Which is safer? An explanation of airspace categories

AN EXPLANATION OF AIRSPACE CATEGORIES

The above chart shows the current US airspace system.

The key to the National Airspace System (NAS) is to allocate the six categories of airspace (see diagram above) using strictly scientific and objective criteria. It is similar to the road system - there are dirt roads in areas where there is little traffic (and therefore a lower risk of a collision), then bitumen roads, then highways with a simple separation line, multi-lane highways and toll roads. You know when an airspace system is objectively and scientifically based when the airspace categories step up from Class G to Class B as the traffic densities increase. This is not the present situation in Australia.

When I initially started the airspace reforms as the CAA Chairman in 1991, there were only two categories of airspace – controlled and uncontrolled. Imagine if Australia only had dirt roads and toll roads instead of five or six road categories! It would result in a lowering of road safety and more accidents because the resources would not be effectively spread to reduce risk over the whole road system.

Quick guide

Class A (equivalent to a multi-lane tollway) is the most restrictive and expensive, Class G (equivalent to a dirt road) is the least restrictive and least expensive. Note that the airspace categories step up using objective criteria from Class G to Class A depending on traffic mix and density.

A
Only Instrument Flight Rules (IFR). Generally airline pressurised aircraft.

B
IFR aircraft separated from all aircraft. VFR (Visual Flight Rules) aircraft separated from all aircraft.

C
IFR separated from all aircraft. VFR separated from IFR aircraft and given traffic information on VFR aircraft.

D
IFR separated from IFR aircraft and given traffic on VFR aircraft.

E
IFR aircraft separated from IFR aircraft.

G
No separation service is provided at all but all aircraft can request radar advisories and "do it yourself" aircraft to aircraft radio calls.

Click here for further information on the US airspace system http://www.asy.faa.gov/safety_products/airspaceclass.htm

Class A

• Typically used in Australia, the USA, Canada, and many other countries for for enroute high level airspace.
• Also used in terminal airspace at Heathrow (London) Airport.

Class B

• Typical LA, New York, and other busy airports
• Typically to 10,000’ AGL (Above Ground Level) - but only to 7,000' AGL at New York/La Guardia
• Not yet used in Australia but Sydney would probably comply and would be upgraded under NAS
• US FAA Class B establishment criteria:
  • The primary airport serves at least 3.5 million passengers enplaned annually (Note: enplaned passengers not passenger movements); or
  • The primary airport has a total airport operations count of 300,000 (of which 50% are air carriers)
  • Although an airport meets the minimum passenger and air traffic operations criteria for a Class B designation, other factors must be considered, such as: would a Class B designation contribute to the efficiency and safety of operations in the area; and is there a current situation or problem that cannot be solved without the designation of Class B airspace?

Class C

• Burbank and over 120 busy US airports, typically to 4,800’
• Also utilised in Australia for Sydney, Melbourne, Brisbane, and other large airports.
• In Australia Class C goes to 18,000’ and adjoins Class A.
• US FAA Class C establishment criteria:
  • The airport must be serviced by an operational airport traffic control tower and a radar approach control; and
  • One of the following applies:
    • An annual instrument operations count of 75,000 at the primary airport
    • An annual instrument operations count of 100,000 at the primary and secondary airports in the terminal area hub
    • An annual count of 250,000 enplaned passengers at the primary airport. (Note: enplaned passengers not passenger movements)

Special note: Since November 2004 Class C airspace has been placed in enroute airspace in Australia and also above airports such as Albury and Coffs Harbour. This is clearly a misallocation of resources.

Class D

• Non-radar towers
• Typical 4.3 nm radius and up to 2,500’ AGL. Typically used at places like Juneau, the capital of Alaska, and at 356 other locations in the USA. 
• Also used in Australia at places like Hobart up to 4,500’ AGL to around 20 nm.
• In the USA, the FAA Class D establishment formula applies. Australia also uses the same formula, however the figures are often manipulated so that towers remain or are not established.

Class E

• Enroute and terminal airspace for IFR aircraft
• Covers all IFR airways in the USA, normally to 1,200' AGL but in some places as high as 14,500' AGL.
• In the USA used for all instrument approaches at non-tower airports.
• Great for preventing controlled flight into terrain as a full radar service is given where radar is available - including minimum safe altitude alerts.
• Without doubt the most versatile airspace of all. When being used correctly, it operates identically to Class A airspace when IMC (cloud) exists, and when VMC (no cloud, visual conditions) exists it can operate identically to Class G. This means it is a highly efficient airspace. Note that Class E is not being used correctly in Australia because of a dogged resistance to change by people in CASA and Airservices Australia.

Class G

• Used in the USA normally at 1,200'AGL and below, and 14,500' and below in less densely trafficked areas
• In USA separate IFR from IFR, even though not legally required. US controllers do everything they can to avoid collisions - especially collisions with the ground!
• In Australia Class G controllers only give a traffic information service for IFR - and if you are in luck, a safety alert about other aircraft. Aussie controllers are not trained to tell a pilot if he or she has gone below the legal minimum altitude and is about to collide with the ground.

You be the judge

Many Australian aviators know that Qantas and others pressured the Airservices Board to reverse the Government decision to move to the US FAA NAS airspace system.

In the USA, all airports have Class E above the airport terminal area – see diagram. This is to facilitate overflying enroute traffic to fly where collision risk is lowest - that is, above the airport.

Let’s look at the actual US situation

In the USA, a light aircraft intending to fly from south to north enroute over Los Angeles is encouraged to fly through the Los Angeles Class B at between 3,500 and 4,500 feet in altitude. This is of course extremely safe as there are no airline aircraft flying directly over the top of the Los Angeles airfield at this level.

Los Angeles has Class B airspace designed for a very high traffic density, whereas Sydney has the lower category Class C. A typical equivalent Class C airport in LA would be Burbank Airport where Class E Airspace starts at the relatively low level of 4,800 feet. This means that light aircraft tend to fly over the airport in the Class E airspace above 4,800 feet where the collision risk is minimal.

The Australian situation

Rather than small aircraft being encouraged to overfly Sydney Airport in the safe US way, it is virtually impossible. For example, in my 30 years of flying, I have never yet heard of a Visual Flight Rules (VFR) aircraft being able to obtain a direct enroute clearance overhead Sydney from south to north, or vice versa.

Instead, pilots of light aircraft are trained to converge together and then fly extra distances to track via a narrow light aircraft corridor over the suburbs. Rather than remain at cruising altitude, where the collision risk is obviously minimal, the pilots are trained to descend below the Class C steps (using more fuel and therefore resulting in higher costs) and then fly in a “light aircraft lane” from Parramatta to Hornsby following flashing lights on buildings. (The system used by Charles Lindbergh in the USA in the 1920s.) The pilots are then trained to track over Hornsby at 2,500' before flying north.

It just so happens that airline aircraft from the north are also tracked over Hornsby - normally at 3,000'. Can you believe this? This dumb system results in the separation distance between passenger carrying airline jets and VFR aircraft at Hornsby typically being 180 metres.

Compare the two

In the USA small aircraft track over the airports at high levels where the collision risk is minimal. Mandatory transponder is required so a full radar traffic information service is provided to the airline aircraft. In Australia we focus the light aircraft to specific points where they are extremely close to airline aircraft. No transponder or radio is required as the light aircraft is supposed to be in uncontrolled Class G airspace. A slight mistake in setting an altimeter or in navigation could cause a collision.

It should be pointed out that many incidents already occur in this vicinity - so the writing is on the wall. If we don't move to the US system we keep the increased risk of collision.

Entry last amended 6/10/05.