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Aeronautical Studies for OLS Penetrations under ICAO Annex 14 and CAA NZ

AOIWEB LTD · 4 December 2025

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Aerodrome safeguarding plays a critical role in protecting aircraft operations and public safety. A core part of safeguarding is the management of obstacles around airports, particularly those that penetrate Obstacle Limitation Surfaces (OLS) defined under ICAO Annex 14, Volume I.

In New Zealand, the assessment and control of these obstacles is governed primarily by Civil Aviation Rule Part 77 – Objects and Activities Affecting Navigable Airspace, with direct implications for instrument flight procedures (IFPs) managed under Rule Part 173.

When a proposed structure penetrates an OLS and cannot reasonably be avoided or redesigned, an aeronautical study is required to determine whether aviation safety can still be maintained.

Obstacle Limitation Surfaces (OLS) – ICAO Annex 14

Obstacle Limitation Surfaces are a set of imaginary three-dimensional surfaces surrounding an aerodrome and its runways. Their purpose is to:

  • Protect aircraft during take-off, approach, and landing

  • Define height limits for structures near flight paths

  • Provide baseline obstacle protection for both visual and instrument operations

ICAO Annex 14 specifies the geometry and extent of these surfaces based on runway code, approach type, and aerodrome characteristics. A penetration of an OLS does not automatically mean a proposal is unsafe, but it is a clear signal that formal aviation assessment is required.

The New Zealand Regulatory Framework

CAA Rule Part 77 – Objects and Activities Affecting Navigable Airspace

In New Zealand, Rule Part 77 provides the legal framework for managing objects and activities that may affect navigable airspace.

Under Part 77:

  • Persons proposing to construct, alter, or extend objects that may affect navigable airspace must notify the Director of Civil Aviation

  • The Director assesses whether the proposal constitutes a hazard to aircraft operations

  • Where necessary, the Director undertakes an aeronautical study to support the hazard determination

👉 CAA Rule Part 77 https://www.aviation.govt.nz/rules/rule-part/part-77/

While OLS penetrations are a common trigger, Part 77 aeronautical studies are not limited to OLS. They may address any aeronautical issue arising from a proposed object or activity, including effects on flight paths, procedures, navigation aids, or pilot situational awareness.

Aeronautical Studies

An aeronautical study is a structured study of an aeronautical problem to determine its impact on aircraft operations and aviation safety.

In the context of Part 77, an aeronautical study may be used to assess:

  • Obstacle penetrations of OLS

  • Impacts on visual or instrument flight paths

  • Effects on existing or future instrument flight procedures

  • Cumulative obstacle environments

  • The operational significance of temporary or permanent hazards

Rather than asking only whether an OLS penetration can be accepted, an aeronautical study asks a broader and more fundamental question:

Does this object or activity create an unacceptable hazard to aircraft operations, and if so, can that risk be mitigated to an acceptable level?

This distinction is important, as not all aeronautical studies involve OLS, and not all OLS penetrations result in unacceptable safety outcomes.

Part 77 and Instrument Flight Procedure (IFP) Impact

A key element of the Part 77 aeronautical study process is the assessment of impacts on instrument flight procedures.

As part of the hazard assessment, the study considers whether a proposed object:

  • Infringes obstacle clearance surfaces used in IFP design

  • Affects existing published procedures

  • Restricts the design of future procedures

If an impact on IFPs is identified, the aeronautical study does not end with the OLS assessment. Instead, it considers the operational consequences for IFR operations.

CAA Rule Part 173 – Instrument Flight Procedure Service Organisations

Rule Part 173 governs organisations authorised to design, validate, and maintain instrument flight procedures in New Zealand.

👉 CAA Rule Part 173 https://www.aviation.govt.nz/rules/rule-part/part-173/

Where a Part 77 aeronautical study identifies an impact on IFPs, the IFP service provider, operating under Part 173, will typically need to:

  • Raise approach or departure minima

  • Modify procedure trajectories or design parameters

  • Apply operational limitations

  • Disestablish procedures if no acceptable mitigation exists

These outcomes can significantly affect aerodrome accessibility, particularly in marginal weather, and may have long-term operational and commercial consequences.

An obstacle may therefore be acceptable from a purely geometric OLS perspective, yet still be operationally unacceptable due to its effect on instrument procedures.

When Is an Aeronautical Study Required?

Aeronautical studies are typically required when a proposal introduces a potential aeronautical problem, such as:

  • Penetration of an OLS by a permanent or temporary structure

  • Encroachment into protected airspace by terrain or vegetation

  • Activities that may affect flight paths, visibility, or pilot workload

  • Developments that may affect navigation, surveillance, or procedures

From a safeguarding perspective, aeronautical studies are a risk assessment tool, not a workaround for poor planning.

Key Elements of an Aeronautical Study

1. Definition of the Aeronautical Problem

Including:

  • Nature of the object or activity

  • Location and dimensions

  • Duration (temporary or permanent)

  • Relevant operational context

2. Airspace and Operational Impact Assessment

Identifying impacts on:

  • OLS and other protected airspace

  • Visual and instrument flight paths

  • Missed approach and contingency operations

  • Existing obstacle environment

3. Safety Risk Assessment

Consistent with ICAO safety management principles:

  • Severity

  • Likelihood

  • Existing controls

  • Residual risk

4. Mitigation and Operational Outcomes

Including:

  • Obstacle marking and lighting

  • Design changes or height reductions

  • Operational or procedural mitigations

  • IFP changes via the Part 173 service provider

  • Monitoring or time-limited approvals

Using Technology to Support Safeguarding Decisions

Early identification of potential aeronautical problems is critical.

Tools such as AOIWeb Pro allow rapid evaluation of obstacles against ICAO Annex 14 OLS, helping to:

  • Identify potential Part 77 triggers early

  • Visualise penetration depth and affected surfaces

  • Support early design decisions

  • Reduce late-stage redesigns or unintended IFP impacts

By highlighting safeguarding risks early, AOIWeb Pro supports more efficient aeronautical studies and better planning outcomes.

Why Aeronautical Studies Matter

Well-prepared aeronautical studies:

  • Protect aircraft operations and public safety

  • Preserve long-term aerodrome and IFR capability

  • Prevent incremental erosion of protected airspace

  • Provide defensible, transparent regulatory decisions

Poorly framed studies, by contrast, can lead to unsafe outcomes, raised minima, or loss of instrument procedures.

A Core Safeguarding Principle

An obstacle penetration that appears geometrically minor can be operationally significant, particularly where instrument procedures are affected. Conversely, a larger penetration may be acceptable if it does not materially affect how aircraft operate.

This principle, central to ICAO Annex 14, CAA Rule Part 77, and Part 173 practice, is why aeronautical studies remain a cornerstone of effective aerodrome safeguarding.