Understanding Obstruction Clearance and Runway Length

The question asks about a specific point defined by the intersection of the obstruction clearance line and the extended plane of the runway surface. This point is crucial for determining the usable length of the runway for aircraft operations, considering safety margins required to clear any obstacles.

Defining the Intersection Point

In aviation, obstacle limitation surfaces are imaginary surfaces designed to protect aircraft during takeoff, landing, and other critical phases of flight. An obstruction clearance line is associated with these surfaces. When an obstruction exists near the runway end, an obstruction clearance line slopes upwards from the runway end or a point near it, ensuring that aircraft maintain a safe vertical distance above the obstruction.

The question describes the point where:

• The obstruction clearance line intersects.
• The extended plane of the runway surface meets this line.
• This intersection is considered relative to the other end of the runway.

This point essentially marks the limit from which an aircraft can safely operate while clearing the obstruction. Any portion of the runway beyond this point towards the obstruction might not be usable for the full operation (like takeoff or landing roll) because the required obstruction clearance cannot be guaranteed.

Analyzing the Options

Let's look at the given options:

1. base width of hanger: A hanger is a building for storing aircraft. Its base width is a structural dimension and has no direct relation to runway obstruction clearance lines or effective runway length. This option is incorrect.
2. effective flight width: This term is not a standard technical term related to the geometric intersection point described. While flight paths have defined widths (like protected airspace or corridor width), this is different from a point defined by runway surface and obstruction clearance lines. This option is incorrect.
3. effective length of runway: The effective length of a runway refers to the portion of the runway that is declared available and suitable for the ground run of an aircraft taking off or landing. Obstructions near the runway ends can reduce the declared distances (like Take-Off Run Available - TORA, Take-Off Distance Available - TODA, Landing Distance Available - LDA). The point where the obstruction clearance line intersects the extended runway surface at the far end (relative to the direction of operation) effectively determines the limit of the usable or 'effective' length for that particular operation. This point marks the beginning of the segment that is no longer safe due to the obstruction requiring a climb gradient starting earlier. This option is consistent with the description.
4. effective length of taxiway: A taxiway is used by aircraft to move between the runway and parking areas. Obstruction clearance exists around taxiways, but the point described in the question is specifically related to the runway surface and takeoff/landing clearance, not taxiway operations. This option is incorrect.

Based on the analysis, the point described is directly related to determining the usable or effective length of the runway when obstructions are present.

Conclusion

The point of intersection of the obstruction clearance line and the extended plane of the runway surface at the other end of the runway is a key determinant in calculating the declared distances, which define the effective length of the runway for different operations.

Revision Table: Aviation Terms

Additional Information: Declared Distances

The effective length of a runway is formalized through declared distances. These distances provide precise information to pilots about the length available for various phases of takeoff and landing. The point of intersection described in the question is fundamental in calculating these distances.

• Take-Off Run Available (TORA): The length of runway declared available and suitable for the ground run of an aeroplane taking off.
• Take-Off Distance Available (TODA): The length of the takeoff run available plus the length of the clearway, if provided.
• Accelerate-Stop Distance Available (ASDA): The length of the takeoff run available plus the length of the stopway, if provided.
• Landing Distance Available (LDA): The length of runway declared available and suitable for the ground run of an aeroplane landing.

Obstructions penetrating the obstacle limitation surfaces at the end of the runway will typically result in a reduced LDA or TORA, effectively reducing the 'effective length' available for operations in that direction. The point described in the question is key to determining this reduction.