Descent speeds

[BOAC]

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?

[Jerricho]

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!" ;)

[coracle]

If not given a speed to fly by ATC then it doesn't really matter what you do as long as you obey the SLP's!!!!!!

[Del Prado]

I'll often give you no speed control, I expect anything from 330-180 kts.

In these days of Mode S radar it gives us an added tool to monitor speeds.

[javelin]

.81 to the changeover, 340kt to downwind, activate the approach, config, land, decamp to Mrs Miggins Tea and Pie shop :ok:

[rolaaand]

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:ok: Cheers,rolaaand.

[Doug E Style]

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!"

[Del Prado]

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.

[DirtyPierre]

Especially if you're asked "GLF!"

Ah, that takes me back to shrimp boats and the old Thomson/CSF Bright display. Jerricho, your old man was a legend as a FLOW controller. And when he'd lean over your shoulder and write in red on the end of your strip "GLF" it really meant max speed!

[haughtney1]

BOAC, having checked your profile I see you fly for Astreus.
In my experience you guys fly faster than anyone else

Is that on the 73 or 75? (when I was at AEU it was 290-340 on the 75:E )

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 ?

[Del Prado]

Is that on the 73 or 75? (when I was at AEU it was 290-340 on the 75 )

My experience is they're even 'worse' on the 738!

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.

[Jerricho]

Heh. Glad you caught the reference DirtyP :ok:

[30W]

In my experience you guys fly faster than anyone else.

If in general they're flying descents on same type continually faster than other larger operators, I would suggest they're doing so less economically......

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

[Del Prado]

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.

[30W]

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

[OBK!]

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.

[BOAC]

OK- I'll take that as a "no there is no 'ATC standard descent' speed" Boac, then:oh: . 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.

[Jerricho]

Sure, if you could hold that to a four mile final ;)

[Monarch Man]

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:uhoh:

[danceswithsheep]

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!!!!!

[CARD]

Somebody mentioned Ryanair, there's one thing that I have been wondering a long time by myself. Why do they always wan't to start their descent so early? One could think that it would be a lot more economical to start descent pretty late and make an idle-power descent to join the gp directly, but instead they start descent really early with a pretty big ROD and then they find themshelves flying at 2-3000 ft almost 10 minutes and as far as I know that is not an economical or fast way to fly a B737...


Sorry about the OT.

[30W]

OBK,

You're right - thankyou for the formulae correction - will teach me to type in a rush!!:(

30W

[Del Prado]

Thanks for the info, all great responses.

[throw a dyce]

If you're operating into class D,E,F,G airspace,then there is a speed limit of 250kts below 10000ft.With modern radar systems we can put the ground speeds on the targets and see roughly whats what.BA will normally ask to keep high speed.Ryanair just do it.
The ones that are tanking around over 250kts are tricky to vector on the ILS.Even though you turn them earlier,they still go through the localiser,as they are too fast.
Helicopters are a whole different thing.Get behind them,which is all the time at ISZ and it's 160 til 4 early.Just the joys of working here.Ain't the pay thats for sure..:D

[jumpuFOKKERjump]

In Oz we had an 'agreement' that those that could (domestic anyhow) would descend at cruise into 300K. Our major airline decided one day to bump up the cost index and not tell us, just all of a sudden they were doing anywhere between 260 and 300K. Nobody died, but the sequences looked crap until somebody asked a pilot what was happening, "What? Nobody told you!" was the response:ugh: After it all settled down the result has been that everybody is treated the same: if you want somebody to go fast you explain it patiently and comprehensively, or, if they are 'profile' you assume 280 and: slow them down if they exceed expectations inconveniently, or, leave holes in the sequence and shaft everybody if the reverse is true.

[separator]

I have posted these before and they are still relevant.

Rules for successful flow control

1. Don’t trust pilots.
2. Don’t trust sector controllers.
3. Don’t trust tower controllers.
4. Don’t trust anyone else.
5. Develop a thick skin.
6. Never back a Twin Otter to beat anything, even another Twin Otter.
7. Never worry about departures, they are the tower's problem.
8. Pray that the management pilots have good FOs.
9. Computer Derived Threshold Times will be accurate only if the captain throws the computer out the storm window as he overflies the threshold in the go-round after being too intimate with the aircraft in front.
10. All pilots think that they should be Number 1.
11. Cardboard Bandeirantes can be used to hide unexplained gaps in the sequence.
12. Unless they build more runways the maximum number of aircraft that can land at an airport in a given time will remain the same, despite the wetdreams of airline schedulers.
13. Have faith in your own ability, no-one else does.
14. Same money, right or wrong.
15. If you believe the tower controller who says that the runway works will be finished prior to the start of the next sequence, you deserve all you get.
16. Your are only as good as your last sequence.
17. If your last sequence was not good, see above.
18. Keep slipping the odd joke to the approach controllers, it keeps their mind off what is coming up.
19. One day you will get a trainee who merely triples your workload.
20. Never sit in on approach and work your own sequence.
21. Always keep a false nose or wig in the car, an angry mob waiting outside the carpark is not a pretty sight.
22. The flow controller is never wrong. The flow controller may merely be acting on information that may now be superseded.


sep

[BOAC]

- jFj - now we are getting somewhere? 280 is the figure the check Captain mentioned. Where does this 'profile' originate?

[Del Prado]

now we are getting somewhere? 280 is the figure the check Captain mentioned. Where does this 'profile' originate?

Australia, by the sounds of it.

[BOAC]

Origin of Check Captain too!:)