Crosswinds

 

Something that student pilots (and many Private and Commercial) pilots struggle with is crosswinds during takeoff and landing.In a recent review of accident statistics for light aircraft, and a staggering 60% of accidents occurred during takeoff and landing, and often not under very challenging conditions.Obviously this needs to be worked on!

 

What makes this worse is the complete risk-avoidance of many flight schools, which wonít allow their students to fly with more than 5 knots of crosswind component.I am not making this up.The intentions of the flight schools are good Ė to reduce accidents Ė but at what cost?When you combine nosewheel trainers and no crosswind training, you end up with a pretty poor product, with very poor stick and rudder skills, which go on to have accidents after their training is complete.Not a good strategy!

 

Letís start with the basics.Any wind can be divided into two components: wind directly down the runway, and wind directly across the runway.There are charts to help you do this, or you can memorize a couple simple trigonometry values (for 30, 45 and 60 degrees) and do the arithmetic in your head.Whatever works for you.

 

For example, letís look at a 10 knot wind at 45 degrees across the runway.I happen to know that the sine and cosine of 45 are both 0.7 which means that I have 7 knots down the runway, and 7 knots across the runway.

 

The 7 knots of wind directly down the runway is good, because it reduces our groundspeed and thus our takeoff and landing distances.Itís important to realize that the airplane moves in the air mass, not with respect to the ground.If you had 70 mph of wind down the runway, you could take off and land vertically in a light trainer aircraft, if you could tolerate the severe mechanical turbulence associated with that much wind.

 

The 7 knots of wind across the runway is what causes people problems.You should know that any certified aircraft is required to demonstrate handling a crosswind of 20% of the stall speed.So with a 50 knot stall speed, it must be able to handle at least a 10 knot crosswind, and likely will handle much more.Itís important to know that the maximum demonstrated crosswind during certification (which will be in the POH/AFM) is NOT legally a restriction Ė that might be all the crosswind they could find during the aircraft certification.

 

Takeoff

 

A takeoff with the above 10 knots crosswind at 45 degrees to the runway, would be pretty straightforward in most light trainer aircraft, despite their slow stall speeds.Remember that the higher the stall speed, the more crosswind you can handle Ė itís a smaller percentage.A high wing loading eats up the runway but makes the crosswind a cinch.

 

Anyways, back to the takeoff.In either nosewheel or tailwheel aircraft, be sure to roll the yoke or push the stick into the crosswind to spoil the lift on the upwind wing during the takeoff.As you speed up, reduce the amount of aileron deflection because when you are going faster, there is more mass of air passing by the aileron, and hence being deflected by it.

 

In a nosewheel aircraft in a crosswind, donít get the nose up in the air if at all possible, as you would for a soft or rough field takeoff.If you do, you can weathervane.Better to keep the aircraft level, hope the nosewheel doesnít shimmy too badly, and when a good speed is attained, a positive rotation is performed to get the aircraft well clear of the runway.Then, a co-ordinated turn is performed into the wind, and the wings are leveled and the ball is centered with right rudder, to hold a crab which will result in a track of the extended runway centerline.You want to perform a positive rotation at a slightly higher speed than normal, to avoid touching down again with a downwind drift, which is really hard on the landing gear Ė you donít want to sideload them.

 

In a tailwheel aircraft in a crosswind, if you maintain the three-point attitude during the takeoff with the tail down, the extra angle of attack will get you airborne early into ground effect, and you donít want that, because you donít want to risk touching down again and sideloading the gear, same as a nosewheel aircraft.With a crosswind, you really want to raise the tail of a tailwheel aircraft to a level attitude Ė most people donít raise the nose anywhere near far enough Ė to decrease the angle of attack, reduce lift and stick the main tires on the runway until you have a good speed, and then perform a positive rotation to avoid touching down again.Once youíre in the air, do exactly the same thing as a nosewheel aircraft above, because in the air there is no difference between a nosewheel and a tailwheel aircraft.

 

Circuit

 

In the pattern (circuit in Canada Ė how very British!) you want to adjust the legs to compensate for the wind aloft, which is often stronger than the wind on the ground, even at circuit altitude (eg 1000 AGL).If you donít, youíre going to be really screwed up when you turn final.

 

For example, lets say you are flying left traffic, and there is crosswind from the left on the runway.Itís going to be a tailwind on base, and is going to cause you to be very high turning final, if you donít do something to compensate for it.Similarly you may need to compensate on the crosswind and downwind legs.

 

Landing

 

99% of pilots prefer a crosswind from the left, and thatís probably a mistake.Letís say you arrive at a quiet uncontrolled airport.No other traffic, and the windsock is directly across the single runway, at 15 knots.Would you take the crosswind from the left or right?Most pilots choose to take the crosswind from the left, and thatís the wrong choice.

 

Not only will it make you high on final due to the tailwind on base, lets say you weathervane left into the wind (from the left) during the landing.It gets dicey, so you add power to increase the effectiveness of your rudder.Good choice, but the problem is that if you are flying behind a Lycoming or Continental, the added power is going to swing the nose even further left, making the weathervaning worse.Itís terrifying to have full right rudder on, and watch the nose swing left.And that the choice that 99% of pilots make.Sigh.

 

Always choose a direct crosswind from the right.So, if you weathervane right, and left rudder isnít doing it, add power and the nose will swing left out of the wind and straighten you out.This is so simple, and completely ignored by most pilots, which is probably why 60% of accidents of light aircraft occur during takeoff and landing.

 

Back to the landing.Letís start on long final.Establish the aircraft on the extended centerline.If youíve got a tailwind on base, your groundspeed is higher so start your turn onto final earlier to avoid using excessive bank.If youíve got a headwind on base, start your turn onto final later than normal with your usual angle of bank, to establish yourself on the extended runway centerline promptly.If there is lots of wind down the runway, turn base a little earlier than normal to give yourself a slightly shorter than normal final, which will allow you to use your normal rate of descent on final which will be flown at a slightly higher than normal angle.You can fly the normal glidepath Ė itís not the end of the world Ė but you will have to drag the aircraft in with power.Yuck.

 

Anways, weíre on final now.Crab as required, ball in the center, to track the runway centerline inbound.Do not let yourself get blown downwind.Perform co-ordinated turns as required to maintain the extended runway centerline.Stay on or slightly above the normal glidepath, as dictated by the headwind component as described above.Donít get low, into the mechanical turbulence.

 

Now for a little unpopular truth.You can actually land a nosewheel light aircraft in a crab in a crosswind.The tires will howl, and so will the instructor in the right seat, but gosh, the center of mass ahead of the mains will straighten the nosewheel aircraft out.I know that people donít like to talk about this, but look at an Ercoupe sometime.

 

However, if you try that stunt in a tailwheel aircraft, you will regret it because you will be off the runway and rolled into a ball shortly after touchdown.On short final, you must transition from a crab to a sideslip, dropping the wing into the wind and perhaps adding a touch of power to compensate for the drag of the sideslip.A little opposite (top) rudder to stop the aircraft from turning, and you touch down with the aircraft aligned precisely with the direction of travel.

 

The problem is that many people are busy enough during the flare, and they forget to keep the wing down and they drift downwind toward the runway lights, which makes me unhappy.So, I just tell them to put 5 degrees of bank on during the flare to keep it simple, which will compensate for most crosswinds.10 degrees of bank is only required for sporty crosswinds in most aircraft.

 

Now here is probably the most important part of this article: after touchdown, as you roll out on the runway and slow down, you MUST roll the aileron fully into the wind as soon as possible, and ALWAYS end up using full aileron into wind during the rollout.

 

This is not just figure skating and good form chickensh1t.Yes, the raised aileron on the upwind wing spoils lift and helps keep the upwind wing down Ė you donít EVER want to let the wind get under the upwind wing Ė but there is a far more important consideration.

 

The aileron on the downwind wing goes down when you roll the yoke (or push the stick) into the crosswind as you slow down and very importantly, creates adverse yaw which uses the wonderful long wing to produce lots of torque to oppose the natural weathervaning tendency of the aircraft, as it slows down.

 

This actually determines the true maximum, limiting crosswind that the aircraft is capable of handling, as it rolls out at the end of the landing.I use this technique not only in the tailwheel aircraft that I fly, but in piston twins and jets that I fly.They have enormous vertical fins, and badly want to weathervane into strong crosswinds.

 

Story time,Gather round the camp fire.About 20 years ago, I went flying in a 25G40 knot direct crosswind as an experiment in my Maule M4-210C.The wind was measured at Ottawa, and was directly across the single paved runway at my home airport, after a strong cold front passage.So naturally I went flying.The takeoff was easy, with the powerful prop slipstream over the rudder to help keep me straight, but the landings were another story.I couldnít slow it down without weathervaning!With the power off, there was decreasing air over the rudder as I slowed down, and I could add power to stay straight, but that didnít help me stop during the landing.It was a fascinating experiment, and actually determined the true maximum crosswind capability of the airframe.

 

Most of the time, the pilot is the limiting factor for crosswind landings, not the airframe.Bluntly, his technique sucks, and this is something that most pilots need to work on.

 

One subtopic I glossed over was airspeed on final, for a crosswind landing.A common error amongst low-time pilots is to approach far too fast with a crosswind because it ďfeels goodĒ.

 

In a nosewheel aircraft, the excess speed results in a reduced angle of attack, and often the pilot will end up touching down first on the nosewheel, trying to force the aircraft onto the ground.This is called ďwheelbarrowingĒ and is horrible.Donít do that.

 

People have complicated formulae which I ignore, for increasing airspeed on final.

 

Letís look at the basics.We donít need to add airspeed on final for the crosswind component.Itís the headwind component thatís going to cause trouble.

 

Remember that the wind aloft is almost certainly stronger than the wind on the surface.Letís say you have 10 knots of headwind component on the surface, and 20 knots of headwind component at pattern altitude.You do not need a PhD from MIT to realize that at some point during final youíre going to lose 10 knots of headwind component, and thatís going to cause the aircraft to sag.Best add some power to try to regain your airspeed when this happens.This is annoying in a light aircraft, but in a heavier aircraft with more momentum, itís going to cause you heartburn.

 

A really simple way to precisely compensate for this ďloss of headwind shearĒ is with your GPS.Letís says your final approach speed is 60 knots, and you know there is 10 knots of headwind component.This means that you want a ground speed of 50 knots at touchdown, so to have a stabilized approach, set a GPS groundspeed of 50 knots on long final.With the 20 knots of headwind, you will end up indicating 70 knots of airspeed, and when you go through the loss-of-headwind shear, your airspeed will drop to 60 knots without the aircraft having to change itís groundspeed.Simple.You will have to check back slightly to increase the AOA, but thatís all.

 

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acboyd@gmail.com†† Sept 2011