Thanks for taking the time to add your thoughts enicalyth.
The ACN for my 767 example does initially appear low, and this is due to something which I found out about the hard way - the computation of ACN for multi-wheel gear changed recently, with new alpha factors.
The original airport pavement design method is the deflection-based ESWL method which overstates the damaging effect of multi-wheel gears. This led to the introduction of pavement thickness reduction factors, or 'load repetition factors' called alpha factors. The design thickness of the pavement is obtained by multiplying the ‘standard’ thickness, T, from the ESWL design curve by alpha.
Boeing felt penalised by this on their 4 wheel gear and especially the 6 wheel gear of the 777. They/FAA developed new alpha values based on the mid-2000s full-scale tests at the FAA National Airport Pavement Test Facility using four- and six wheel undercarriages (I have a photo of one of the test sections at the bottom of this post).
In 2007 the ICAO Air Navigation Commission released approval (via State Letter SL AN/420.1-EB/07/26 dated 16 October 2007) directing that permanent changes to be made by the aircraft manufacturers in the way that they calculate aircraft classification numbers (ACNs). The revised alpha factors affected the ACNs of all four-wheel and six-wheel main landing gear aircraft. The new Alpha Factors are 0.800 and 0.720 cf. the existing values of 0.825 and 0.788.
I have got the Boeing updated ACN charts released pursuant to that, and from those I interpolate ACN of 45 rigid and 43 flex for your B767-200, 141520kg version (except at 190 psi which is inconsequentially different).
Most of the difference in your ACN and mine for the B767-200, 183psi, 141520kg version lies in my using the 2007 ICAO alpha factors. There are also minor differences of less consequence in the interpolation of ACN charts and the pavement software I use, and in rounding ACNs to the nearest whole number.
Staying with the C subgrade [low] as an example:
Your records: rigid ACN is 44 and the flex is 45
Original ICAO ACNs: rigid ACN is 45 and the flex is 47
2007 ICAO Alphas: rigid ACN is 45 and the flex is 43
Your Herc numbers are pretty close to the ones from my software. I was wondering at the difference between that and the A400M and what the Airbus engineers had done. Then I twigged – they have put A400M 141 tonnes onto main gear with 12 wheels = 11.2 tonnes/wheel (assuming main gears take 95% of the weight). The C130 Herc puts 73 tonnes onto main gear of 4 wheels, which is 17.3 tonnes/wheel. That drops the ACN quite a bit.
Your PM must still be in transit because it doesn’t show up yet.
FAA TEST
