Base-to-Final Stall Margin in a 45° Bank
You are returning to a 500 m grass strip in an ultralight. The stall speed at maximum takeoff mass in level flight is 34 kt IAS. You are at 400 ft AGL, turning base to final in a 45° bank. The indicated airspeed is 48 kt and decreasing. There is a 10 kt tailwind on base and a 12 kt gust spread; density altitude is 3,500 ft. You feel light buffeting. Trees are at the threshold, your target approach speed is 1.3 Vso, and flaps are set to 30°. You are high and drifting past the centreline. You lower the nose and add power; the airspeed recovers to 52 kt, but you are now fast and high. Decide whether to continue the landing or go around.
1. Calculate the load factor and stall speed in the 45° bank, and explain the buffet.
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Load factor in a 45° bank: n = 1/cos 45° = 1.41. Stall speed increases with √n: 34 × √1.41 ≈ 40.4 kt. At 48 kt you have only about 7.6 kt margin, and the speed is decreasing while the gust spread is 12 kt. The buffet may be stall warning or turbulence, but either way it is a warning not to pull. The target approach speed in the bank should be 1.3 × 40.4 ≈ 52.5 kt, so 48 kt is already below the recommended margin.
2. Why was 48 kt inadequate, and what does the tailwind do to the approach?
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The tailwind raises ground speed, so the aircraft covers the base leg quickly and tends to overshoot the centreline; the natural reaction is to steepen the bank, which raises load factor and stall speed further. At 48 kt in a 45° bank, you are close to the accelerated stall speed, and a gust can put you below it. The 3,500 ft density altitude reduces engine response if you need power. The correct response is to reduce bank, lower the nose, add power, and go around rather than trying to salvage the landing.
3. State the correct go-around sequence and the reason for each action.
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Go-around: apply full power, reduce bank to level or shallow, lower the nose to maintain at least 52–55 kt, establish a positive climb, then retract flaps in stages and trim. Do not add back pressure to “stretch” the glide; that increases angle of attack and load factor. Announce the go-around, fly clear of the trees, and rejoin the circuit for a stabilized approach with the correct speed. If the aircraft is fast and high, accept the go-around; a 500 m strip with a tailwind is not the place to force a landing.