Page 287 - Planning and Design of Airports

248 Airp o r t D e sign

The required fillet radius F is given by

2
0.5
2
F = (R + d − 2Rd sin A ) − 0.5u − M (6-11)
max
where u is undercarriage width, that is, the distance between the out-
side tires on the main gear and M is the minimum distance required
between the edge of the outside tire and the edge of the pavement,
that is, the edge safety margin.
The length of the lead-in to the fillet is given by
(4 d tan . 0 5 A max )
L = ln − d (6-12)
d
u
W −− 2 M
where W is the taxiway width on the tangent and ln represents the
natural logarithm.
These equations may be solved for a given aircraft tracking a
curve of radius R to find the necessary lead-in and fillet radius to
maintain a minimum edge safety margin between the tire and pave-
ment edge. If the value of the maximum nose wheel steering angle,
B , exceeds 50° it is recommended that the radius of the centerline
max
curve R which the nose wheel is tracking be increased.
The use of these equations in determining the critical taxiway
curve design parameters is shown in Example Problem 6-3.

Example Problem 6-3
Determine the minimum lead-in and the radius of the fillet to maintain the
cockpit over the centerline for an aircraft with a 156.1-ft wingspan, a wheelbase
of 64.6 ft, undercarriage width of 34.25 ft, and the distance between the main
undercarriage and cockpit equal to 72.1 ft. The aircraft is moving between two
parallel taxiways through a connecting taxiway which has a centerline perpen-
dicular to the parallel taxiways.
With a wingspan of 156.1 ft this aircraft is in airplane design group IV. From
Table 6-23, the taxiway width is 75 ft, the minimum safety margin is 15 ft, the
recommended centerline radius is 150 ft, the recommended length of the fillet
lead-in is 250 ft and the recommended fillet radius is 85 ft.
To verify that these recommended values are acceptable, we can use Eqs. (6-10)
through (6-13). From Eq. (6-10), we have
A = sin (d/R)
−1
max
or
A = sin (72.1/150) = 29°
−1
max
From Eq. (6-11),

−1
B = tan (w/d tan A )
max max
or
−1
B = tan [(64.5/72.1) tan 29°] = 27°
max