Page 10 - Feb24T

Instrument Departures by Ed Verville
Approach to Rifle, CO
Whether I am doing an FAA Instrument Checkride, CFI-Instrument Airplane, 61.58, 135.293, 135.297, or a 121.141 proficiency check, I see pilots struggle with Standard Instrument Departures (SIDs) and Obstacle Departure Procedures (ODPs). The standard instrument departure is based on a minimum climb gradient of 200 feet per nautical mile (NM). How- ever, when a SID or an ODP shows a minimum climb gradient of, say, 310 feet per NM, I ask the student if that is higher or lower than the standard climb gradient. I have been dismayed that nearly half of the responses are wrong. So, I thought we should take a closer look at this topic. This subject gets even more confusing when we discuss One Engine Inoperative (OEI) or engine out procedures.
8 • TWIN & TURBINE / February 2024
Standard Instrument Procedures
Standard Terminal Instrument Procedures (TERPS) typically assume a climb gradient of 200 feet per nautical mile (unless a greater climb gradient is specified). This means that for every one NM (6076 feet) the airplane moves forward horizontally, it needs to climb 200 feet vertically. During development, departure procedures first look for an obstacle clearance based on a 40:1 ratio. This is about one foot of obstacle rise for every forty feet of horizontal distance or the length of a school bus. At mile one, the highest obstacle can be 152 feet. Then, the FAA TERPS adds 48 ft/NM to come up with the standard climb gradient of 200 ft/NM. At two nautical miles horizontal distance, the highest obstacle may be 304 feet (adding another 152 feet to the obstacle height for each NM of horizontal distance)