Obstacle clearance is based on the aircraft climbing at 200ft per nautical mile, cross ing the end of the runway at 35ft AGL, and climbing to 400ft above the airport eleva tion before turning unless otherwise specified in the procedure. This is the basic obstacle clearance specification. A slope of 152ft per mile, starting no higher than 35ft above the departure end of the runway, is assessed for obstacles. A minimum of 48ft of obstacle clearance is provided for each mile of flight. If no obstacles penetrate the 152ft per mile slope, IFR departure procedures are not published. So far so good. If nothing is specified on the SID or in the takeoff section of the airport plan chart, then all you need to do is meet the above climb criteria. If obstacles penetrate the slope, obstacle avoidance procedures are speci fied. These procedures may be a ceiling and visibility to allow the obstacles to be seen and avoided; a climb gradient greater than 200ft per mile, detailed flight maneuvers, or a combination of the above. In extreme cases, IFR takeoff may not be authorized for some runways. Unless you have some really bad problems, any aircraft capable of IFR flight is able to meet the standard obstacle climb gradient.Climb gradients are specified when required for obstacle clearance. Crossing restrictions in the SIDs may be established for traffic separation or obstacle clearance. When no gradient is specified, the pilot is expected to climb at least 200ft per mile to MEA unless required to level of fly a crossing restriction. Perhaps we should get our heads together here. We are used to thinking in terms of climbing and descending at a certain feet per minute rate. These climb gradients are based on climbing so many feet per mile. To meet these restrictions, the rate of climb will be a function of both airspeed and ground speed. The faster you fly across the ground, the higher your rate of climb will have to be to meet the climb gradient. Climb gradients may be specified to an altitude/fix, above which the normal gradient applies. Some procedures require a climb in visual conditions to cross the airport at or above an altitude. The specified ceiling and visibility minimums will be enough to allow the pilot to see and avoid ob stacles near the airport. Obstacle avoidance is not guaranteed if the pilot maneuvers farther from the airport than the visibility minimum. This is a very important point. If you are giv en an IFR clearance with a two-mile visibility restriction, it is your responsibility to stay within two miles of the airport until above the ceiling specified in the same clearance. You must remain in visual conditions to see and avoid any obstacles. That segment of the proce dure which requires the pilot to see and avoid obstacles ends when the aircraft crosses the specified point at the required altitude. Thereafter, standard obstacle protection is provided.1.The basic obstacle clearance specifies that aircraft turns when （ ） ．