---
title: "CS ADR-DSN.H.420 Inner horizontal surface — Certification Specifications and Guidance Material for Aerodrome Design (CS-ADR-DSN) — Issue 7, Aerodromes · Avioverse"
description: "Applicability: The purpose of the inner horizontal surface is to protect airspace for visual manoeuvring prior to landing."
url: "https://avioverse.aero/regulations/easa/aerodromes/cs-adr-dsn/chapter-h-obstacle-limitation-surfaces/cs-adr-dsn-h-420"
---

# CS ADR-DSN.H.420 Inner horizontal surface

Certification Specifications and Guidance Material for Aerodrome Design (CS-ADR-DSN) — Issue 7 · Regulation (EU) No 139/2014 · EAR revision 13 Mar 2026

IR Implementing rule

## CS ADR-DSN.H.420 Inner horizontal surface

(a) Applicability: The purpose of the inner horizontal surface is to protect airspace for visual manoeuvring prior to landing.

(b) Description: A surface located in a horizontal plane above an [aerodrome](https://avioverse.aero/glossary/easa/aerodrome) and its environs.

(c) Characteristics: The outer limits of the inner horizontal surface are defined by a circle centred on the geometric centre of the [runway](https://avioverse.aero/glossary/easa/runway), by a convex contour composed of circular arcs centred on the intersections of the extended RWY centre line with the end of the RWY strip, joined tangentially by straight lines parallel to the runway centre line, as shown in Figure H-1, or on other points established for such purpose.

(d) The height of the inner horizontal surface should be measured above an established elevation [datum](https://avioverse.aero/glossary/easa/datum). The elevation datum used for the height of the inner horizontal surface should be:

(1) the elevation of the highest point of the lowest [threshold](https://avioverse.aero/glossary/easa/threshold) of the related runway; or

(2) the elevation of the highest point of the highest threshold of the related runway; or

(3) the elevation of the highest point of the runway; or

(4) the [aerodrome elevation](https://avioverse.aero/glossary/easa/aerodrome-elevation).

\[Issue: ADR-DSN/3]

IR · CS ADR-DSN.H.420 — Regulation (EU) No 139/2014 · ED Decision 2016/027/R · Aerodromes Easy Access Rules · EAR revision 13 Mar 2026

[All rules in CHAPTER H — OBSTACLE LIMITATION SURFACES](https://avioverse.aero/regulations/easa/aerodromes/cs-adr-dsn/chapter-h-obstacle-limitation-surfaces)

Consolidated from the EASA Easy Access Rules (revision 13 Mar 2026, extracted 13 Oct 2026) for convenience. Not the official publication — verify against the Official Journal of the European Union and the EASA publications before operational use.