In a tragic accident, Missouri Governor and Senatorial hopeful Mel Carnahan died October 16 when a twin-engine Cessna 335 impacted terrain just south of St. Louis in nighttime IMC conditions. Shortly prior to the crash, the pilot reported that he was experiencing trouble with his primary attitude indicator and stated that he would try to use the co-pilot's attitude indicator. The final report that the NTSB will issue many months from now may or may not shed any light on the accident cause but those in the know understand that the accident pilot may have taken an E-ticket ride on "The Parallax View." AVweb contributor R. Scott Puddy fills you in.
July 10, 2002
|About the Author ...
R. Scott Puddy was an ATP, CFI, CFI-I, MEI who taught out of the
Buchanan Field Airport (CCR) in Concord, California. Scott was type-rated
in the Beech/Raytheon King Air 300 series but regularly flew a V35 Bonanza
and practices law in San Francisco.
On the morning of June 18, 2002, Scott perished doing what he
loved: practicing aerobatics in a Yak-52, in the mountains of Brentwood,
He contributed many articles about flying to AVweb in recent
years and also worked as our features editor. His enthusiasm for
aviation and his intensity in pursuing it were simply extraordinary.
Even more extraordinary was his dedication to sharing his passion for
flying with others, by teaching and writing. He touched a lot of lives,
undoubtedly saved many, and his legacy of written words will continue
to do both for many years to come. Scott's warmth, wit, and keen
intelligence will be missed by all who knew him and worked with him.
occurs when you view an instrument or any other object from an angle so
that the instrument indication is distorted. The severity of parallax depends
on the extent of the gap between the instrument pointer and the instrument
face and the angle from which you are viewing the instrument.
CFIs in aircraft with only one set of gages deal with parallax every day,
having to use the instruments on the left side of the panel while sitting in
the right seat. Instruments that read in a vertical direction will give skewed
indications when viewed from the side. For example, the directional gyro (DG)
has its index pointer at the top of the instrument. When viewed from the right
side, the indication will be distorted to the left so that the right-seat user
will perceive a heading indication of approximately six degrees when the
instrument is indicating 10 degrees. If there is a more narrow gap between the
index and the instrument face (for example because there is a heading bug
installed) the distortion will be less. (In aircraft with heading bugs, the
distortion of the directional gyro's indications may be only one or two
Parallax exists only when you are viewing the instrument across the angle
of the instrument's indication. The directional gyro reads in a vertical plane
so parallax will occur when the instrument is viewed from the side but not
when viewed from above or below. You could raise the DG three feet on the
panel, placing it above the CFI, and suffer no distortion because the
instrument is read in the vertical plane. Similarly, the instruments on the
panel that give readings along a horizontal orientation (such as the turn
coordinator and the vertical speed indicator) do not give distorted readings
when viewed from the side. Other instruments such as the airspeed indicator,
the altimeter and (most significantly) the attitude indicator are a mixed bag
because they can give readings in either a vertical or a horizontal
Dealing With Parallax
attitude indicator is the most important instrument for the instrument pilot.
(Remember: "Power + Attitude = Performance").
It is the only instrument that gives instantaneous indications of both the
airplane's pitch attitude and its bank attitude. The pitch indication is along
a horizontal perspective because you are comparing the vertical alignment of
the miniature airplane with the horizon line on the instrument face. The bank
indication, in contrast, is along a vertical orientation because you are
comparing the position of the fixed pointer with the index markings at the top
of the instrument. Hence, the bank indications will be distorted (if the index
pointer is used as the reference) whereas the pitch indications will not.
If you are forced to view the instrument cluster from the side, you have
two means of dealing with the parallax. If you choose to use the instrument
indications along a vertical perspective you must continuously add or subtract
from the indicated values to determine the true values. For example, if you
use the index at the top of the DG to read headings, you must mentally add
four degrees to the indicated heading to determine the true heading.
The alternative is to use substitute indexes that are on a horizontal
alignment and therefore do not give distorted indications. For example, the
horizon line and the 90-degree bank marking on the attitude indicator would
appear to be aligned when viewed from the side in straight and level flight.
Although the index at the top of the instrument would indicate that the plane
was in a slight bank to the left you could use the 90-degree bank reference
point as an alternate index and defeat the consequences of parallax.
When flying IFR
from the right in a single-panel plane, it is easy to suffer the mental math
of adjusting for the distorted indications on secondary instruments like the
directional gyro. However, it is too confusing to make continuous corrections
to the indications that you must continuously rely on to hold the plane in a
constant attitude. You may therefore choose to ignore the misleading bank
indications at the top of the attitude indicator altogether. You can use the
attitude indicator's miniature plane and horizon line (which give accurate
readings in a horizontal orientation) for pitch information. For bank
information, you can use the turn coordinator (primarily) and the 90-degree
bank mark on the attitude indicator (secondarily). (Don't use the attitude
indicator's 90-degree bank mark as the main bank reference because it can be
somewhat confusing, particularly in a non-level pitch attitude.)
Flying IFR using the co-pilot's panel is harder than it looks. For the
uninitiated, the instrument indications will be confusing. Some of the
instruments (e.g., the turn coordinator and vertical speed indicator) will
give accurate readings. Others (e.g., the directional gyro) will give
inaccurate readings. Still others (e.g., the attitude indicator) will give
readings that are either accurate or inaccurate depending on what part of the
instrument you're looking at. If you combine confusing instrument indications
with instrument meteorological conditions, throw in a dash of turbulence and
level spoonfuls of fear and apprehension, you have the perfect recipe for
Parallax And Vertigo
Don't Try This At Home...
I have a vivid
recollection of my first attempt at instrument flight from the right seat. I
was very comfortable flying solely by reference to instruments and it had been
years since I had experienced spatial disorientation that persisted for more
than five seconds. I was well along in my CFI training and had been flying
from the right seat in VFR conditions for quite awhile. A flying buddy and I
were out shooting approaches under the hood and he suggested that, given my
developing right-seat skills, I could save 0.4 on the Hobbs meter if I just
flew my approaches from the right rather than landing to switch seats.
"Sure Bill. How hard could it be?" I responded.
What followed was a 20-minute E-ticket ride, just like the ones at
Disneyland, and only slightly more expensive. I managed two survivable ILS
approaches but was experiencing spatial disorientation the entire time. I had
to call it quits after the second approach because I was exhausted from
continuously having to overcome the sensation of being in a turn in order to
fly a straight line. It was quite entertaining, but then again fear and
apprehension were not part of the equation that sunny afternoon. I knew that
all I had to do was doff the hood and the world would stop spinning. When I d
had enough, I did and thankfully it did.
...What Is Vertigo?
Vertigo, or spatial disorientation, occurs when your mind does not receive
the clear visual cues that will cause it to suppress the continuous confusing
signals being transmitted by your inner ear. The beginning instrument student
will usually have the opportunity to experience vertigo until interpreting the
instrument indications becomes routine. From that time on, the mind accepts
the instrument indications as substitutes for real world visual cues and
ignores the inner ear's contradictory signals.
If you attempt
to fly in IMC using the co-pilot's instrument cluster, you must clear two
hurdles in order to avoid spatial disorientation. First, the confusing signals
generated by the inner ear will be more pronounced. The semi-circular canals
of the inner ear are aligned straight ahead. In order to view the co-pilot's
panel, you will hold your head at an angle to the plane's direction of motion
and will alter that alignment of the semi-circular canals to the side.
Turbulence and the like that would otherwise generate signals like:
"you're speeding up" or "you're climbing" will instead
generate the signal "you're turning."
Second, your defenses to vertigo will be compromised. For all the reasons
discussed above, the instrument indications will be confusing because of the
parallax view. Unless you have acclimated to that condition, just as occurs
with beginning instrument students, your mind will not accept the instrument's
indications as adequate visual cues. Instead, it will listen to the erroneous
signals from the inner ear and you and your passengers will be on the E-ticket
What's A Pilot To Do?
In any emergency situation you need to go with what you know unless trying
something new is the last resort. Just because you find yourself in a box
canyon, it doesn't mean that you should attempt your first-ever Immelman Turn.
Configure for slow flight and execute a 180-degree turn using the least amount
of space possible. If there's not enough room, aim for something soft.
In the specific circumstance of an instrument failure in a plane with dual
instrument quadrants, do not attempt to use the co-pilot's quadrant unless you
have been trained to use those instruments. If your co-pilot is
instrument-rated, you could delegate your responsibilities as PIC. Otherwise,
fly partial panel using the instruments you are accustomed to.
If you happen to own a plane that has dual instrument quadrants and want to
realize the benefit of the redundancy you have paid for, you need to train
using the co-pilot panel. Purchase half a dozen of those four inch rubber
disks with the little suction cups on the back. Apply one to each of the
primary flight control instruments on your side of the plane and give it a go.
You'll be surprised by how difficult it is. However, you will have an easier
time of it if you devote some advance thought to selecting instrument
indications that have a horizontal orientation to rely on for basic pitch and
bank information. Select a CAVU day for the first outing and fly with an
instrument-qualified safety pilot. When vertigo sets in, enjoy the E-ticket