The phenomenon is known by many names — death spiral, graveyard spiral, suicide spiral, vicious spiral. It has been with us since the Wright Brothers, and over the years has claimed many pilots and airplanes, heavy iron and flibs alike. Legendary NWA captain Paul Soderlind looks at two classic cases, both four-engine transports flown by seasoned airline crews. He then discusses how and why these spirals develop, how to avoid them, and what to do if you find yourself in one.
May 11, 2000
|About the Author ...
A. Soderlind took
his first flight lesson in 1935 at age 12, earned his private certificate on his
18th birthday (which was then the CAA minimum age), and earned his commercial
and instructor ratings three months later. In 1946, on his 23rd birthday, he
checked out and became the nation's youngest airline captain at Northwest
Airlines, where he developed many of the standards and procedures that were
adopted by the airline.
In 1954, Paul was named Northwest's Director, Flight
OperationsTechnical. In that job he flew the inaugural acceptance flight for
every new airplane that Northwest ordered from the DC6 through the 747. In
his 30,000+ hours, he has flown about 30 different types of airliners, about 20
different types of bizjets and turboprops, the Lockheed F104, and about 400
designs of GA aircraft from seaplanes to helicopters to Molt Taylor's Aerocar
and ultralights. He retired from Northwest in 1973, and since then has given
lectures at ICAO, IATA, Boeing, NBAA, ALPA, IAA, Embry-Riddle, the U.S. Air
Force, and the FAA Academy.
He has served as a consultant to FAA Administrators
Alexander Butterfield, Langhorne Bond and David Hinson, Associate Administrator
Richard Skully, and turned down an offer from the Reagan administration to head
the FAA. His list of bizjet consulting jobs reads like the Fortune 500, and his
list of airline consulting jobs reads the the Dow Jones Transportation Index. He
has flown over most of the Northern Hemisphere (and some of the Southern), from
Hong Kong on the west to Paris on the east, including the Philippines, Taiwan,
Japan, Alaska, Canada, Hawaii, and most of the contiguous United States. He has
won bookcases full of awards, including the 1964 ALPA Air Safety Award, the
first FAA Citation and Gold Medal for Extraordinary Service to Aviation Safety,
the 1979 Laura Taber Barbour Air Safety Award, the 1985 General Billy Mitchell
Award, a 1994 FAA Special Recognition Award for Lifelong Commitment to Aviation
Safety, and in 1997 was inducted to the Minnesota Aviation Hall of Fame.
In 1999, Aviation Week and Space Technology honored Paul and his partner,
Northwest's Chief Meteoroligist Dan Sowa, for the Turbulence Plot program they
developed in 1965.
There is a unique kind of destructive spiral that I'll
call "The Deadly Spiral" (TDS), although it's often referred to by
other names, including Suicide Spiral, Death Spiral, Vicious Spiral, Graveyard
Spiral. While pilot disorientation may sometimes be involved, the more common
cause is much simpler, as we'll see. Although this phenomenon has been with us
since Orville and Wilbur first did their thing, many pilots have a limited
understanding of why it occurs, how to avoid it, and how to recover from it.
Wolfgang Langweische, author of the classic Stick and Rudder, once said
that there are only a few dozen men in the whole world who fully understand an
airplane's spiral behavior. While I don't claim to be one of those, it does seem
to me that the time is ripe to discuss the things that are known about this
To illustrate that TDS can happen even with highly experienced pilots,
consider the following two case studies that involved high-time airline crews
flying large transport aircraft:
Case Study A
The airplane involved was a four-engine turboprop in airline cargo service.
The only occupants were the captain, co-pilot, flight engineer, and a
deadheading pilot in the jump seat. The captain and flight engineer were highly
experienced and both had many hours in type. The co-pilot was relatively
inexperienced but had received all required training and was certified and fully
The flight was cruising at 22,000 feet at night, between layers, in smooth
air. The co-pilot was doing the flying, navigating, communicating everything
and the flight engineer was attending to normal duties at his panel. (The
captain's activities are relevant to what happened, and will be discussed
shortly.) To appreciate what happened to the flight during the next 30 seconds,
look at this plot of data from the flight data recorder (FDR) and cockpit voice
Click for full-size image
Times of significant events (in seconds) are read against the horizontal
scale at the bottom. The triangular symbols mark the time of relevant CVR
comments and other key events. The FDR's heading trace was inoperative.
The flight was entirely normal up to Time 08 (i.e., eight seconds into the
plot above), and had been on autopilot in "altitude hold" mode for
over 30 minutes. There were some earlier comments on the CVR about a problem
with the co-pilot's attitude indicator, but after switching its souce to the
captain's vertical gyro, operation was normal. Here's a transcript of the final
22 seconds of the CVR:
||First departure from cruise
deviation from normal first noticed
||Unidentified voice, CAM?
||Co-pilot's voice, CAM2
||Increasing wind noise (airspeed
||"You got it?"
||Co-pilot's voice, CAM2
||Captain's voice, CAM1
||Landing gear warning sounds
(power levers pulled to IDLE)
||Overspeed warning sound begins
||Sounds of structural breakup
||Unidentified voice, CAM?
NOTE: In the CVR transcript at left, "CAM" denotes cockpit area
microphone. "CAM1" is on the left side of the cockpit, while "CAM2" is on the right
side. "CAM?" denotes that the location from which a sound came was
The first anomaly begin at Time 08 when the flight first departed from the
altitude it had held for over 30 minutes, probably when the autopilot was
disconnected. This wasn't discovered for some six seconds as noted by the CAM?
statement "Altitude" at Time 13.4. 1.9 seconds after the co-pilot said
"What's happening here?" he asks "You got it?" presumably
asking the captain if he had taken control of the aircraft. The captain's
"No" was without any particular inflection or concern in his voice.
The descent from Time 18 (Point A on the diagram) to Time 30 (Point B)
averaged over 32,000 feet per minute, which is consistent with TDS
Most shocking was that it took only 14 seconds to go from normal flight to
structural breakup, in good weather and smooth air, with an experienced crew and
no evidence of mechanical failure.
The fact that the co-pilot was the sole pilot flying/navigating/communicating
was related to the captain's probable activity during an extended period
preceding the initial departure from cruise altitude. Specifically, other pilots
this captain had flown with reported that he had a long-standing habit of
reading what was euphemistically termed "non-operational material"
while flying. Indeed, one could hear pages turning on the CVR recording.
The primary cause of this accident was "nobody watching the store,"
a factor that has preceded every TDS incident on which relevant data is
available. Complicating matters was a relatively inexperienced co-pilot getting
no help from either the captain or the flight engineer.
Case Study B
Another classic incident occurred in the earliest years of U.S. airline jet
operations. A four-engine airliner on a moonless night in smooth air over the
ocean got into TDS, diving from 36,000 feet to 6,000 feet before recovering.
Recovery load factor exceeded 6g, and despite a wing spar that was permanently
bent, the crew managed to land the aircraft successfully. This near-catastrophe
started while the captain was in the passenger cabin, the co-pilot was doing
paperwork, and the flight engineer and navigator were working behind an
anti-glare curtain between themselves and the pilot stations. The captain (in
the passenger cabin) was the first to realize something was amiss. The loss of
control began shortly after the autopilot disconnected unexpectedly.
Another no-one-watching-the-store case.
After a detailed inspection by the manufacturer's engineers, the airplane was
declared structurally safe, and it went on to fly out the rest of its years
without serious incident.
Aerodynamic and Control Aspects of TDS
basic problem is an airplane's spiral mode, which is inherent in the airplane's
shape. Virtually all airplanes have weak spiral stability and "want"
to start turning, however slowly. The typical airplane, if left unattended, will
simply not go straight for long. To believe that it will implies that it has
heading stability i.e., once headed west, it will keep heading west. But it
won't. There's no such thing as inherent heading stability.
Assume for the moment that:
the airplane is perfectly shaped
it is perfectly rigged
it is perfectly trimmed for straight-and-level cruise
fuel is perfectly balanced between left and right wings
if multi-engined, power is perfectly balanced between left and right sides
there is not a hint of the slightest wind shear
the air is smoother than a mouse's tummy
You will never find such perfection in the real world, but let's pretend it's
all true for purposes of the discussion that follows.
With a perfect airplane in perfect conditions and nobody attending to the
controls, one could conceivably continue straight and level for several minutes,
but more likely the time is measured in seconds. In any case, sooner or later a
wing will drop it may be either left or right, the direction being entirely
random. When the wing drops, the nose will go down and the airspeed will
increase just a little if the bank angle is small. But with one wing down,
the airplane will start to turn. The higher wing being on the outside of the
turn is moving faster than the lower wing, producing more lift, causing bank
angle to increase, the nose to drop further, airspeed increase even more ... and
on and on, the situation feeding on itself ... a "vicious spiral" in
more ways that one.
Once the turn starts, one of two things will happen if the turn is not
Case 1. The Stable Spiral
When a wing drops, the airplane will begin to turn, the nose will go
down and the airspeed will increase. After a relatively short time,
airspeed will stop increasing and remain a few knots above the original
trim speed, and bank angle will remain constant at 20° to 30° or so.
The spiral has reached a stable mode and the airplane will continue in a
descending turn as long as altitude remains.
Case 2. The Unstable Spiral
Once the turn starts, airspeed and bank angle will continue to
increase, a stable state will never be reached, and the spiral
ultimately will develop into a near-vertical dive at airspeed and bank
angle far beyond all normal limits. Our "perfect" airplane
perfectly rigged, trimmed, etc. will usually go "all the
way" no matter which wing drops initially. (No two airplanes, even
if they are of exactly the same type, will react exactly the same.)
Now so far, we've been discussing a "perfect" airplane flying in
"perfect" conditions. Neither of these ever occur in the real world,
where secondary effects make things worse.
Consider the airplane with only a "bendable" rudder tab adjusted to
counter "torque" in cruise. (Never mind that the turning tendency that
pilots often refer to as "torque" is not torque at all ... that's a
discussion for another time.) Though the tab has been adjusted to offset the
left-turning tendency attributable to "torque," it will do so only for
one particular combination of altitude, power, indicated airspeed, etc. With any
other condition, the tab's anti-turning force will no longer balance the
"torque" and a turn will start. The same is true with
cockpit-adjustable rudder trim. Anything that unbalances the airplane a tiny
fuel or power imbalance, for example will require the trim to be readjusted.
Such a small imbalance almost always will go unnoticed by the crew until the
airplane reacts by starting to turn, at which point the trim is readjusted.
But what if it's not readjusted? Will the airplane enter a stable spiral or
an unstable spiral? Let's take another look.
our airplane is cruising serenely along in smooth air, two opposing turning
forces are at work "torque" tending to turn the airplane left, and
the rudder tab tending to turn it right. What happens then depends on which of
the opposing turning forces is dominant. If, for example, power increases (a
temperature decrease can cause that) and if nothing else is changed, the
increased torque becomes dominant and the airplane wants to turn left. But if
power is reduced, the effect of rudder trim becomes dominant and the airplane
wants to turn right. If the right wing drops first, the increased airspeed
strengthens the right-turning effect of the tab and the airplane goes "all
the way." If the left wing drops first, the tab's right-turning forces will
dominate when speed increases, and the airplane will reach the stable mode.
Every airplane has a built-in turning tendency even brand new ones
usually due to misrig or mistrim in roll or yaw or both. The initial turning
tendency is usually small, usually difficult to detect at first, and extremely
difficult to isolate the specific underlying cause. The onset is usually
insidious, beginning very slowly, usually with little or no seat-of-the-pants
clues strong enough to alert an inattentive pilot that something's awry. An
airplane cannot suddenly "snap" into a spiral unless it's grossly
out-of-trim in the yaw or roll axis. Nevertheless, a well-developed spiral often
develops with astonishing rapidity, as we saw in our first case study where it
took only 14 seconds for a large transport aircraft to go from controlled flight
to structural breakup.
The only true "cure" for TDS is avoidance, avoidance, avoidance.
Someone must be watching continuously what's going on, and be prepared to
initiate recovery from an incipient spiral without delay.
Recovering from TDS
One widely published recovery procedure involves seven steps, several
of which are either unnecessary or can actually be detrimental. Considering the
time-critical circumstances under which it might be needed, a seven-step
procedure is far too complicated. There is a better one, and it involves only a
Level the wings with slow,
gentle rudder pressure,
keeping hands off
Applying rudder produces yaw, which produces roll, and the airplane will
unbank. Relax rudder pressure as the wings approach level, then continue to hold
them level with the rudder only. Keep hands off the yoke or stick. Do not fret
that aileron and rudder are not coordinated in the recovery coordination is
As the wings begin to unbank, the nose will come up. There is no need to
apply back pressure to recover from the dive. The nose will come up by itself
with no great increase in load factor (i.e., g force). If the airplane was in
trim at a reasonable airspeed before the spiral began, it will return to the
same airspeed by itself ... provided the pilot doesn't interfere by
applying pitch inputs!
When the nose comes up, it will momentarily overshoot the original attitude,
then pitch down again. This pitch-up-pitch-down cycle will continue, the pitch
excursions decreasing with each cycle (engineers call this a "phugoid
oscillation") until the airspeed settles down at or within a few knots of
the original trim speed. (This assumes, of course, that the airplane was trimmed
to a reasonable airspeed before the spiral began, and that the C.G. is within
The airplane will "take care of itself" in pitch. It
"wants" to seek and hold the airspeed (actually, angle of attack) that
it had been trimmed to. Its natural speed-keeping stability will return it to
that speed. Let it do so on its own.
Every pilot should get an appropriate demonstration from a knowledgeable
the airplane will begin a turn if allowed to fly hands-off
the bank angle and airspeed will increase once the turn starts
the rudder-only, hands-off-the-yoke procedure will work admirably to
A Caution to Pilots and Instructors
For a realistic, conservative and safe demonstration, allow the airplane only
to begin a spiral dive. Don't let the bank angle increase beyond 25°
to 30° and don't let the airspeed get anywhere near redline. Begin at least
5,000 feet above the terrain, since the demonstration will take you both below
and above the initial altitude. Do the demonstration in smooth air othherwise
any gustiness or shear may hide the true effects of the spiral mode. If a spiral
dive is allowed to get too steep meaning with excessive airspeed the
recovery pitch oscillations would be quite large at first, and in the first
pitch-up the airspeed may drop to near the stall. Not to worry, the airplane
will not stall, though you might get a temporary beep or two from the stall
warning system. Excessive airspeed must be avoided, and the demonstration
can be accomplished effectively without getting to a high airspeed.
As with all simulated emergency procedures, caution is the word.
An airplane left on its own will sooner or later begin to turn, and
airspeed and bank angle will increase. All other factors being reasonably
normal, a spiral cannot develop to a dangerous degree suddenly, but
it can do so more rapidly than many pilots might imagine ... in a matter of
Avoidance is the best medicine. A dangerous spiral cannot develop if
someone is continuously "watching the store." This is the only
guaranteed method of TDS avoidance. Do not depend on an
"unsupervised" autopilot, since it may disconnect unexpectedly,
and the disconnection may go unnoticed until a dangerous spiral has
If a spiral develops, use the single-step recovery procedure: Level the
wings with slow, gentle rudder pressure, and keep hands off the controls.