AirspaceandtheRules

Aviation regulation looks arbitrary until you know what each rule was written after. Almost none of it was designed in advance. It accumulated, one accident at a time, and most individual rules exist because something specific happened and somebody decided it should not happen again.

That is worth holding onto, because it changes how you read the rulebook. A regulation is not a hoop. It is the compressed record of a failure, and the reason it seems excessive is usually that you have not met the failure.

The international framework came out of the 1944 Chicago Convention, which created what became the International Civil Aviation Organization and established that each country has sovereignty over the airspace above its territory. That is why the rules in this chapter are national rules rather than global ones, and why an airline flying from London to Tokyo complies with a different rulebook every few hours.

Airspace is divided into six classes, labelled A through G with no class F used in the United States. The classification tells you one thing above all: how much air traffic control involvement is required before you may be there.

Class A is everything from 18,000 feet MSL up to and including flight level 600. Every flight in it is on an instrument flight plan under ATC control — there is no VFR flying in Class A at all. This is where the airliners are.

Class B surrounds the busiest airports. Entry requires an explicit clearance: not a radio call, not an acknowledgement, but a controller saying you are cleared into the Bravo. Its shape is often described as an upside-down wedding cake, with a small cylinder at the surface and progressively wider shelves stacked above it, so that the protected volume matches the shape of arriving and departing traffic rather than a convenient cylinder.

Class C surrounds moderately busy airports with a control tower and radar service. Entry requires two-way radio communication to be established — which has a precise meaning covered below. The standard shape is a five-nautical-mile core from the surface up to 4,000 feet above the airport, with a ten-mile shelf above 1,200 feet.

Class D surrounds airports with an operating control tower and nothing more elaborate. Two-way radio communication again, typically within about four nautical miles up to 2,500 feet above the airport.

Class E is all the other controlled airspace — the great majority of it — and requires no clearance and no radio call for VFR flight. It usually begins at either 700 or 1,200 feet above the ground, depending on what is beneath it, and the chart says which.

Class G is uncontrolled. No clearance, no radio, no ATC service. It is generally the layer beneath Class E, and it is where a great deal of low-level flying happens.

Flying under visual flight rules means you are responsible for seeing and avoiding other aircraft. That only works if you can see far enough to do something about what you see, which is what the VFR weather minimums are for.

The general case, in controlled airspace below 10,000 feet MSL, is three statute miles of visibility and a set of cloud clearances: 500 feet below the cloud, 1,000 feet above it, and 2,000 feet horizontally from it. Those distances exist because an IFR aircraft may emerge from that cloud at any moment, at speed, and the clearance is the reaction time.

Above 10,000 feet MSL the numbers grow, to five statute miles of visibility and a mile horizontally from cloud, because aircraft up there are permitted to fly faster and the closing speeds are higher.

Class B is the exception that looks like a loosening and is not. Inside Class B the requirement is three miles of visibility and simply to remain clear of cloud, with no distance specified. That is only safe because every aircraft in Class B is known to and separated by ATC, so the cloud clearance that protects against an unseen IFR arrival is not needed.

The right-of-way rules live at 14 CFR 91.113 and are short enough to learn properly. An aircraft in distress has right of way over everything. After that there is a hierarchy by category, and the logic behind it is manoeuvrability: the aircraft least able to get out of the way has priority.

A balloon has right of way over a glider. A glider has right of way over an airship. An airship has right of way over an aeroplane or helicopter. An aircraft towing or refuelling another has right of way over other engine-driven aircraft, because it cannot manoeuvre freely.

Between two aircraft of the same category, three geometries cover it. Converging: the aircraft that has the other on its right gives way. Head-on: both alter course to the right. Overtaking: the aircraft being overtaken has the right of way, and the overtaking aircraft passes to the right.

There is one rule that overrides the hierarchy, and it is the one that catches people. An aircraft on final approach to land, or already landing, has right of way over other aircraft in flight or on the surface — but an aircraft may not use that rule to cut in front of another aircraft that is already on final approach.

Minimum safe altitudes are at 91.119, and the first clause is the one that matters most: everywhere, an aircraft must be at an altitude allowing an emergency landing without undue hazard to persons or property on the surface if the engine fails. That is a judgement, not a number, and it applies over every square foot.

Then the numbers. Over a congested area — a city, a town, an open-air assembly of people — 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet. Over anything else, 500 feet above the surface. Over open water or sparsely populated areas, no closer than 500 feet to any person, vessel, vehicle or structure, which allows genuinely low flying provided nothing is near.

Speed limits are at 91.117. Below 10,000 feet MSL the limit is 250 knots indicated. Beneath the lateral limits of Class B airspace, or in a VFR corridor through it, 200 knots. Within four nautical miles of the primary airport of Class C or D airspace, at or below 2,500 feet above the surface, 200 knots.

The 250-knot rule below 10,000 exists because that is where the light aircraft are, and a limit on the fast traffic buys reaction time for everybody. It is also why airliners noticeably accelerate as they climb through 10,000 feet.