A400M ACN charts

[OverRun]

Trying to check the Airbus A400M for an airport pavement, I could find no ACN/PCN guidance or charts anywhere. So I have prepared some.

The A400M has a tridem undercarriage - two main gear legs with 6 wheels each. Main gear tyres are 43 x 15.5-17, at 1012 kPa pressure. Current MTOW is 141,000kg and empty weight is 76,500kg.

I calculated and drew ACN charts for the A400M, and they can be found here:
http://profemery.info/aviation/A400M_flex.gif
http://profemery.info/aviation/A400M_rigid.gif

These charts are preliminary. There is no official data from Airbus or any other source, so I had to estimate gear spacing and load on main gear. There might also be some additional variability because of the unique kneeling landing gear. But at least these can serve as an interim guide.

Checking the ACNs against those for another 140tonne-class aircraft, the Boeing 767-200, I find that 767-200 on a C subgrade at 141 tonnes is ACN 43, and the A400M is ACN 30. That seems about the right order of difference.

[enicalyth]

It is nearly a decade since Airbus and I parted company.

At 110psi the 73030kg Herc Mk3 was tabled as rigid 32/35//38/40; flex 30/33//35/39. The company line was to repeat the "30" as often as necessary for it to be imprinted upon the mind and I have the brochure upon which the claims were based.

If you mean a B767-200 of the 183psi variety the 141520kg version could be about rigid 32/37/44/51 and flex 36/38//45/63.

Apart from the one Airbus doc I kept the rest has log gone in the skip. Wishful thinking and nightmares galore, Magrathean engineering allied to Golgafinchan management... what stuff.

I'd contact the person named below in the first instance but for what its worth your C figure as an ACN is low by my recall.

My PM refers

Thought I'd heard the last of that!

Saw your letter regarding EDI. I have some figures, wonder where they are? I'd ask KO Sally but I fear her wrath.

Anyway the man you need is.......

[OverRun]

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
http://profemery.info/FAA_test_sm.jpg

[enicalyth]

Prof!

I appended a further note via PM.

Moving on... There is a difference between max logistic TOW and max tactical TOW and even MTOW. Imagine wing area 221 sq m, Aspect Ratio 8.1, 842 rpm on a 5.33m dia 8-blade prop. In the same breath imagine 400ktas at 9.5kg/nm. And in the next breath range circles centred on Paris 2400nm/30 tonnes; 3450nm/20 tonnes; 4750nm/ferry. These claims are not simultaneous. For example take 400ktas - that's Mach 1.05 tip speed so shed weight or slow down or take a bigger pinch of salt. [Any resemblance to an A400M claims is purely coincidental]. So if MTOW has gone up, in my day 136500kgf @ 2.25g [127500 @ 2.5g] it is not hurrah but rather because the ship is overweight and a burner.

By the bye the C-17A was projected with a 160000lbf payload [2.25g] and 2400nam at M0.77 cruise with LCN [paved surface] coyly quoted as "better than 49". Try an AUW of 265800kgf at 144psi rigid 53/53/53/70 and flex 52/59/71/95... see what you think.

[OverRun]

Hi enicalyth

All received, thanks. Useful insight into the A400M. I took a long look at the C-17A after your post and found a little trap which I hadn't known existed (due to my limited and doubtless ignorant horizons).

Its flexible ACN does drop quite a bit using the 2007 alpha factors, which is to be expected with 6 wheels per main gear. The trap is in the rigid pavement ACN. The 'rigid cutoff' is limited in software to 3.00 times rrs to comply with ICAO's ACN calculation. This gives odd ACNs for rigid pavements, which only become sensible with the Rigid cutoff set to say 5.

FAA says "the standard ACN cutoff for rigid pavement stress computation is 3 times the radius of relative stiffness (rrs). This gives inconsistent results with large complex gear configurations such as the C-17 (high-strength ACN higher than low-strength ACN)."

Ok - now I know. I think this might be quite humorous to let loose on my students and see who picks it up :ok:

[enicalyth]

I am glad to have opened a box of delights! The C17 rigid example is a good 'un isn't it.