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S-Turns Across a Road

A boat pointed straight at the far bank still steps ashore downstream. S-turns are where you stop steering by heading and start steering by what the ground is actually doing.

Ground Reference Maneuvers — FI.X.B

Why you fly it

Roll out on a heading in a crosswind, pick a point on the nose, and watch it slide slowly sideways across the windshield. Nothing has changed inside the airplane — same heading, wings still level — but the ground underneath is going somewhere you did not choose. Someone rowing across a river makes the same mistake by holding the bow on the far bank the whole way over: they steered by heading when the thing they actually cared about was their track across the water. Until you have watched it happen from the cockpit you will keep flying headings and wondering why the ground ends up somewhere else.

That is what the ground reference maneuvers are for. Some pilots do exactly this for a living — pipeline patrol, aerial survey, search and rescue — holding a track over the ground at low altitude while the wind works against it. Everyone else meets the same skill in the traffic pattern, where a downwind that drifts, a base that overshoots and a final that will not line up are all one failure repeated: not correcting for drift.

What the maneuver is

Two 180-degree turns in opposite directions, flown either side of a straight line on the ground, so your track draws an S across it. Each half of the S should be the same size and shape as the other, you cross the line wings level and perpendicular to it every time, and the altitude does not change from entry to the end.

The line grades you. Heading and bank are inputs; the shape you leave over the ground is the result, and you can see it from the cockpit while you are still flying it.

Setting it up

Find a straight road or section line running as close to perpendicular to the wind as you can. Exactly perpendicular is ideal and rarely available, so take the road that works best. Work out the wind before you commit — from the surface report you departed with, from smoke, dust, water or crop rows, or from the drift you can see on the way out. If you are pulling the wind off a METAR, remember it is a surface number and the air you are maneuvering in is a few hundred feet above it.

The line also has to be somewhere you are allowed to be, and somewhere that leaves you an out. Choose it over open, sparsely populated ground and keep away from congested areas, dwellings, livestock and people. 91.119 sets the floor: an altitude that would let you make an emergency landing without undue hazard to anyone on the surface, and, away from congested areas, no closer than 500 feet to any person, vessel, vehicle or structure. You will spend this maneuver low and turning, so pick the field you would put it in before you start rather than after the engine gets your attention.

• Descend to the entry altitude — 600 to 1,000 feet AGL. Just pick 1,000; it is an easier number to hold and it buys you room.
• Clear the area above, below and on both sides, and keep clearing it throughout.
• Configure the airplane and settle on a power setting that holds altitude at your maneuvering airspeed, then leave it alone. There is no number to look up — ground reference maneuvers do not appear in the POH — so the setting is the one your instructor works from in your airplane. Changing power mid-maneuver is one more variable you do not need.
• Pick a reference on the cowl or wingtip so you have a fixed mark to judge pitch and bank against while your eyes are outside.

Flying the turns

Enter perpendicular to the road, flying downwind, and roll into the first turn as you cross. That crossing is where your groundspeed is highest, so it is where the bank must be steepest — the airplane is being carried over the ground faster there than at any other point, and it takes more turn rate to hold the radius.

From then on the bank comes off progressively. As the nose swings toward the wind the groundspeed falls, and by the time you complete the 180 and arrive back at the road you are heading upwind, at your slowest groundspeed, in the shallowest bank of the maneuver. Roll wings level, cross the line perpendicular, and roll into the opposite turn — shallow, because the groundspeed is still low — then steepen steadily as you come back around toward downwind, arriving at the road steep again.

There is no bank angle to set and hold anywhere in this maneuver. It changes continuously from entry to finish, while the airspeed you read stays constant and the groundspeed underneath it does not.

The size of the semicircles is set by the bank you pick at entry, so pick one you can shallow out of. The Airplane Flying Handbook puts the steepest bank of the maneuver at somewhere around 45 degrees, and it offers that as guidance rather than a placarded limit — but the practical point holds, because entering at 45 leaves you nowhere to go. Somewhere in the twenties or thirties gives margin on both sides.

Fly the track, not the heading. If your arc is bulging away from the shape you want, add bank; if you are cutting inside it, take some out. Look outside — the corrections come from what the ground is doing, not from the heading indicator. Glance in to confirm altitude and airspeed, then get straight back out.

Hold altitude within 100 feet and airspeed within 10 knots, stay coordinated the whole way, and fly at least one complete set, or as many as your instructor or examiner asks for.

What goes wrong

Misjudging the wind is the root of most of it. If you have the direction wrong, no bank schedule will save the shape, and you will spend the maneuver chasing an error you cannot name.

Running the schedule backwards is next. The schedule is keyed to your heading relative to the wind, not to which side of the road you happen to be on, and it runs the same way through both halves of the S: steepest at the downwind crossing where the ground is going by fastest, shallowest at the upwind crossing where it has slowed. Reverse that — shallow as you cross heading downwind, steep as you cross heading upwind — and you draw a lopsided S that is obvious from the air. So is working the controls in steps: rolling to a bank, holding it, then noticing you are wrong and snatching it back. The inputs here are smooth and never quite finished.

Altitude wanders because the pitch that held level flight at one bank will not hold it at another. Trim for level flight before you enter, then correct with pitch as the bank varies rather than letting it drift and fixing it in lumps.

The dangerous error is using rudder to hurry a turn that is not coming around fast enough. That is a skid, at low altitude, in a bank, and if you add back pressure to hold the nose up you have assembled the classic cross-controlled stall — the same one that kills people on the turn from base to final. That stall is an instructor-demonstrated maneuver and nothing else: you see it set up deliberately at a safe altitude, with an instructor, so that you recognize the feel and never meet it by accident at 800 feet. If the arc is not tight enough, the answer is bank and coordination, never bottom rudder.

Where you meet it next

On the checkride this is Ground Reference Maneuvers, and the tolerances above are the ones the examiner is holding you to. Turns around a point is the same skill wrapped all the way around a single point instead of split across a line, and the rectangular course is the same skill flown in the shape of a traffic pattern; the cross-controlled stall lesson covers, at altitude, the thing the skid at the bottom of this one leads to. All three sit alongside this one in the lesson index.

You will use it far more often than you fly it. Every pattern you enter, every base leg the wind pushes you through, every final that needs a crab is this maneuver with the road taken away. The crosswind calculator gives you the component you will be flying against down there.