Wake turbulence and the PSTAR: the wingtip-vortex physics behind every question

We are number two. Ahead of us on final is something with a widebody's wingspan, settling toward the numbers a mile and a half out; we are stepped down behind it in a trainer that weighs less than one of its main tyres. The tower reads us our landing clearance and adds three words that change the picture — "caution, wake turbulence." Nobody is going to fly the next ninety seconds for us. The separation, the profile, the choice to keep coming or go around — all of it just became ours.

Wake turbulence is the section of the PSTAR that punishes intuition, because the honest answers run against the eye. The aeroplane that looks the most violent isn't the one that throws the strongest wake; the calm day isn't the safe one; and the tower's caution isn't the tower taking responsibility. Every one of those inversions is a keyed question in the bank, and every one falls out of the same physics. Learn the physics once and the section stops being a list to memorise.

The vortex is the bill for lift

An aeroplane holds itself up by carrying lower pressure over the wing than under it. At the tip, where the two meet, the high-pressure air underneath spills up and around into the low pressure on top, and the flow leaves the tip as a tight, rotating cylinder of air. A wing in flight sheds two of them, counter-rotating, trailing back like the wake off a boat.^[2] That is the first thing the bank tests, and it tests it by trying to pin the wake on the engines. Here is one question as published:

Wake turbulence is produced by

— and the keyed answer is "all fixed and rotary wing aircraft" (TP 11919E, question 7.08 — Transport Canada's question and answer key, reproduced with attribution).^[1] Not jets. Not the heavy iron only. Anything with a wing making lift — a Cessna, a helicopter's rotor disc, a sailplane on a ridge — is trailing vortices, because the vortex is simply what lift costs. The distractors the section leans on — "turbo-jet powered aircraft only," or blaming "jet wash" — are checking whether you think the hazard comes out of the tailpipe. It comes off the wingtip.^[1]

Heavy, clean, and slow: why the fiercest-looking jet isn't the worst

Now the recalibration the whole section is built on. Ask a student which aeroplane throws the worst wake and the hand goes to the airliner on short final with everything hanging out — full flaps, gear down, boards ready. It looks the most disturbed, so surely it's the most dangerous. The AIM says otherwise: vortex strength is governed by the wing's shape, the aircraft's weight, and its speed — and of those, weight matters most. The strongest vortices come off an aeroplane that is heavy, in a clean configuration, and slow.^[2] That is the classic PSTAR answer, and the trap sitting beside it is heavy, landing configuration, and slow — deliberately close, because it matches the eye.^[1]

The reason is angle of attack. A heavy wing flown slowly has to work at a steep angle to make the lift it needs, and that drives the sharpest pressure difference at the tip and the tightest vortex. Extend the flaps and drop the gear and you spread the lift across a dirtier, more disturbed wing — the vortex loosens slightly. So the moment a large aeroplane is most benign to follow is the moment it looks busiest, and the moment it is cleanest and slowest — just after lift-off, gear travelling up — is the moment its wake is meanest.^[1][2]

Down, behind, and sideways with a little wind

Wingtip vortices don't hang where they were made — they sink. From behind a large, heavy aeroplane they descend at four to five hundred feet a minute and settle roughly a thousand feet below the path that made them, and they do it slowly: less than a thousand feet down in the first two minutes.^[3] That one behaviour writes the whole playbook — the hazard is the air below and behind the aeroplane ahead, so the safe place to be is above and behind its track.^[3]

Sideways, they drift. Left alone the two vortices spread apart at about five knots, which is why a light crosswind does something the eye doesn't expect: it holds the upwind vortex still. A wind of three to seven knots can park that upwind vortex over the very runway you're aiming at, while it hurries the downwind one toward a parallel.^[2] In calm or light wind the AIM tells pilots to stay especially alert for vortices that linger in the touchdown area, drift in from a nearby runway, or sink into the path of VFR traffic at five hundred feet and below.^[2]

And the calm day — the one that feels safest — is the dangerous one. Vortices only break up when atmospheric turbulence tears them apart, so the smoother the air, the longer they hold together.^[2] With no wind to disturb them, the hazardous wake of a large aircraft can persist for five minutes or more.^[1] The bank states it that flatly, and the physics is why: no turbulence, no break-up.

From rotation to touchdown

The bank also wants you to know exactly when the wake switches on and off, because that timing is what you fly against. Here is the second question worth holding word for word:

Wake turbulence caused by a departing large aeroplane begins

— keyed "with rotation" (TP 11919E, question 7.09).^[1] The wing starts trailing vortices the instant it starts carrying weight — at rotation, as the nosewheel lifts — and it keeps trailing them until the nosewheel of a landing aeroplane touches back down.^[3] Between those two points there is wake on the runway and along the departure or approach path; before rotation and after touchdown there isn't. That is why every avoidance rule below is written around someone else's rotation point and touchdown point.

The avoidance you actually fly

Every practical rule is the same idea — stay out of the sinking air behind a bigger aeroplane — applied to the geometry in front of you.^[4]

• Departing behind a departure. Plan to be airborne before the point where the aircraft ahead rotated, and climb while staying above its departure path — or ask for a turn to get off that path entirely.^[4]
• Departing behind a landing. The opposite geometry: plan to lift off beyond the point where the arriving aircraft touched down, since its wake ends at touchdown and lies on the runway behind that point.^[4]
• Landing behind a landing. Stay at or above the aircraft's final-approach path, note where it touches down, and land beyond that point if it's safe to do so.^[4] The exam's version of this keys to exactly that — land beyond the other aircraft's touchdown point, never short of it.^[1]
• Landing behind a departure. Touch down before the point where the departing aircraft rotated, staying above the wake it left climbing out.^[4]
• En route. Avoid flying below and behind a large aircraft, and if one is on your track — meeting or overtaking — offset to the side, preferably upwind.^[4]
• On the ground. Holding near an active runway, expect wake; before crossing behind a large aircraft that has just taken off or landed, wait a couple of minutes.^[4]

The through-line is worth saying plainly: above-and-beyond on landing, before-and-above on departure. The AIM notes that the largest number of dangerous encounters on its record happened in the last half-mile of final — exactly where a light aircraft is slow, low, and tempted to duck under to make the runway.^[4]

The decision is yours, not the tower's

The last inversion is the one that matters most on the day. When a controller says "caution, wake turbulence," it's easy to hear reassurance — the tower has it handled. It doesn't. That phrase is a caution, not a clearance to relax, and the AIM is blunt about where the responsibility sits: it is the pilot's job to adjust the flight path to avoid the wake.^[4] The bank tests the point head-on, and the keyed answer is that avoiding wake turbulence is the sole responsibility of the pilot — not ATC, not shared.^[1]

That responsibility comes with a power students forget they hold: a clearance is an authorisation you may refuse. Cleared for take-off right behind a large aircraft that has just made a low approach and overshoot, the correct move is neither to launch into its descending wake nor to sit on the runway hoping — it's to decline the clearance and tell ATC why.^[1] A clearance binds you only once you accept it, and that acceptance is the pilot-in-command's to give or withhold.^[6]

None of which means ATC does nothing. Controllers apply timed separation — commonly two minutes — behind a known heavy aircraft, and they won't hand you a take-off clearance into a heavy's wake without the cautionary attached.^[5] But the AIM says plainly that the pilot is in the best position to judge the separation actually needed, and that in spite of every measure, ATC cannot guarantee you won't meet wake.^[5] The margin between "cleared" and "safe" is the part of the flight the controller can't fly. That part is ours.

Back on final, number two behind the widebody: the physics is the whole brief. Its wake is sinking away below the path it flew and trailing back from where its wheels will touch. Stay above that path, keep the aiming point beyond its touchdown, carry the power to hold altitude instead of dragging yourself low, and the vortices are settling into air you've already left behind. The tower cautioned us; the separation is ours. That is the entire section — and it's one of the few afternoons on the PSTAR that, flown wrong, doesn't give you a second question.