Airport obstacle surfaces

Journal of Advanced Transportation, Fall, 2009 by Scott Litsheim, Xiaosong Xiao

[FIGURE 5 OMITTED]

3.5 Departure

In the case of an instrument departure, an OCS is applied during the climb until at least the minimum en route value of ROC is attained. The minimum OCS is a 40:1 surface based on the minimum rate of climb of 200 feet/NM. Figure 6 shows the classic 24% rule that the 40:1 is derived from, where ROC=0.24*CG. The minimum ROC supplied by the 200 feet/NM CG is 48 feet/NM (0.24 x 200)=48). Since 48 of the 200 feet gained in 1 NM is ROC, the OCS height at that point must be 152

feet, or 76% of the CG. Thus, the slope of a surface that rises 152 feet over 1 NM is 40. The purpose of ROC is to ensure that a certain clearance between the aircraft and the obstacle is maintained.

[FIGURE 6 OMITTED]

4. One Engine Inoperative (OEI) Surface

This surface is used to establish the takeoff climb limit weight. It is not to be considered a clearance surface and information concerning penetration to this surface is provided for information only and does not take effect until January 1, 2008 [FAA(2006b)] Aircraft climb regulations are specified in FAA Order 8400.10 [FAA(2004a)], Air Transportation Operations Inspector's Handbook, and Federal Aviation Regulation (FAR) Part 25 [FAA(2004b)], 121 [FAA(2005a)] and 135 [FAA(2006a)]. The takeoff weight of large, turbine-powered airplanes must be limited to allow the aircraft to climb at a specified gradient through each of the defined climb segments of the takeoff flight path.

The obstacle clearance net takeoff flight path for FAR Part 25 airplanes is derived by subtracting an increment from the actual flight path the airplane can fly (gross flight path). The net flight path begins at the point the airplane reaches 35 feet above the runway and must pass not less than 35 feet over each obstacle, as shown in Figure 7.

Because the minimum climb gradient required for two-engine aircraft is less than the minimum climb gradients for three-engine or four-engine aircraft, the most critical situation for obstacle clearance is that for two-engine aircraft. And for aircraft with two engines the subtracted increment is 0.8%. So, the slope of the surface will be the minimum two-engine required actual (gross) flight path of 2.4% minus 0.8%, which equals 1.6%, or a 62.5:1 slope. Then, for each specified altitude and temperature, a maximum allowable takeoff weight is calculated to achieve a net takeoff flight of at least 1.6% which assures that all obstacles are cleared by the required 35 feet.

It specifies the obstacle-free minimum climb gradients required for turbine-powered aircraft with one engine inoperative. Accordingly, the 1.6% (62.5:1) slope has been adopted by the FAA as the one-engine-inoperative surface, starting at the runway threshold elevation. It is now referenced in FAA Advisory Circulars and Orders.

[FIGURE 7 OMITTED]

However, for penetrations, and in order to maintain the 35 feet of required clearance, a steeper than 1.6% net takeoff flight path is needed which will further reduce weight.

5. Runway Threshold Sitting Surface