Holding patterns -why?

[dkatwa]

Hi all...another stupid question....why do planes circle in holding patterns before landing? With modern computers etc, can't they land right away (subject to, say, bad weather over the airport etc)?
Also, how much does it cost to do an orbit in, say, 737 or A380?

[El Bunto]

Do you have a particular airport in mind?

Certainly in my part of the World ( Ireland, north and south ) holding patterns are exceptionally rare and only occur due to weather or an incident on the runway or taxiway. Most aircraft are cleared straight-in with only an occasional dog-leg to slot them in behind others on approach.

Edit: I note you're from London. Heathrow holding patterns... now there's a mess resulting from congestion. Not only aircraft waiting for their slot but also ATC arranging aircraft approaches by wake-category, so that an E-170 doesn't end-up behind an A380.

The alternative, such as practised within the USA, is to hold aircraft on the ground at departure airport so that there is less holding at destination. But that doesn't work across ATC regions and is really just another hack around the fact that we don't have free-flight navigation, by which method all aircraft would arrive exactly on time every time.

[Crazy Voyager]

Aircraft are already Held on the ground due to congestion, it's Done through slots. Different system maybe than the us but same principle.

The Heathrow Holds are an effect of it being extremely congested, you will find the same at other places but not constantly in use like at Heathrow

[dkatwa]

Yes, El Bunto...I was thinking of Heathrow....on a 50 min flight from Lux, we would spend maybe 10 mins going round in circles....
Not being a commercial pilot, I thought maybe planes could slow down/ speed up in flight slightly, to coincide with a straight in approach and landing. Seems like a huge waste of fuel just going round in circles...though I don't know how much fuel is wasted by this

[DaveReidUK]

Heathrow holding patterns... now there's a mess resulting from congestion.

Well that's one way of looking at it.

The other way would be to understand that, for many years until recently, using holding stacks was the only way to guarantee a sustained landing rate at the level that Heathrow achieves. The stacks may eventually become redundant, but for the time being they are a legacy of how things have always been.

It's not rocket science, just basic queuing theory.

[HEATHROW DIRECTOR]

dkatwa... have a look at this web site and it will give you some idea of the amount of traffic in the skies. Flightradar24.com - Live flight tracker! (http://www.flightradar24.com)

[Airbanda]

dkatwa

As HD's link will show you Heathrow has four stacks:

Bovingdon NW
Ockham - SW
Biggin - SE
Lambourne - NE

Routing via those points is usual, even if there's no need to actually enter the holding pattern. Probably the only practical way to meet need to separate LHR inbound from outbound and both from traffic using the other London airfields as well as providing a clear starting point to sequence them onto approach respecting wake turbulence etc issues.

[dkatwa]

thanks guys...the only remaining question in my mind is the cost of one orbit, in a typical small jet (737) and a biggie (A380)....

[HEATHROW DIRECTOR]

Probably quite a lot. However, just consider how much fuel is wasted every day by millions of cars in traffic jams. There is no real answer.

[Crazy Voyager]

Aircraft are already sped up and slowed down to manage the holds and reduce the holding times, here's an article on slowing down inbounds

Cross Border Arrival Management (XMAN) Trial | NATS (http://www.nats.aero/newsbrief/cross-border-arrival-management-xman-trial/)

With regards to the cost I'll leave it to one of the pilots to answer in detail, but a full hold is 4 minutes flying time.

Also if you're in the hold 10 minutes you're unfortunate, the average in June 2014 according to the article above was 8 minutes, I think it's reduced even further now with xman although I'm not sure what the latest figure is.


Also remember that Heathrow is so busy that if you start missing landing rates you will have delays and/or cancellations later. Heathrow needs to consistently land at aircraft the entire day, if they don't it will cause issues later, which is down purely to mathematics really. That's how full Heathrow is in terms of capacity.

[oyviv]

Many years ago I frequently flew as SLF to LHR, but hardly ever landed in the morning rush hours without first being stacked above the green and pleasant land in the Home Counties or thereabouts!
One of the few exceptions was a fuel emergency in B-747! I still remember the eerie approach we made from the Channel Coast and the sighs of relief at touch-down!
I think I'll prefer a couple of turns in the stack any time!:ok:

[Talkdownman]

It's not rocket science, just basic queuing theory
dkatwa, imagine the sands of a simple egg-timer (http://img.ehowcdn.com/615x200/ehow/images/a04/pd/3t/make-simple-minute-timer-800x800.jpg)…the upper sphere is analogous to the holding patterns. The system needs such a reservoir to achieve optimum flow onto a limited resource…ie. the runway. There's even a book about it (http://mason.gmu.edu/~jshortle/FQT4_Cover.jpg)...

[Qwerti]

As a rule of thumb, average fuel consumption figure on the 737 is 40kg/min

[philbky]

Crazy Voyager, not sure why you think the average hold reflects what you should expect. It all depends on the time of day and, more importantly the crosswind which, in my experience of over 50 years of flying into the Row, has become more prevalent in recent years and, of course, runway 23 has long gone.

Using the first flight out of Shannon to Heathrow which used to be at 07.30 we would regularly be held at Shannon waiting for a slot release. Best departure was 07.55, worst was 08.35. On the 08.35 departure with an in air time scheduled at one hour we actually touched down at 10.15 with a slow crossing and a hold of over 30 minutes. Reason cross wind.

The flight has now been rescheduled for 08.45 but we regularly still have a short on ground hold, a slow crossing and up to 15 minutes over the Surrey countryside.

Now you might think that an 11.25 scheduled arrival might fare better but coming in from Houston on the BA196 we often get a ten to fifteen minute hold. If the winds on the pond are favourable we can arrive over the UK coast up to forty minutes early only to be told by the flight deck that we have been asked to slow and then we get in the hold.

It gets worse, having landed we often have a ten to twenty minute ride around the Heathrow concrete before we can get on a gate.

[Crazy Voyager]

Sure, you will sometimes have holds over the average. But the pure mathematics are that if say flight A holds for 15 minutes, then flight B holds for 7 minutes, creating an average of 8. Obvously you need to apply this to all the inbounds, but roughly 50% of the time you should hold less than the avergage time. Therefore my comment that to hold more than 2minutes over the average could be seen as "bad luck". Obviously as you say yourself, in periods of high inbound demand hold times are incraesed, so if you reduce the time scale from the entire day the statistics change. In the morning the average hold might very well be more, say 12 minutes (just a figure with no facts to base it on), but then the hold time during some other tmie of the day will be less, to once again create the 8 minute average.


Also on days with delay the hold times are often increased, part of this is because it seems most airlines rather depart and then hold for extended periods rather than sit on the ground. Why I don't know but if we ever get the option to ask someone "do you want to go now and hold for 30 mintues plus or wait half an hour" they always seem to pick the option of going now. Why I don't know (airline policy?)

[philbky]

Of course the statistics are there but we all know there are lies, damned lies and statistics and averages only work if your situation is average or better. For passengers travelling at peak times the delays seem to be getting worse.

Whilst traffic has obviously increased since my first flight into LHR in 1963, the increase in hold times, the number of in flight slow downs and the number of pre take off on ground holds on a number of routes seems to have increased at an accelerating rate in the last few years.

The use of one runway for take off, the other for landing, has stymied the reduction in delays for years. Having had professional contact with the CAA and NATs for many years I know the "reason" but can point to many airports around the world where runways with similar proximities work in parallel for landing safely and satisfactorily.

The reason for going and having to hold on arrival stems, it seems, from the FAA's edict that if you are off the gate on time, it doesn't matter how long it takes to make the runway, or if you have to hold at the destination, your flight will be logged as departed on time. This seems to have become a world wide norm.

[MidlandDeltic]

Sure, you will sometimes have holds over the average. But the pure mathematics are that if say flight A holds for 15 minutes, then flight B holds for 7 minutes, creating an average of 8.

Eh? If one flight has a hold of 15 minutes, and another 8, the average hold time of the two is (15+7)/2=11 minutes, not 8............

[Crazy Voyager]

Oops, maybe that's what I get for dropping out of uni and starting work in aviation instead :O Not sure how I managed to come to that conclusion since it's obviosuly wrong but might as well leave it there now for the internet to laugh at.

Hopefully though the rest of the post makes some sort of sense? Yes I'm aware it's a slightly blunt statistic, but at the end of the day, if the average delay is 8 minutes a random flight at a random time will have a delay of roughly that and a hold of more could be seen as "bad luck".

Anyhow, might be time to move on.

I think though, philkby, that you touch on a very interesting and good point. To measure on time performance the way it's done now does create some very "interesting" results, as you said yourself, lies, damned lies and statistics.

[HEATHROW DIRECTOR]

<<If one flight has a hold of 15 minutes, and another 8, the average hold time of the two is (15+7)/2=11 minutes, not 8............>>

That still baffles me, but I never was much good at maths! 15 and 8 suddenly become 15+7...?

[fujii]

First, the terminology. Planes don't circle, the patterns are "racetrack"
Another thing that can result in holding is the weather. Down my way if a line of thunderstorms hits the airport, numerous things can occur. There may be a need to change runways and aircraft will be requesting vectors clear of cells. This makes it very busy in the terminal area so arriving aircraft outside the terminal area mal be placed in outer holding stacks. When thunderstorms are in the area, ground staff are not permitted out on the aprons (WHS requirement) so aircraft cannot park.

Similar to above, a fast moving front may require a runway change. A tower tries to stay ahead of the change but sometimes they get caught by the speed.

Rapidly deteriorating weather below the CAI I ILS minima can also cause problems. Greater spacing is required between approved CAT II and CAT III aircraft and if there are any CAT I one aircraft in the mix, they have to be put somewhere.

[DaveReidUK]

The use of one runway for take off, the other for landing, has stymied the reduction in delays for years. Having had professional contact with the CAA and NATs for many years I know the "reason" but can point to many airports around the world where runways with similar proximities work in parallel for landing safely and satisfactorily.

The fact that Heathrow operates its runways in segregated mode, with the runway roles alternating at intervals, has nothing to do with safety considerations.

[Gonzo]

The fact that Heathrow operates its runways in segregated mode, with the runway roles alternating at intervals, has nothing to do with safety considerations.

Well, there is the fact that independent parallel arrival is not currently a procedure approved for LHR by the CAA

[DaveReidUK]

Well, there is the fact that independent parallel arrival is not currently a procedure approved for LHR by the CAAChicken and egg.

There has been no reason up to now to develop the safety case for independent parallel arrivals, given that runway alternation has been enshrined in government policy for the last 40-odd years.

But with the watered-down alternation scheme that will result if a third runway is built, simultaneous landings on adjacent runways would happen around 50% of the time and the CAA have already published their initial assessment of SOIR at LHR.

[philbky]

I said I knew the reasons for the segregated mode which doesn't imply I thought it was a safety issue. The point I made about other airports was to pre-empt replies about the safety issue. Perhaps I should have put that point in a different sentence.

Staggered parallel arrivals are used from time to time. The distance between the runways is just 50 metres short of the international standard mentioned by LookingForAJob.

At SFO the distance between runway centre lines is less than 250 metres with the added complication of parallel take off on two runways cutting across the two landing runways at ninety degrees. Taking off at night towards Oakland Bay, sitting on the right hand side of an aircraft is always an interesting experience looking at two sets of lights heading towards your aircraft and being conscious of another aircraft also taking off next to you.

It would be interesting to know just how much more difficult that sort of operation is compared to slotting in take offs at LHR between parallel landings.

If and when a third runway is built at LHR it will be interesting to see just how the government of the day handles the flack that will be heading its way when parallel operations become a norm.

In the meantime, given climate changes and the increasing frequency of stronger cross winds, delays will continue and there is absolutely no prospect of a runway angled to deal with the problem.

[Gonzo]

Dave Reid,

You are conflating independent approaches with mixed mode.

They are different, one does not lead to the other.

LFAJ,

Taken in isolation, the runways are far enough apart.

However, I can see that others here know far more about this matter than I, so I shall bow out of the conversation.

Next time I'm in my office I shall pay more attention, there are obviously more people working there then I notice! :}

[DaveReidUK]

IIRC, the runways are not far enough apart to comply with international standards for independent operationsYes, but that doesn't necessarily preclude them. The CAA's view:

"The current ICAO separation for allowing independent approaches with no mitigations is 1525m.The existing runway separation is approximately 1450m and the proposed northern runway separation is 1035m from the existing northern runway. The proposer must demonstrate that the design meets the requirements of ICAO Document 9643 The Manual on Simultaneous Parallel or Near Parallel Instrument Runways (SOIR) for all possible modes of operation, in particular given the stagger of the proposed northern runway. Appropriate mitigations will need to be provided."

[DaveReidUK]

You are conflating independent approaches with mixed mode.

They are different, one does not lead to the other.No, I'm not. I'm perfectly aware that they aren't synonymous.

But the proposed R3 concept of operations of necessity implies both:

a) mixed mode on one runway for the bulk of the day (because arrival and departure demand is fairly evenly balanced, except for early mornings and late evenings) with the other two operating segregated arrivals and departures

and, therefore

b) simultaneous approaches on the mixed mode runway and the arrival one, which 50% of the time will mean two adjacent runways (hence the CAA's comments above)

[Talkdownman]

I recall we trialled mixed mode once and it was an absolute nightmare for GMC, especially getting N/S departures to the correct holding point, all that head-on traffic on the outer. Went in the same bin as the tower periphery furniture and CCF 'Tunnels in the Sky'. Also better leave some room in the bin for 27RW. Do you switch off SMF when doing staggered parallels?

[HEATHROW DIRECTOR]

I presume this discussion refers to parallel approaches in IMC? I've done hundreds of parallel approaches in VMC, and that's going back many years.

[Talkdownman]

Me too, pre-SMF. nats were happy at the time to turn a blind eye to it but didn't like it once their SMF was installed. Remember the fuss over the parallel Concordes? All consenting adults sensibly coordinating during an auspicious and spectacular occasion, yet those involved were seriously criticised because separation was 'lost' during parallel approaches, even with two complicit Concorde captains. Of course, the ATCO came off worst...

[HEATHROW DIRECTOR]

I had a couple alongside each other one day, Traffic in front and behind were yellow and the two on the approach were red. The Chief Wizard asked what was going on and I said "I'm trying to find a green one for a set of traffic lights".

[pax britanica]

Some airports hold and some-Frankfurt is a good example just extend the downwind leg -before they got the extra runway that extension could be very long indeed so you could see the 'field' off to the side at about 10,000ft but probably didn't land for another 20 minutes while you went half way to Stuttgart and back . Not possible at LHR due to all the other airports and traffic but works well at Frankfurt where people were spared the going around in circles experience and don't in general realise whats going on.
Arlanda in Stockholm has situation where because of geography almost all flights arrive from the south and so rather than hold they slow everyone down over along distance and separate them in trail as the head for the turn onto finals like long drawn out queue . Singapore have their own version where you go for another trip around the island when its busy and again its less obvious the racetrack patterns. Horses for courses as they say but a lot depends on geography/traffic patterns and nearby airports or lack of.

When its extremely busy LHR (Someone will no doubt correct me if I am wrong on this) sometimes use high level or 'en route' holds where inbounds are held at points on the approach path like Southampton or one of the Lambourne inbound waypoints at say 15000 ft and above because the normal holds can get full and for ATC reasons can only operate between certain altitudes, as the stacks empty aircraft are released for these points only to pick up another hold-the normal one closer to the field.

On 'the' Sept 11 I spent 6 hours in one such hold over Lands End before returning to Gatwick from where we had originally departed-a nice sunny day and some nice scenery on part of the track but the novelty, and the view, did wear off after a while-like the first hour lol

PB