Base Turn

Runway 27 · right-hand circuit, downwind 4,500 ft north · 1,000 ft · 120 KIAS, 110 at 50 ft · sea-level aerodrome

Key figures

Downwind, level Base turn descent 3° glidepath Wind Correct AOB, this wind Correct AOB, still air Other mode

Assumptions

Runway 27 (headings and wind are true, no variation). Downwind 1,000 ft, 4,500 ft north of the centreline, crabbed to hold that track.
The turn starts when the threshold is 53.5° past abeam, measured at the threshold from the line square to the runway, over the ground.
Every change of bank is made at 15° per second, and every change of rate of descent at 500 ft/min per second. At the base turn point the aircraft rolls into one bank angle, chosen so the turn rolls out on the centreline on the heading that holds the centreline track; the roll-out starts early enough to be wings level on that heading. The descent starts 1 s after the base turn point. The change from 5° to 3° starts early enough to meet the glidepath without going below it.
AOB error: the base turn is flown at the correct angle of bank plus the error (never more than 45°), uncorrected until the last moment a correction still works, and then corrected there: roll (at 15°/s) to the bank required to reach the centreline and hold it. A correction works if it needs no more than 45° and no opposite bank, and is wings level on the centreline (within half a runway width) by 200 ft. The run pauses at that point with the AOB indications red. "AOB req" is the bank that, rolled onto at that moment, would roll out on the centreline; a turn already heading for a roll-out within 30 ft of it needs none. If no correction works even at the base turn point, the run pauses there. "Correct now" (right of the HUD) makes the correction at that moment instead: the approach is re-flown from there and playback carries on; going back before that point, or letting the run loop, restores the default run. The green and blue paths are flown at the correct constant bank in this wind and in still air, and the yellow path is this approach flown in the other descent mode, with the same bank error (and its own last-point correction); all three are drawn 25% opaque. A striped pole marks where the 3° glidepath passes 400 ft.
Two modes for the descent in the base turn. CDM: 5° over the ground, so the rate of descent follows groundspeed. Visual attitude: a constant rate of descent, the still-air 5° rate at the start of the turn. Either way it is held until the 3° glidepath is reached, then the glidepath is flown. The 3° glidepath is fixed to the ground, so its rate of descent follows groundspeed.
IAS 120 kt until the roll out onto final, then reduced evenly with distance to 110 kt at 50 ft over the threshold, and held there. TAS from the air density at each height: temperature falls at the ISA lapse rate (1.98 °C per 1,000 ft) from the aerodrome temperature entered, pressure falls from QNH through that temperature column, and density follows from pressure and temperature. IAS is treated as equivalent airspeed (compressibility is under 0.1 kt at 120 kt) and the air as dry (humidity would add up to about 0.5% to TAS on a hot, humid day). Heights are true heights above the aerodrome: 1,000 ft is a true 1,000 ft, not 1,000 ft on the altimeter, which in non-ISA temperatures would be about 4% lower per 10 °C colder than ISA (higher when warmer). Wind is the same at all heights.
The 3° glidepath crosses the threshold at 50 ft and meets the runway about 950 ft beyond it. No flare.
Runway 6,000 × 150 ft is illustrative. The aircraft is drawn larger than scale in the wide views.