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The Base to Final Turn

One of the most important and critical turns in the circuit pattern, and in your flight, is when you are ‘low and slow.’  The base to final turn is a critical manoeuvre that when done uncoordinated, can lead to an increased risk of an unrecoverable stall-spin accident. Because we are so low in that phase of flight, recovering from a stall-spin from that altitude is not possible.

When we overshoot this turn is where the problem can become critical. In an attempt to get the airplane back on the proper approach path to make the runway, many pilots add lots of aileron and find this doesn’t get them back on track enough. So, they add rudder in the same direction and though this turns the nose to where the pilot wants to go it also puts them in an uncoordinated flight profile.  Watch what happens here:

Overshooting often the cause

After overshooting the base to final turn, forcing the plane into a normal approach can become tragic.  The airplane can stall without having enough altitude to recover: once the stall spin develops, there is not much that can be done to bring it back.  A lot of the time why this happens is because the pilot turns base, not anticipating a tailwind, and before they know it, the tailwind has blown the plane through base.  So in order to get back on track the pilot will attempt to force the plane back on track using lots of aileron and rudder.

Other times the turn will be too timid, some people only turn 10-15 degrees in the pattern, which often does not get enough to get the plane where it needs to go.  Then people will push to past 30 degrees, but if this is coordinated, there is no need to bank the plane that far.

The problem comes when we use some aileron to get back on track but that doesn’t work, so we use more rudder to tighten the turn. This results in us being uncoordinated.  This increases the bank angle and rate of descent.   Many will use opposite aileron to soften the bank angle and pull back on the yoke to check the rate of descent.

This will increase the angle of attack on the inside wing – a stall and spin on the inside wing can come quickly.

How to avoid

If you notice that you are being blown closer to the runway on the downwind, anticipate that this wind will blow you through your base turn. So, turn early, watch your angle of bank and keep coordinated. If you cannot regain your track in a coordinated fashion, simply overshoot.  There is no shame in a go-around.

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Illustrative example of wing stall using yarn

This video shows what happens to the air flow over a wing as it stalls.  Yarn, placed all over this airplane wing is pushed backward by laminar airflow during regular flight.  When approaching stall speeds,  the laminar layer moves forward more and more until it is all the way at the leading edge of the wing and drops off, causing the wing to stall. The yarn shows the turbulent air moving forward very clearly.

Note where the turbulence starts as the wing starts to stall, and how it reorganizes as the wing recovers from the stall.

This video was made by Harv’s Air in Stenbach, Manitoba, where the pilot taped four rows of 4.5 inch long pieces of yarn over the entire wing of a Diamond DA40. The flight was over Southern Manitoba.

This video does a good job of illustrating what happens to the airflow as it goes over the wing during stall. Read more about stalls and angle of attack here.

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81 Turn World Record Inverted Flat Spin

This video of a world record breaking inverted flat spin will make you dizzy just watching it.

The Pitts S-2B just drops like a rock! Watch as he plummets, upside down over 21,000 feet.

Air show performer Spencer Suderman makes an 81 turn, three minute long inverted flat spin in his Pitts S-2B that took him from 23,000 feet to less than 2,000 feet over the California desert. You can watch the altimeter spinning as he’s descends, upside down.

Number of spins: 81.7 – now that’s a precise count.

This was his third attempt to beat the previous record, which is 78 turns set by airshow legend Wayne Handley in a Giles 202 in 1999. In his previous attempts at beating that record, he was able to do 64 spins and 77 spins.

Suderman credits ElectroAir, who makes FAA-certified, variable-timing, electronic ignition systems for this success, allowing his engine to operate at that altitude and throughout the spin.  He is helping the company collect engine data for FAA certification in six-cylinder Lycoming engines.

Suderman had to apply for FAA permission to fly the VFR biplane, built in 1984 over the California desert. Over El Centro, in the Salton Sea, he climbed up to 23,000 which took him approximately 30 minutes. He wore gloves, an oxygen mask and several layers of clothing as the outside temperature was 9 degrees F.   The event was recorded with three onboard cameras.


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Stalls and angle of attack: a very important relationship

The relationship between stalls and airspeed is often misunderstood.   It is not actually the airspeed of the aircraft that will determine when the wing will stall, but rather the angle of attack.

Stall recognition is generally taught with reference to airspeed only.  Students are taught to pull up to stall the aircraft and continue doing so, watching the airspeed bleed off, watching the needle go from the green arc, through the white arc where this ends, and the airspeed at which that aircraft (in that particular configuration) is known to stall.  Instructors drill into us the importance of angle of attack, and that the aircraft can stall at almost any airspeed. In fact, my instructor and I stalled the aircraft at full power settings. It was surprising and intense, and very important to recognize that this can happen.

There is no instrument to show the relationship between angle of attack and airspeed determining stall due to the complex forces that determine when an aircraft will stall, including weight of aircraft, load factor, the aircraft’s center of gravity, and other factors such as altitude, temperature, aerofoil contamination (frost or ice on wings) and turbulence.   The airspeed indicator alone cannot measure when a stall will occour.

Your POH will give you stall speed with flaps up and flaps down configurations, for a certain weight. We learn in ground school that stall speed increases with weight forward center of gravity (which acts like an increase in aircraft weight), load factor (such as in a turn) and when there is surface contamination.  Once you know the basic stall speeds, it is up to you, the pilot, to be able to recognize when you are increasing or decreasing the stall speed.

It’s not the airspeed, it’s angle of attack

A typical lift curve, showing where lift angle is reached, which is about 16 degrees in this example. Image from wikipedia.org
A typical lift curve, showing where lift angle is reached, which is about 16 degrees in this example. Image from wikipedia.org

Angle of attack is the angle at which the relative airflow meets the wing. This is what determines when a wing will stall. It’s important to understand relative wind – this is the way the air flows over the wing – when this is disrupted, air can no longer flow the way it’s designed to over the wing, and lift decreases.  The critical angle of attack is reached when the maximum lift coefficient is obtained, after which lift will drop off when the angle is exceeded, and the aircraft will loose lift. After the critical angle of attack is reached, the aircraft is said to be approaching a stall.

The aircraft will always stall at the same angle of attack, called the critical angle. Many modern jets have an instrument that prevents the pilot from increasing the angle of attack past the critical angle, this is called the angle of attack limiter or alpha limiter.

Dangers of being low and slow

However this type of tool, or similar instrument is not in most general aviation planes. This leads to the pilot having to be very careful in making sure they don’t push their airplanes in into this flight envelope. When is a stall most dangerous? When you are low and slow.  Typically, the base to final turn can be very hazardous, and this is corroborated with the amount of stall-spin accidents that happen during this circuit sequence (for general aviation airplanes).  On this turn, you are low, and your airspeed is decreasing since you are on approach. When you turn, you increase the load on the aircraft, and if you push it into a stall (say, by executing a steep turn) you can enter a deadly stall-spin from which recovery is difficult due to the proximity of the ground.

ICON Aircraft GuageRecently I’ve discovered the aircraft manufacturer Icon Aircraft. This company has created an angle of attack instrument for general aviation airplanes.  This instrument measures angle of attack and presents it to the pilot showing when they are flying within the proper range.  This is a very interesting development that should go a long way into increasing safety.

I recommend watching the video about the concept below. Very cool!