2.3 The 120 m (400 ft) AGL Height Limit

The 120 m (400 ft) AGL Height Limit

CASR 101.085 sets the maximum height for RPA operations at 400 ft above ground level (AGL), which is 120 m AGL. This is one of the most examined numbers in the RePL syllabus, and the rule has several subtleties the exam targets: the difference between AGL and MSL/AMSL, how to measure over altered surfaces and objects, and the approval pathway for going higher.

AGL vs MSL vs AMSL

Three altitude references appear in aviation, and confusing them is a common source of errors:

• AGL — Above Ground Level: height above the surface of the earth (or water) directly below the aircraft. The 400 ft RPA limit is an AGL limit.
• MSL — Mean Sea Level: the average level of the sea, used as a datum for altitude. MSL and AMSL are used interchangeably in Australian practice.
• AMSL — Above Mean Sea Level: the same reference as MSL; altitudes expressed as AMSL are heights above the average sea level.

Manned aircraft altitudes are usually reported in feet AMSL on charts and in radio calls, whereas the RPA height limit is in feet AGL. A manned aircraft at 1500 ft AMSL over terrain 600 ft above sea level is 900 ft AGL — well above the 400 ft RPA ceiling. Comparing an RPA's 400 ft AGL ceiling to a manned aircraft's charted AMSL altitude requires converting one to the other using the local terrain elevation. For situational awareness near manned aviation, you must know both the terrain height and your own AGL height.

Over rising or sloping terrain, the AGL reference moves with the ground. An RPA climbing a ridge at a constant 100 m AGL is legal even though its AMSL altitude is increasing — AGL is always measured from the point on the surface directly below the aircraft at that moment. The same logic means that if the terrain drops away (e.g. flying off a cliff edge), the RPA's AGL height jumps upward relative to the new lower surface, and the pilot must descend to stay within 120 m AGL. A common scenario question describes an RPA flying level at 100 m AGL that then passes over a 40 m drop in the ground; the AGL height becomes 140 m and the flight is now outside the SOC.

Measuring the 400 ft Over Altered Surfaces and Objects

CASA's rule for measuring the 400 ft AGL limit has three scenarios the exam tests:

1. Natural, unaltered surface — measure from the natural ground (or water) directly below the RPA. Straightforward.
2. Altered surface (e.g. a mining pit, quarry, or excavation where the ground level has been lowered) — measure from the current surface, not the original pre-alteration ground. A drone 100 m above the floor of a 30 m deep pit is 100 m AGL, not 130 m.
3. Object on the surface (e.g. a building, tower, or stack) — measure from the base of the object (the earth surface), not the top of the object. This is the rule candidates most often get wrong.

Worked Example: The 50 m Building

Suppose a 50 m tall building stands on flat ground. The exam question: How high above the building's roof can an RPA (no approval) fly?

• The 400 ft (120 m) AGL limit is measured from the ground at the base of the building, not the roof.
• 120 m AGL minus the building's 50 m height = about 70 m above the roof.
• Therefore the RPA may fly up to roughly 70 m above the roof (120 m AGL total), not 120 m above the roof (which would be 170 m AGL and illegal without approval).

This is the canonical exam trap: candidates add the building height to 120 m. The rule is the opposite — measure from the base.

The Above-400 ft Approval Pathway

Operations above 400 ft AGL are possible but require a specific approval. The pathway is:

1. The operator is an included operator — in practice, a ReOC holder. AC 101-01 states that included operators may apply to CASA to operate above 400 ft AGL using Form 101-09.
2. Submit Form 101-09 — titled simply RPA Flight Authorisation in the CASA forms table — with all supporting documentation the form requests.
3. CASA assesses the application against the safety case, the documented practices and procedures, and the airspace concerned.
4. Include a review of local aviation traffic. CASA explicitly expects an above-400 ft application to include a review of local traffic and the implementation of appropriate mitigators, because operating higher increases the chance of conflict with other airspace users.
5. Expect conditions. The approval may specify a maximum height, a geographic boundary, a time window, observer or radio requirements, and — where CASA or the air navigation service provider requires it — a NOTAM.

One planning point AC 101-01 raises that candidates miss: operating higher also increases the ground hazard area. If the RPA fails at height, a strong wind carries it a much greater horizontal distance before it reaches the ground, so the area you must keep clear grows with altitude.

The approval is not a blanket licence to fly anywhere above 400 ft; it is usually specific to a location, altitude band, and time, and may carry conditions (e.g. observer requirements, radio monitoring, maximum altitude).

Why 400 ft Is the Ceiling

The 400 ft ceiling exists because most manned aircraft operations near the surface — the circuit, approach, and departure phases — occur at or above this altitude. Keeping RPA at or below 400 ft AGL provides a vertical separation buffer between unmanned and manned aircraft, supporting see-and-avoid. Above 400 ft the risk of conflict with manned aircraft rises sharply because that is where manned aircraft routinely transit. The 400 ft figure is not arbitrary; it aligns with international RPA conventions and with the lower bound of typical manned-aircraft circuit patterns.

A second worked example reinforces the altered-surface rule. An open-cut mine has excavated a pit 40 m below the surrounding natural ground. An RPA flies 100 m above the floor of the pit. Because the surface has been altered, AGL is measured from the current surface (the pit floor), so the RPA is 100 m AGL — within the 120 m limit. If the same RPA flew 100 m above the original natural ground level (which is now 40 m above the pit floor), it would be 140 m above the pit floor and therefore 140 m AGL — outside the SOC. The rule consistently anchors measurement to the surface directly below the aircraft at the time of flight, whether that surface is natural, excavated, or built upon.