All this talk about proficiency: we never seem to give it a rest, do we? For example, let’s pick on vacuum failures. The system is being eliminated by modern dual independent electrical system aircraft offered by Cirrus and Lancair, but are a major contributor to fatal accidents in instrument conditions for most of us who must still fly much older aircraft. Although I was never terribly won over by the clever pun, as it turns out it happens to be appropriate for me to make one now, because most of those arguments about partial panel proficiency have, in a way, been made inside one (a vacuum, that is).
LET THE RECORD SHOW…
At least five years ago (in 1998), the FAA announced an initial first step to achieve meaningful reductions in fatal accidents by the year 2007. Looking at the most prevalent causes of aircraft accidents, the goal of the Safer Skies program is to find the best actions to break the chain of events that lead to them. Both the FAA and the aviation industry identified vacuum failures in IMC as a major contributor. The FAA’s Civil Aeromedical Institute and the AOPA Air Safety Foundation have also done empirical studies (i.e., live volunteers in simulated failure scenarios) proving how pervasive and insidious vacuum failures actually are, in real life.
THE ACHILLES HEAL
Now, admit it: what is the most prominent and self-evident instrument on your panel when you’re flying in the clouds? Well, aside from its somewhat counter-intuitive presentation, which I have discussed in an earlier article about them (“Requiem For the Misguided,” February 2002), it’s the attitude indicator. Why? Well, it’s obvious: it gives you quick and (usually) easily interpreted feedback on both pitch and bank. Most of us use it a heck of a lot. In general aviation aircraft, both the attitude and heading indicators are powered by the forced movement of air from higher pressure to lower pressure (the “vacuum“) over a tiny windmill much like the water wheel run by a stream in a storybook sawmill. (My junior high school science teacher admonished our class way too many times for me to make the mistake of using the word “suck” when it comes to “pulling” air through a pipe, so I’m passing that psychosis on to you.) Note that it’s a single source; many aircraft do not have a standby vacuum system (or alternate source for supplying this pressure differential), nor a second attitude indicator. (I guess it’s no surprise that airliners aren’t among that group, huh? I don’t think it takes much not to need to wonder why.) They’re air driven, and powered by what we call a vacuum pump. Unlike the venerable Electrolux vacuum cleaner that usually lasts for decades, these things are good for only a few hundred hours. In a typical FBO or flying club airplane, that can potentially mean just months.
THE BENEFIT OF SHIFTY EYES
Because “vacuum failures” are so slow and insidiously hard to recognize when they occur, cross-checking of other instruments for pitch and bank information must be done constantly while in IMC. Either a turn coordinator or turn-and-bank indicator (which are usually electrically powered) are often used to cross-check the vacuum-powered instruments for bank information. We’ve covered some useful techniques pretty handily in earlier articles. (It would be great if we all had an FAA mandated blinking red light in the center of our panels to warn us of waning vacuum pressure but, perhaps foolishly, we don’t.) Unfortunately, the rate-of-turn instrument doesn’t tell you what your actual bank angle is; it just shows your rate of turn.
The Danger Of Shifty Eyes: Even when a pilot knows (or at least suspects) that something is amiss, it still takes time to decide which instrument went South, and unless the pilot actually covers the misleading instrument, loss of control can still occur. That’s especially true during subsequent partial-panel flight (particularly during the higher workload period of an instrument approach).
SO WHAT DOES UNCLE SUGAR SUGGEST?
Here’s where I’m coming from with regard to that witty (well, maybe halfway) reference to our being inside a vacuum. First, the FAA tells us what instruments are required for instrument flight in 14 CFR Part 91. In 91.205(d)(3), it says that for IFR flight, we need to have a gyroscopic rate-of-turn indicator (except for aircraft with a third attitude instrument system, usable through some pretty wicked attitudes). Also, 91.205(d)(4) states that we must also have a slip-skid indicator. But, get this: that same FAA is now suggesting that there is a “rationale for change” in substituting a second attitude indicator. To me, that’s kind of like the FAA is coming out of the closet, really. Although there are some die-hard purists who will no doubt frown on what I am about to say, my reaction is: good riddance to the limits inherent in having only the needle and ball to fall back on! Although I sometimes shake my head over the number of trees Sporty’s has killed to send me all those catalogs, it was Hal Shevers who submitted the original petition for rulemaking to the FAA, suggesting this.
Having a second attitude indicator (electrically powered, or at least with a power source separate from the primary one) instead of a rate-of-turn indicator could make an order of magnitude difference in a pilot’s peace of mind (and emotional equilibrium, when under duress). It provides more information (turn direction, bank angle, and pitch attitude, and not just turn direction and turn rate). It’s also less confusing, because, in plain English, it looks like the original picture. That’s not to say that the other (vacuum-powered, in most cases) attitude indicator could no longer lead you down the garden path, if it went Tango Uniform and you didn’t cover it up, because it could still be every bit as distracting.
THE PRICE OF PROGRESS
The down side? Well, aside from the obvious one regarding money, if you pull your rate-of-turn indicator out of your panel, you’ll lose your ability to make standard rate turns. (True, keeping a given bank angle for a given speed will yield that standard rate turn, although most of us, in a pinch, won’t be likely to recall that dividing your true airspeed in knots by 10 and adding five degrees will tell you about what that bank angle is.) But truth be told, there is little need in today’s airspace for constant rate turns, anyway. I hasten to add though, in the hope of mitigating any foaming-at-the-mouth rejoinders, that as far back as 1970 the FAA itself admitted that the rate-of-turn indicator had outlived its usefulness. And my final preemptory volley is again from the FAA’s own pen: Advisory Circular 91-75, “Attitude Indicator“, dated June 25, 2003. In it are the specific requirements (applicable to Part 23 aircraft under 12,500 pounds operating under Part 91, and having a required crew of one) spelling out just exactly what is now allowed.
THE BOTTOM LINE: The fix for safe IFR flight isn’t simple, and it isn’t cheap. An electric attitude gyro will cost well over a thousand dollars, and there’s the time and a few other things, but admit it: wouldn’t it be great to have one?