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provide obstacle protection for departing aircraft. They
also allow for efficient routing of traffic and reductions
in pilot/controller workloads. These procedures come in
many forms, but they are all based on the design criteria
outlined in TERPS and other FAA orders. The A/FD
includes information on high altitude redesign RNAV
routing pitch points, preferred IFR routings, or other
established routing programs where a flight can begin a
segment of nonrestrictive routing.
DESIGN CRITERIA
The design of a departure procedure is based on TERPS,
a living document that is updated frequently. Departure
design criterion assumes an initial climb of 200 feet per
nautical mile (NM) after crossing the departure end of
the runway (DER) at a height of at least 35 feet. [Figure
2-15] The aircraft climb path assumption provides a
minimum of 35 feet of additional obstacle clearance
above the required obstacle clearance (ROC), from the
DER outward, to absorb variations ranging from the
distance of the static source to the landing gear, to differences in establishing the minimum 200 feet per NM
climb gradient, etc. The ROC is the planned separation
between the obstacle clearance surface (OCS) and the
required climb gradient of 200 feet per NM. The ROC
value is zero at the DER elevation and increases along
the departure route until the appropriate ROC value is
attained to allow en route flight to commence. It is
typically about 25 NM for 1,000 feet of ROC in nonmountainous areas, and 46 NM for 2,000 feet of ROC
in mountainous areas.
Recent changes in TERPS criteria make the OCS lower
and more restrictive. [Figure 2-16 on page 2-14]
However, there are many departures today that were
evaluated under the old criteria [Figure 2-15] that
allowed some obstacle surfaces to be as high as 35 feet
at the DER. Since there is no way for the pilot to determine whether the departure was evaluated using the
2-13
previous or current criteria and until all departures have
been evaluated using the current criteria, pilots need to
be very familiar with the departure environment and
associated obstacles especially if crossing the DER at
less than 35 feet.
Assuming a 200-foot per NM climb, the departure is
structured to provide at least 48 feet per NM of clearance above objects that do not penetrate the obstacle
slope. The slope, known as the OCS, is based on a 40 to
1 ratio, which is the equivalent of a 2.5 percent or a 152-
foot per NM slope. As a result, a departure is designed
using the OCS as the minimum obstacle clearance, and
then by requiring a minimum climb gradient of 200 feet
per NM, additional clearance is provided. The departure
design must also include the acquisition of positive
course guidance (PCG) typically within 5 to 10 NM of
the DER for straight departures and within 5 NM after
turn completion on departures requiring a turn. Even
when aircraft performance greatly exceeds the minimum
climb gradient, the published departure routing must
always be flown.
Airports declaring that the sections of a runway at one
or both ends are not available for landing or takeoff publish the declared distances in the A/FD. These include
takeoff runway available (TORA), takeoff distance
available (TODA), accelerate-stop distance available
(ASDA), and landing distance available (LDA). These
distances are calculated by adding to the full length of
paved runway, any applicable clearway or stopway, and
subtracting from that sum the sections of the runway
unsuitable for satisfying the required takeoff run, takeoff, accelerate/stop, or landing distance, as shown in
Figure 2-16 on page 2-14.
In a perfect world, the 40 to 1 slope would work for
every departure design; however, due to terrain and manmade obstacles, it is often necessary to use alternative
requirements to accomplish a safe, obstacle-free departure design. In such cases, the design of the departure
may incorporate a climb gradient greater than 200 feet
per NM, an increase in the standard takeoff minimums
to allow the aircraft to “see and avoid” the obstacles,
standard minimums combined with a climb gradient of
200 feet per NM or greater with a specified reduced runway length, or a combination of these options and a specific departure route. If a departure route is specified, it
must be flown in conjunction with the other options. A
published climb gradient in this case is based on the
ROC 24 percent rule. To keep the same ROC ratio as
standard, when the required climb gradient is greater
than 200 feet per NM, 24 percent of the total height
35'
152'
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Instrument Procedures Handbook (IPH)仪表程序手册上(49)
