rejoining the circuit/pattern

The prime aim in the landing sequence is to perform the operation safely, the secondary aim is to touch down usually with minimum vertical speed and minimum horizontal ground speed, consistent with maintenance of controllability particularly in gusty wind conditions. The touchdown should be made without excessive side forces affecting the undercarriage. A not so important objective, is to touch down close to a pre-selected position. The student pilot usually finds, of all normal flight procedures, the techniques for landing an aircraft in varying conditions, are the most difficult to master, because of the greatly enhanced effects of air movement when close to the surface and the fine judgements and control movements involved.

We will look at the common factors to be considered in landing a normally configured, 3-axis, fixed undercarriage, nosewheel or tailwheel aeroplane which may, or may not, be flap equipped. Aircraft designed with full 'short take-off and landing' STOL capability will use slightly different techniques in some parts of the approach and landing. There are differing landing procedures or techniques, or combinations thereof, applicable to airfield dimensions and surface conditions:

normal landing
short field landing
soft field landing.

The basic landing sequence is varied, according to prevailing conditions (and there is a varying degree of alignment correction to allow for the crosswind component of the wind velocity), but it usually has four parts:

  • Joining the circuit pattern of the airfield, during which the aircraft is decelerated from cruise speed to circuit speed, the airfield is visually checked for serviceability and obstructions, surface wind direction ascertained from observation of the windsock/s, the whereabouts of other traffic is established, the landing direction and approach is planned and the pre-landing cockpit checks are carried out in a logical sequence.
     

  • The approach to the landing, during which the aircraft is decelerated from circuit speed to the reference indicated approach speed [Vref ], configured for landing, established in a constant rate of descent and aligned so that the flight path traced over the ground during the final approach is on the same line as the intended ground roll-out path. The flight path passes over an imaginary 50 feet high screen, placed at a distance before the airstrip threshold.
     

  • A transition period where both the rate of descent and the forward speed are slowed during a 'round-out' prior to touchdown.
     

  • The touchdown and subsequent ground roll after which the aircraft is turned off the landing area at an appropriate taxiing speed. The landing is then complete when the aircraft is securely parked, the engine is off and any passenger is safely disembarked.

The most favourable conditions for optimum landing performance at maximum weight are:

a pilot who exercises sound judgement, follows the rules and the recommended procedures
a surface, of ample length, that is dry and level, or with a slight upslope
a low density altitude i.e. low elevation and low temperature
a smooth, full headwind of reasonable and constant velocity.

Apart from the pilot's condition and capability, landing performance is limited by the following constraints, all of which should be carefully assessed both within the pre-landing procedure and at the flight planning stage to establish whether a safe landing is viable. Generally most of the engine effects and other phenomena affecting take-off performance, have no significant effect on landing performance except with both tailwheel and nosewheel aircraft, the inertial effect of the cg position. However when a landing attempt is aborted then any of those phenomena may be present during the initial go-around.

  • Demonstrated landing distance. Landing distance is the total distance required to clear an imaginary screen, 50 feet or 15 metres high, placed before the airstrip threshold; then touch down and bring the aircraft to a halt with normal braking in nil wind conditions. It should be borne in mind that the manufacturer's 'demonstrated' landing distance has been achieved by a very experienced test pilot in very favourable conditions, during the type certification tests. The landing distance required by the average recreational pilot may be considerably greater.
     

  • Airfield dimensions and slope. The usable length of runways or strips must be ascertained, as well as the degree of slope both with and across the direction of landing. Landing downslope will reduce deceleration and lengthen the ground roll. Slope across the landing path makes the touchdown and subsequent ground roll more difficult to control. At a 'one-way' airstrip a combination of airfield slope and rising terrain at the high end mandates landing upslope, no matter what the wind direction.
     

  • Airfield surface and surrounds. A short dry grass or rough gravel surface might decrease the ground roll by 10% compared to that for a smooth sealed surface. Wet or long grass might decrease the ground roll by 30%, however there is a possibility that a wet surface can induce aquaplaning, adversely affect braking and/or result in a groundloop. Long grass can catch a wingtip resulting in a groundloop. A soft or waterlogged surface might greatly decrease the roll but increase the possibility of the aircraft tipping over during the roll, or might prevent take-off if such is attempted during the landing roll. The location and height of man-made obstructions, trees and local topography must be assessed.
     

  • Airfield density altitude. A critical factor which is often not correctly assessed; the density altitude has a major effect on the approach speed (i.e. the true airspeed is significantly greater than the indicated airspeed) and thus the ground speed at which the aircraft touches down and the length of the subsequent ground roll. Density altitude also affects the aircraft's climb-out performance should the landing be aborted.
     

  • Wind velocity and turbulence. Wind strength, direction, downflow, gust intensity, surface turbulence and the potential for wind shear events are normally the major considerations in landing performance, for a properly maintained aircraft.

The pilot-in-command of an aircraft must assess all the foregoing factors and conditions to ascertain the total distance required for obstacle clearance and landing; judge if the landing can be safely conducted; and ascertain a safe escape route if the landing should need to be aborted. All the foregoing assumes that the height of the cloud base allows sufficient visibility and appropriate terrain and obstruction clearance within the circuit. The problem for the less cautious pilot, if the airfield conditions are found to be unsuitable, is that an eventual landing is mandatory and, if flight planning was poor, there may be no acceptable alternative airfield within range.

the overhead join

A standard procedure has been adopted for any piston-engine light aeroplane approaching to land at an airfield. This procedure is called the standard circuit pattern and is adopted by convention rather than laid down by regulation. In regulated airfields, the pilot should follow the instructions of the air traffic controller who will probably request that you make a call some distance from the airfield.  The circuit requires that an aircraft should track over at least three legs of a rectangular course aligned with the runway or landing strip which is most into-wind. Turns, once established within the circuit, will all be in the same direction, usually to the left unless otherwise stated; and the downwind leg will be flown at moderate speed, adjusted to avoid overtaking preceding aircraft, and holding a constant height normally 1000 feet above the airfield level. If the traffic circuit is left hand, approach the airfield keeping it to your left. You should approach the airfield keeping it to your right if the circuit is right hand.

The pilot must then manoeuvre so that he/she crosses the threshold of the landing runway before descending from 2000 feet on the 'dead' side of the active runway, tracking close and parallel to that runway. This is the upwind or into-wind leg. This gives 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. Care should be taken when joining overhead as other aircraft can also be joining.

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 optimum traffic spacing will be achieved. 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.

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 make a downwind call to ATC and hold altitude and appropriate traffic spacing. Set power and trim the aircraft to maintain an airspeed which 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 so as 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.

Planning time! Select an intended touchdown target on the airstrip. This should be far enough into the strip 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 most light aircraft the latter requirement is probably inconsequential for most runways at public aerodromes. The figures will vary according to the aircraft's drag characteristics in the landing configuration.

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

The time spent flying base leg is most important, providing the opportunity to set up the aircraft in the approach attitude; to establish a power and flap setting [and trim] for the required rate of descent; to check for conflicting traffic both airborne and on the ground and particularly any traffic on a straight-in approach or very wide circuit; to assess the crosswind component along the landing path; to decide the touchdown technique appropriate for the conditions and to 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. A significant difference between the two indicates wind shear will be encountered during the final approach; which 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 negating all the 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 speed must be reduced and/or the downwind leg must be extended further downwind, altitude maintained and the start of descent, and some actions, delayed until the aircraft is well into the base leg or even established on final approach.

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. If satisfied with the initial approach lower full flap if the wind speed is fairly high partial flap may suffice adjust airspeed to the recommended final approach speed [Vref] and re-trim. On controlled airfields, an RT call of 'final' is given, and permission to land is then granted by ATC.

In the final approach, once flaps are set, the airspeed and the rate of descent are controlled with both elevator 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.

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 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 nose wheel. In a tailwheel aircraft be very wary of any brake application during the roll-out.


Variations on joining the circuit

The foregoing is the full circuit pattern which should be adopted when inbound to an unfamiliar airfield. However when inbound to an airfield which is well known to you, and 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 degree angle from outside the pattern.  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; but it is good practice to fly a nice, tight circuit. This also allows a forced landing to be safely accomplished on the airfield should power be lost.