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.