ACN and PCN

[Porrohman]

Thanks once again for your replies OverRun.

I was a bit surprised when I checked the population of Edinburgh, and it showed as only 450,000. And 9 million pax/year. From the airline economist's viewpoint, that is at the bottom end of traffic needed for a decent sized airport, and I would think it might even be getting below that needed to have commercially viable B777-300ER operations on a daily basis. There is not much scope for bigger planes at smaller airports because it drops the frequencies down and the punters prefer higher frequencies. The medium sized 250 pax A330-200 is better fit at a place like Edinburgh than a 350 pax B777-300ER.

Although the population of Edinburgh is only 450,000 the catchment area for the airport has a much higher population. The majority of the Scottish population live within 2 hours drive of Edinburgh Airport and Edinburgh is a major business and tourist destination.

To quote from Wikipedia;

Edinburgh is the most competitive large city in the UK according to the Centre for International Competitiveness.

Edinburgh is the UK's second financial centre after London and Europe's fourth by equity assets.[68] In world terms, it ranks ahead of Dubai, Amsterdam and Washington in the Global Financial Centres Index.

Tourism is an important economic mainstay in the city. As a World Heritage Site, tourists come to visit such historical sites as Edinburgh Castle, the Palace of Holyroodhouse and the Georgian New Town. This is augmented in August of each year with the presence of the Edinburgh Festivals, which bring in over 4.4 million visitors,[67]

Add to this the fact that there are far more Scots and people of Scottish descent living outside Scotland than inside, many of whom return to Scotland quite regularly, and there is a large untapped potential for long-haul flights from EDI.

Over the past decades, some would argue that it suited BAA and BA to try to route as many pax as possible via their LHR and LGW hubs but many in the business community in Scotland feel that the lack of direct long-haul flights has harmed Scotland's competitive position. It appears that this situation may well improve in years to come if we believe BAA’s master plan for EDI. This ( http://www.edinburghairport.com/asse...nv2_single.pdf ) predicts passenger numbers will increase to 13.7m by 2013. The forecast for 2013 to 2030 predicts that "Passenger numbers could grow to 26 million a year, with over half travelling to and from international destinations."

At the moment, millions of passengers every year travel from EDI via European hubs, such as London Heathrow, London Gatwick, Paris, Amsterdam and Frankfurt, to long-haul destinations. UK CAA statistics show, for example, that there are enough passengers from Scotland to Hong Kong per annum to justify a daily direct flight. At the moment, EDI's ambition is to add some extra long-haul flights to North America and the Middle East. I haven't seen the detailed statistics, but I expect that there are enough passengers to and from the Far East and Australasia to justify a regular direct flight to Singapore.

The ACN at that weight is 79, which is pretty close to the 74 PCN of the 06/24 runway, and using the 10% rule, the pavement engineer is almost certain to grant a concession. A final check is still needed by the aircraft performance engineer that it will hit nothing in the climb on a hot windless day with an engine out. JT can do that [IMG]file:///C:/Users/Colin/AppData/Local/Temp/msohtml1/01/clip_image001.gif[/IMG]

Looks like it could be a viable operation to me. Except I'd prefer to fly A330s and make more money from it.

Given your comments in an earlier post about the damage that 777-300ERs are causing to pavements at some airports, how likely is it that the 10% rule would be applied for this aircraft type? With the 10% rule, medium range routes from EDI with a 777-300ER might be possible but without it they would not. I agree with you regarding the A330. It is better suited to EDI's PCN and runway length than a 777-300ER and better suited to testing the market potential for long-haul from EDI.

It is still possible that the PCNs of taxiways Lima and Mike (unknown but possibly 31F/C/X/T) are constraints for long-haul ops from EDI. If anyone reading this thread knows the PCNs for these taxiways then please let me know.

[Porrohman]

I have managed to establish that Taxiways Lima and Mike at EDI have the same PCN as runway 12/30 i.e. they are 31F/C/X/T. This means that the PCNs of the SE Apron (72/R/C/W/T) and the South Cargo Apron (79/R/D/W/T) are currently pretty academic because they can only be reached via pavements that have a PCN of 31F/C/X/T. Any operations from these two aprons or from Block 33 (the turning area at the SE end of runway 12 which is often used as an overflow parking area) by aircraft that exceed an ACN of 31 on a low strength subgrade on a flexible surface therefore require an exception to be granted under CAP168. (NB. The source of this data is the NATS website and it was confirmed to be correct by BAA ops at EDI.)

For detailed information about EDI such as detailed specifications and layout charts, see the NATS web site.

This means that the only passenger apron at EDI which currently has stands large enough to accommodate any aircraft larger than a B767 without winglets (i.e. the SE Apron) can’t be used for regular long-haul operations because it can only be reached via links that are rated as 31F/C/X/T.

AFAIK, there are currently only two international stands at EDI on the main apron (stands 2 and 4) that can accept a B752W or a B763 without winglets and these are already heavily utilised so accommodation for long-haul aircraft at EDI is currently extremely limited. These are currently the largest international stands on the main apron at EDI.

Now that the Competition Commission are forcing BAA to sell EDI, the new owners will need to decide whether to strengthen the PCNs and increase the number of long-haul stands or not.

The cost to provide higher PCNs leading to the SE Apron should not be too high as it just involves the strengthening of a few hundred metres of taxiways. It’s somewhat strange that BAA did not strengthen these taxiways when they created the SE apron.

The costs of additional long-haul stands I imagine will just be a supply and demand issue for the new owners; if the demand is there they will be built and if it’s not they won’t.

The cost to upgrade part of 12/30 to allow aircraft with a higher ACN to reach the South Cargo Apron would be more substantial. Approximately another 600 metres or thereabouts of runway would need to be strengthened. Strengthening this part of 12/30 would also allow aircraft with higher ACNs to use Taxiway Hotel to/from the SE end of the SE Apron so it wouldn’t just benefit the South Cargo Apron.

The simplest way to strengthen these would presumably be to add an additional layer(s?) of asphalt to the top of the existing surface. Given the existence of alternative taxiing routes, I imagine that these works could be carried out without causing too much disruption to airfield operations.

I would therefore be surprised if the new owners do not, as a minimum, upgrade the PCNs linking the SE Apron to Taxiway Alpha and thus enable the three large wide-bodied sized stands on the SE apron to be used for aircraft (e.g. A332, A333, B752W, B763W, B764, B787 and B744) at the sort of ACN levels that would permit regular use for long-range operations. I'm somewhat surprised that BAA has left this weakness to persist for so long, especially given the number of press releases they have made in recent years saying how they are trying to attract additional long haul flights from Asia, the Middle East and North America.

Hypothetically, if the new owners make a decision at some stage in the future to strengthen the main runway (06/24), the main taxiways and the main aprons to bring them up to the strength necessary to enable operations of B773ER and B777F aircraft without weight restrictions how, in practical terms, would that work be accomplished when the runway is so heavily utilised and when the secondary runway 12/30 has so many shortcomings? (it has no ILS/NDB/LOC/DME, weak PCNs, poor alignment to prevailing winds, a shorter length than 06/24, no parallel taxiways for most of its length and noise issues for the local communities to name a few).

In an earlier post, OverRun said;

Cost to upgrade Edinburgh
The EDI pavement PCN rating has been done on a technical basis (the "T" in the rating), so I'm guessing the EDI probably have a reasonable idea of what is needed and the cost. I can give a ballpark estimate, but it is very much subject to adjustment based on the actual pavement. To move the runway rating for the 777-300ER from 274.4 tonnes to 352.2 tonnes, simplistically needs another 200mm of asphalt. The runway, parallel taxiway, stub taxiways and some of the apron all need to be overlaid. The cost will be loaded for (a) night work, constructing in limited hours to MOWP, temporary ramps and grooving, etc; (b) many runway lighting changes and re-cabling, matching in the geometric levels at all shoulders and all intersections, new markings; and (c) compliance with noise, environmental, and all manner of constraints.

I'm guessing that the whole project might run to GBP150 per tonne of laid asphalt. That would put the project at about GBP 15 million. However these projects can take on a life of their own (especially some of the UK ones I've seen), so figures going skywards up to GBP 50 million could be thrown around.

In practical terms, would it be possible to add another 200mm of asphalt to runway 06/24 by means of night time closures of say 6 or 8 hours? Wouldn't this extra 200mm create a "step" in the runway surface which, even if smoothed, would constitute too much of a grade change for an aircraft travelling at high ground speed? Or are there well proven methods to carry out such strengthening works using overnight runway closures?

[OverRun]

I see this thread has come back to life. Let me pick up on an earlier point made by Porrohman, before I reply shortly on the asphalt overlay question.

I seem to recall reading that EDI was one of the V-Bomber dispersal airfields during the cold war so runway 12/30 must presumably have been much stronger than 31F/C/X/T back then.

I got curious about that, so I derived the ACN of a Vulcan MkII bomber (such information is not normally available). At its normal maximum operating weight of 81.6 tonnes, the ACN is about 20 on a “C" subgrade; at its war overload takeoff weight of 90.8 tonnes, the ACN is about 24 on a “C” subgrade. I say ‘about’ because the exact undercarriage and tyre spacing is not available to me. However it does show how effective the 8 tyres per main gear leg were in reducing the ACN; modern civil aircraft at that sort of weight (A321) with only two tyres per main gear leg have ACNs around 58/C.

So the Vulcan bomber would be OK to operate on the 12/30 runway with its 31/C rating.

[OverRun]

Despite all the discussions about PCN, I think that the 06/24 runway length which was raised early in the discussion needs revisiting.

The short 06/24 runway length at Edinburgh is an issue, and as I take a look at a satellite photo of it, I see that both runway ends have displaced thresholds which means that there are obstacles at both ends. It is a bit unusual to have both ends displaced at a major airport – it becomes sort of like landing in a goldfish bowl. The AIP shows me that the airport has been pro-active in trying to squeeze out every available last metre of runway, and it has probably succeeded. A closer look on the photos shows hills at both ends, which prevents any decent length increase from what is there now. This runway has an average ASDA of 2615m now which is the length of a middle-haul runway, and it is clearly going to be a middle-haul runway forever – it is not and can never be a long-haul runway. That alone will affect future international traffic.

Having spent the time studying the satellite photos, there is another point that leaps out at me.

It would be churlish and unfair to characterize Edinburgh Airport as a tight little place, quart-into-a-pint-pot, developed higgle-dee piggle-dee, compromise heaped upon compromise, jammed into a small valley surrounded by hills, with encroaching town development crowding the one runway, and limited growth potential. Unfair - because the airport has clearly done an excellent job and a lot of hard work in developing its assets to their limits despite the constraints. Churlish, because I am not Jeremy Clarkson and this is clearly not Top Gear, so these comments are boorish.

Nevertheless, the reality is that this is never going to be an attractive airport for future development. Any sort of development there is going to need large amounts of money thrown at it, for which there are not the passengers to pay. And after spending all that money, well - to paraphrase Winston Churchill – in the morning when the airport owner is sober, the airport will still be ugly.

In terms of strengthening, adding 200mm asphalt during overnight works. Yes it can be done, and such overnight work is done at airports around the world. It is complex, difficult, slow and expensive. For a single runway airport (like Edinburgh effectively is), everything is harder again. Operations are affected. The asphalt is added in multiple layers, one layer at a time, with ramps at the end of each night’s work to make it acceptable for aircraft. The runway closures are preferably 8 hours or 10 hours, rather than 6 hours. The amount of work takes months to do rather than days. The investigation and design alone will take 12 months. There is excellent guidance in the UK CAP 781 about the planning of such work, learning in part from the lessons of the monumental cock-up at Bristol Airport.

I have never been to Scotland and so cannot comment about local issues. I took a look at Glasgow Airport from the satellite, and that also looks a pretty tight airport. The two airports are pretty close together, and if you are after a genuine long haul international airport for the region, then maybe what is needed is a brand new giant-sized greenfields airport located between the two. Space for triple runways (1 x 3800m, 2 x 2750m), space for 2 terminals, multistorey carparks, rail links to Edinburgh and Glasgow, hotels, cargo/industrial areas. Get the English to pay for it as part of Scottish independence

[Porrohman]

Thanks for your replies OverRun. They are very helpful and I really appreciate you taking the time to respond to my questions.

Typically, how many layers would be needed to build up an extra 200mm of asphalt?

When strengthening a runway in this manner, does the surface ever get reclassified from being rigid to being flexible and, if so, what thickness of asphalt does there need to be for the surface to be considered to be flexible? Or will it always be considered to be a rigid surface if there is concrete under the asphalt?

A few years ago, runway 06/24 at EDI was resurfaced and I seem to recall they tried to do that in a seven hour window each night with the runway closed from 11pm to 6am. During these works, I think the runway was strengthened very slightly from 72/R/C/W/T to 74/R/C/W/T. I’m surprised BAA didn’t take the opportunity to upgrade the PCN by more than that, given all the press announcements they have made in recent years about trying to attract additional long-haul flights. If 200mm of asphalt would take the runway from PCN 74 to 108 then an increase from 72 to 74 sounds like a negligible increase to the thickness (perhaps 12mm?). Are there technical reasons why BAA couldn’t have strengthened it more at that time and, if so, how are these reasons likely to affect any future strengthening?

As regards the length of runway 06/24 at Edinburgh, it seems to be adequate for many long-haul operations, although longer would be better. If necessary, it can be extended at both ends using starter strips according to the BAA Masterplan. It would still have a displaced threshold at both ends (on 24 for noise/PSZ reasons I suspect and on 06 because of the adjacent railway embankment) but the LDA should be fine for most aircraft. The Masterplan also reserves land for a second parallel runway to the north of the River Almond which would be a similar length to the proposed extension to the existing 06/24.

At the moment;
Runway 06 has TORA of 2,556m, TODA of 2,616m, ASDA of 2,616m and LDA of 2,344m.
Runway 24 has TORA of 2,553m, TODA of 3,002m, ASDA of 2,614m and LDA of 2,347m. (447 m of clearway declared within TODA)

The maps in the Masterplan indicate that the starter strips they have in mind would result in a total surface length of perhaps 3,300m (I’ve manually scaled the extensions versus the existing runway length on the Masterplan maps) which I think might give approximately;
Runway 06 TORA of 2,800m, TODA of 2,800m, ASDA of 3,300m (but no over-run as there is likely to be jet-blast deflectors and then there is a railway embankment at the stop end), LDA 2,600m.
Runway 24 TORA of 3,300m, TODA of 3,300m, ASDA of 3,300m, LDA 2,800m

Edit: On reflection, perhaps these distances will be less due to obsticle clearance and/or other considerations.

To extend the runway beyond that would require relocating the railway line at the east end of the runway. It wouldn’t be economic to extend it the other way as it would involve re-routing the M9 motorway and clearing a lot of properties. The possible runway extensions mentioned above should be reasonably adequate for the type of long-haul routes / aircraft that EDI might attract and the main restriction for long-haul is likely to remain the PCNs unless they are strengthened.

Even without extending the runway, the length is okay for a fair number of different types of aircraft to operate the long-haul routes that EDI might attract. By “long-haul” I mean transatlantic, Middle East or further afield.

I looked at the detailed performance characteristics for the A332 on the Airbus web site and even without a runway extension this aircraft should have a range from 06/24 on a standard day of about 4,200nm at MZFW, about 5,000nm with 237 pax (typical 3-class) plus 20,000lbs of cargo and about 6,000nm with no cargo (assuming that a suitable stand can be provided). That should be fine for a good range of destinations.

A B744 has a reasonable payload/range from EDI too as does a B763 and A333. The biggest problem for EDI is the B777 series, and especially the B773ER. This pretty much rules Emirates out of EDI in the immediate future and perhaps explains why they chose GLA and NCL rather than EDI. The PCNs at these airports allow a B773ER to lift about an extra 60,000lbs or more compared to EDI, assuming ACN=PCN. This represents a lot of payload/range.

The main constraint to additional long-haul flights from EDI at the moment is not the runway length or even the PCNs of the runway and main aprons. It is the weak PCNs between taxiway alpha and the SE apron, because this is the only apron at EDI currently capable of taking any aircraft larger than a non-wingletted B763. It seems very strange that BAA would invest in three large stands on the SE apron and then not upgrade the relatively short taxiways to be sufficient strength to use the stands for long-haul operations.

In summer 2010, Japan Airlines flew charters into EDI using B744s. Having arrived with a full load of passengers, they departed empty on a short hop (for a B744) to Frankfurt. I analysed the detailed airfield performance and payload/range charts for the B744 on the Boeing website, and as far as I can tell, it should be able to depart EDI for Japan with a full load of pax. Range with 412 pax and no cargo from EDI, given the existing PCNs of the aprons and runways, is about 6,400nm for a B744ER and about 5,750nm for a standard B744; EDI-NRT is 5,010nm.

The only reason I can think that they didn't do that is because the low PCNs of the links to the stands on the SE apron prevented a high ACN departure. The ACN of a B744 operating at that weight would have been about double the PCN of the taxiways leading to the stand.

I hope that one of the first things the new owners will do will be to strengthen these taxiways to something more suitable. It’s only a few hundred metres of taxiway so the costs should not be excessive.

Since this thread started, BAA have produced a revised Masterplan for the Edinburgh Airport which substantially reduces the previous growth forecasts and pushes back the date for a second runway by 20 years. Here is a brief summary from the AirportWatch web site (source: AirportWatch | Edinburgh Airport Master Plan released – barely changed from the draft )

The airport Master Plan is for the next 30 years, up till 2040. They anticipate passenger numbers will grow from 9 million per annum now, to 12.3 million (central forecast) by 2020. (The central forecast in the 2006 Master Plan was 17.6 million by 2020). They anticipate 20.5 million passengers per year by 2040 (the central forecast in the 2006 Master Plan was 23 million by 2030). They expect 141,300aircraft movements per year by 2020 and 200,600 per year by 2040. Cargo and mail might grow to 56,300 tonnes by 2020 and 81,900 tonnes per year by 2040. They do not anticipate “needing” a 2nd runway until 2040, but have plans to set aside land before 2040 for such a runway.

The 2006 Masterplan also anticipated procuring a large chunk of land to the SW of the existing terminal, currently occupied by the Royal Highland Agricultural Society of Scotland (RHASS) as their showground, and add further aprons and a terminal there. That plan has also been indefinitely delayed, not least because of the huge amount of money the RHASS were demanding to cover the cost of relocation.

It’s hard to know which forecast to believe; the 2006 Masterplan seemed to be very optimistic but the 2011 Masterplan seems very pessimistic. I can’t help suspecting skulduggery on the part of BAA in one or both of these Masterplans.

The ACN for the Vulcan is surprisingly low, but I was forgetting about the sixteen wheel main undercarriage (eight per leg). As you say, it spreads the weight much better than modern undercarriages. Given the runway length at EDI at that time (12/30 is about 5,900ft), the MTOW and therefore the ACN would have been limited by the runway length. Presumably if they ever needed to operate from EDI they would have taken off with a partial fuel load and topped up from a Victor or Valiant once airborne.

[OverRun]

Adding 200mm asphalt to the runway is often done in 3 layers – 70mm / 70mm then topped with 60mm. But there are variants, depending on time available and the type of friction treatment.

After the asphalt is added, the runway structure will be a hybrid of concrete/asphalt. The behaviour of the pavement will be somewhere between flexible and rigid. Depending on the strength of the rest of the pavement structure beneath the concrete, it could be classified as either F or R. For common runway structures, it would possibly be classified as F, but that would be ultimately be the choice of the designer.

Resurfacing of asphalt runways is required every 7-20 years (the actual period of time depends on climate and pavement condition). The extent and design of the last resurfacing at Edinburgh are not known by me. If it was simply an overlay, often the pavement rating is not changed after such a resurfacing, so if it was changed then this could either reflect a (slight) increase in strength or perhaps the removal of a weak area during the resurfacing. If it was a ‘mill and fill’ approach rather than an overlay, then it was most likely the removal of a weak area led to a small increase in PCN. It could also have been increased because in the process of investigation/design, the designer realized that the PCN could be increased on the basis of the new data available to him. I have seen this recently at several other airports.

With or without starter strips, the runway length is such that this is still only a middle distance runway. Heathrow has genuine long distance runways: Runway Length 09L/27R: 3902m x 50m; Runway Length 09R/27L: 3658m x 45m. Gatwick has one: Runway Length 26L/08R: 3316m x 46m. Glasgow Prestwick Int'l just about has one: Runway Length 13/31: 2987m x 46m. But the existing Edinburgh runway is already extended close to its limit given the displaced thresholds at both ends.

[Porrohman]

Thanks once again for your replies OverRun.

As regards a "Central Scotland Airport", this has been considered on a number of occasions in the last half century or so. There are very few suitable sites in Central Scotland because of terrain and weather. In addition, the costs of such a project would be extremely high and therefore difficult to justify on economic grounds for the foreseeable future.

Four possible sites have been considered. These were at Slammanan, west of Kirkintilloch, west of Stirling and Airth. Of these, the only one that was considered to be suitable was at Airth which is between Grangemouth and Stirling but it's doubtful that a runway much longer than 3,000m could be shoe-horned into the site unless the village was demolished. The rest of the possible sites were discounted because of terrain, geology, surface access and a variety of other reasons.

A study in 2002 by academics from universities in Glasgow and Edinburgh summarises the possibility of a Central Scotland as follows;

Executive Summary
• There does not seem to be a case for constructing a new airport in central
Scotland.
• Suitable sites for constructing a new airport in central Scotland are rare, but an
area around Airth has been identified as a possibility (section 4).
• The costs of such a project have been estimated to be anything from
£5.8billion to £7.4billion (Tables 7, 8 and 9 and section 5.1.6.2).
• The costs of operating a large new airport would be significantly lower than
experienced at Edinburgh or Glasgow and would bring producer benefits
(section 5.1.6.4 and Appendix-B-Figure 9).
• Travellers would experience benefits of a greater frequency of flights and a
wider range of destinations. These benefits are offset somewhat by the
reduced convenience of the Central Scotland Airport’s location (section
5.1.6.3).
• Surface access distances to a Central Scotland Airport would increase by 56%
to 66% over current levels (section 5.1.6.7).
• Transferring to a central Scotland location would mean over 3000 of the
existing workforce at the two airports retaining their jobs, but 4,000 losing
their jobs in Edinburgh and Glasgow (section 5.1.3 and Table 3).
• These lost jobs would be concentrated in the lower skilled occupations.
Glasgow would be more badly affected than Edinburgh (section 5.1.3).
• Despite firms claiming that access to air transport is important, this indirect
employment is widely spread and unlikely to change much locationally in
response to the creation of a Central Scotland Airport (section 5.1.3).
• A Central Scotland Airport would be largely neutral to planning in west
Edinburgh but would sharply conflict with Glasgow City and Clyde Valley
Joint Structure Plans (section 5.1.4).
• The scope for creating a hub operation at a Central Scotland Airport is
distinctly limited, in part owing to the success of Copenhagen Airport (section
3.2).
• The cost of such a construction significantly outweighs the estimated benefits,
with benefit-cost-ratios of 0.45 to 0.58 being typical estimates (section 5.1.6.8
and Table 14).
• The benefit-cost ratio of a Central Scotland Airport is markedly less that other
much more promising airport investment possibilities, such as an additional
runway at Edinburgh with a benefit-cost-ratio of 2.93 (section 5.1.6.8 and
Table 13).
• Were such a project as a central Scotland Airport ever to be viable, it would
certainly be beyond the 2030 planning period.
• For an economy on the geographic margins of Europe, good air transport
linkages are vital for growth (section 5.1.1).
• The above conclusions regarding a Central Scotland Airport imply that the
responsibility for developing central Scotland’s vital airlinks essentially rests
with Edinburgh and Glasgow Airports, currently under the common ownership
of BAA plc (section 5.1.2 and section 6).
• Serious consideration should, therefore, be given to maximising the potential
of these assets – for example, by linking Edinburgh Airport directly to the M8
and by providing direct through rail links from Glasgow Airport through the
City of Glasgow to Edinburgh Airport and the City of Edinburgh (section 6).

Source; http://homepages.ed.ac.uk/mainbg/Files/csa%20study.pdf

The conclusion of this and other studies has been that the best option is to develop Edinburgh and Glasgow airports rather than consider a Central Scotland Airport.

[Porrohman]

Ten days after GIP's purchase of EDI was announced, NATS published revised PCNs for EDI. Amongst these revisions, the centre (original) section of Taxiway Alpha had its PCN revised from 72/R/C/W/T to 70/F/A/W/T despite no significant works having been carried out. The "/F/A" part of this revised PCN represents a very substantial increase in the strength of the pavement. The slight drop from 72 to 70 is relatively unimportant from a strength perspective; I think it just reflects the fact that the pavement needs to be resurfaced. I believe the resurfacing works will commence on 29th July according to a recent NOTAM.

My assumption is that GIP have carried out a new technical analysis of the pavement, and reclassified it from having a rigid surface to it being flexible and have re-analysed the subgrade and reclassified it to be high strength (A grade) rather than low strength (C grade).

Taxiway Alpha at EDI was built in the mid-70s and was originally concrete. At some point it was topped with asphalt. Is it normal for a pavement's subgrade to be reclassified to be a higher grade after a period of use / settlement / consolidation and, if so, how long typically does it take for a subgrade to settle / bed down to become a higher sub grade?

Typically, what depth of asphalt on top of a concrete pavement (lets say 72/R/C/W/T) is needed before the pavement’s characteristics become flexible rather than rigid? Is there a rule of thumb or are there too many variables?

Having revised the PCN of taxiway alpha from 72/R/C/W/T to 70/F/A/W/T how likely is it that a revised technical analysis of Runway 06/24, which was constructed at the same time as taxiway alpha and had the same original PCN, would result in a similar PCN increase?

[OverRun]

The rating 70/F/A/W/T should be enough to carry any civil aircraft.

The highest ACN civil aircraft in the world is the A340-600 @ 381t: ACN on A subgrade flexible is 66 (Airbus say 70 but I am having some difficulty matching their numbers), followed closely by the Boeing 777-300ER @ 352t: ACN on A subgrade flexible is 64.

Hint - COMFAA 3.0 has a little trick to it for airport engineers – the library A340 models only go up to the -300 and so when you have to derive the -600 from first principles, the % main gear gets set to represent only the wing gear at 63.5%, and ignores the 110 tonnes load on the centre main gear.

The reclassification of pavement strength occurs periodically at airports following a technical evaluation without any construction work necessarily being associated with it. Technical evaluations should happen periodically (ranging from 3-10 years), and are typically done with non-destructive FWD testing of the pavements. They may or may not be done in conjunction with some construction work. Often an airport may get engineers in to design and rehabilitate some piece of pavement which requires them to bring in testing equipment before the design, and as an inexpensive extension to the work, some other pavements will get tested.

Because some airports can have many sections of pavement – presenting different sections along the runway(s), and the various taxiways and aprons – the testing may be limited to key areas only. I have seen international airports with over 100 pavement sections – each managed individually – and I am sure there are airports with many more than that.

I have just done one such technical exercise for an international airport, and this resulted in reclassifying the airport pavement strength upwards.

The normal process in such technical analysis is to estimate the subgrade strength using various techniques, and then strength of the pavement layers, and finally the PCN. The decision rigid/flexible would come from consideration of the layer depths, the concrete (rigid) condition, and the data from the FWD test back-calculation; such a decision would require a couple of days of analysis.

It is not possible to draw any inference about the PCN of other pavements, such as 06/24, from this analysis. These are very high loads at these sort of PCNs and extrapolation is unwise.

[Porrohman]

Thanks for your reply OverRun. I find this to be a fascinating subject and I have learned a lot from your posts.

It's pleasantly surprising to find just how much difference a new technical assessment can make to a PCN. As far as I know, the PCNs for taxiway alpha, runway 06/24 and the main apron at EDI have remained relatively unchanged since they were built in the mid 70's so I presume that the previous owners, BAA, never carried out a full re-evaluation of the PCNs, otherwise they would presumably have discovered what the new owner, Global Infrastructure Partners (“GIP”), have just discovered. It would appear that one of first things that GIP did (either as part of the due diligence / survey process while they were bidding for the airfield or in the few days immediately after the purchase was announced) was to re-assess the PCNs of some sections of the airfield and the results have included some very substantial upgrades to PCNs.

The previous PCN of the centre section of taxiway alpha (72/R/C/W/T) limited the maximum weight of a B773ER to around 595,000lbs assuming that ACN<=PCN. The re-assessed PCN enables the same aircraft to operate at its maximum take-off weight of 775,000lbs. (Note however that runway 06/24 is currently too short and has too low a PCN for that weight; Boeing’s detailed performance charts indicate that circa 720,000lbs would be the maximum take-off weight from a dry 8,600ft runway on a standard day if the PCN is suitably increased but the current PCN of 74/R/C/W/T would limit take off weight to 605,000lbs assuming that ACN<=PCN. If a 10% overload is permitted then take-off weight could be higher.).

The remaining two sections of taxiway alpha were upgraded from 72/R/C/W/T to 120/R/C/W/T (without any works being undertaken), and taxiway mike was upgraded from 31/F/C/X/T to 37/F/A/W/T (without any works being undertaken). My suspicion is that, if a new technical assessment is carried out on the rest of runway 12/30 (taxiway mike is part of 12/30) it might also be 37/F/A/W/T and this would go a long way towards explaining why the occasional large wide-bodied aircraft has been allowed to taxi and park on this runway in the past.

It'll be interesting to see whether GIP also revises the PCNs of the aprons and 06/24 at EDI in the coming months. Our previous discussion in this thread about how much additional asphalt would be needed to increase the PCNs at EDI might become irrelevant if it turns out that the subgrade on 06/24 and the aprons is re-assessed as high strength and the surface is re-assessed as flexible.

In the context of the costs of operating an international airport, and based on the re-assessment processes that you have described, it doesn’t sound like re-assessing PCNs is a particularly expensive task, yet the benefits can be very substantial.

[issakov]

Dear All
Can I ask What is realationship Between PCN Restriction & Regulated Takeoff weight Of An aircraft

[OverRun]

issakov,

I am not sure I quite understand you. Your phrases are not quite standard phrases so I am not quite sure exactly what they refer to i.e.

Regulated Takeoff weight Of An aircraft

could be maximum TOW or it could be the weight limit for the runway (instead of using a ACN/PCN system). I see that Baltimore use such a limit.

PCN restriction

might be the published PCN for a runway, i.e. 34/F/B/X/T

Can you give the actual examples please for both or explain what situation they are being used in so I can better understand. And the aircraft type/airport please.

This is not a comment on your English, which is acceptable and undoubtedly much better than my Russian (and my French and German), but rather trying to be very precise about your definition so as not to give a wrong answer.

[Porrohman]

Can I ask What is relationship Between PCN Restriction & Regulated Takeoff weight Of An aircraft

I have an amateur interests in these issues and will try to give an answer to your question but there are experts in this forum, such as OverRun, who will be able to correct any misconceptions that I may have and fill in any gaps.

Many factors affect the permissible take-off weight of an aircraft from an airfield. These include;

The ACN of the aircraft. For a brief explanation of ACN, see Aircraft classification number - Wikipedia, the free encyclopedia

The PCN of all the pavements that the aircraft will use for parking, taxiing and take-off. For a brief explanation of PCN, see Pavement classification number - Wikipedia, the free encyclopedia

The performance of the aircraft, including any self-imposed restrictions such as de-rated engine performance (often used to to extend engine life) or any serviceability issues. Some quite detailed aircraft performance data is available on the Boeing and Airbus web sites e.g.
Boeing: Airport Technology
Aircraft Characteristics *| Airbus, a leading aircraft manufacturer

The prevailing weather conditions; low air pressure (as a result of altitude and/or low atmospheric pressure) reduces engine performance as does high ambient temperatures. Wind conditions (direction, strength and gusts) and the available friction on the runway also need to be taken into account. e.g. a wet runway increases stopping distances in the event of a rejected take-off.

The runway length, runway surface (e.g. ice, asphalt, concrete, gravel, grass) and surface condition (wet, dry, snow, ice, other contaminants, degradation).

Airfield operator, aircraft operator and regulatory restrictions. e.g. some airfields have noise restrictions that might affect take-off performance of some aircraft and some aircraft operators have policies that require reduced engine performance to be used in order to extend engine life and times between overhauls. Also, some airfields might allow some aircraft types to operate regularly at an ACN that exceeds the PCN of a pavement. A 10% overload is often permitted on a regular basis and for occasional operations higher overloads might be permitted, subject to regular inspections of the pavement by suitably qualified engineers. Here is an example of the data that is published by an airfield operator and by a regulator;
NATS | AIS - Home (see the "Textual Data" link)
http://www.caa.co.uk/docs/33/CAP168.PDF

In order to calculate the weight that any particular aircraft can depart at from any particular airfield, all of these factors, and probably some others, need to be taken into account.

If we take Edinburgh Airport as an example, let's look at the effect that PCNs can have on the maximum permissible take-off weight for a Boeing 777-300ER.

Currently, the only stand that is capable of accommodating an aircraft of this size has a PCN of 72/R/C/W/T. Until last year, the main runway had a PCN of 74/R/C/W/T but, at that time, the apron and runway were connected by a relatively short section of taxiway (taxiway Mike) which had a PCN of 31/F/C/X/T and another taxiway (taxiway Alpha) that had a PCN of 72/R/C/W/T. For this aircraft type at that airfield at that time, the biggest constraint on take-off weight was not the runway length but the PCN (strength) of taxiway Mike.

At the time that Edinburgh Airport was bought by new owners last year, the PCN of taxiway Mike was re-assessed as having a PCN of 37/F/A/W/T. Some time later Taxiway Alpha was re-assessed as being 70/F/A/W/T. and the main runway has since re-assessed as having a PCN of 87/R/C/W/T. The limiting factor for take-off weight for a Boeing 777-300ER remains taxiway Mike with a PCN of 37/F/A/W/T, albeit, the A-strength sub-grade allows a significant increase in the permissible weight of a Boeing 777-300ER.

Taxiway Mike had some significant works carried out on it earlier this year but the PCN has not been revised yet. I am assuming that the airfield owner is allowing the surface to bed down before carrying out a technical re-assessment of its PCN but I cannot say for sure whther this is the case. If, by way of example, a re-assessment in the coming months increases the PCN of taxiway Mike to, let's say, 70/F/A/W/T then the limiting factor for Boeing 777-300ER operations at Edinburgh would be the PCN of the apron where the aircraft parks which is currently 72/R/C/W/T.

If that apron is re-assessed / refurbished and attains a PCN of, let's say, 70/F/A/W/T (this is the PCN that Taxiway Alpha attained after it was re-surfaced and re-assessed) then the limiting factor for Boeing 777-300ER operations would become the runway length and/or weather conditions.

As you will see, aircraft take-off weight at any particular airfield is a complex subject and requires careful calculations by suitably qualified people. As I said at the start of the the thread, I am a self confessed amateur but if you read some of the information in the links that I have provided you will hopefully get an initial understanding of this complex subject.

[EMB170]

Somebody was asking , where to find PCN classifications for taxiways and aprons.
Please refer to, in case of EDI and for that matter all UK civi airport to UK AIP .
Every ICAO country should have an updated AIP which is a legal document unlike Jeppesen.
Everything is explained, even obstacles with heights and distances.

[Porrohman]

Somebody was asking , where to find PCN classifications for taxiways and aprons.
Please refer to, in case of EDI and for that matter all UK civi airport to UK AIP .
Every ICAO country should have an updated AIP which is a legal document unlike Jeppesen.
Everything is explained, even obstacles with heights and distances.

Information for UK commercial airports is available via this link; NATS | AIS - Home