descent clearance

[fourthreethree]

This is a question that has been much discussed between my colleagues at Maastricht recently, and I thought I would try and get a pilots perspective on it.

It is quite a simple clearance, but one which is regularly misunderstood. I personally dont care if its an ICAO standard or not, as yet I have not checked if it is, and if so what its official meaning is. I simply want to know what you pilot chaps understand when you hear the following clearance.

"XYZ123 descend flight level 260 two thousand feet per minute or greater"

I would greatly appreciate your feedback, thanks in advance.

[5milesbaby]

433, from a London ATC point of view, not many use descent rates as you suggest, but only 'expedite'. Very few on even rarer occasions use rates specific to maintain separation, I think because it isn't ever taught, so we try other methods.

Therefore to me your statement suggests:-
1. Leave for FL260 now.
2. Immediately increase to at least 2000ft/min descent rate.
3. If unable to comply within a reasonable time (ie1000ft), inform ATC and possibly state maximum rate at current level, and FL where 2000ft+ can be achieved.

That what you were after?

[Notso Fantastic]

Agree with 1 & 2, but to make 3 a reality, use speedbrake to crank the descent rate up to 2000fpm at least. There's another trick to pull out of the hat to increase descent rate- accelerate.

[ecj]

This sounds like old style procedural separation where rates were used to maintain the prescribed standard separation.

Still used at airfields like Stornoway, where there is no airfield radar available. Two departures same outbound track.
Faster first hopefully. [Area: RSYD clearance]

No 1:climb greater than 1000 fpm until passing flight level 100.

No 2: climb less than 1000 fpm until level fight level 90.

Otherwise, using radar, specify a point by which the ac is required the achieve the cleared level.

[fourthreethree]

Thx for replies so far.

5milesbaby

I'm not after any particular response. I know what I mean when I give the clearance, but recent experience has shown me that pilots don't necessarily understand the same thing.

notsofantastic

I understand your point re: speed, but if the desired speed profile is affected by a given descent rate would you be happy to make the sacrifice in order to stick to the restriction, and if not would you inform atc of your unwillingness/inability to keep the restriction? Would you assume that an average ROD would suffice?

[Notso Fantastic]

No pilot I fly with would not happily change speed/descent rate/descent profile whatever in order to comply with such a request. We look on it as instructions to stop us bumping into somebody else, which for the sake of our mother's lovely little baby boy is very close to our heart!

[M.Mouse]

433

OK, a question from a pilot. Why do Maastricht generally ask for specific descent rates?

I would imagine that most of us would comply but Maastricht seem to be the only place in the world where giving a descent rate is routine.

From a pilot's point of view it comes across a being dictatorial about the way we fly the aircraft. It also means that we have to overide our company SOPs that ask us to fly the most economical speed/profile.

I certainly comply but must admit it always irritates to be given a descent rate for no APPARENT good reason.

I am not trying to be controversial but am generally interested in the situation from your point of view.

[fourthreethree]

M Mouse

Showing my inexperience here, 5milesbaby hinted at it but you have confirmed it for me. I never knew that a rate of descent was used only in Maastricht, not having visited too many other centres. It is taught to us through abinitio training and OJT that it is an easy way to assure a certain flight level at a certain time. The fact that we are the only people who use such a clearance would tend to explain the confusion it sometimes causes.

To answer your question, why do we use it? Difficult to explain without a radar picture in front of me. I do not use as a rule, I am aware that you guys have speed/descent profiles to adhere to as much as possible, and although I do not know the intricacies of your job or of aircraft handling, I know that vertical movement affects speed. So I try whenever possible to leave it in the hands of the pilot, using

"descend when ready to be level in x minutes/x miles before xyz"

But it is getting busier up there, and such a clearance is not always possible. Lets take a hypothetical situation. I have an inbound at F380, which I need to be level F260 in 8 minutes, in order to comply with an LOA restriction. He has just cleared traffic at F370, but still has opposite, crossing in 2 minutes at F350. So I give descent F360, expect to be level F260 in 8 mins. Crossing his track are two flights, F300 in 6 mins and F270 in 8 mins.

This would not be an uncommon scenario.

I could give a step descent, get a lower level, say F230, and after crossing the guy at F300 descent to F230 to cross F260 in 2 mins. Lots of work, lots of monitoring, and several descent clearances for one of maybe 30 aircraft on my frequency.

Or, I could put three aircraft on headings, all needing own nav later, if traffic allowed this, and if military status allowed this. This would require six transmissions for hdg/own nav, and six readbacks.

OR I could simply give the following clearance, after the target clears the opposite tfc at F350

"Continue descent F260(or lower if arranged) rate of descent 2500fpm or greater"

Now, this is the crux of my question. IF the pilot does as I intend, and descends with a rate never falling below the given rate, then I have my separation with all traffic with just one simple clearance. Efficiency achieved, I can move on to other problems.

However, if the pilot takes option two, ie: 2500fpm for 4 mins = 10,000ft,so I'll do 1500fpm for 2 mins then increase to 3500fpm for 2 mins, there goes my separation, pass me the incident report forms Mr. Supervisor.

And that is why I am asking this question. I realise that the clearance may be restrictive, and notsofantastic I am reassured to read your last post on this thread, but sometimes we have to be restrictive in order to assure safety. However if my clearance can easily be misunderstood, then maybe I should change the phraseology I use, and suggest the training department do the same. Maybe iso "rod 2000fpm or greater" I should say "continuous rod 2000fpm or greater.

So, clear as mud? :cool:

Thx again for your replies...

[eagleflight]

For your information only:

I have noted that several other ATC centers (i.e. Zurich, Milan, Munich etc) also issue descent rates in order to avoid step descents, so this "problem" seems not to be "restrected" to Maastricht only...


433, may I turn the question around ? Why do some of your collegues leave us flying at FL100 for several miles / minutes and then say: "Climb to FL 260, reach within the next 4 minutes" ?

It seems that some of you like to do this especially when we have ISA +25°C... ;)


Take care

eagleflight

[Notso Fantastic]

433- Not to be forgotten is the fact that it takes time for a descent to reach VSI demanded levels. Setting cleared altitude, then VS and punching the 'do it' button, the thrust levers pause, then slowly start retracting. That can take 10-15 seconds or so. Falling speed is detected, so the autopilot starts gently nosing down so that people in the cabin don't even feel it. Then the rate of descent very gradually increases. I think in the heat of European control, you may not allow for such finesse- much of the first minute can be taken up with getting the descent up to the rate requested. Allow for the descent to stabilise and see that it is still not quite the demanded level and time to think what to do........ Just don't expect high speed jets with pax to respond rapidly.

[have another coffee]

433,
I come across these clearances alot when approaching SPL from the south. We never have doubt about the meaning of the clearance. Start decent to FL.... with the given rate of descent.
Approaching SPL form the south it seems to us most of the times it is issued to meet some restiction at brussels. As it is always 2000 ft/ min to FL 250 Then switch to Brussels. There runs a tape (it seems) that says descent 220 switch Adam. If 06 or 01 in use your about 2000-3000 ft below profile. For other runways it has you easily about 6000-8000 below profile.
A lot of pilots in my company usually let the airspeed build up with 2000 ft/min to intercept profile sooner (ever noticed?). Say from normal M.79-300kts to .82-330 kts which gives you a less steep descent path (with given rate) and you intercept the descent profile sooner.
The instructions which say reach FL ... in ... minutes are much more confusing. Do I need to start the descent now? Some people start a shallow descent till a point where they can do a idle descent to comply with the clearance etc....
Depending on aircraft type most aircraft fly Mach numbers till about FL 260. Above that you easily reach 4000 ft/min descent rate as airspeed increases (speed of sound increases whilst descending). The aircraft is in effect continiously pitching over the keep the same mach number (idle descent). Below the switch it keeps the same indicated airspeed and rate of descent falls back to 2500 ft/min (depending aircraft type) (and gradually reduces as idle power from engines is increasing with lower altitudes). When reducing speed to say 250 kts it will not be much more than 1300 ft/min.
The controllers are not always aware of the above effects.

An exceptions seems to be the london controllers which are extremely good in planning the descent for you as they are constant aware of the distance to run versus altitude and speed you fly.

Care to comment on the restrictions mentioned above with Brussels?

[fourthreethree]

eagleflight

Well I'll start by saying Maastricht airspace is F250 and above, so the clearance in your post will not apply to us.;)

However I take your point, and will answer it as best I can. There are a million possible reasons why this could happen. It is not only other aircraft we have to keep you clear of, there are also restricted/danger areas, military airspace, adjacent sectors etc. None of these are obvious to you as pilots, even if you check your map you do not have the activation times or levels at your mercy. There are also LOA restrictions to adhere to, some of which you may well become familiar with if you fly the same route several times. However these too are liable to change, again for no reason apparant to you as pilots.

All I can really say is we never give restrictions for no reason, and giving the reason every time on the freq would be impossible. If for whatever reason you are unable to meet the given restricyion you are obliged to let us know. That way we can do something about it, arrange something with the military, arrange a later descent etc. But this is all extra work for us, and is not always condusive to an orderly flow of air traffic. Personally if I need to give a "tough" restriction, high rate of descent or whatever, I have it in mind that it might not be possible and I try to have a plan "b" ready just in case.

However it sounds to me that the clearance given as an example in your post is rather excessive, and could fall into the "controller mistake" category!! We're all human, even me:eek: , and of course we might simply forget to climb/descend/turn in time, which will lead to a more stern restriction. Again, on a personal note, if I have screwed up I will try to make amends and organise the necessary as outlined above without prompting from the pilot.

As has been done to death in other threads, atc is a fine balance between what pilots want and what controllers need, a bit of understanding on each side goes a long way. That is the whole purpose of this thread.

have another coffee

Like the name:) , and thanks, I will!!

Ok first off I am a Deco controller, so don't really know about approaching SPL from the South, you're with AMS by the time you reach my airspace.

But interesting to hear your comments re descent clearances. The meaning of what you see as a more confusing clearance is just as simple. Descend to be level in x mins/miles etc, means descend when ready, as long as the restriction is met I don't care when you start descent. (If I wanted you to start descent now, for whatever reason I would include it in my clearance.) Such a clearance would usually be issued if there is no traffic to effect the descent, so why would it matter to me when you start down? What really pi$$es me off mind you is when I give a clearance to descend when ready, the pilot stays level for 5 mins and then says he cant keep the restriction!!:mad: Still, as I said earlier, we're all humans, I guess a bit of Karma wouldn't go amiss!!

A lot of pilots in my company usually let the airspeed build up with 2000 ft/min to intercept profile sooner (ever noticed?).

Noooooo really? :oh: Nothing like giving your company away is there???? Its well known and doesnt give us cause for complaint.

The thing I would add in defense of our Brussels sectors is that they do have a very busy and very small airspace to get you down and meet the restriction. I don't really know how they work but they dont really have too much time to plan your descent. The other option would I guess be for the French to take you down to say FL300 or so and leave less for Maas to do. Maybe this is done in busy periods.

Once again thanks for the responses guys keep em coming:ok:

[EuroATC]

To add another controllers point of view, I have worked in Toronto ACC, Geneva ACC and now Bahrain ACC. I have used rates in all 3 places. Moreso in Geneva because of airspace contraints. You ask.. why are we stuck low then given climb to be level in 4 mins? Or why are we given a rate all the way up. There are many reasons, some are airspace restrictions, some are due traffic and others are due to the controller not wanting to get burned. I have been burned way too many times giving a descent and especially a climb.."XXX climb to FL 340 and reach by..." Aircraft being too low, or on a descent aircraft being too high. Yes some pilot might view this as "US" flying the plane but this is not so.. This is ATC providing separation and adhearing to agreements with other control facilities. In periods of light traffic I would normally give descend to.. and reach by XXX. When I am busy, I use rates all the time. It also shortens the number of transmissions.... How many times have I seen this : I ask an aircraft to start the descent. I am asking for a reason, then the pilot is giving me a couple hundred feet per minute.. It happens ALL the time.. So if i ask for descent, I am ALWAYS going to give you the rate if I am busy. Someone on this site suggested that rates are more of a procedural thing? ... A busy radar environment, you cannot do without.. unless of course you want to be non stop talking ...transmitting to the same aircraft 4 times

[PPRuNe Radar]

433

There are some pitfalls in using the rate of descent method to ensure that an aircraft is level by a specific time. It does not make the method right or wrong, as we can use many different methods of ATC to achieve the final aim. But those with experience will take them in to account when formulating their plan.

Firstly, as explained by our pilot brethren, the rate is not achieved instantaneously. The autopilot system (and presumably the hand flyers as well !!) will be looking to manouevre the aircraft gently in a constant 1G manner. It may take the first 1000' of climb/descent to get everything in the right ball park.

Secondly, the rate will also start to bleed off well before the cleared level to ensure another gentle transition to level flight. You cannot expect someone to maintain a rate of 2500fpm or greater through FL270 for FL260 as per the example you give. The pilot will need to start reducing to around 500-1000 fpm at least 1500' before the cleared level. Possibly even before that depending on the aircraft type and its handling or automation characteristics.

Remember also that the advice given in RVSM airspace is to also aim for a maximum rate of 500-1000 fpm between 1500' and 1000' before the cleared level. Otherwise (as per the thread in ATC Forum which is talking about TCAS), you are very likely to get RAs starting to muck up your day.

I'd agree with the fact that UK controllers tend not to use this method very much. Our preference seems to be more geared to specifying levels required at positions and then letting the pilot work it all out (or Honeywell or Sextant). Just the way we've been brought up to do it I suppose.

[fourthreethree]

PR

Fully agree, and precautions are taken when such a clearance is given, safety buffers and all that, and I am aware of the fact that a rate is neither instantly achieved nor instantly abated, all facts that I am trying to drum into my trainees!! Also, if giving a relatively high rod, I would never expect the pilot to keep the rate until maintaining. If this could be the case I would obtain a lower level from our subjacent sector and tell piolt to maintain rate untill passing F260, as in my example above.


But, to get back to my original point, what is understood by pilots by the clearance with a rate of climb or descent? I have had several very interesting answers but the more the merrier, keep the opinions coming.

[EuroATC]

"Secondly, the rate will also start to bleed off well before the cleared level to ensure another gentle transition to level flight. You cannot expect someone to maintain a rate of 2500fpm or greater through FL270 for FL260 as per the example you give. The pilot will need to start reducing to around 500-1000 fpm at least 1500' before the cleared level. Possibly even before that depending on the aircraft type and its handling or automation characteristics"

This is basic ATC knowledge and anyone who doesn't understand this should not be sitting behind a radar...

I agree that letting the crew of the aircraft fly and manage the flight is the best way to go. Just tell them where they need to be a xxx level and let them do it... of course as you say with the safety buffer built it...

BUT?? what do you do when they miss the restriction? Then you are screwed ? How do you teach your trainees to get out of that situation?

I have worked in 3 different places now and I have learned so much since my initial validation in Canada. Best way to say it actually is "experience abroad was an eyeopener".. For many different reasons, working methods, traffic patterns, complexity of airspace etc. As my job as an ATCO is to provide a service, I do my best to make it expedition at the same time as ensuring safety but bottom line is that .. I am the one who gets hauled into the office when i lose 5 miles / 1000 feet, my plan might be perfect but when just 1 pilot misses a restriction I am the one getting screwed.. Sorry guys but I am there to protect myself and will keep using those climb and descent rates :)

Good posts on here, good to see different views

[fourthreethree]

Ok I want to claim this topic back from the dead......

Once again, and with the same airline:* I had the situation whereby the pilot decided, 5000' from his cleared level, to reduce his rod from the assigned 1000fpm (hardly excessive) to 400fpm, smack opposite and then at the same level as the restricting traffic. After being told to expedite through safe level, I asked why he had reduced his rod. His answer, once again, was quite astounding. He said...

"oh, no reason"

So I ask the question again. Pilots, when given a rate of descent in a clearance, do you take it as an average or continuous rate??

As EuroATC said in the last post on here, there are some interesting views here, so lets keep it going.

[fatboy slim]

433 - I always fly the ROD given when assigned (rarely in UK, often by German stations) and would insist that the guy next to me does too. Same with speeds - I figure you guys are blessed with the big picture!! If you are in the UK file a CHIRP to let people know about this.

[decimal86]

i fly thru your airspace often and i have no problems at all adjusting to constraints/restrictions given by ATC. sometimes there is an airspeed restriction coupled to the V/S.

and in icing conditions as well! i might add...but if given sufficient time to conform then all will be well. we take it as a challenge esp after sitting there for 6 and a half just monitoring CRT!

if the instructions are, 'to leave now and give me a rate of....' i presume the ATC has very good reasons to get me out of that band of air.

may i ask a Q?

if given a normal descend clearance, do i need to ask,'pilot's discretion?' or is it asap. is this an ICAO standard? thks.

[fourthreethree]

decimal86

Good question, been covered before, but always brings up a variety of answers. Here is MY take on it.

Depends on what yo mean by a "normal" clearance. If you mean simply "descend FL280" this is an instruction to leave now but with no other restriction. If you mean "descend FL280 be level at XYZ" it is less clear. My opinion, and i stress it is my opinion is that top of descent is pilots discretion, descent profile is at pilots discretion also, as long as you meet the restriction I dont care how you do it. I give such a clearance when there is no other traffic to effect your descent.

But........

Many controllers disagree with me on this point. They believe, and could probably quote you section of an official ICAO document to back themselves up, that descend means descend and do it now because now is when I am issuing the clearance. They will say that the minimum rate of descent allowed by ICAO without notice from the pilot is 500fpm, and if that is enough for the traffic then they are covered.

Thats fine, and it is a personal choice, and I'm sure when they explain it to the incident investigators they will be proved right.

If there is traffic to effect your descent profile I will give you some form of control to keep you clear of it, be it a rate of descent, a heading or a speed restriction, indeed I normally clarify a clearance with no descent restriction with "descend when ready...." or "pilots discretion descend.....".

But the old favourite of this discussion is this. If in doubt......ask . It is potentially a safety issue so surely if you are unsure of what is meant by a clearance it is best to clarify.

Hope this answers your query, and I look forward to restricting you the next time you fly through.:E :ok:

[decimal86]

thks for that. i always ask before descending but if as a matter of rules, then it's leave now. not a problem.

on another matter, does CPDLC actually reduce your workload over your airspace? how so? wouldn't a voice instruction be faster?

[Bigmouth]

I´ll dial in 260 on the alt. sel. and 2000 or better on vs.
If given a rate of climb on the other hand I might go for the average, as I have always assumed (wrongly?) that´s what you want. It´s obviously harder to maintain a constant 2000fpm climb than a descent, especially heavy and at altitude. But if needed and if we´re talking 3-4000 feet I can always reduce climb speed and do a little zoom climb.
European controllers seem to do their own thing a lot, it´s often hard to know what is expected in different regions.

[millerman]

decimal86

CPDLC does increase our workload (once you have clicked through the various menus) and voice is much quicker!
Which is why we can only give routings,new squawks,check mike and frequency change instructions via data link at the moment.
Anything else is classed as safety critical and must be given by voice.

[Guy D'ageradar]

433

As another who uses specific rates I fully agree with your comments.

Although I have been know to use "expedite" in the past, I personally feel far more secure if I allocate a specific rate to acheive the required separation. The gulf between company SOP's seems to be so great these days that it is the only way to ensure that I don't get stung by someone doing a few hundred feet per minute (did someone say Nigel?) . If I have multiple climbing/descending/crossing tracks, it is the only way to avoid multiple level-offs, which I consider to be part of the service.

EuroATC

Likewise, fully in agreement. Time to change that monicker though!! Hope it's not too hot out there (although I think we beat you a few times this summer!)

Regards,

Guy.:cool:

[fourthreethree]

decimal86

CPDLC.........don't get me started!!!! No it does not decrease my workload, indeed we are told that if traffic levels are high we should not use CPDLC for exactly this reason.

However it should be pointed out that the datalink possibilities we have now are at a very early stage of development, and this is really a very long term project. I am by nature a fairly cynical minded person, I am very wary of anything new in my job. But even I can see the potential for the future and the impact on our working lives. Whether the benefits to be had are worth the time, effort and money being poured into the project is not a decision for mere mortals such as myself!!

The other thin I dislike about datalink is that the silent control which is foreseen takes away the possibility of any situational awareness for the pilots, much discussed on the BBC's 'Crowded Skies' program, and subsequently in threads about the program in these forums.

bigmouth

This was the response I was looking for. I am very grateful to you for posting. Now I know that my clearance is ambiguous.

If I give a climb clearance with a rate of climb I would expect that rate of climb to be met throughout the whole climb, and I will explain why.

Situation: you have come to my freq F245 to F250 requesting climb F350. I give you "Climb F350 1500fpm or greater " The reason for this clearance is, 99% of the time, traffic. The rate will mean in

1 min you pass F260
2 mins F275
3 mins F290
4 mins F305
5 mins F320
6 mins F335
7 mins F350

These are the levels I expect you to achieve to keep clear of opposite tfc @ F270 crossing in 3 mins and tfc no.2 @ F320 crossing in 6 mins. If you carry out the climb in accordance with my clearance separation is assured.

But if you take it as an average, then your climb profile may be as follows

1 min pass F278
2 mins F296
3 mins F303
4 mins F308
5 mins F312
6 mins F330
7 mins F350

This would mean that at exactly 6 vmins you would achieve seperation with tfc at F320. But 10 seconds earlier at 5 min 50 sec you had a loss of separation with this traffic.

The second profile would be in line with a clearance such as "climb F350 be level in 7 minutes".

I am fully aware that there is a possibility that your a/c capabilites might prevent you from achieving a continuous roc, but if this is the case I would expect you to tell me.

Incidentally the example I give above is very similar to the incident I had a few weeks ago which prompted me to start this thread. 1000' seperation was achieved with 4.9NM opposite direction converging at 900kts plus. Very scary.

As a post script I would add that this is not intended as a wrist slap for bigmouth, I truly appreciate your comments, and shall take them on board. This was my point, that what I thought of as a simple clearance, unambiguous because that was what I had been taught, is actually open to misinterpretation.

Thanks to all for your views, keep em coming!!:ok: