Vortex Wake and the 757
 

I'm puzzled... :confused:

I clear a 757 (black and yellow with a big M on it) for take off one minute behind another 757. He says "need another minute for vortex behind the heavy." I remind him that it was a 757 and he says he knows!

This is going too far!

If you consider a 757 a heavy and you're in a 757 following there is surely no vortex wake requirement on departure.

As it is, its bad enough that some airlines need two minutes behind a 757 and don't tell you until you clear them for take off, but this just takes the biscuit.

What is going on?

Prepares to be enlightened...

the 757 has exceptionally severe wake turbulance that has been a factor in many aircraft crashes. As a result, the heavy category was extended to the 757. Its wake turbulence is worse than the 767.

Its climbing performance is so great that you cant get above it either.

Cheers
Wino

Wino

I'm aware of the problems associated with the 757, but my point here is that in the UK it is NOT classed as a heavy by ATC. Airlines have their own minima for take off behind a 757 that differ from those of ATC.

A 747 behind a 747 does not require a vortex wake separation, so why should a 757 behind another 757 require one?

Are the vortices so dangerous that we should be allowing two minutes for all aircraft behind 757s?

It is the lack of consistency in this issue that causes confusion and frustration.

In the US I beleive it is mandatory but can be waived by the crew of the following aircraft.

Is it a threat? I think so.

So are CBs, yet some people fly right through them and nothing happens.

On a windy day i dont worry about wake. But on a calm day? you bet!

cheers
Wino

BTW did yoou ever go to a Yank pilot house party in Lymm two summers ago?

[ 30 July 2001: Message edited by: Wino ]

Heard about them but never went. Far too lively by all accounts! :D

Perhaps it's time that the vortex wake categories were revised!!
I must admit, I've fallen foul of this one and had a/c line up behiind a 757, with one short final, only to be told "we need two minutes behind the 757" then the inevitable go around happens!!

I've now got into the habit of asking pilots if they'll accept 1min behind a 757, seems to work.

What puzzles me though, is if the 757 vortex is worse than a 767, why do the ATP's the Dash 8 and such like gladly accept the 1 min departure, they will fly through the vortices at some point, surely they will be more likely to suffer the ill efffects of flying through them.

Has anyone done a study on this, I believe that NATS have been doing some research into vortex wake encounters but not sure if there is any definitive study on the matter.
All yu pilots out there could help us overstretched tower bods by telling us when you come on frequency if you'll need 2 mins behind the 75 ahead of you on the taxiway.

Safe flying

Wino, that's the first time I've ever heard someone say the 757 has a high climb rate. ;) ;)

It climbs like a broken box and only then if you're not going anywhere. :( :(

[ 30 July 2001: Message edited by: max_cont ]

Isn't the 757 in the upper medium vortex class?

Upper medium is only a classification on arrival not departure, requiring a smaller aircraft to be an extra mile behind it on approach.

4 instead of 3 in the case of lower mediums and smalls, and 6 instead of 5 for lights. These distances are still "on trial" at the busier UK airports.

Cossack

The prob here is the difference between the wake category and the actual vortex produced by the 757. In my experience as a BA 737-200 Capt I had one too many encounters at one minute spacing so always then insisted on two mins. The effect of catching a 757 wake is to be rolled at about 90 degrees a second and that needs FULL opposite aileron and a touch of rudder. Try it at about 400 feet for a real adrenaline rush! If your clients want 2 mins behind a 757 I'd let them for safety's sake alone.....

Cossack

You are right. There is no additinal spacing required for a 75 following another 75 either inbound or outbound, but Wino is correct; the 75 does cause vortex which is inconsistent with its weight and so perhps we do need to look at this. KLM and a few others are now insisting on the 75 being treated as a heavy, if they are following it (it is a requirement under Dutch law.) A further problem with the 75 is its very slow typical Vref which means that following traffic (particularly 734s) will often catchup by over a mile during the safety critical final part of the approach.

It probably is time that the wake vortex catagories were looked at to allow for such things. In TC we are developing a tool called FAST which will eventially allow us to go to time based spacing on final. This will permit us to have an individual wake vortex spacing for each possible combination of aircraft types. When/if mode S ever makes it in we will be able to further adjust that for the relative Vrefs using the downlinked data. At present, no alterations are made for surface conditions (either way) and if there is no headwind the time interval between vortex pairs is often much less than it is intended to be.

Wake vortex is a killer and we mess with it at our peril. The danger isn't just confined to airframe loss either, there is also the issue of serious personal injury to crew or pax. I have been involved in countless very serious incidents, often invovling the 75 and a close friend once had a terrifying encounter going into Paris. (The airframe had to be structurally tested before it flew again because it had been on its back. There was 'toilet blue' all over the ceiling.)

We did a laser trial at LL about 7 or 8 years ago to see if we could cut the spacings further. I am led to believe that far from reducing the spacing, the results showed that under some circumstances we should be increasing them; so it all went quiet.

This is where I have a problem. We are simply not prepared at the political level to address the issue of runway capacity shortage. The Heathrow runways are so oversubscribed that the idea of increasing the spacings is a political non-starter and there is pressure on us to be on the legal minimum. The lack of redundant runway capacity is leading us into a "push it" culture where safety is not coming first.

Don't the politicians always say safety is paramount?

[ 30 July 2001: Message edited by: ZeroNine Left ]

[ 30 July 2001: Message edited by: ZeroNine Left ]

[ 30 July 2001: Message edited by: ZeroNine Left ]

Recomended spacing,

I think your average turboprop will accept redced spacing because he will rotate much nearer the threshold. If he is on a departure which gives him an early turnout, not behind the 757, then it shouldn't be a prob.

Max_cont.

The 757 has a superb rate of climb because in the UK at least they are generally at least ten tons below max certified take off weight. This is because it saves the airline massive amounts of nav charges if they are registered at 103 tonnes and will still go anywhere in Europe. Some are still registered at 113 tons to allow them to go to Israel, Egypt, Bangor etc.

Bally, I appreciate your point, but where I work, the jet Sid's go straight ahead, the props go straight ahead til 4d, therefore there is no "early turn", just an earlier turn. They still fly through the vortices generated by the 75, but the pilots don't seem to have a problem with that at all.
Not being a pilot I don't fully understand the logic behind this, if it's unacceptable dor one company's 757 to depart 1 min behind another 757 or safety reasons, then why i it acceptable for an ATP, Dash 8, 737, MD80 etc???

Recommended Spacing

My own view is that 1 min spacing is sufficient for most medium to heavy aircraft. The wake from any aircraft drifts downwards and outwards from the aircraft in the worst case (Still air) Thus any aircraft following, with aproximately the same or better climb angle is going to be well above the vortex. The only time I have encountered wake has been on approach prior to G/S intercept or on NAT tracks and airways. That said, it is up to the aircraft captain to make his own decision. As long as he lets you know before he lines up :mad:

Bally Heck. Does the term "tongue in cheek" mean anything to you?

Check your company forum and you'll see some of my posts there. So now you know I know the B757, I still say it climbs like a broken box :D :D

If you need a graphic demonstration of a good climb rate, jump into AA or AB

The laser trial at heathrow showed that under some circumstances vortex will CLIMB. One can no longer assume they are going to descend.

There is real danger in the using higher climb gradients in order to cheat on the spacing. Imagine he has an engine failure as he rotates; he will then be slow, with no spare power and so the authority of his control surfaces will be limited at a time when he would need maximum if he gets into vortex.

I restate my previous point: Vake vortex is a killer and we mess with it at our peril. If there is a capacity problem at airports which is pushing us into this mode of ops. then we need to address that, not cut corners on safety.

In the UK the categories for vortex wake are as follows:

Heavy 136,000kg or greater
Medium less than 136,000kg and more than 40,000kg
Small 40,000kg or less and more than 17,000kg
Light 17,000kg or less.
Note: Light is a category only used in the UK.

As was mentioned earlier, the 757 comes in at around 113,000kg which is considerably less than 136,000kg needed to make it a Heavy using this table from MATS Part 1.

It has been recognised,however, that the 757 produces vortices far stronger than its weight would indicate and so has been placed into another group at certain airports, namely Upper Medium.

The separation required for arriving aircraft are as follows:

Heavy followed by:
Heavy 4 miles
Upper Medium/Lower Medium 5 miles
Small 6 miles
Light 8 miles. (Reduced to 7 at busier airports on a trial basis)

Upper Medium followed by:
Heavy/Upper Medium 3 miles (radar separation, not vortex wake)
Lower Medium/Small 4 miles
Light 6 miles

In the USA, they have categorised the 757 on its own in with Heavies. The separation for arriving aircraft are:

Heavy behind heavy 4 miles.
Large/Heavy behind B757 4 miles.
Small behind B757 5 miles.
Small/Large behind heavy 5 miles.

It can be seen when comparing these tables there are some noticeable differences. More space is given when following a 757, even to Heavy types and less space is given to Light aircraft.

Surely it is time, as 09L has said, that the issue of vortex wake categorisation is looked at closely and operating criteria are harmonised so that we are all doing what is expected of each other. If a 757 generates such strong vortices that even a Heavy may be affected then a recategorisation is required sooner rather than later. Up until then we will continue to be perplexed with late requests for extra time (up to a total of 3 minutes from the same departure point) when following 757s. On departure only runways this is an inconvenience that only slows down the departure rate. On mixed mode runways it will cause havoc.

Max Cont

If you are comparing an aircraft's climb rate to AA or AB then relative comparisons are unfair until you get into types pre-fixed with an F. Sorry I thought a broken box was a shed on 1 donk. :eek:

So if a 757 becomes a Heavy a/c then does that mean that it can go 1 minute after another heavy on departure? Eg. 747 followed by 777 is 1 minute sep, then the 777 follwed by 757 is also 1 minute split. I dont think that many 75 pilots would be happy with this and I wouldnt be happy doing this as a controller.

But after all arent the Vortex separations the recommended MINIMUM? So therefore any pilot is within his rights to ask for more time.

Correct, Slim Shady and the same applies to ATCOs. Nobody should feel they have to tuck a 73 4 miles behind a 75, I use 4.5 as standard because that is what I believe is safe. In still wind I add extra to everything. :)

COSSACK...The guys have a point, sometimes departing behind a B757...or...following one on approach can be an interesting experience . For us the cutoff is the defined 136,000kgs making an exception for the B757,generally, as one makes their way out for departure at LHR and the subject a/c is departing ahead, I would endevour to advise ATC of the 2 mins or even 3 if using B16 or B79,sometimes I've felt the lash of ATC's razor tongue but that a/c does churn up the air a lot ...Please fogive us our...

09L,

We can only play by the rules given to us.

If everyone decided to fudge them to their own personal criteria, where would that leave us? Be a bit of a shambles I suspect.

You don't happen to tell every aircraft that is following a heavy that superfluous piece of info as well do you?

Burp.

Well Burp, appropriate handle!

Correct, and the rules permit any controller to increase separation when they deem it fit. For example, following an aircraft type such as the 75 that is known to produce more vortex than expected from its weight and which has caused many vortex incidents. Safety before movements doesn't seem unreasonable to me.

Superfluous? Well, wake vortex is unpredictable. With your time in the job you will have had traffic behind a heavy get into a vortex problem at some stage, even when he's comforably over the prescribed minimum. It seems to me that to give the guy a warning that he is following one would be a most professional thing to do; and that is what we are about here isn't it?... professional ATC?

Here's a piece of superfluous info. for you: A tree is in one of two states; it is either growing or it is dead. Now if all human beings were open to growth the world would progress. That would be nice, wouldn't it?
Night night :)

A Slim Shady
You have just re-inforced the point I was making in my first post. If pilots consider the 757 to be a heavy if they are following it, but not if they are in it, it opens up a can of worms where ATC don't know what the separation should be because its all at the discretion of the following pilots or their company.

09L
I'm not suggesting a 757 go 1 minute behind a 777 but there has to be a laid down procedure for this as in the US. We are, like it or not, under commercial pressure to move aircraft at a contracted rate to and from our runways. As burp implies, if every pilot wants another 30 seconds or a minute before departure or you provide and extra half mile on every gap because you think it appropriate, I think you will soon find that targets are not being met and BAA or whoever will want to know why.

TR3
Is it less safe to depart 1 minute behind a 757 than 2?
I don't know, but there are people in offices paid to make these rules on the basis on research and fact. They have said that you can depart 1 minute behind a 757 so 1 minute it is. If you want/need more, tell us early, it may mean you depart before the 757.

In 14 years of controlling I have not yet been notified of any wake vortex incidents from aircraft under my control. Are pilots keeping quiet or just no incidents to report? I do pass type and distance of preceding traffic when separated by the minimum, maybe that helps I don't know.

Until the rules are modified I will continue to use the separations laid down.

Once again, if you want more than the ATC prescribed minimum time before departure, tell us early.

Here in OZ the 757 is classed as a Heavy if leading and as a Medium if following.

Ah, a common sense approach...and from Oz too!!! :D

Also on the subject of wake. When I was completing my command training we landed at LHR on 27R with a very light tailwind from the rear right hand quarter. The training captain then related a story to me of landing in a similar situation with nother command trainee where the preceding aircraft's wake had been blown back to the threshold/landing area. He said that it was very exciting during the flare!

Sme months later it happened to me in not quite so dramatic a fashion but noticeable.

Apologies if I am preaching to the converted but something to bear in mind.

09L,

You may recall that the Upper Medium group was introduced to cater primarily for the 757. This following a fair amount of study into the subject.

If you now have some non anecdotal evidence that the spacing should be increased even further, I'd be very pleased to hear it.

In the meantime, I'll just carry on applying the recommended spacing. Perhaps the only saving grace here is that the number of 757's operating into EGLL is gradually reducing.

Thanks for the philosophy lesson :D

Burp.

Cossack

Sorry, I may have misunderstood you! I wouldn't suggest a 75 goes 1 min behind a 777 either!! But with respect, I cannot agree about the contracted rate. We declare a cpacity which we would expect to meet, all other things being equal but we are not contracted to that (well not at LL anyway). Sometimes, in ideal conditions, we smash that rate and at other times we don't get near it. I guess what I am trying to say is I don't believe any individual should feel that they HAVE to make targets. That will just lead us further into this "push it" culture which will get people into trouble, particularly the less experienced.

As far as the 75 is concerned, it is not just the high vortex, the darned thing is also very slow on short final; yes, I add a bit on behind but a take a bit off in front and that way you don't find 73s only 3 miles behind it as it touches down. Safety is enhanced but the landing rate doesn't change. I think that is a reasonable and professional way to go about it.

As far as BAA are concerned: We all know that the runways are oversubscribed and that there is no spare left to cater for problems but that isn't our problem. It is not our job to make up for the unwillingness of government/BAA to provide the runway capacity that we need. Our primary task is safety and if on a still morning when 76s are being bumped around by 75s then it is unwise to remain stuck to the rules. "...the obedience of fools etc..."

Burp. No, as I am sure you know, I connot give non-anecdotal evidence of the 75s nature but I can point you to the evidence on this forum and that found in other countries such as the US, Germany and Holland. You are of course quite entitled to take the stance you do and nobody could criticise you for it but what is more important, the words of the rule or the spirit it enshrines? Are we forbidden to use our common sense? If we know that what is written doesn't really suffice aren't we honour bound to address that?

Thanks for listening. :)

[ 01 August 2001: Message edited by: ZeroNine Left ]

[ 01 August 2001: Message edited by: ZeroNine Left ]

TR3 and any other EGLL pilots.
I and many of my colleagues promise not to give you a lashing with our tongues (fnarfnarf) if you want more than the prescribed minima, but please tell us before we give the runway to you
:)

DITTO!

Fair enogh "YELLOW SNOW"

What a lively debate!

09L
What would happen at TC if one day a Heathrow Director (not the HEATHROW DIRECTOR obviously ;) ) was feeling a bit off and decided that they would add a bit, say a mile, to every gap?

Answer: The holds would fill up, the achieved landing rate would be looked at, the reasons for it would be sought and said Director would probably be asked to pull the finger out or be replaced.

What I am trying to discover here is, is the recommended spacing still appropriate? If it is we should all be willing to use it. If it is not, then the powers that be need to do some more work and find spacing requirements for both arivals and departures that ATC and airlines are happy to accept and then any confusion is removed.

I'm all for using common sense, but common sense, or the lack of it, varies from person to person. We should have a system that works well in normal day to day use without the necessity to resort to common sense. It is just another variable.

There's obviously something wrong with what is happening at the moment because different countries have different rules for the same problem. Who is right? No wonder pilots get confused. Is it 4 miles, 2 minutes or will 1 do? :confused:

Morning Cossack
Yes, it's a great debate, enjoying this one.

I completely agree that the system does need to be standardised because a KLM will come onto the frequency and insist on 5 miles if he is put behind a 75, because of his national law, when everybody else has to take 4. Also, this level of anecdotal evidence is quite sufficient to tell me that this needs to be looked at. How should we go about starting that?


A couple of other things: at LL, unless there is no headwind, the holds are full for a significant part of the day anyway.

We have a PC which constantly monitors the landing rate, giving us a reading for each of the last quarter hours and predicting, on the basis of the last 6 landers, what we should achieve in the hour. It helps the traffic manager with flow.

As far as spacing is concerned; well firstly, THE Heathrow Director never has an off day, he is always on top form! But there is a lot of bu*****t about LL and how perfect everything and everybody is. The truth is that it isn't like that. It does happen that sometimes you don't feel like packing 'em in and you don't. I have seen even "the best" doing that but I have also seen the same people breaking wake vortex spacing in a misguided effort to reduce delays. A quarter mile or so and not corrected, even on still days.

A senior manager said to me about 3 years ago that one of his worries was that the LL team clearly felt they HAD to land huge numbers all the time and that exposed them to error. It is not our job to make up for capacity shortages and every time we try we take pressure off other people to do what they should do.

[ 02 August 2001: Message edited by: ZeroNine Left ]

09L
We have the same readout on our EAT machine too. Interesting reading somedays! We can see how you, Gatwick, Stansted and Luton are getting on too.

As for starting a review, I guess its a word in Unit Management's ear and take it from there. Maybe the powers that be read this. You can only hope.

There is no doubt there is pressure to perform to the best all the time, but this pressure has to be tempered with knowing that what you are doing is safe in everybody's mind, not just your own.

Spot on!

I had no idea we were national viewing!

Have you been to have a look at FAST? BIG debate going on here about it. We won't need it until we get to time based separation but SASS would be a real benefit now, making the job much easier and it should shorten training times too.

How are you for staff up there? :)

I saw a FAST demo at the ATMDC a couple of years ago. Won't it take the "skill" out of it a bit?
When you get a "quiet" few minutes I'm sure someone will let you see how we're doing to! Up to 36/hr yesterday on westerlies with all those crossers! The departures were a bit slack though!

09L,

I'm intrigued a little by the above. You increase separation behind the 757 but reduce it in front. Do you only do this when you've agreed 2.5nm separation with the tower in advance or would you do it anyway because it is another of your own rules?

I honestly have to say that no KLM has ever asked me for increased spacing behind a 757. They used to ask all the time on departure when I did the tower, but never on arrival.

Our senior manager must have been very worried, especially as every year for the last three years the declared capacity has gone up :confused:

B.

cossack,

Currently you almost have to be more skillfull to use FAST so that you can recognise when it is trying to sell you a pup :eek:

B.

Now there's progress! :D