Jim Macklin wrote:
A normal IDP requires 152 ft/nm, about 350 ft. min @ 120
KIAS.
Many airports have obstacles that require a steeper climb,
so they publish "non-standard TO minima"
Some airports cannot be departed safely in IMC due to local
terrain.
Altitudes required to clear an obstruction include a margin,
2,000 feet in mountainous regions and 1,000 feet in other
areas.
The IDP gradient at Aspen is based on the angle from the
departure end to the critical obstacle, a rate that is well
above the engine-out performance of most airplanes. [460
ft/nm will be close to 1,000 ft/min and most turboprops
won't do that on one engine, it is the worst your plane can
do that is the performance you need to consider.]
Once past the critical obstacle, a climb at standard rates
will be safe. [Again, those rates may be above the
performance of many smaller, low powered aircraft.]

From AC 120-91:

7. TERPS CRITERIA VERSUS ONE-ENGINE-INOPERATIVE REQUIREMENTS.

a. Standard Instrument Departures (SID) or Departure Procedures (DP)
based on TERPS or ICAO Procedures for Air Navigation Services—Aircraft
Operations (PANS-OPS) are based on normal (all engines operating)
operations. Thus, one-engine-inoperative obstacle clearance requirements
and the all-engines-operating TERPS requirements are independent, and
one-engine-inoperative procedures do not need to meet TERPS
requirements. Further, compliance with TERPS all-engines-operating climb
gradient requirements does not necessarily assure that
one-engine-inoperative obstacle clearance requirements are met. TERPS
typically use specified all-engines-operating climb gradients to an
altitude, rather than certificated one-engine-inoperative airplane
performance. TERPS typically assume a climb gradient of 200 feet per
nautical mile (NM) unless a greater gradient is specified. For the
purposes of analyzing performance on procedures developed under TERPS or
PANS-OPS, it is understood that any gradient requirement, specified or
unspecified, will be treated as a plane which must not be penetrated
from above until reaching the stated height, rather than as a gradient
which must be exceeded at all points in the path. Operators must comply
with 14 CFR requirements for the development of takeoff performance data
and procedures. There are differences between TERPS and
one-engine-inoperative criteria, including the lateral and vertical
obstacle clearance requirements. An engine failure during takeoff is a
non-normal condition, and therefore takes precedence over noise
abatement, air traffic, SIDs, DPs, and other normal operating
considerations.