The standard circuit pattern

[Admin]

12.3 The standard circuit pattern

 

For at least the past 65 years, a standard procedure has been adopted for any piston-engine light aircraft approaching to land at a non-controlled public airfield. This procedure is called the standard circuit pattern and is adopted by convention rather than laid down by regulation. Following the pattern requires that an aircraft should track over at least three legs of a rectangular course aligned with the runway or landing strip that is most into-wind. Turns, once established within the circuit, will all be in the same direction, usually to the left unless terrain or ground habitation dictate otherwise. The downwind leg will be flown at moderate speed (adjusted to avoid overtaking preceding aircraft) and at a constant height — normally 1000 feet above the airfield level is recommended, but some primarily ultralight airfields may have a lower standard circuit height. And, of course, the aircraft must be operating in visual meteorological conditions [VMC] — clear of cloud and in sight of the ground at all times, if at or below 1000 feet agl. Check the visual meteorological conditions for aircraft operating under the visual flight rules.

 

Consistency

 

The height of the circuit is particularly important for ultralight pilots. Ultralight engines are not renowned for their reliability and the circuit height should be sufficient that, following power loss, an aircraft flying a reasonably tight circuit has every chance of gliding to a safe landing area on the airfield.

 

Pilots should adopt their own personal circuit procedures, to be used wherever possible; the principle being that consistency improves performance. Do not automatically apply the procedure utilised at a training airfield when operating elsewhere. The skills involved can only be assimilated by repeated practice at many airfields — not by reading books or web pages. Consistency is the key. Every circuit and landing should be performed to the best of the pilot's ability; such consistency makes the occasional difficult landing easy.

 

The diagram below (adapted from the Sydney Basin Visual Pilot Guide, courtesy of the Australian Civil Aviation Safety Authority's Aviation Safety Promotion program) demonstrates the full routine for a piston-engine aircraft inbound for landing at a public airfield.

 

 

 

 

 

The routine

 

1. The first stage is an overflight at a height not less than 1500 feet agl (preferably with Local QNH set, but if this is not obtainable, use Area QNH) to determine the airfield serviceability, the surface wind direction, the runway/strip being used by other traffic and confirmation of the circuit direction; or if no other traffic, to select the strip to be used. While in the circuit, keep monitoring the relative position and the movements of other traffic at all times. Note that the 'circuit area' is taken to cover the area within a radius of three nautical miles from the 'airfield reference point'. Assume that the latter is the runway intersection.

 

If the airfield is unfamiliar, the overflight also provides the opportunity to examine the circuit area for safe escape routes from each runway following a late go-around. Also check the area for suitable forced landing sites and associated hazards should the engine fail during a go-around or after take-off. See the Coping with Emergencies Guide.

 

2. The second stage is to manoeuvre so that a let-down from 1500 feet is commenced on the 'dead' side of the active runway, tracking close and parallel to that runway. This is the upwind or into-wind leg. The first and second stages provide the opportunity to carefully check the airfield area and boundaries for hazards — animals, power lines and other wires, ditches, obstructions, and to ascertain the whereabouts of other traffic in, or joining, the circuit and to be seen by them*. All manoeuvring should be done so that the airfield activities always remain in sight; i.e. don't turn away for a short time and then follow with a reversed turn onto downwind.

 

*The official term for this latter procedure is 'unalerted see and avoid', but it has its limitations. See the Australian Transport Safety Bureau research report 'Limitations of the see-and-avoid principle'. The report was first issued in 1991 when mid-air collisions in Australian general aviation averaged about one per year but collisions have increased slightly since then. Most — or nearly all — general and powered recreational aviation mid-airs occur in the circuit area, generally when one aircraft descends into another from behind.

 

3. When circuit height is reached and the upwind end of the runway has been passed, choose an appropriate position to turn onto the crosswind leg so that there will be no conflict with traffic on the crosswind and downwind legs, and to achieve optimum traffic spacing. You are now entering the traffic side of the circuit. Watch for aircraft joining the circuit on crosswind and for aircraft taking off; ensure that you provide adequate clearance. Maintain circuit height and, allowing for drift, track at 90° to the runway.

 

4. Turn 90° onto the downwind leg at an appropriate distance past the runway (after checking for aircraft joining the circuit on the downwind leg), check the crosswind drift against selected landmarks and adjust heading to track parallel to the runway, perform the appropriate downwind cockpit checks, and hold altitude and appropriate traffic spacing. Set power and trim the aircraft to maintain an airspeed that allows time to plan the landing without unnecessarily delaying other traffic — probably around 1.7 × Vso.

 

Note that although we call these legs 'upwind', 'crosswind' and 'downwind', they are only nominally named so, because the surface wind is unlikely to be exactly aligned with the 'into-wind' runway or a single airstrip, and the wind at circuit height might vary considerably from that at the surface.

 

5. Planning time! Pick an intended touchdown target on the airstrip. This should be far enough into the strip so that an undershoot on approach will still allow normal roundout and touchdown on the runway, or an overshoot on approach will still allow ample runway to bring the aircraft to a halt. For all ultralights and most light aircraft, the latter requirement is probably inconsequential for most runways at public aerodromes. A touchdown target maybe 400 feet from the threshold is about the norm; never target the beginning of the runway or strip for touchdown. Now choose another point, say 200 feet back from the touchdown target towards the threshold; this is the aiming point. Of course, it may be difficult to identify such positions at a featureless airstrip; also, the figures will vary according to the aircraft's drag characteristics in the landing configuration.

 

We are presuming here that we are operating at the average recreational aviation airfield where the strip length may be 2000–3000 feet. It can be a little embarrassing for the light aircraft pilot who touches down 400 feet past the threshold of a 6000 feet runway and then has to taxi a kilometre to the first exit. At a licensed aerodrome, the runway centre-lines are 100 feet [30 m] long with a 100 feet gap in between, and the 'piano keys' which normally mark the threshold are also 100 feet long. There should also be touchdown marks at 500 feet [150 m], 1000 feet [300 m] and 1500 feet [450 m].

 

6. At an appropriate distance past the aiming point, turn 90° onto the base leg, and hold airspeed but reduce power so that a descent is started during the turn. Lower the first stage of flap if so equipped. Reduce airspeed (but not less than 1.5 × Vso), and trim.

 

The time spent flying base leg is most important, as it provides the opportunity to: set up the aircraft in the approach attitude; establish a power and flap setting (and trim) for the required rate of descent; check for conflicting traffic both airborne and on the ground and particularly any traffic on a straight-in approach or very wide circuit; assess the crosswind component along the landing path; decide the touchdown technique appropriate for the conditions; and review the pre-landing checks.

 

Hold an accurate heading on base to carefully monitor drift, comparing the wind velocity at that height with the surface wind indicated by the windsock(s). A significant difference between the two indicates wind shear will be encountered during the final approach — this may erode the safety margin between the approach speed and Vso, or cause other difficulties. Never be tempted to fly a semi-circular base with a short final approach — it is very poor airmanship and negates all the safety check features of the square base leg.

 

It may be that preceding traffic conditions preclude a turn onto base at the optimum position — in which case you must reduce speed and/or extend the downwind leg further downwind; maintain altitude; and delay the start of descent, and some actions, until the aircraft is well into the base leg or even established on final approach.

 

7. Start a 90° descending turn onto the final approach so that, on completion of the turn, the aircraft is lined up with the extended notional centre(line) of the landing strip. During the turn, be aware of the reversal height phenomena and confine external scanning to the intended flight path and to the check for conflicting aerial traffic particularly ahead of and behind you. If satisfied with the initial approach, then lower full flap (if the wind speed is fairly high, then partial flap may suffice), adjust airspeed to the recommended final approach speed [Vref] and re-trim.

 

Once stabilised in the final approach, control the airspeed and the rate of descent with small movements of flight controls and throttle. The power setting should be such that it allows small power reductions, or power increases, in order to maintain the approach path. This can't be done if the approach is set up with the engine at idle power. In addition, the thrust response is not that effective from an idle setting and, for many aircraft, an approach at idle power will entail a high sink rate, which may be difficult to manage. Also, an idle power approach tends to over-cool the engine and may promote carburettor icing, both of which may result in high power not being available when needed — such as in a go-around.

 

8. Continue tracking down 'final', whilst correcting for the crosswind component, and watching the position and apparent movement* of the aiming point relative to the windscreen. Then at 50 feet or so, substantially reduce the rate of descent, reduce thrust to zero, touchdown and roll-out until it is safe to turn off the landing strip.

 

If so equipped, and in a nosewheel aircraft, brakes may be applied to slow the aircraft during the latter part of the roll-out — but only if the aircraft is moving in a straight line on a firm surface and the elevators are raised to keep excess weight off the nosewheel. In a tailwheel aircraft, be very wary of any brake application during the roll-out. The braking systems in ultralight aircraft are generally only provided for light use in ground manoeuvring.

 

* If the aiming point appears to be moving up the windscreen you are undershooting (too low) and will touch down before the target. If the aiming point appears to be moving down the screen you are overshooting (too high) and will touchdown past the target. If it appears to be motionless in the screen the approach slope is good and touchdown will be close to the target. The foregoing presumes that all of the runway is visible through the windscreen during the final approach. However, there are some aircraft where the forward visibility over the nose is inadequate at approach speeds and special techniques, such as side-slipping, may be required.

 

Variations on joining the circuit

 

The previous discussion outlined the full circuit pattern that should be adopted when inbound to an unfamiliar airfield. However, when inbound to a familiar airfield of which you are aware of the current runway in use and its serviceability, it may not be necessary to overfly the airfield, and the circuit may be joined anywhere on the green path; i.e. on the upwind, crosswind or downwind leg. Downwind joins are normally made at a 45° angle from outside the pattern. You should not join the standard circuit on base or final — the red shaded path in the diagram. When joining crosswind or downwind, you should already be at the circuit height.

 

Note that only the pattern of the standard circuit is fixed. Its dimensions; e.g. the length of the downwind leg or its distance from the runway, are variable. It is good practice to fly a nice, tight circuit. This also allows a forced landing to be accomplished safely on the airfield if power is lost.

 

However, for operational reasons, not all aircraft will fly a standard pattern or even base their circuit on the same runway. The turning radius of regular passenger transport [RPT] aircraft is too large to conduct the normal circuit pattern, so they perform either a 'circling approach' or a 'straight-in approach'; the latter being much safer for RPT aircraft. Agricultural aircraft reloading at a public airfield tend to use a runway and circuit pattern which best suits the job conditions.

 

CASA have produced two new (2010) advisory publications to support procedures and provide guidance on a code of conduct to allow greater flexibility for pilots when flying at, or in the vicinity of, 'non-towered' aerodromes; i.e. airfields in Class G airspace. These Civil Aviation Advisory Publications are: CAAP 166-1 'Operations in the vicinity of non-towered (non-controlled) aerodromes' and CAAP 166-2 'Pilots responsibility in collision avoidance in the vicinity of non-towered (non-controlled) aerodromes by 'see and avoid'.

 

Please read the combined CAAP 166-1/166-2 document. Note that the 'ultralight' term used in the CAAPs when recommending a 500 feet circuit height, refers only to those RA-Aus aircraft which have a normal cruising speed below 55 knots, or thereabouts.

 

CASA have also produced an online interactive learning tool titled 'Operations at, or in the vicinity of, non-towered (non-controlled) aerodromes' which is now available at casaelearning.com.au/M02/index.htm.

 

Source: http://www.recreationalflying.com/tutorials/groundschool/umodule12.html#landing_performance