PCN

[CEJM]

Done a search on this subject and it came up with several results but not really what I am looking for.

How important is the PCN for a given airport?

As an example take Corfu, Greece (CFU). The PCN for this airport according to the books is 33/F/B/X/U

Our airline and many others operate A321's into CFU which have a ACN of 25 (47000kgs) and 52 (89400kgs). The flights are normally full and take off weights are 80+ tons. As a result the ACN is higher then the PCN.

Now the Aerad supplement says that aircraft with a higher ACN then the PCN may operate into these airports as long as they are authorized by the airport authority.

As the operation of A321's into Corfu is not a one off why isn't the PCN increased? As the U in the code means that the number is confirmed by actual use.

In addition to the A321 we also operate widebodies to CFU who's empty ACN is already way above the PCN (33).

Would be nice to see what your views are about this.

CEJM

[AerocatS2A]

My view would be that operating with a higher ACN than PCN will ultimately mean the aerodrome operator needs to perform more maintenance on the runway than otherwise. The benefit to the aerodrome operator of not seeking to increase the PCN is that they maintain more control over the overweight operations on to the runway and can therefore have more control over the way that may affect their maintenance costs. At some point they may see more benefit in having more unrestricted access to the airfield by larger aircraft and may either update their runway data or perhaps make modifications to the runway itself to provide for a higher PCN.

On the subect of the "U" meaning it's confirmed by actual use, actual use may suggest that there is an impact on maintenance costs and therefore ops need to remain restricted. Flight crews wouldn't be aware of this, all they see is that they land and the runway doesn't fall apart, it doesn't mean it's not suffering minor damage though.

This is purely a WAG from me, but it makes some sense.

[MrFixer!!!]

yeah SASKATOON you are right......that was Indian Airlines actually and their's were the only A320s with the Bogies fitted.....I heard they are going back to the standard fits though!!

[Dani]

I was into this question a decade ago and came to the conclusion that it's not really relevant to the pilots.

What I gathered from reading the sources and talking with the people who know is that PCN can be below your ACN. It's more something for the airport and that they will have damage to their runway in the long run if they allow aircraft with higher weights.

I think the Air India case was more because they worried about damages on some airports not that the PCN was too high.

Dani

[mutt]

Dani, how did you come to this conclusion? We actually use PCN as a weight limitation and will either reduce the aircraft weight or change aircraft type. Having personnally inspected some runways with low PCN values, I really dont like the idea of trying to land an aircraft on them. Who will take responsiblility if you damage your aircraft?

We had a situation years ago where an aircraft landed in the wrong airport, it tore up the runway which actually had to be rebuilt by the airline!

Mutt

[CEJM]

Thanks all, for your replies.

Dani, I am interested why you think that it is not really relevant to the pilots.

What happens in case we taxi an aircraft which at take off weight has an ACN which is twice as high as the PCN and as a result damages the runway/taxiway/apron?

We have got the information on the flightdeck and in such a case I would be surprised if they don't place any blame with the pilot, i.e. bad flight planning/preparation.

As with regards to the Air India with bogies, they still operate them.

[Dani]

No runway will go immediatly broke just because you landed with a higher weight - if you are not completly off the mark. You cannot distroy a runway just by landing there once. Ah, you are talking about damages to the aircraft! Well that shouldn't really be the case, if you are not way too heavy. The aircraft will not sink on a runway with ACN > PCN.

Your friendly ops office always looks into the PCN number before it approves your airport. As long as your aircraft is within the limit, it will approve that you land there. Of course also this airport has to agree that you land there.

If your company wants you to check the PCN number for every flight, that's ok.

AerocatS2A is pretty much correct on what he says, imho, if the airport approves that you land there they are responsible for runway damages. If you land however on a different airport that might be a different case.


Dani

[CEJM]

Dani,

I am aware that no runway will break just because you land at a higher weight. That on itself is not the issue, neither do I believe that the aircraft will sustain any damage.

However the PCN is valid for 'all' paved surfaces which an aircraft can use.

We are going to operate an aircraft to an airport where the ACN (take off weight) will be twice as high as the PCN. What happens when the aircraft sinks into the apron? (seen it happen in Mombasa)

As you say the operations department should check if an airport if suitable for a certain aircraft type. But as you and I know they are only human and mistakes are made. Do the authorities place any blame on a pilot when the aircraft causes damage to any paved surface while either landing/take-off/taxying when the book says that the ACN is higher then the PCN? We don't carry any documents in the flight deck that the airport authorities have given an authorization to use the airport while the ACN is higher then the PCN.

[Dani]

double the number is wwwwaaayyyy too much! Never do such a flight!
52 is not the double of 33. And wide body don't need necessarily need to have a higher ACN than a small one. They have more wheels!

If your "friendly" ops office really insist, let them give you the written approval. As an airline you always need also a letter from the airport you want to operate, i.e. you need an approval. You simply cannot go to any place you want. If you receive it, then everything is fine!

It's correct when you say that it's not only a question of the RWY but all apron surfaces.

I'm pretty sure that your board documents say which class of aircraft you are, maybe even the PCN number is listed. With that in hand you should easily prove that you are allowed to fly - if the ACN match the PCN...

Maybe CFU just has some very remote areas with a low PCN and to protect themselves, they give the PCN for every surface as the number 33? Why not drop in the CP office and ask him? He wont bite (well, sometimes...)

Good luck,
Dani

[kijangnim]

Greetings
The Airport Authority, can give a dispensation, for few landings above the PCN, it is common practice.
Yes it does shorten the runway life, but the time scale is more a matter of years than months.
Now if you do the computation you will see that the total weight is on the main wheel, and if you look at the table you will see that the nose wheel is taking around 7% of that total weight, so you have margin equal to what ever the nose wheel takes.

[john_tullamarine]

There is a significant degree of finger-in-the-wind in PCN values, especially if not assessed by a technical review. Overload operations are not at all uncommon.

I have sent a note to Overrun - he will be able to provide technically competent guidance on what's what with PCN.

[CEJM]

Dani,

You are right that double 33 is not 52. However the aircraft which is going to operate the route (different airfield) is a widebody and the max take-off weight (which it regularly operates at) ACN is exactly double the PCN. We got more airfields in our schedule were the ACN is much higher than the PCN.

The documents on the aircraft do indeed state the ACN. However we never (as far as I know) carry any specific approval documents (from the airport) to operate a certain type to an airport were the ACN is higher then the PCN.

The CFU case was just used as an example as more carriers operate A321's into that airfield so they will have the same issue.

Already sent an email to the CP.

John, thanks very much. Looking forward to his input.

[Alteburger]

CEJM, I also encountered this issue with the A321 with reference to a lot of the Greek airfields my company visits during the summer. I was particularly concerned with the liability of the crew if any subsequent aircraft or airport damage occurred.

Having spoken to our flight ops technical department, I discovered that the airport authority is approached prior to operation into this airports, and agreement is reached based on the frequency of operation, limitations on apron availability for parking, taxiway limitations and, in the case of Skiathos, the height of the water table would you believe?! I approached our guys with a list of airports I'd found where we exceeded the PCN, (which included CFU, KGS and maybe even the odd Spanish airport like MJV I think), and unrestricted operations had all been approved by the airport operator.

[CEJM]

Alteburger, I expected something like that. I am sure that we got the same approvals as you got.

The same as you, I am interested in the liability issue. We operate a fair amount of ad hoc flights on very short notice. I wonder how these approvals are arranged in such a case.

CEJM

[OverRun]

Dani – I’d like to correct you on one of your assumptions which is dangerously incorrect

No runway will go immediatly broke just because you landed with a higher weight - if you are not completly off the mark. You cannot distroy a runway just by landing there once. Ah, you are talking about damages to the aircraft! Well that shouldn't really be the case, if you are not way too heavy. The aircraft will not sink on a runway with ACN > PCN.

I have broken the runway several times. The worst was a rut so big that the airport inspector put his briefcase upright in the rut, and the top of the briefcase was below the pavement surface. The offending C130 was landing at a much higher weight than I had permitted. It had the back door down and as it slowed and sank into the runway, the loadmaster saw it and advised the pilot, who immediately accelerated and took off without coming to a stop (and sinking in further).

A rut of 20mm is considered broken. I have done that in one pass of a 737-800 a couple of years ago and I have got a rut of 12mm with one pass of a 767-200. It doesn’t sound like much, but that has serious implications for breakup of the surface, FOD, early damage in wet weather, and aquaplaning. It can trigger the runway being immediately closed for repairs. Plus we had to ban the 737-800s for about a year until the runway was strengthened, which didn’t help the airline. It should take 20 years of trafficking to reach that rut depth.

Pavement concessions
An aircraft can sink into the runway if the ACN is > PCN – it just depends on how much greater. A small overload, like 10% above, is usually OK and is common. I have gone to a larger overload occasionally but only with a slow build-up over several years and the runway being inspected by an engineer after each overweight flight. An overload of 25%+ is considered an emergency at my airports. And most international airports operate to the same or more conservative standards. Don’t forget to get a concession for tyre pressure as well if needed.

Can you get detected if your ACN > PCN? Yes. That’s one of the things I do for airports. I back-calculate the flight operation and get a pretty close idea of your takeoff weight. And if it is above the published PCN or the airport operator’s PCN concession for your flight, you are liable.

Most concessions are arranged by Ops Department with the airports and most are long-standing concessions. The number of flights per year, weights, etc are considered by the airport before giving the concession. Short notice flights are usually handled by an urgent e-mail or phone call and a concession given/refused in a day or so. Some airlines have their own database of PCNs and have a stack of 'one-off' approved concessions from all the airports they are ever likely to use - I think QF have something like this.

The PCN is complex to calculate. Most airports calculate it (and the concessions) because the cost of a runway asphalt overlay is quite a few million dollars, and they want to balance out cost and life and income. The big worry is liability though, since the overload might cause damage to the aircraft or cause loose material on the runway and FOD could occur (think Concorde). A few million in repairs can be budgeted for, but the lawsuits for a megadeath crash can’t.

There is some ‘finger in the wind’ work with PCN as JT says. Sometimes the airport uses the PCN as a tool to control who lands there (think Jandakot), and it is falsely low. Sometimes the airport has good experience with a certain operation and simply sets the PCN based on that aircraft. But generally it has been calculated by the person who designed the runway back before it was built.

Corfu
The thread opened with CEJM considering Corfu and 80+ tonnes. Now I’ve written all the above, I’m going to pull out my slide rule. The ACN on a B subgrade for 80t in an A321 is 46. The runway PCN is 33. That is an overload of 39%. If the airport has not given a concession, and you operate at that, you are clearly liable.
ACN chart for A321 - http://www.geocities.com/profemery/aviation/A321.htm

If a significant number of those flights are already operating there, then the actual runway strength is very likely greater that the published strength. I see they have given an ‘X’ tyre rating which is <1500 kPa and that covers all the heavy passenger jets, so something in the runway has some strength.

You can make a visual check of the runway strength yourself. Next time you operate there in wet weather, look for water standing in wheel ruts in the runway and taxiway. If the thing looks like an old farmer’s track with deep ruts, there is a genuine problem with weakness. If there is no discernible rutting, then the runway is probably strong. And if you’re really worried, get a 30m length of string, go out to the taxiway or runway, and get two people to hold the string taut across half the runway, from the edge of the runway to just past the centreline. If you measure a rut > 15mm, you have a problem. If not, you probably don’t.

CEJM's widebody
The widebody issue for CEJM with the ACN being double the PCN is a bigger concern. Overloading is not a forgiving thing, and the damage is proportional to the overload to the power 4 – the bigger the overload the very much worse it gets. Let me give an example:

Assume PCN 32, B subgrade, which matches the ACN of a 737-300 at 60 tonnes. That is 14.25 tonnes per tyre.

Now double this to a widebody at ACN 64. That is a 747-400 at 395 tonnes, which is 23.4 tonnes per tyre. The increase in damage is (23.4/14.25)^4 = 727%.

Look – that is way outside of anything acceptable. I cannot over-emphasise my concern. I would suggest an urgent meeting with the airport and their engineers (and you with your engineers). Some airlines I know would themselves get civil engineers in to inspect the airport and independently check the PCN before the operation. I’ve personally experienced this with SQ, (the old and lamented) Ansett, and SAA.

The ACN of the 321 creates an obscure and very technical problem in terms of the calculation of PCN. It is not at all relevant to the concern of CEJM though, so I’ll pull out the data and make a second post about it shortly.

[OverRun]

Peculiarities of PCN for the A321

The Airbus 321 series creates an obscure and very technical problem in terms of the calculation of PCN. It is not relevant to pilots (except for arcane technical interest), but it is to airport authorities and airlines.

It became known in 2008 that there is an anomaly in the calculation of ACN/PCN. The procedure for computing PCN on rigid pavements given in the FAA AC 150/5335-5A sometimes gives unreasonable PCN values when the aircraft mix contains:

• a large number of narrow body aircraft (A321, etc.).
• a relatively small number of large wide bodies such as the Airbus A330 or Boeing 747 or 777.

The procedure tends to produce a PCN for one of the narrow bodies as the critical aircraft, indicating that the wide body operations should be restricted. This is not a true representation of the situation. The anomaly exists at several airports.

As an example, for a 400mm thick PCC pavement, the ACN of the A321-100 is 54 at maximum weight of 83.4 tonnes and the A330-300 ACN is 62 at maximum weight of 230.9 tonnes. The design for A330 at 239.5 tonnes which is an ACN of 65 is for a PCC slab thickness of 399mm (about equal to the 400mm PCC slab). Therefore a PCN of 65 is acceptable, even though the strict application of the method would find the A321-100 as the critical aircraft and thus a PCN of 54 should be used. The published PCN should be 65, not 54.

This problem seems to have reared its head with the A321 particularly because its undercarriage is so heavily loaded and it is such a popular aircraft.

[Dani]

Thanks, OverRun, for this insight information.

I'm pretty sure that PCNs in Greece are not what they seem to be. You cannot compare it with Australia. These runways have been built in WW II and have sustained a long history. Everyday a swarm of Airbus' and Boeings land on them, and if there would be the slightest damage to them, they would immediatly start a no-fly. There is much politics in there.

Question:
Ruts happen on Asphalt? Can they also be created on concrete? I thought that concrete is not being bent but breaks. Most of Greece's island's runways are concrete.

If you look at i.e. WSSS you see ruts way above 1cm. But mainly on the taxiway. I thought that's mainly a temperature problem and not (only) a weight problem.

Dani

[OverRun]

Dani,

Ruts happen on asphalt, and the 33/F/B/X/U rating for Corfu says it is asphalt. Ruts don’t happen on concrete, but the edges/corners of much overloaded concrete slabs can spall and break and that can give you fist size or even wine-bottle size chunks of concrete lying on the surface. What is an FOD possibility with asphalt is an FOD certainty with concrete.

I don’t know WSSS but I suspect it presents a slightly different problem which is outside the earlier discussion. Starting point is that there are a first class bunch of airport engineers there, so whatever happening would be under close surveillance.

My Google Earth shows the taxiways to be asphalt. If there is rutting greater than 1cm on an asphalt taxiway, then there could be an issue or maybe it is just that the taxiways are coming to the end of their life. I can see the pattern of pavement repairs on the airport, and they certainly could be related to slow speed and viscous flow of the bitumen in the asphalt. As well as a weight problem, that is a low speed problem and a high temperature problem (and the two are interchangeable in describing the behaviour of asphalt: low speed is equivalent to high temperature. So the combination of low speed + high temperature is a double whammy. My own belief is that hot humid places are even slightly worse again, and as I type, I’ve just worked out why – the temperature stays high at night as well as during the day. Does that make WSSS taxiways a 2½ whammy?).

The WSSS problem could also be due to the extra damaging effect of the 777-300ER aircraft. These aircraft are much harsher on the pavement than the older 777-200 and 747-400 due to the significantly higher weight per tyre. I’ve seen some asphalt early rutting problems in the Middle East due to these aircraft, and the Middle East has temperatures which are hot, so problems would be expected to occur there first. I suspect those aircraft are quite common at WSSS, and so I wonder if WSSS is also being affected by this.

This is all part of the equation that is worked out when the PCN is determined by the airport, and the WSSS engineers would have predicted the end of the asphalt life years before it was actually reached.

[Dani]

Thanks a lot! Great posts

[CEJM]

Thank you OverRun for a very interesting post.

It does explain how it all works and your input is really appreciated. Next tiime when I operate one of these flights I will most certainly pay attention to the tell tale signs you have mentioned.

Thanks again.

CEJM