You're about to depart an airport under IFR with low ceilings. There's no SID and no ODP. When can you start your on-course turn, and why? It's called a "diverse departure assessment", and here's how it works.

Airports Complete An ODP Assessment

When the FAA analyzes an airport for instrument departures, it completes an obstacle departure procedure (ODP) assessment. TERPS criteria are fairly complicated, so we'll break down some of the most important facts about instrument departures. The initial climb area (ICA) is the segment of an instrument departure procedure that starts at the DER (departure end of runway) and proceeds along the runway centerline extended to allow the aircraft sufficient distance to reach an altitude of 400 feet above DER elevation and to allow the establishment of positive course guidance by all navigation systems.

A typical straight departure ICA extends 2-5 NM from the DER along the runway centerline extended. It is 500 feet wide each side of the runway centerline at DER, then spreads out at 15-degrees. The obstacle clearance surface (OCS) is a slope that extends at 152 feet per nautical mile (that's a 40:1 ratio).

Mountainous terrain, tall buildings, and towers create unique hazards for aircraft departing under IFR. If an obstacle penetrates the 40:1 obstacle identification surface, then the procedure designer chooses whether to:

• Establish a steeper than normal climb gradient (More than 200' per NM)
• Establish a steeper than normal climb gradient with an alternative that increases takeoff minima to allow the pilot to visually remain clear of the obstacle(s)
• Design and publish a specific departure route
• A combination, or all of the above.

Here's an example of an ODP using a few of the steps listed above:

Diverse Departure Assessment

According to the FAA in IPH Chapter 1, "if an aircraft may turn in any direction from a runway within the limits of the assessment area and remain clear of obstacles that runway passes what is called a diverse departure assessment, and no ODP is published."

A diverse departure assessment ensures that a prescribed, expanding amount of Required Obstacle Clearance (ROC) is achieved during the climb-out until the aircraft can obtain a minimum 1,000' ROC in non-mountainous areas or a minimum 2,000' ROC in mountainous areas.

Like all other instrument departures, unless otherwise stated, obstacle clearance is based on the following criteria:

Don't Turn Until You Hit 400 Feet (DER)

In flat areas of the country, you might not find an ODP published for departure because there simply isn't a reason to have one. Plus, if there's a light load on ATC in the area, you probably won't find a SID published either.

That means it's your responsibility as PIC to remember the regulations governing your instrument departure. Obstacle protection is still guaranteed, but you must follow the "non-charted procedure." To remain within the initial climb area , you'll need to maintain runway centerline to 400 feet (above DER). This keeps you safely clear of obstacles.

Once you've reached 400 feet above the departure end of the runway, you can initiate your on-course turn (assuming ATC hasn't restricted your turn at that point).

At Some Airports, Obstacle Avoidance Is 100% Up To You

Some airports have no guaranteed "safe zones" for aircraft departing IFR. When an airport is assesed for an instrument approach, the FAA also performs an ODP/diverse-departure assessment. If the airport hasn't been assessed for an approach or specific departure procedure (nothing is published), obstacle avoidance is the pilot's responsibility.

There are thousands of small, private, and uncharted airports all around the country, and the FAA can't get to them all. Departing from these airports in low weather isn't a good idea if you can't see all of the obstacles and terrain around you.

Have you flown any complex departure procedures? Tell us in the comments below.