Page 312 - Planning and Design of Airports

270 Airp o r t D e sign

Because many of the latest generation aircraft require more com-
plex landing gear configurations that are provided in Table 7-5, a spe-
cial consideration for very heavy aircraft were made by assigning a
gross takeoff weight of 300,000 lb and a dual-tandem landing gear
configuration to any aircraft with maximum gross takeoff weight
greater than 300,000 lb. This rough approximation was, in part, moti-
vation, to develop an entirely new assessment of fleet mix with
respect to airfield pavement design and evaluation.

Cumulative Damage Failure Method
The current method of airfield pavement design and evaluation now
considers each type of aircraft that uses the pavement explicitly. The
“design aircraft” concept has been replaced by design for fatigue fail-
ure expressed in terms of a “cumulative damage factor” (CDF). The
CDF for a given aircraft is a value between 0 and 1 which expresses
the contribution to ultimate pavement failure of the projected number
of uses for each aircraft type that use the pavement. Based on Miner’s
rule, a traditional theory which estimates the amount of use until fail-
ure of a pavement, the CDF for a given fleet of aircraft is determined
by Eq. (7-5).

CDF = ∑ (n /N ) (7-5)
i i
where n is the expected number of annual departures of aircraft i and
i
N is the number of departures of aircraft i that would lead to pave-
i
ment failure for each aircraft i in the mix.
When CDF meets or exceeds 1, the cumulative predicted number
of operations for each of the aircraft in the mix will lead to failure of
a given pavement system. Any value less than 1 represents the frac-
tion of pavement life that has been effectively “used up.” For exam-
ple, a CDF of 0.75 would indicate that the pavement has used 75 per-
cent of its useful life, and has 25 percent of its life remaining under
the predicted traffic usage before fatigue failure.
For both the design of flexible and rigid pavements, the current
FAA pavement design method applies a computer software model to
estimate the appropriate thickness of designed pavement layers,
given the Young’s modulus E value of the subgrade and the expected
aircraft fleet mix, such that the CDF of the pavement equals 1 after a
20-year life of the designed pavement.
The FAA approved software, FAA Rigid and Flexible Iterative Elas-
tic Layered Design or FAARFIELD comes equipped with a library of
aircraft, their maximum gross weights, landing gear configuration,
and contribution to CDF for the given pavement design. Figure 7-4
illustrates the “aircraft” window of FAARFIELD with user inputs of
each given aircraft’s estimated departures for the to-be-designed
pavement system.