Wake turbulence on final approach
You are on a 3° ILS final at 140 kt groundspeed, 4 NM from touchdown, cleared to land behind a heavy four-engine jet that has just crossed the threshold. Tower reports the preceding aircraft as a heavy. Wind is a light 5 kt tailwind component on final. You are flying a light twin at 1,800 kg. The runway is 2,600 m long and dry.
1. Where is the greatest wake-turbulence risk, and how should you manage spacing?
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The heavy jet generates counter-rotating vortices that sink and drift with the wind. With a tailwind component, vortices are carried toward you and linger on the approach path rather than being blown clear. The greatest risk is at and just above the flight path, roughly one wingspan below the generating aircraft, and near the threshold where the jet was low and slow (high vortex strength).
Management: maintain at least the prescribed separation (typically 4–5 NM behind a heavy for a light aircraft), stay above the preceding aircraft's flight path, and plan to touch down beyond its touchdown point. With a tailwind, add extra spacing because the vortices drift toward you.
2. You feel a sharp roll and buffet at 500 ft AGL. What do you do?
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This is likely wake encounter. Actions: maintain or increase power, hold the aircraft level with positive control inputs, do not fight the roll abruptly, and go around if you cannot stabilise by ~300 ft AGL. Do not attempt to land from an unstable, buffeting approach. A go-around is the correct decision - a wake encounter close to the ground leaves no margin to recover after touchdown.
3. Why does aircraft weight matter for wake encounter?
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A lighter aircraft has less inertia and is displaced more by the same vortex-induced velocity, so the roll rate and altitude loss are greater. Your 1,800 kg twin is far lighter than the heavy jet, so the same wake that would barely disturb the jet can roll you rapidly. This is why wake separation categories are based on the generating aircraft's weight.