You Can’t Trust Your Instruments Except You Have To

One of the toughest things about learning to fly is learning to trust the instruments, especially when they disagree with what we see and feel. However, we have no choice; our eyes, our inner ear, and our general kinesthetic (a fancy word for “seat of the pants”) senses can all be fooled. This is one of those things that’s impossible to believe until you’ve experienced it — and then it’s impossible to forget.

Whether you’re really experiencing disorientation in the clouds, or, hopefully, just practicing under the hood with an instructor, the first time you catch your body lying to you is jarring, indeed. Given the right sort of input, everything from your brain on down will line up in lockstep agreement that your airplane is flying straight and level when, in actuality, you might be in a steep spiral.

That’s powerful stuff, and has to be taken absolutely seriously; luckily for us, we can learn to ignore those sensations and trust our instruments.

But there are times when we shouldn’t.

Consider what I call the basic seven pack: the airspeed indicator, attitude indicator, altimeter, turn coordinator, heading indicator, vertical speed indicator (VSI), and, number seven, the magnetic compass. Each of these gauges has quirks and is subject to untrustworthy behavior under certain circumstances, usually involving a systems failure but not always.

The airspeed indicator has errors built in that you have to account for when determining calibrated and then true airspeed, and the pitot-static system that powers it is subject to failures due to ice and other blockages. The altimeter and VSI are also subject to problems with the static half of that system. The turn coordinator needs electricity, so electrical problems can cause it to start lying to you. The heading indicator wanders over time, and must be regularly reset to match the magnetic compass, just not when you’re accelerating, decelerating, or turning, because that’s when the compass has problems of its own. The attitude indicator is one of the most trustworthy, but, because its rare failures are usually gradual, leading you into a spiral descent or something similar, when it does let you down it’s extremely serious.

If every gauge has some propensity for error or at least some risk of all-out failure, however unlikely, then what do we do about this?

Since you can’t completely trust anything, you have to trust everything.

That is, you have to trust the total of what every instrument and all of your senses are telling you. If you’re descending at 500 fpm, your power is set at 2400 rpm, and your airspeed indicator reads zero, you know it’s time to stop trusting the airspeed indicator (and, probably, turn on the pitot heat.) If your attitude indicator shows you straight and level, but your airspeed is increasing, your altimeter, VSI, and your inner ear tell you that you’re descending, and your compass, turn coordinator, and the view out the window all tell you that you’re turning, it’s time to stop trusting the attitude indicator.

You get the idea.

Review, analyze, and compare, and then, if you find that one thing disagrees with all the others, trust it less, or not at all. The concept is usually called crosschecking, and it’s one of the most important skills you can develop as a pilot.

This seems complicated and even a little counterintuitive at first, but I think it’s one of the most interesting things about flying. The process of scanning the instruments, looking outside, asking yourself if everything makes sense, then repeating if it does, and investigating further if it doesn’t, is part of what makes flying one of the most “in the moment” activities I’ve ever come across. The constant challenge/response cycle of asking yourself, “Is everything good?” and then answering, “Yes, it’s all good,” keeps you engaged. Anyway, after you’ve flown for a while, it all becomes absolutely second nature. 

Trust me.

Except when I’m wrong.

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