Controlling Dimension (CD) refers to a critical dimension or characteristic of a part, assembly, or system that directly affects its fit, function, or performance. It is a dimension that has a significant impact on the overall operation, safety, or reliability of an aircraft or spacecraft.

Controlling Dimensions are typically identified during the design and development phase of a project and are carefully monitored and controlled throughout the manufacturing, assembly, and testing processes. These dimensions are often subject to strict tolerances and specifications to ensure that the part or system meets the required performance and safety standards.

Examples of Controlling Dimensions in aviation and aerospace include:

1. Engine components: Dimensions such as bore diameter, piston stroke, or valve clearance can affect engine performance, efficiency, and reliability.
2. Airframe structures: Dimensions such as wing span, chord length, or fuselage diameter can impact the overall aerodynamic performance and stability of an aircraft.
3. Landing gear: Dimensions such as wheel diameter, brake diameter, or strut length can affect the aircraft's ground handling, braking performance, and safety.
4. Avionics and electronics: Dimensions such as antenna placement, wiring harness routing, or connector spacing can impact the performance and reliability of communication, navigation, and control systems.

Controlling Dimensions are critical because they can have a significant impact on the overall safety and performance of an aircraft or spacecraft. Any deviations from the specified dimensions can lead to reduced performance, increased maintenance costs, or even catastrophic failures.

To ensure that Controlling Dimensions are accurately controlled and maintained, aerospace manufacturers and regulators often use a range of techniques, including:

1. Dimensional inspection and measurement
2. Tolerance analysis and stack-up studies
3. Geometric dimensioning and tolerancing (GD&T)
4. Quality control and assurance processes
5. Certification and testing protocols

By carefully controlling Controlling Dimensions, the aviation and aerospace industries can ensure the safe and reliable operation of aircraft and spacecraft, as well as compliance with regulatory requirements and industry standards.