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Killer Stability

There’s a maneuver -- it's both easy to enter and easy to recover from -- that, when left unchecked, almost always ends in death.There’s a maneuver -- it's both easy to enter and easy to recover from -- that, when left unchecked, almost always ends in death. Almost nobody teaches this maneuver to aspiring private pilots and many advanced pilots don’t even fully understand why a stable airplane naturally “wants” to enter this regime.

Practice steep turns and you’ll learn something about this tendency. Get some altitude, clear the airspace, and roll into a steep turn (45 degrees bank angle will do it). Now, let go of the controls, and see what happens. If your airplane is dynamically stable (virtually all certified airplanes are), it will:

  • Roll into a steeper bank;
  • Pitch downward; and
  • Accelerate toward the ground.
You’ve entered what’s often called the “graveyard spiral.”

NO, IT’S NOT A SPIN
Stability
: Although there are visual similarities, there’s a big difference between a spiral and a spin. A spin is the result of allowing the airplane to stall while in uncoordinated flight, or (as usually occurs in spin training) forcing the airplane into uncoordinated flight after the wings are stalled. In a spin, one wing is above its critical angle of attack, and therefore produces no lift; the other wing is developing lift. The contrasting forces create the rotation. A spin is a stable maneuver, meaning that the airplane will develop a pitch attitude, a rotational rate, a vertical speed, and an indicated airspeed (all sometimes deceptively low) and hold it so long as the pilot does not forcibly recover. In many, many airplanes, simply letting go of the controls and the airplane will cause the aircraft to recover from a spin -- in fact, this was the preferred method of spin recovery in military training back in the Stearman/T-6 era. Of course, it only works well if there's enough empty space below the aircraft.

Instability: A spiral, on the other hand, is an unstable maneuver -- when left unchecked, an airplane in a spiral will develop progressively more aggressive pitch attitudes, bank angles, vertical speeds, and airspeeds. Letting go of the controls is exactly the wrong thing to do in a spiral; in fact, most pilots’ natural reaction is to do exactly the wrong thing to try to recover from a spiral, with disastrous results.

Problem: Instructors and examiners briefly show pilots “unusual attitudes” (including spirals) during instrument training, but this hood work doesn’t adequately explain why a spiral is “normal” for a stable airplane, or what needs to be done to avoid spirals in the first place.

HOW AND WHY STABILITY CAN WORK AGAINST YOU
Part 1
: Let’s look back at that steep turn. In most airplanes, once the bank angle exceeds about 35-40 degrees, it will continue to increase unless you apply some opposite aileron. CFIs generally do teach this -- they call it the “overbanking tendency.” But just imagine that you don’t notice the overbanking tendency, or for some reason you allow the aircraft to continue overbanking. As the wing rolls away from the horizontal, there’s less and less lift holding the airplane up. Without wings to generate lift, the aircraft will begin to act more like a thrown dart with the weight out in front and the light tail surfaces at the back. If you’re banked to the left in (an airplane with an American engine), the effect is even more pronounced as the left-turning tendency of the propeller pulls you “downhill.”

Part 2: To understand this part, stop thinking about “up” and “down” relative to the earth. Instead, consider “up” to mean “up” relative to your seat in the airplane. As the airplane's nose begins to point toward the earth it loses altitude, but it will also gain airspeed. Since a properly trimmed airplane will attempt to maintain a constant airspeed, it will now pitch “up” -- relative to your seat. Trouble is, “up” is now pointed toward the center of the spiral. From this position, pitching toward the center merely becomes a steeper the bank angle, which makes the spiral ever more pronounced.

You don’t have to be close to stall speed, or exceed the critical angle of attack to enter a spiral.

Result: In a spiral, the airplane will develop a very high rate of speed and loss of altitude. If you don’t correct it soon, you’ll either

  1. hit the ground because of the tremendous vertical speeds, or
  2. break up in midair because you will exceed the airplane’s structural limiting airspeed.
Of course, on the way to the break-up or the ground, the airplane will continue to make matters worse --precisely because it’s designed to be “stable,” and seek a trimmed airspeed. All of this can (and does) happen in a matter of seconds.

WHERE AND WHEN
Spiral-type accidents (like the NTSB's interpretation of JFK Jr.'s) are almost always the result of loss of spatial orientation -- night flight into a “dark hole” airport; distraction in the clouds, especially in turbulence; or the classic “attempted visual flight into instrument meteorological conditions.” A spiral accident can also occur in a circling instrument approach or while practicing ground reference maneuvers, in a “botched” steep turn or even in a day, visual traffic patterns when a pilot gets distracted and can't recover in time. Quite likely, a large percentage of what we call “stall/spin” accidents in the traffic pattern, and virtually all the “impossible turn” attempts at getting back to the runway if an engine quits on takeoff are, in reality, spiral-type mishaps.

DEFENSE
That said, it’s easy to avoid spirals and to recover from one if you don’t.

  1. Be aware of the overbanking tendency -- know that, if you bank beyond about 30 degrees, you’ll need to be ready to apply opposite aileron to maintain the bank angle.
  2. Avoid bank angles beyond about 30 degrees when you don’t have a lot of room to recover, such as in the airport traffic pattern (most pilots won’t even let it get that steep so close to the ground).
If you enter a spiral, the first thing you need to do is to level the wings. Once the wings are level and “up” relative to the seat is the same as “up” relative to the earth, the airplane’s stability will pull the nose “up” and effect a recovery from this “unusual attitude.” Plus...
  • In most airplanes, it’s better to level the wings first with opposite rudder. Opposite aileron in a steep bank may actually aggravate the situation, making recover take longer. In a spiral, you don’t have extra time to waste!
  • Pull the power. Power is accelerating you downward, especially in a left-hand turn. Reduce your rate of descent and lower stress on the airplane by pulling the throttle(s) to idle.
  • In an emergency, extend the landing gear, if you’re flying a retractable. Forget about maximum gear speeds -- this is to save your life! At worst, you’ll lose gear doors to the slipstream, or maybe cause minor damage to the gear. If you’re nearing or beyond “red line” airspeed, though, throw the gear out to slow you down!
  • Once the wings are level, guide the airplane smoothly into a level attitude. It will actually want to radically pitch up as it seeks a trimmed speed. Let the nose come up, but not so high that your spiral recovery turns into a stall. Ease the plane into level flight... gently.
BOTTOM LINE: Spirals kill far more pilots than spins ever have, but we don’t go out of our way to show spirals to aspiring pilots, explain why and how they develop, or teach ways to avoid or recover from spirals. Get some practice with an instructor familiar with the spiraling tendencies of your airplane and be ready to recover -- properly -- if you ever face this unknown killer.

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About This Author:
Tom Turner is a widely published author and regular forum speaker at EAA's Oshkosh/Airventure and American Bonanza Society. Tom holds an M.S. in Aviation Safety with an emphasis on pilot training methods and human factors. He has worked as lead instructor at FlightSafety International, developed and conducted flight test profiles for modified aircraft and authored three books including: Cockpit Resource Management: The Private Pilot's Guide and Instrument Flying Handbook (both from McGraw-Hill). His flight experience currently spans 3000 hours with approximately 1800 logged as an instructor. Tom's certificate currently shows ATP MEL with Commercial/Instrument privileges in SEL airplanes.
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