Basic Instrument Panel


What are those gauges and needles telling you?

By Peter A. Bedell

At first sight, the instrument panel of even the smallest airplane can be an overwhelming sight. An array of gauges, dials, knobs, and digital displays can make newcomers wince at what faces them. It's comforting to note, though, that most of the instruments found in training aircraft are surprisingly simple and user-friendly.

The most important instrument in the initial training phase is the big transparent one over the glareshield, or dashboard if you will. The windshield is the primary flight instrument for visual flight rules (VFR) flight. In VFR conditions, the airplane's instruments are used as references rather than a reliance in maintaining proper flight attitude. Although it's tempting to stare at all those little needles and gauges winding away as the airplane climbs and turns, the windshield, with its built-in horizon, provides the best reference to the airplane's attitude.

When the weather gets bad and the windshield becomes opaque with clouds, we rely solely on the information that the instruments provide in order to maintain the proper flight attitude and desired course to get us to our destination.

Let's take a look at the cockpit layout of a typical trainer. The center column in a Cessna 172, for example, has the fuel selector valve at the bottom just above the floor, the elevator trim wheel halfway up, and the power controls (throttle and mixture) at the top of the center column near the pilot's right knee. Unlike the car, the throttle is operated with your hand, and ground steering is performed by your feet. Taxiing an airplane is a little awkward at first but comes easily with time.

Above the power controls, in the center of the panel, is the radio stack. In basic trainers, there is generally a communications (com) radio and a navigation (nav) radio. In aircraft equipped for IFR (instrument flight rules, which means able to fly in the clouds we just mentioned) flight, there are generally two com and two nav radios as well as a host of other navigational equipment.

Directly in front of the pilot are the flight instruments. There are six principal instruments: the altimeter, directional gyro or heading indicator, attitude indicator, vertical speed indicator, turn coordinator, and airspeed indicator. These instruments ensure that the airplane is at the desired pitch and bank whether it be a descending, left turn or just straight-and-level flight. These instruments are so important that they are found in virtually every powered aircraft, from the smallest trainer to the space shuttle.

The attitude indicator, or artificial horizon, displays your flight attitude, or what you should see out the windshield if the weather were to allow it. It has blue shading on the top, depicting an artificial sky, and a black or brown bottom, representing the ground. In between is the horizon bar. A fixed miniature airplane lies in the middle of the instrument, giving the pilot a tail view of what the airplane's attitude is. Markings along the rim of the instrument depict degrees of bank. If the miniature airplane's wing is in line with the third mark, the airplane is in a 30-degree left or right bank. This instrument runs on gyroscopic, vacuum, or electric power, and in the most sophisticated aircraft, the pilot may be looking at the depiction on a TV-style, miniature cathode-ray tube.

In most airplanes, the altimeter lies to the right of the attitude indicator. The altimeter is user-friendly even to the newcomer. It reads just like a clock; the big hand marks hundreds of feet, and the little hand marks thousands of feet, just like minutes and hours on a clock. For those training in the Rockies, or those who happen to get some stick time in a high-flying airplane, there is a thin marker needle that depicts altitude in tens of thousands of feet. Some altimeters have a combination digital and analog display, but most altimeters in training aircraft will be strictly analog.

Underneath the altimeter is the vertical speed indicator (VSI). It tells the pilot what rate the airplane is climbing or descending in feet per minute (fpm). Although the VSI in our general aviation trainers reads up to 2,000 fpm, we'll be lucky to see the needle sweep past 1,000 fpm in a climb.

To the left of the VSI, and under the attitude indicator, lies the directional gyro (DG), often called the heading indicator. This instrument simply tells us what our heading is. Because the magnetic compass, another very important primary flight instrument (usually attached to the glareshield or windshield), floats in fluid and sloshes around during turbulence and maneuvering and naturally lags or leads turns, depending on direction of the turn, it's hard to get an idea of what your heading really is. This is when it is nice to have the DG. Not only is it easier to look at, it doesn't lead, lag, or slosh around like the compass. The DG cannot substitute for the compass, though. It does not have a north-seeking magnet and therefore must be set to match the compass heading before takeoff. When on the end of the runway, for example, if you are lined up with the centerline, the runway numbers should approximately match your heading, so set the DG to the runway numbers before starting your takeoff run. The DG is subject to precession, which causes it to wander over a period of time. About every 15 minutes, if the airplane is straight and level, check and reset the DG to the compass heading.

Usually to the left of the DG is the turn coordinator. It has two components. One looks like a miniature airplane viewed from behind. In a turn, the miniature airplane banks in the direction the real airplane is turning. You may ask, "Why have an instrument that shows the airplane's bank when the attitude indicator already does the job?" Good question. The answer is that by banking the real airplane so that the miniature airplane's wings are lined up with the turn coordinator's lower reference marks, you can use the instrument -- along with a stopwatch -- to make timed turns. Line the little airplane up with the marks, and you'll turn at 3 degrees per second. Hold that bank, and in 15 seconds, your heading will change by 45 degrees; in 30 seconds, you'll make a 90-degree turn.

Why go through all these gyrations? In case the heading indicator and/or attitude indicator fails. With the turn coordinator, a stopwatch, and the trusty old magnetic compass, you can turn to specific headings with a fair degree of accuracy. So the turn coordinator -- which is electrically powered -- is a backup in case the vacuum-driven attitude and heading indicators give up the ghost.

The other component of the turn coordinator is the inclinometer, better known as the rudder ball. This is a black ball in a liquid-filled, curved and sealed tube. The ball is there to help keep your turns coordinated. That is, to prevent the airplane's tail from skidding to the outside of a turn or slipping to the inside. In a slip, the ball will roll opposite to the direction of the turn; In a skid, the ball moves to the inside of the turn. To prevent slips or skids, pilots are told to "step on the ball," meaning apply pressure to the rudder corresponding to the ball's location. Ball left? Step on the left rudder until the ball centers.

Our final important flight instrument, the airspeed indicator, is easy to relate to. Much like your car's speedometer, it tells you how fast you are going in either miles per hour or knots. If knots confuse you, just remember that for every 100 knots, add 15 to get your speed in miles per hour. The average Cessna cruises around 100 knots, or 115 mph. Unlike cars, an airplane's groundspeed is affected by winds, and our actual speed over the ground will vary, depending on the winds aloft.

Airspeed indicators are color coded for quick recognition. The green arc depicts normal airspeed operations. The yellow arc is a danger zone and can be penetrated only in smooth air. A red line depicts the "never-exceed speed," the speed at which structural failure may occur. Lastly, the white arc depicts the safe speed zone for deployment of flaps.

Finally, there are engine instruments, and once again, like your car, they are designed for a quick scan to make sure the needles are in the green. Most general aviation trainers have an analog tachometer mounted near the primary instruments as well as an amperage meter, fuel gauges, and oil pressure and oil temperature gauges.

Although a guided tour of a particular airplane by a certificated flight instructor is the best way to become accustomed to the instrument panel, this quick and dirty description will undoubtedly help decipher some of the initial shock of seeing all those gauges for the first time.


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