The Parallax View: It's an E-Ticket Ride

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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.

Parallax 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 degrees.)

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 perspective.

Dealing With Parallax

The 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 vertigo.

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 ride.

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 ride.