February 1, 2004
By Barry Schiff
Retired TWA captain Barry Schiff has been writing for AOPA Pilot for more than 40 years.
Every applicant for an instrument rating learns to control an airplane following the failure of two gyroscopic instruments, the attitude and direction indicators. The resultant procedure is called flying partial panel. It is what old-timers refer to as flying needle-ball-airspeed. In most airplanes, the turn coordinator has replaced the turn needle.
Few pilots have difficulty demonstrating their ability to fly instruments using only partial panel because the simulated failure of the vacuum pump and, therefore, the attitude and direction indicators is both expected and obvious. After all, when an instructor covers these instruments and says to control the aircraft using the remaining gauges, you have no choice but to shift your attention and do as instructed.
Unfortunately, such training is unrealistic and poor preparation for what can happen in the real world of instrument flight. Failure of gyroscopic instruments usually occurs gradually and insidiously. It often does not become obvious that something is awry until the pilot is confronted with a bewildering display of contradictory flight information that often is accompanied by a confusing combination of G forces and airspeed noise. Such events can befuddle the senses and lead to a predictable number of fatal accidents every year.
Better training could include an airplane equipped with a concealed valve installed, say, on the right side of the right seat cushion — much as the AOPA Air Safety Foundation did for a study in 2000 (see " Safety Pilot: The 90-Percent Solution," February 2002 Pilot). In this way, an instructor could simulate failure of the vacuum pump without the student knowing about it. The attitude and direction indicators would gradually fail and truly test the ability of the pilot to recognize and cope with the problem.
When one or two gyroscopic flight instruments fail, a pilot may have precious little time to determine specifically what has failed and then to eliminate it from his scan. He cannot afford to waste time searching the instrument panel in a disorganized attempt to determine the cause of his problem. Instead, he needs to use a procedure that he knows will resolve his difficulty with a minimum of effort.
Unfortunately, I do not believe that such a procedure has been formalized. I could not locate an organized method of coping with the confusion and chaos that can result from insidious instrument failure. Each instructor seems to teach this subject in his own way. This led me to develop the following three-step plan of attack. I make no claim that it is a magic bullet, but it is better than no plan at all and is one that I have taught for years.
Step 1: Disconnect the autopilot. If an autopilot is engaged and the airplane is doing something other than what you want or expect, disconnect the autopilot immediately. There usually is more than one way to do this. Be certain that you are familiar with all of them just in case the primary disconnect switch on the control wheel fails to do the job.
Step 2: Execute a quick systems check. This involves looking at the vacuum gauge (or pressure gauge in some aircraft) and the turn coordinator's fail flag. This probably will expose the problem. If the vacuum is excessively low, the air-driven gyros are unreliable and attention must be shifted toward the remaining instruments. If the fail flag shows on the turn coordinator, then this instrument must be ignored. Be certain, of course, that you know where the fail flag is located. Familiarize yourself with it when on the ground and before turning on the master switch. Because the turn coordinator provides roll guidance for most general aviation autopilots, a fail flag may be the reason for the autopilot taking you for an unexpected ride.
For those who have an electrically powered HSI (horizontal situation indicator), a systems check is unnecessary. Electrical failure becomes obvious long before an electrical gyro spins down.
Step 3: Vote and veto. The most confusing situation develops when vacuum pressure is normal and the turn coordinator's fail flag is not showing. This means that either the attitude indicator or the heading indicator has failed independently of the vacuum system (usually for mechanical reasons). The best way to determine which of the three roll instruments (AI, DI, or TC) has failed is to use them to vote on the direction of bank. In other words, the one that disagrees with the other two should be considered to have failed and excluded from your scan. It is extremely unlikely that two of these instruments have failed simultaneously and independently in a way that would cause you to veto the only available and valid roll information.
Airline operations utilize the vote-and-veto method in a variety of ways. During an autoland approach in a Boeing 767, for example, if one coupled autopilot disagrees with the other two, it is automatically taken offline so that it cannot interfere with the safe continuation of the Category III approach and landing.
Finally, confirm that the behavior of the wet compass agrees with your choice of valid instrumentation.
It is easier to tell someone to ignore a failed attitude indicator than it is to do so. A failed AI is so prominent that trying to ignore one can be difficult and distracting. Instrument covers should be available during every instrument flight; those inexpensive little things can save your life.
Visit the author's Web site ( www.barryschiff.com).
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