Minimum ROD
 

As an ex ATCO who defected to the other side I'm hearing some strange ideas from some of my pilot colleagues. As far as I was aware the min. rate of descent is 500fpm as per UKAIP. If you guys want anything else you'll tell us - right? Lots of people I work with seem to think that you want us to descend much more quickly, especially in the holding patterns, just in case there's someone descending on top of us! But what about if the guy below is descending at the legal minimum? As far as I'm concerned if we're going to be holding for ages then there's no rush to get down - but I'd like to hear your views.

I can't speak for the folks on that side of the Atlantic, but here we want someone to come down at a normal rate of decsent for a jet and not glide down. If we give you lower, we want you to go down so that we can get other aircraft down or miss some other traffic... Or as we sometimes say when we ask say rate of decsent, and they say what do you want <sigh>, [not what we asked] and then we say we want you to go down like it was YOUR idea and not ours <G>...

regards

Scott,

Sorry and all that, but if you want the 'normal descent rate for a jet' but you don't want it to 'glide down', how do you propose we should lose height?

The ideal jet profile for descent is a glide from ToD to about 1500ft AAL. The only means of going down faster is to use speedbrakes, and that's inefficient.

As for asking about RoD and not being given a straight reply, that's because aircraft don't normally descend at a constant rate, but, rather, a constant speed. The rate varies. We can give you a specific rate, by using Vertical Speed mode, but there are problems with this, as selecting too high a rate will result in the speed getting too high. Some airlines don't allow use of V/S with the thrust levers closed, either, so they will only give you a constant rate that it less than their normal rate, which by the tone of your post, is not what you want.

So, what those pilots are saying is, 'we can't tell you our rate of descent because it isn't constant, but if you want us to give you a constant one, we'll try to fly it'.

allthenines, you're spot on about 500fpm. I always use 1000fpm and V/S in holding situations, to give a definite rate but avoid nuisance RAs. Sadly, a lot of aircraft have autoflight systems which don't know about the 500fpm bit, and will almost fly level to decelerate.

[ 22 August 2001: Message edited by: NorthernSky ]

althenines,
normally 500 fpm is fine, but do please remember it's a minimum. sometimes you come over at a release level of 120 but because the aircraft below have just been vectored off the hold you are cleared down to 80 straight away. in this case we do need a high rate of descent so the aircraft above don't miss their place in the queue. if you are cleared down through two or three stack levels, why not pick up the rate to nearer 1000 ?

at the end of the day if we do want a high rate of descent we will ask you to EXPIDITE

How many times do we see this type of discussion here? It is good to have it but lets face it, it is Air Traffic Control not Air Traffic Consultation! Positive control is the catch phrase and there are only three types of descent clearance in my book.

1) 'Request Descent', Decsend .....
2) When Ready Decend....

1 & 2 Have no vertical restrictions, be that traffic or otherwise.

3) Descend..... Be level .... By ....
Descend..... with xxxxFPM

If a controller wants a rate, let him/her specify it, otherwise let the driver drive the acft.


A thought for the future, would it be easier to take the pilot out of the flight deck or the controller out of the centre?
:confused:


cos i cant spel

[ 22 August 2001: Message edited by: TrafficTraffic ]

Now that's an excellent question. Many issues arise. Which job is more readily automated? You only need look at in-trail climb, or the new Heathrow vectoring advice thingy (someone post here please with its correct name!), plus TCAS, and other systems, to see which job is already being automated.

Also, if the centre guys have a big problem, those of us airborne would probably get onto the ground somewhere without an accident. If the aircraft has no pilot, and runs into difficulty, then it's 'Goodnight Vienna'.

Passengers don't even know about en-route ATC, in the main (how many Joe Publics think an Air Traffic Controller stands on the ramp with a pair of ping-pong bats?), but they'd be seriously worried about boarding an aircraft without two guys in the front wearing white (or blue!) shirts.

I have a feeling that this discussion could get seriously interesting.... Let's keep it going.

vertigo, bear in mind that, in the situation you describe, we will be trying to get the height off rapidly too, because we are on or above the profile. It's only when we're asked to go down early without an explanation or a clear restriction that these problems generally arise.

[ 22 August 2001: Message edited by: NorthernSky ]

Actually my response was the 500 fpm that was mentioned in the first post. That is more in the order of a float down... 1500 fpm I have no problem with what so ever. If I need more I will say so. If I ask what the rate of decsent is I ask for what you are doing NOW! If I need more I will ask for it, if I don't I leave you alone. Your mode C doesn't always show up on our display and when it does it shows up every 10 second in the enroute places here...

Now as to getting into the bit about piloting or controlling <G>. I don't know if you really want to go there yet <BG>. There are a LOT of UAVs out there right now. If the airlines thought that they could get away with it, y'all would be gone <G>... As of now, there is NOTHING out there that can replace the human for ATC. They have tried a LOT of computer solutions and none of them have worked yet. To many variables to consider for a computer or a program to get it right. The only way that it would work today is if all of you stayed on your route AND there were no deviations at all.

All of this said light hearted. I too am a pilot with instrument, commercial and multi ratings, so I do know what you are doing up there <G>. I also spend a bit of time at SimuFlite teaching some of the instructors stuff and spending time in the jet sims...

regards

OK Scott, but you must be careful with your words - to do 500fpm in a jet, you'll need plenty of thrust - again, you won't be 'floating' down.

We've also hit on a human factors problem, especially in Europe. It usually goes like this.....

ATC 'Blockhead 123, what is your Mach number?'
B123 'decimal 83, Blockhead 123'
ATCX 'roger'.

Now, in lots of Europe where English is not the first language, doubt will exist as to whether Blockhead 123 must now maintain .83 or is still on 'free speed'. Similarly with requests to 'say your heading' which are not folowed by 'roger, radar heading, or 'continue present heading until advised'.

Here, ATC need to tighten up their phraseology, or clarify things by saying, for example, 'for my information, say your heading', followed by 'roger, own navigation to ABC VOR'.

Clarity is vital (even here on PPRuNe), so let's make ourselves understood. :D

Northern Sky - as far as I'm concerned, if ATC asks for my speed or heading and then just says "roger" I assume I'm still free to do what I like. If he/she wants a specific hdg/speed he'll ask for it. If he doesn't ask for it he won't get it - simple!!

tired,

You're 100% right. But, to continue the example above (and this has happened to me more than once usually in France or Spain), what do you make of the next instruction?

ATC 'Blockhead 123, report your speed to XXX on 123.45'.

This illustrates that the controller does expect the aircraft to fly the speed. In the first exchange, had Blockhead 123 concluded the conversation by saying,

'Blockhead 123, confirm we may vary our speed?',

or something similar, the problem does not arise. It's slack R/T on the part of the controller, but the crew should ealise this and take steps to clarify their clearance.

Yep, I'm with "tired" on this one. "N.sky" raises a valid point, but a question is not an instruction and visa versa. In a busy R/T environment, it is essential to strike the right balance between clarity and brevity. I would never wish a pilot to feel inhibited in seeking clarification, but please only if reasonable grounds for doubt exists. Let me cite another example (not a moan). ATC clear pilot for a Visual Approach. Frequently, the pilot will then ask if he may descend. Quite unnecessary. If ATC require a level restriction, it must be specified with the clearance. If you are asked to report your speed to XYZ, then please do just that. If a particular speed is required, it too must be specified. Hope this helps.

Back to allthenines original post.

The min rate of descent *is* 500fpm in the UK.
That accomodates unpressurised aircraft as well as others when descending in a stack and allows the use of procedural separation for stack management.

Enroute , pressurised aircraft that descend at that rate will penalise themselves in anything other than a quiet environment.

Unpressurised aircraft may expect their routes to be extended when the ideal descent profile is blocked by other traffic.

That's all there is to it. Anything else has to be a contract between controller and pilot to acheive a desired game plan.

[ 23 August 2001: Message edited by: hatsoff ]

Allthenines, I'm agreeing too. No need to go belting down (apart from the aforememntioned several levels at a time scenario)- RA's involve far too many heartbeats and paperwork for my liking, but there's a number of skippers I fly with that think along the lines of what you were saying. Sounds like we're on the same fleet/airline!

NorthernSky- I'm with the other respondant on that too- if you're asked your speed and all you get is a roger and you're unsure whether you have actually just received a command- ask. Can you imagine the amount of wasted time if the control had to apend "I'm actually just asking this for info-...could you tell me your speed" every time they were asking a question.

Spose everyone's got foibles tho. (that spelt right?)

Alf

Alfie,

Don't get me wrong, I'm with him too!

My point (which relates mostly to non-UK ATC) is that there is reasonably frequent use of this less-than-specific form of words. Yes, I do seek clarification - but others don't, and I share the sky with them (we all do). The solution is for ATC to recognise this, and to use an expression like 'just for my information' or to confirm 'free speed'.

Why, in any case, would you be interested in knowing the instant speed of an aircraft in your sector, unless to decide what speed to impose?

In particular, and despite the recent recognition of this issue, many ATCOs don't understand energy management clearly, and don't realise that in a typical descent between FL350 and FL150, say, I may vary the speed between 340kts and 210kts several times, in order to fly efficiently and to manage the descent. Some aircraft do this automatically, others rely on the pilots, some of whom do use the technique and some don't.

However, I don't want to bog us down in a single issue, and I must say that this doesn't frighten me half as much as 'Descend FL80 and contact.....' or 'Cleared for Take-off rwy 14' whilst the previous landing aircraft is still on the runway, or..... shall I go on?

HF issues are real ones, and we need to be conscious of them.

BTW 'foible' is spot on (from the French faible), and good to see the dust blown off a great word :)

[ 23 August 2001: Message edited by: NorthernSky ]

Hi it's me again. Well I'm glad that we got that issue sorted out? I think we're basically agreeing that the legal figure is 500fpm but in some situations a higher rate is useful (clearing SEVERAL stack levels from the en-route release - good point Vertigo). The reason for my querie is that it's much better, from a passenger comfort and noise point of view, to keep a constant rate of descent all the way down. There's nothing worse than the thrust going up and down all the way from 45 North of BNN to when we drop the wheels. It makes ME nauseous never mind the PAX down the back!

Cheers guys. :cool: