9.3 Airspace Classes, Control Zones and CTA Steps

Airspace Classes Relevant to RPA

Australia uses the ICAO airspace classification system. For a Remote Pilot Licence (RePL) holder operating under the standard operating conditions, three classes matter day to day:

• Class G — uncontrolled, non-towered. The default airspace outside controlled zones and upper airspace. Most standard RPA operations occur in Class G. No Air Traffic Control (ATC) clearance is required, though you must still observe the standard operating conditions, keep clear of manned aircraft, and use CTAF procedures at non-towered aerodromes.
• Class D — controlled, typically around regional towered aerodromes. Entry requires ATC clearance. An RPA generally cannot enter Class D without that clearance and, for operations outside the standard conditions, a Remote Operator Certificate (ReOC) approval.
• Class C — controlled, typically around major capital-city and high-density terminal areas. Entry likewise requires ATC clearance and, outside standard conditions, ReOC approval.

Rule of thumb: if the airspace is Class C or Class D, treat it as closed to a standard RPA operation unless you have explicitly obtained clearance and any required approval.

Controlled vs Uncontrolled Airspace

Control Zone (CTR)

A Control Zone (CTR) is the surface-to-boundary controlled airspace around a towered aerodrome. It is usually Class D or Class C. The defining feature is that controlled airspace reaches from the ground up to the CTR upper limit. Inside a CTR:

• The CTA step altitudes printed on the chart are not the controlling factor — the controlled airspace already starts at the surface.
• An RPA cannot operate inside the CTR without ATC clearance (and, outside standard conditions, ReOC approval).
• The CTR boundary is the first thing to check when planning a flight near a towered aerodrome.

CTR Dimensions and the Surface-to-Boundary Concept

CTR shape and size vary by aerodrome. A typical regional Class D CTR is a circle or polygon of roughly 5 to 8 NM radius centred on the aerodrome reference point, extending from the surface (SFC) up to a published upper limit such as 2,500 ft or 4,500 ft AMSL. Capital-city Class C CTRs are larger and more complex, often with several sectors and stepped upper limits. The charted boundary is what matters: any point inside the boundary, at any altitude from the ground up to the upper limit, is inside controlled airspace. "Surface-to-boundary" means there is no low-level gap below the CTR — the controlled airspace touches the ground throughout the zone.

CTA Steps

Control Area (CTA) steps are the stepped bases of controlled airspace that sit above and around a CTR. As you move outward from a towered aerodrome, the base of controlled airspace rises in defined steps — for example, 1,000 ft AMSL close in, 2,000 ft AMSL further out, 3,000 ft AMSL beyond that. Each step is published on the VTC or VNC in feet AMSL and applies from the step line outward until the next step line.

Reading a CTA Step Table

The chart margin often prints a CTA step table summarising the stepped bases around each major aerodrome. A typical entry looks like:

Read the table together with the chart: find your operating distance from the aerodrome reference point, read across to the CTA base, then convert that base to AGL using the charted terrain elevation at your site. A step base of 1,000 ft AMSL over 600 ft terrain gives a 400 ft AGL base — tight for a 400 ft AGL RPA, and likely inside controlled airspace if the RPA reaches the full 120 m.

Example: Transit Near a Class D Aerodrome

You plan a survey 8 NM from a regional Class D towered aerodrome, outside the CTR. The CTA step table shows a 2,000 ft AMSL base at that distance; terrain is 800 ft AMSL. The AGL base is 1,200 ft, well above your 400 ft AGL operating height, so the standard flight sits below the CTA step. You are also beyond the 5.5 km (3 NM) controlled-aerodrome buffer. The flight is legal under the standard conditions, provided you remain in Class G, stay clear of the CTR, and monitor the CTAF for any nearby non-towered traffic.

To know whether an RPA flight sits below a given step:

1. Read the charted step base (AMSL).
2. Read the charted ground elevation at the operating point (AMSL).
3. Subtract to get the step base in AGL: step base (AMSL) − terrain (AMSL) = step base (AGL).
4. Compare the step base (AGL) against your planned RPA height (AGL).

If the operating point is inside the CTR, this calculation is irrelevant — controlled airspace already covers the surface. Always resolve the CTR question first, then the CTA-step question.

The 5.5 km (3 NM) Controlled-Aerodrome Rule

CASA's standard operating conditions prohibit an RPA from being operated within 5.5 km (3 NM) of a controlled aerodrome without approval. A controlled aerodrome is one inside a CTR. This distance rule is a buffer that protects arriving and departing manned traffic and works alongside the CTR boundary:

• The CTR boundary itself may be smaller or larger than 5.5 km depending on the aerodrome.
• Even if a point is outside the charted CTR, if it is within 5.5 km of the aerodrome reference point, the standard conditions still bar the flight unless approval is obtained.
• Approval pathways typically require a ReOC holder to apply and ATC to coordinate.

Entering Controlled Airspace vs the 5.5 km Rule

These two rules are often confused but govern different things:

• Entering controlled airspace (CTR or CTA) is about altitude and location — whether your RPA, at its operating height over that point, is inside Class C or Class D airspace. It requires ATC clearance (and ReOC approval outside standard conditions).
• The 5.5 km / 3 NM rule is about proximity to a controlled aerodrome — a horizontal distance buffer that applies even when the RPA is below the CTA base and outside the CTR. It protects traffic patterns and circuit areas.

A flight can be below the CTA base and outside the CTR and still be barred by the 5.5 km rule. Conversely, a flight can be more than 5.5 km from the aerodrome and still be inside controlled airspace if it climbs into a CTA step. Both checks are independent and both must pass.

Putting It Together — Pre-flight Airspace Check

1. Plot the operating area on the current VTC or VNC and mark the operating altitude.
2. Check whether the point sits inside a CTR (surface controlled). If yes, the flight is a controlled-airspace operation requiring clearance.
3. If outside the CTR, check the 5.5 km / 3 NM buffer to any controlled aerodrome reference point.
4. If clear of the CTR and the buffer, check the CTA step base above the point using the step table or charted step lines, convert to AGL, and confirm the planned RPA height stays below it.
5. Confirm the airspace class (G is the target for standard operations).
6. Record each result as part of your pre-flight planning so the reasoning is auditable.