL360.
So to keep it simple let's just say that aircraft get the most miles per gallon in the cruise.
Therefore the simple answer is that the most efficient way to fly a jet aircraft is to climb as quickly as possible to the cruise level and stay there as long as possible.
From this you will gather that in order to stay at cruise level as long as possible requires a steep descent. Therefore the most efficient possible end to a flight would be to remain at ideal cruise level until such a point that the throttles are closed and the steepest possible path followed to landing. Being a relatively clean aircraft, steep descents usually result in high speeds
In other words,
Steep dive throttles closed from cruise level to a point where the dive is made less shallow and the speed bleeds off in time for the aircraft to be stable and configured on the approach with eht power at the correct setting.
Good in theory. Impossible in UK airspace!!
In reality, the ideal speed is always dependent on the wind.
If the aircraft NATS are talking of are descending at 275Kt into a 275Kt headwind they are going to use all their fuel going nowhere. Flying at 300Kt will improve the situation somewhat depending on time available.
A tailwind may make a slower speed more efficient.
Basically, such a broad statement is only going to work on a ideal day in the specific aircraft with the specific loading and the specific C of G position and the ideal descent profile.
Do NATS expect ATCOs to worry about all that?
Finally having said all of the above, for many operators, the cost of fuel is often less than the cost of maintenance. Therefore spending a few extra pounds of fuel to fly faster and save engine life can save the operator lots of £s over time with a large fleet.
Put simply, if it was that simple lots of people would be out of a job and the cost index would be a thing of the past.
Jet engines are most efficient at relatively high power settings.
Airframes have certain speeds which are most efficient.
to have the two happening at once usually requires the aircraft to be operating high - say FL300 to FL360.
So to keep it simple let's just say that aircraft get the most miles per gallon in the cruise.
Therefore the simple answer is that the most efficient way to fly a jet aircraft is to climb as quickly as possible to the cruise level and stay there as long as possible.
From this you will gather that in order to stay at cruise level as long as possible requires a steep descent. Therefore the most efficient possible end to a flight would be to remain at ideal cruise level until such a point that the throttles are closed and the steepest possible path followed to landing. Being a relatively clean aircraft, steep descents usually result in high speeds
In other words,
Steep dive throttles closed from cruise level to a point where the dive is made less shallow and the speed bleeds off in time for the aircraft to be stable and configured on the approach with eht power at the correct setting.
Good in theory. Impossible in UK airspace!!
In reality, the ideal speed is always dependent on the wind.
If the aircraft NATS are talking of are descending at 275Kt into a 275Kt headwind they are going to use all their fuel going nowhere. Flying at 300Kt will improve the situation somewhat depending on time available.
A tailwind may make a slower speed more efficient.
Basically, such a broad statement is only going to work on a ideal day in the specific aircraft with the specific loading and the specific C of G position and the ideal descent profile.
Do NATS expect ATCOs to worry about all that?
Finally having said all of the above, for many operators, the cost of fuel is often less than the cost of maintenance. Therefore spending a few extra pounds of fuel to fly faster and save engine life can save the operator lots of £s over time with a large fleet.
Put simply, if it was that simple lots of people would be out of a job and the cost index would be a thing of the past.
Spink Pots
Join Date: Jan 2003
Location: Up in the air
Posts: 255
Likes: 0
Received 0 Likes
on
0 Posts
I prefer to inform the pilots of the delays and if there are no separation issues, let them fly whatever speed they want. It''s their aeroplane and they know a hell of a lot more about flying it efficiently than I do. If there's no delay, most seem to prefer to maintain over 300Kts right up to the holding fix so that would suggest management are full of ****.
It wouldn't be the first time.
Let the pilots fly their planes - we'll keep them apart. It's that simple.
It wouldn't be the first time.
Let the pilots fly their planes - we'll keep them apart. It's that simple.
Spink Pots
Join Date: Jan 2003
Location: Up in the air
Posts: 255
Likes: 0
Received 0 Likes
on
0 Posts
On another note, I have been tempted to put a poster up around Swanwick saying...
"DID YOU KNOW...
Keeping aeroplanes separated (even B747s!) saves the airlines £Billions every year"
"DID YOU KNOW...
Keeping aeroplanes separated (even B747s!) saves the airlines £Billions every year"
Join Date: Dec 2000
Location: on the golf course (Covid permitting)
Posts: 2,131
Likes: 0
Received 0 Likes
on
0 Posts
DFC wrote:
The last part of this (the steep dive from cruise flight) is frankly b0llox!
Yes, aircraft get good economy at cruise flight levels, but should then descend at Vmd. The point at which you leave cruise altitude should be where a line drawn from the threshhold (allowing deceleration and running the flaps etc) intersects the cruise altitude when descending at Vmd. Remember, continuing the cruise for another 20 miles before descending balls out still uses 20 miles worth of engine power and hence fuel at cruise fuel flows vs 20 miles of idle thrust all to save about 2 minutes.
Therefore the simple answer is that the most efficient way to fly a jet aircraft is to climb as quickly as possible to the cruise level and stay there as long as possible.
From this you will gather that in order to stay at cruise level as long as possible requires a steep descent. Therefore the most efficient possible end to a flight would be to remain at ideal cruise level until such a point that the throttles are closed and the steepest possible path followed to landing.
From this you will gather that in order to stay at cruise level as long as possible requires a steep descent. Therefore the most efficient possible end to a flight would be to remain at ideal cruise level until such a point that the throttles are closed and the steepest possible path followed to landing.
Yes, aircraft get good economy at cruise flight levels, but should then descend at Vmd. The point at which you leave cruise altitude should be where a line drawn from the threshhold (allowing deceleration and running the flaps etc) intersects the cruise altitude when descending at Vmd. Remember, continuing the cruise for another 20 miles before descending balls out still uses 20 miles worth of engine power and hence fuel at cruise fuel flows vs 20 miles of idle thrust all to save about 2 minutes.
Join Date: Dec 2000
Location: on the golf course (Covid permitting)
Posts: 2,131
Likes: 0
Received 0 Likes
on
0 Posts
12W Tim wrote:
Fleet of 20 aircraft operating 6 sectors per day / 365 days per year at £80 per sector = £3.5 million pa. You bet it really matters
.
100 kg of fuel costs about £80. a 737-800 can carry 179 passengers. So the airline has to charge 50p extra per seat. Does it really matter?
.
Join Date: Mar 2002
Location: Euroland
Posts: 2,814
Likes: 0
Received 0 Likes
on
0 Posts
TopBunk,
You will find that there is no mention of "Fuel" in the piece you quote.
You forget that even if the fuel flow is higher, covering the ground at 8nm or more per minute copared to 4nm per minute has an effect.
There is also the issue of engine overhaul costs which have to be factored. For many cases these days, time is more expensive than fuel.
Therefore I recomend that you try playing with the CI in your machine and see if the descent is always at Vmd regardless of which (fuel or time) is set to be more influential.
It is a compromise and as I said in simple no wind situations, ther most efficient operating practice is to descend as quickly as possible.
Have a look at your descent tables and see what descending at say 2000ft per minute burns from FL350 to SL and then check what the burn is at say 3000ft/min.
In the second case the total fuel used during the descent will be less. It will be even smaller at a ROD of 4000 ft per minute.
The time to descend will be less so for the descent, a steep high speed descent will use both less fuel and less engine time / airframe time / crew time etc etc etc. As you say, small saving per flight but over a large fleet over a year it can mount up.
As for the extra time in the cruise?
20 miles at cruise speed (480 TAS) will take about 2.5 minutes.
In that 2.5 minutes, the aircraft descending at Vmd will only be about 3 or 4000 ft lower but now behind due to the reducing TAS (further behind if it has encounted and increasing headwind / decreasing tailwind). So your descent at Vmd keeps you in the air longer - using fuel and using hours (engine / airframe / crew / schedule).
So Vmd will not always be the most efficient.
Don't forget that the largest airline operator in the UK (the one that makes a big profit) uses high frequency and low costs as major tools to make money. The frequency of flights is a big factor and consequently, time may be quite a significant cost issue.
You will find that there is no mention of "Fuel" in the piece you quote.
You forget that even if the fuel flow is higher, covering the ground at 8nm or more per minute copared to 4nm per minute has an effect.
There is also the issue of engine overhaul costs which have to be factored. For many cases these days, time is more expensive than fuel.
Therefore I recomend that you try playing with the CI in your machine and see if the descent is always at Vmd regardless of which (fuel or time) is set to be more influential.
It is a compromise and as I said in simple no wind situations, ther most efficient operating practice is to descend as quickly as possible.
Have a look at your descent tables and see what descending at say 2000ft per minute burns from FL350 to SL and then check what the burn is at say 3000ft/min.
In the second case the total fuel used during the descent will be less. It will be even smaller at a ROD of 4000 ft per minute.
The time to descend will be less so for the descent, a steep high speed descent will use both less fuel and less engine time / airframe time / crew time etc etc etc. As you say, small saving per flight but over a large fleet over a year it can mount up.
As for the extra time in the cruise?
20 miles at cruise speed (480 TAS) will take about 2.5 minutes.
In that 2.5 minutes, the aircraft descending at Vmd will only be about 3 or 4000 ft lower but now behind due to the reducing TAS (further behind if it has encounted and increasing headwind / decreasing tailwind). So your descent at Vmd keeps you in the air longer - using fuel and using hours (engine / airframe / crew / schedule).
So Vmd will not always be the most efficient.
Don't forget that the largest airline operator in the UK (the one that makes a big profit) uses high frequency and low costs as major tools to make money. The frequency of flights is a big factor and consequently, time may be quite a significant cost issue.
Join Date: Dec 2000
Location: on the golf course (Covid permitting)
Posts: 2,131
Likes: 0
Received 0 Likes
on
0 Posts
DFC
Your answer has so many flaws in it that it does not dignify a response.
I maintain that the least fuel used will be as I describe, and rest my case.
Some of what you say has some validity (maintenance related costs as a function of block time etc), but not everyone flies for Ryanair, but there are enough threads about them elsewhere
Your answer has so many flaws in it that it does not dignify a response.
I maintain that the least fuel used will be as I describe, and rest my case.
Some of what you say has some validity (maintenance related costs as a function of block time etc), but not everyone flies for Ryanair, but there are enough threads about them elsewhere
Join Date: Mar 2002
Location: Euroland
Posts: 2,814
Likes: 0
Received 0 Likes
on
0 Posts
TopBunk,
You really need to look at your aircraft's figures.
Again I say that I have not based the situation on fuel alone.
I have done the figures and find that in fuel terms for a certain jet type the extra fuel burnt in making a highspeed descent is 1Lb
How much does 1 pound of Jet A1 cost? Compare that to what you alsone cost per minute and there is a saving in keeping time as short as possible never mind the other cost factors.
So again I say check out the CI in your aircraft and try playing with some figures from the manual.
The point being that how the aircraft is operated is nothing to do with ATC. If operator A wants to try flying slow because fuel is their only major cost then that is perfectly acceptable. If operator B chooses something different then they also are entitled to do that.
The fear is that NATS will loose the run of itself and use it's greener outlook to impose unnecessary restrictions or put restrictions in place which are necessary from a lack of ATC facilities point of view but claim that they are for green issues.
You really need to look at your aircraft's figures.
Again I say that I have not based the situation on fuel alone.
I have done the figures and find that in fuel terms for a certain jet type the extra fuel burnt in making a highspeed descent is 1Lb
How much does 1 pound of Jet A1 cost? Compare that to what you alsone cost per minute and there is a saving in keeping time as short as possible never mind the other cost factors.
So again I say check out the CI in your aircraft and try playing with some figures from the manual.
The point being that how the aircraft is operated is nothing to do with ATC. If operator A wants to try flying slow because fuel is their only major cost then that is perfectly acceptable. If operator B chooses something different then they also are entitled to do that.
The fear is that NATS will loose the run of itself and use it's greener outlook to impose unnecessary restrictions or put restrictions in place which are necessary from a lack of ATC facilities point of view but claim that they are for green issues.
Join Date: Dec 2008
Location: From Despair To Nowhere
Posts: 130
Likes: 0
Received 0 Likes
on
0 Posts
TopBunk
That supports my point. If cost of fuel per seat was the dominant factor in decisions of how to operate the aircraft to the point of saving 50p a seat then most short-haul journeys would be in turboprops, of whatever size. In that case of course there would be 100+ seat turboprops, as there would be a market for them and there is no technical reason why not. However my case does not rely on the possibility of 100+ seat turboprops, only on the fact that for most short-haul journeys the cost per seat, however many seats there are, is lower.
Let airlines operate aircraft, let ATC give them the service they are paid to give, and give so very well in the UK and much of Europe in my non-trivial experience. Oh, and let me descend outside controlled airspace if I want to. If I ask, there is a good reason.
That supports my point. If cost of fuel per seat was the dominant factor in decisions of how to operate the aircraft to the point of saving 50p a seat then most short-haul journeys would be in turboprops, of whatever size. In that case of course there would be 100+ seat turboprops, as there would be a market for them and there is no technical reason why not. However my case does not rely on the possibility of 100+ seat turboprops, only on the fact that for most short-haul journeys the cost per seat, however many seats there are, is lower.
Let airlines operate aircraft, let ATC give them the service they are paid to give, and give so very well in the UK and much of Europe in my non-trivial experience. Oh, and let me descend outside controlled airspace if I want to. If I ask, there is a good reason.
Join Date: Feb 2006
Location: Hants
Posts: 2,295
Likes: 0
Received 0 Likes
on
0 Posts
Scuzi
Agree totally - management are far to quick to interfere nowadays. We even had one manager claim AMAN was a success for airlines at a meeting the other day (was using it as an example of what 'projects' could do for us).
NATS management is very quick to try to impose things on us - the latest is the minumum hold level of FL90... being trialled in the depths of winter in the depths of recession - of course it won't have much effect on operations... come the time when traffic levels rise again though...
NATS CO2 figures are very misleading - they are calculated by assuming aircraft are left on the SID at low level, when in fact climbs to more efficient levels are given as often as possible - to the majority of departures.
All these information posters and operational constraints are designed by management who want to be seen to be doing something by their superiors.
Agree totally - management are far to quick to interfere nowadays. We even had one manager claim AMAN was a success for airlines at a meeting the other day (was using it as an example of what 'projects' could do for us).
NATS management is very quick to try to impose things on us - the latest is the minumum hold level of FL90... being trialled in the depths of winter in the depths of recession - of course it won't have much effect on operations... come the time when traffic levels rise again though...
NATS CO2 figures are very misleading - they are calculated by assuming aircraft are left on the SID at low level, when in fact climbs to more efficient levels are given as often as possible - to the majority of departures.
All these information posters and operational constraints are designed by management who want to be seen to be doing something by their superiors.
All these posters around Swanwick are particularly galling if you've just worked an aircraft who's departed early, inbound to London, who then wants to sit in the hold for 25 minutes because he's early for his runway slot and there's no stand available.
Join Date: Mar 2002
Location: Euroland
Posts: 2,814
Likes: 0
Received 0 Likes
on
0 Posts
NATS management is very quick to try to impose things on us - the latest is the minumum hold level of FL90... being trialled in the depths of winter in the depths of recession
Making the holding levels FL350 in some study will not change the fact that the final reserve fuel is calculated at 1500ft AAL and until something is published in the AIP the extra London holding fuel will still have to be calculated at 7000ft and carried.
Everyone in aviation knows that less fuel is burnt holding at 9000+ compared to 7000. There is no need for a study to prove that. Would it not be better to study the effect of having to carry "London Fuel" all the way from Tokyo.
DFC, there will be more fuel burnt because it takes more miles to lose that extra 1000' and if number 1 needs an extra 3 miles then the all the following aircraft will be 3 miles further back (that's a lot of wasted fuel for a sequence of 50)
If approach can't get the correct aircraft off the stack because there are not enough levels available then
1) the most expeditious landing sequence cannot be followed (more delays for everyone following)
2) Aircraft may 'lose their place'. unfair on the ones at LAM (or holding out at Braso)
3)There may be gaps on the approach if all the aircraft are through one stack. One level at LAM in particular can often be blocked by survey flights, that just leaves 2 levels to vector off the hold to maintain a landing rate of 40+, that's just not possible.
On paper it seems like a great idea but the landing rate will suffer, maybe not a problem when it's quiet but once traffic levels increase a lot of questions will be asked about why the landing rate is so low.
If approach can't get the correct aircraft off the stack because there are not enough levels available then
1) the most expeditious landing sequence cannot be followed (more delays for everyone following)
2) Aircraft may 'lose their place'. unfair on the ones at LAM (or holding out at Braso)
3)There may be gaps on the approach if all the aircraft are through one stack. One level at LAM in particular can often be blocked by survey flights, that just leaves 2 levels to vector off the hold to maintain a landing rate of 40+, that's just not possible.
On paper it seems like a great idea but the landing rate will suffer, maybe not a problem when it's quiet but once traffic levels increase a lot of questions will be asked about why the landing rate is so low.
Join Date: Feb 2006
Location: Hants
Posts: 2,295
Likes: 0
Received 0 Likes
on
0 Posts
Taking just one level away from us (it is mandatory, not to be done at our discretion during quieter times), means that we will have to hold out sooner. (Standing agreements start as low as FL130, giving only 4/5 useable levels at the second busiest EGLL stack)
Holding out causes en-route a lot of problems, the hold areas are huge and the time to take aircraft off and bring them on is big.
Any extra fuel saved will be lost in this process - that's not-withstanding the very pertinent points Del Prado has made.
To keep runway occupancy, you need to have aircraft at the lowest levels in the hold... I can release them to EGLL at FL90, but the first thing they will do is drop it to Min Stack, or min stack+1 depending which hold it is.
Higher level holding is a good idea if the runway is blocked or unavailable - not so good when you are trying to keep occupancy levels up.
You can bang on about fuel burn all you like - the theory is correct, but the putting it into practice is complete fantasy when you are trying to land aircraft!
To make the practice mandatory regardless of traffic scenarios or loading is insane. The GS who suggested this (you know who you are, stack swap king), has just confirmed the belief by many in the ops room that he is clueless about TMA (Area) operations... Of course it's been siezed upon by office workers who see anything with the 'green' label as being good - without thinking of the ramifications.
Holding out causes en-route a lot of problems, the hold areas are huge and the time to take aircraft off and bring them on is big.
Any extra fuel saved will be lost in this process - that's not-withstanding the very pertinent points Del Prado has made.
To keep runway occupancy, you need to have aircraft at the lowest levels in the hold... I can release them to EGLL at FL90, but the first thing they will do is drop it to Min Stack, or min stack+1 depending which hold it is.
Higher level holding is a good idea if the runway is blocked or unavailable - not so good when you are trying to keep occupancy levels up.
You can bang on about fuel burn all you like - the theory is correct, but the putting it into practice is complete fantasy when you are trying to land aircraft!
To make the practice mandatory regardless of traffic scenarios or loading is insane. The GS who suggested this (you know who you are, stack swap king), has just confirmed the belief by many in the ops room that he is clueless about TMA (Area) operations... Of course it's been siezed upon by office workers who see anything with the 'green' label as being good - without thinking of the ramifications.
Join Date: May 2007
Location: swanwick carp lake
Posts: 232
Likes: 0
Received 0 Likes
on
0 Posts
"look at me, look at me, i'm being proactive, aren't i green, look look how green i am, PLEASE don't sack me because you rightly believe that i have done nothing to deserve my continuing employment over the last 5 years."
overheard 3.5 seconds after this scheme was dreamed up.
I have got an idea, the person/people involved in this trial earn a lot of money each and every year. what would be the CO2 saving if we sacked them and spent their "salary" on trees being planted each and every year.
overheard 3.5 seconds after this scheme was dreamed up.
I have got an idea, the person/people involved in this trial earn a lot of money each and every year. what would be the CO2 saving if we sacked them and spent their "salary" on trees being planted each and every year.
Join Date: Feb 2001
Location: Hants, UK
Posts: 1,064
Likes: 0
Received 0 Likes
on
0 Posts
The po-int of the posters as I see them are to make ATCOs think a bit about the speeds they use to stream aircraft. So many people start at 300kts+ and work backwards in 10-kt steps. This may require the one in front to speed up from its Econ speed for no real reason other than a lack of awareness. If you start at 280kts or so then you can achieve a stream just as well and save fuel at the same time.
In addition, many ATCOs seem automatically to use 10kt differences between subsequent aircraft. Notwithstanding the slight differences in TAS as the ones behind are a few thousand feet higher, it is perfectly possible to achieve a stream of aircraft with them all flying the same speed. Speed control to create extra miles does not work on the majority of UK en-route sectors as the distances are too short. It does a good job at stopping them catching up with each other too quickly, though.
Here's an idea: put the information on the posters into your back of tricks and use it now and then. You might find you can still stop the aircraft bumping into each other, provide good presentation to the next sector AND save the planet at the same time. Can't be bad, can it??
In addition, many ATCOs seem automatically to use 10kt differences between subsequent aircraft. Notwithstanding the slight differences in TAS as the ones behind are a few thousand feet higher, it is perfectly possible to achieve a stream of aircraft with them all flying the same speed. Speed control to create extra miles does not work on the majority of UK en-route sectors as the distances are too short. It does a good job at stopping them catching up with each other too quickly, though.
Here's an idea: put the information on the posters into your back of tricks and use it now and then. You might find you can still stop the aircraft bumping into each other, provide good presentation to the next sector AND save the planet at the same time. Can't be bad, can it??