Descent speeds
Per Ardua ad Astraeus
Thread Starter
Join Date: Mar 2000
Location: UK
Posts: 18,579
Likes: 0
Received 0 Likes
on
0 Posts
Descent speeds
My line check Captain was talking about ATC 'expected' descent speeds.
We normally descend at cruise Mach until c/over to IAS and then the FMC produces an IAS based on the 'Cost Index' and the speed can vary from 250kts to 320kts (737) depending on C Index.
Primarily a UK question but w/wide answers appreciated: is there a 'standard' descent IAS that you will expect?
We normally descend at cruise Mach until c/over to IAS and then the FMC produces an IAS based on the 'Cost Index' and the speed can vary from 250kts to 320kts (737) depending on C Index.
Primarily a UK question but w/wide answers appreciated: is there a 'standard' descent IAS that you will expect?
Ohcirrej
Join Date: Feb 2003
Location: This is the internet FFS.........
Posts: 2,921
Likes: 0
Received 0 Likes
on
0 Posts
I think the best answer to this would be I would "expect" you guys to fly whatever speed you need to for profile (how's that for a non-committal answer 
) . Speed certainly is can be black magic, and I do find it amusing sometimes that same types of aircraft are being flown at drastically different speeds. Yesterday afternoon I had two CRJ 100s filed the same route, same IAS, same cruising altitude but different company. The second guy was doing his best to try and smoke the first (of course, I don't know who was carrying more of Winnipeg's world famous ah, err, ahhh ........forget it)
If we need you to fly a specific speed for sequencing or separation, we'll let you know. Especially if you're asked "GLF!"
) . Speed certainly is can be black magic, and I do find it amusing sometimes that same types of aircraft are being flown at drastically different speeds. Yesterday afternoon I had two CRJ 100s filed the same route, same IAS, same cruising altitude but different company. The second guy was doing his best to try and smoke the first (of course, I don't know who was carrying more of Winnipeg's world famous ah, err, ahhh ........forget it)If we need you to fly a specific speed for sequencing or separation, we'll let you know. Especially if you're asked "GLF!"
Join Date: Dec 2001
Location: UK
Age: 45
Posts: 121
Likes: 0
Received 0 Likes
on
0 Posts
BOAC-From an ATC point of view it amazes me how the same aircraft type flown by the same company on the same route will fly at sometimes VERY different speeds. If I need you on a speed for sequencing I'll tell you,if you are given "speed at your discretion" then it is your call,as fast or as slow as you like.I would add however that if you are going to fly at a speed that is particularly slow for your aircraft type during the descent,then let me know if you can. I would "expect" a B737 to descend at 280-310ish knots. I have been caught out before because 737s/A320s etc have hit the brakes and have slowed to 250 or below very early in the descent.My fault really because as Del Prado says,no speed control means exactly that.
To answer your question,apart from adhering to the SLP's then there is no standard IAS.But if you want to fly slowly,gimme a shout
Cheers,rolaaand.
To answer your question,apart from adhering to the SLP's then there is no standard IAS.But if you want to fly slowly,gimme a shout
Cheers,rolaaand.
Since the company reduced our cost index a while back the computer now gives us some very leisurely descent speeds, especially on short sectors, but most crews will now increase that so we're not getting in the way. When it was brought in I asked if anyone had told NATS to expect our aircraft to have slower descent speeds. You can probably guess the answer.
By the way, I like the guy in Amsterdam control who says, when asked about speed control, "Go as fast as you dare!"
By the way, I like the guy in Amsterdam control who says, when asked about speed control, "Go as fast as you dare!"
BOAC, having checked your profile I see you fly for Astreus.
In my experience you guys fly faster than anyone else.
This is NOT a criticism, in fact it helps my planning when I see a flystar barrelling at 300+kts (I know who to make No.1 ) in the same way as all geebees and most easys will come back to 250kts earlier than most.
In my experience you guys fly faster than anyone else.
This is NOT a criticism, in fact it helps my planning when I see a flystar barrelling at 300+kts (I know who to make No.1
) in the same way as all geebees and most easys will come back to 250kts earlier than most.
Join Date: Jan 2004
Location: Brisbane, Australia
Posts: 561
Likes: 0
Received 0 Likes
on
0 Posts
Especially if you're asked "GLF!"
Join Date: Jun 2004
Location: Surrounding the localizer
Posts: 2,200
Likes: 0
Received 2 Likes
on
1 Post
BOAC, having checked your profile I see you fly for Astreus.
In my experience you guys fly faster than anyone else
In my experience you guys fly faster than anyone else
)Just out of interest, do you ATCers take into account the wind up high? (as this makes a big difference to the profile) or is it a case of looking at the mode S MCP speeds, and comparing it to the G Speed ?
Is that on the 73 or 75? (when I was at AEU it was 290-340 on the 75 )
On approach we always take into account the upper wind but I think you mean profile from TOD so I'll let someone from area answer that one.
Join Date: Jun 2001
Location: UK
Posts: 523
Likes: 0
Received 0 Likes
on
0 Posts
In my experience you guys fly faster than anyone else.
As oil prices have surged, Cost Indexes (ratio of fuel costs against direct operating costs) have changed. This has had the effect of generally lowering speeds across the industry. Not all companies either monitor, or produce accurate data often enough (particularly smaller ones), and so a disparity between operators may be apparent.
We are encouraged to operate the aircraft economically - and so this means slower speeds at the moment. If of course ATC want something tactically different on the day, then complying is no issue at all.......
30W
30W interesting point. Higher speeds would presumably reduce flight times, how do the costs/ benefits balance up?
Looking at say Ryanair or Astreus as examples, they are flying faster so presumably reducing journey times and increasing the chance of the next departure being on time whereas other operators (GB/Easyjet to name two) fly more slowly and save fuel.
Do the 'faster' operators feel the benefits outweigh the costs or do you think they don't consider the cost index enough?
Just curious, I have no preference either way.
Looking at say Ryanair or Astreus as examples, they are flying faster so presumably reducing journey times and increasing the chance of the next departure being on time whereas other operators (GB/Easyjet to name two) fly more slowly and save fuel.
Do the 'faster' operators feel the benefits outweigh the costs or do you think they don't consider the cost index enough?
Just curious, I have no preference either way.
Join Date: Jun 2001
Location: UK
Posts: 523
Likes: 0
Received 0 Likes
on
0 Posts
DP,
Increased descent speeds generally mean increased fuel burn. Econ descent speed in an FMC is partially dictated by Cost Index.
CI= Fuel Cost/Direct operating cost
So, as fuel prices have risen, the CI has generally fallen. The lower the CI entered into an FMC, the lower economical speeds it produces (basis being fuel cost saving becomes more important than airframe time cost saving).
On an individual flight, the cost saving is not huge. Multilply that by a fleet on each sector across a flying season though and the cost saving can be quite substantial.
Companies review this with variable quality of course, but the savings are defiantely there at the moment - high oil price=lower speed (up to a point for aerodynamic reasons.....)
Some people just like flying fasy (macho thing?, think they are doing the company a favour?). Those who descend fast in the days of high fuel costs may feel they are doing great service, but in the true cost analysis, are doing their employers little good. I'm not suggesting any of these are necessarily the case with the airlines you mention, but suggesting some pilots have this mentality.
In many cases it might well be a case of companies not providing the right, and accurate, information for crews to use. Some have fuel cells, which regularly review costs and revise CI's accordingly. Some may not keep on top of such issues and hence don't provide the crews with education and accurate information on such areas.....
30W
Increased descent speeds generally mean increased fuel burn. Econ descent speed in an FMC is partially dictated by Cost Index.
CI= Fuel Cost/Direct operating cost
So, as fuel prices have risen, the CI has generally fallen. The lower the CI entered into an FMC, the lower economical speeds it produces (basis being fuel cost saving becomes more important than airframe time cost saving).
On an individual flight, the cost saving is not huge. Multilply that by a fleet on each sector across a flying season though and the cost saving can be quite substantial.
Companies review this with variable quality of course, but the savings are defiantely there at the moment - high oil price=lower speed (up to a point for aerodynamic reasons.....)
Some people just like flying fasy (macho thing?, think they are doing the company a favour?). Those who descend fast in the days of high fuel costs may feel they are doing great service, but in the true cost analysis, are doing their employers little good. I'm not suggesting any of these are necessarily the case with the airlines you mention, but suggesting some pilots have this mentality.
In many cases it might well be a case of companies not providing the right, and accurate, information for crews to use. Some have fuel cells, which regularly review costs and revise CI's accordingly. Some may not keep on top of such issues and hence don't provide the crews with education and accurate information on such areas.....
30W
Join Date: Jul 2002
Location: uk
Posts: 524
Likes: 0
Received 0 Likes
on
0 Posts
30W
Just to correct you there on the formula for CI.... CI = Cost of Time/Cost of Fuel so as you mentioned, fuel price increases, CI decreases.
Many airlines ignore variable cost index, due to either lack of resources or just can't be bothered with the hard work involved in using it on the line and so tell there crews to fly at a fixed index, no matter where they go.
In our company, we've actually fully introduced variable cost index flight planning alongisde optimal scenario analysis, and true enough, cost indexes are low and may add 5 minutes to your journey...the fuel savings at the end of the year are huge. Having an accurate cost index figure in the FMC/FMGC does many favours, it will determine what cruise speed to fly at based on that index, and modify it during the cruise depending on wind, weight, temperature etc. The descent speeds are around 255-270 kts at cross over which is sometimes too slow, and as you say, crews often modify this as they believe it's crazy to descend that slow, not knowing that this is where a lot of fuel is saved. Same in the cruise, pilots will often see M0.76 as the econ speed at a certain cost index, and ignore it, justifying there actions with garble about OTP when they'd be arriving in plenty of time anyway.
I think a lot of airline crews do need educating about the principle of cost index, especially when variable cost index is being used. It's good to know however that many of our competitors just peg around europe at a fixed index.
Just to correct you there on the formula for CI.... CI = Cost of Time/Cost of Fuel so as you mentioned, fuel price increases, CI decreases.
Many airlines ignore variable cost index, due to either lack of resources or just can't be bothered with the hard work involved in using it on the line and so tell there crews to fly at a fixed index, no matter where they go.
In our company, we've actually fully introduced variable cost index flight planning alongisde optimal scenario analysis, and true enough, cost indexes are low and may add 5 minutes to your journey...the fuel savings at the end of the year are huge. Having an accurate cost index figure in the FMC/FMGC does many favours, it will determine what cruise speed to fly at based on that index, and modify it during the cruise depending on wind, weight, temperature etc. The descent speeds are around 255-270 kts at cross over which is sometimes too slow, and as you say, crews often modify this as they believe it's crazy to descend that slow, not knowing that this is where a lot of fuel is saved. Same in the cruise, pilots will often see M0.76 as the econ speed at a certain cost index, and ignore it, justifying there actions with garble about OTP when they'd be arriving in plenty of time anyway.
I think a lot of airline crews do need educating about the principle of cost index, especially when variable cost index is being used. It's good to know however that many of our competitors just peg around europe at a fixed index.
Per Ardua ad Astraeus
Thread Starter
Join Date: Mar 2000
Location: UK
Posts: 18,579
Likes: 0
Received 0 Likes
on
0 Posts
OK- I'll take that as a "no there is no 'ATC standard descent' speed" Boac, then
. It was not my intent to discuss individual pilot/airline ways. Out of interest, the most economic descent profile on a 737 is at Cost Index 0 - and that drifts down at 250kts'ish all the way - which would, I assume, be very popular.
. It was not my intent to discuss individual pilot/airline ways. Out of interest, the most economic descent profile on a 737 is at Cost Index 0 - and that drifts down at 250kts'ish all the way - which would, I assume, be very popular.
A cost index is only one part of it..there are other factors to consider as well.
A high speed descent can avoid a hold, a runway closure, or make a schedule that includes connections on other flights.
The cost of time IMHO is immensly undervalued, simply because accountants dont have any idea what it costs a company in goodwill or reputation for a late arrival/departures (all they use is effectively their best guess)..simply because they only deal in absolutes. i.e. direct operating costs
A high speed descent can avoid a hold, a runway closure, or make a schedule that includes connections on other flights.
The cost of time IMHO is immensly undervalued, simply because accountants dont have any idea what it costs a company in goodwill or reputation for a late arrival/departures (all they use is effectively their best guess)..simply because they only deal in absolutes. i.e. direct operating costs
Join Date: Jul 2002
Location: Its warm here!!!!!!
Posts: 71
Likes: 0
Received 0 Likes
on
0 Posts
We at Air Traffic have to take into consideration all the rules and regulations, cost cutting/timesaving for our customers, management keeping staff to a minimum and the occasional missing aircraft/ mil inside CAS/ runway changing etc. Speed will always be a difficult one to answer but I have tried to allow high speed as often as possible within the Scottish TMA.
If I can I will ask TOD or Descent when ready but its becoming very busy airspace! If I'm sounding busy or you struggle to get in on the frequency, don't ask, you will get told.
I have enjoyed speaking to you all in Scottish airspace but treat it more like London and expect the hold more and more!!!!!
If I can I will ask TOD or Descent when ready but its becoming very busy airspace! If I'm sounding busy or you struggle to get in on the frequency, don't ask, you will get told.
I have enjoyed speaking to you all in Scottish airspace but treat it more like London and expect the hold more and more!!!!!