For ILS obstacle clearance surfaces, pilots are assumed to not deviate from the centreline by more than what deflection?

• A. One full scale deflection
• B. Half a scale deflection
• C. One and a half scale deflection
• D. A quarter scale deflection

Answer: (B)

In ILS obstacle clearance surfaces, the protected area around the final approach centerline is defined assuming the aircraft remains within half of the maximum lateral deflection shown on the guidance instrument. This half-scale tolerance provides a conservative margin for tracking error and instrument limitations, helping ensure obstacle clearance along the final approach path. If you allowed more than half-scale, the lateral deviation could encroach on protected space and risks obstacles; if you required less, the clearance would be unnecessarily tight. The result is that half a scale deflection is the standard assumption for lateral deviation on ILS obstacle clearance surfaces.