Spectators Balcony (Spotters Corner)If you're not a professional pilot but want to discuss issues about the job, this is the best place to loiter. You won't be moved on by 'security' and there'll be plenty of experts to answer any questions.
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.
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.
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.
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.
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.....
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 l@ser 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 ]
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???
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
The l@ser 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.
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.
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.
Vortex Wake and the 757
