Decoding the undercarriage assembly. As well as decoding the various authorities words, I've expressed it in terms of the simple approach of tyres per main gear leg [helps me understand it better. '

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The most common civil aircraft undercarriage configurations are:

S = single wheel = one tyre per main gear leg = DC-3

D = dual = T = twin wheel = double = two tyres per main gear leg = Boeing 737

DT = dual tandem = dual in tandem = B = bogie = TT = twin tandem = 4 tyres per main gear leg = Boeing 767

DDT = double dual tandem = DB = double bogie = 4 tyres per main gear leg, and two main gear legs per side = Boeing 747

The more exotic configurations are (usually military):

ST = Single-tandem = C-130

SBTT = Single-Belly Twin Tandem = KC-10 or DC10-40

TT = Twin-tandem type (includes quadricycle landing gear) = B-52

TRT = Triple-tandem landing gear = C-17

TDT = Twin delta-tandem landing gear C-5

and of course the Boeing 777. I may have been wrong in my comments about TT and TRT above. Boeing themselves call this a tridem gear, and designate this as TD = tridem dual. But you won't find it mentioned as that very often.

Decoding the 'airfield listed maximum AUW dependant upon type of undercarriage assembly' is next. It does apply to most of the airfields in the USA, which means that it is usually MAUW in '000 lbs except for those airports that express it in '000 kgs. Confusing at times.

The interpretation of these ratings is that any aircraft having that designated type of main landing gear can operate up to the allowable gross weight (in 1,000 pounds) indicated by the numerical value of the rating. For example, the rating D145 means that each of the A319, A320, DC9, MD80, F27, F28, BAe146, B727, or B737 aircraft can operate on the rated pavement at gross weights up to 145,000 pounds, regardless of the individual airplane characteristics such as wheel spacing or tire pressure. The common and overriding factor is that these aircraft all have dual-wheel main landing gear arrangements.

The All Up Weight (AUW) method of assessing allowable gross weights is somewhat nebulous in that it is a method that allows the operation of many different aircraft on the pavement without regard to its pavement loading characteristics. This method does not consider factors such as number of wheels on a main gear, differences in wheel spacing, tire pressure, or percent of weight on the main gear. The ACN/PCN method takes much better account of these factors, and is more common outside the USA.

So now we can start to decode the airfields. Do it in two passes – the first is the literal pass, and the second is to check if it makes sense and someone hasn't got pounds and kilograms mixed up:

Rwy 14-32 S75, T185, ST175, TT325.

The first pass of decode gives us:

S = single = 75,000 lbs

T = twin = 185,000 lbs

ST = single tandem = 175,000 lbs

TT = twin tandem = 325,000 lbs.

Now check that against real aircraft to see that it is in lbs and not kgs, and that it makes sense:

S = single = 75,000 lbs. Ignore this – it helps the engineer think in terms of ESWL which is beyond the scope of this discussion.

T = twin = 185,000 lbs. Boeing 737-800 HGM has max takeoff weight (MAUW) of 172,500 lbs – that's pretty close, and I know the A321 is a bit heavier which makes this limit credible.

ST = single tandem = 175,000 lbs. Lockheed L-100 is 155,000 lbs MTOW, and the military allow an overload in their C130s, which makes this limit credible.

TT = twin tandem = 325,000 lbs. Boeing 767-200 is 300,000 lbs MTOW for the regular and 387,000 lbs for the ER version, which makes this limit credible.

I get a sense that the runway structure here is a bit thin, and the larger variants of the 767 are pushing the limit. The 767-400 ER would probably be over the limit, as would the B747.

Next decode:

AUW100S/190D/360DT/550TRT/850DDT

The first pass of decode gives us:

S = single = 100,000 lbs

D = dual = T = twin = 190,000 lbs

DT = dual tandem = TT = twin tandem = 360,000 lbs.

TRT = Triple-tandem landing gear = 550,000 lbs

DDT = double dual tandem = 850,000 lbs.

Now check that against real aircraft to see that it is in lbs and not kgs, and that it makes sense:

S Ignore this as before.

T = twin = 190,000 lbs. Boeing 737-800 HGM (MAUW) of 172,500 lbs; A321 is a bit heavier which makes this limit credible.

DT = dual tandem = 360,000 lbs. Boeing 767-200 is 300,000 lbs MTOW for the regular and 387,000 lbs for the ER version, which makes this limit credible.

TRT = triple-tandem = 550,000 lbs. C-17 is 585,000 lbs MTOW. The Boeing 777 is 506,000-660,000 lbs MTOW. I simply don't know which they are referring too. Is it TRT or TD or TT or what? If there is a National Guard or Air Force base at the airport, then it could be the C-17. If the airlines are operating 777s, then it could be the 777.

DDT = double dual tandem = 850,000 lbs. Boeing 747-400 is around 850,000 lbs MTOW depending on the version, which makes this limit credible.

What are the limits at this runway mean in terms of ACN/PCN? Well I checked the ACN charts for typical aircraft for each category to see what ACN would be equivalent. This is approximate since different aircraft in a single category can have slightly different ACNs, as mentioned above.

Type '000 lbs ACN

D 190 50

DT 360 48

TRT 550 46

DDT 850 62

So I guess this runway would have a PCN of 48-50. The rating for the DDT is higher at 62, which could mean that the pavement can be higher rated at PCN 62,or the runway really is PCN 50ish, and the higher weight limit may have been set for a strategic reason.

Mini,

When you're faced with having to stab in the dark a bit, then there is another method called the "Using aircraft method of determining PCN".

This procedure can be used when there is limited knowledge of the existing traffic and runway characteristics. It is also useful when engineering analysis is neither possible nor desired. Accuracy of ratings based on "Using aircraft" is by nature less than that for a Technical evaluation, but PCNs can be assessed more quickly and with minimal cost. However, airport authorities should be more flexible in the application of a Using aircraft PCN in that the rating has not been rigorously determined. There are only two basic steps required to arrive at a "Using aircraft PCN":

1. Determine the airplane with the highest ACN in the traffic mix currently using the runway. This is the critical airplane.

2. Assign the ACN of the critical airplane as the PCN.

Then check your aircraft ACN is below that PCN. It can be hopelessly over-conservative, but hopefully not too optimistic.