1.H
Aviation Merit Badge
Digital Resource Guide
Explain the purposes and functions of the various instruments found in a typical single-engine aircraft:
Attitude indicator, heading indicator, altimeter, airspeed indicator, turn and bank indicator, vertical speed indicator, compass, navigation, communication, and engine performance indicators.
Let’s look at the functions of these fundamental and important flight instruments.
Attitude Indicator
Function: The attitude indicator or “artificial horizon” lets the pilot get an immediate picture of the airplane’s attitude, which is its position relative to Earth’s horizon. The attitude indicator shows both bank (roll) attitude, which is the relationship
between the wings and the horizon, and pitch attitude, which is the relationship between the nose and the horizon.
How it works: Displays a symbolic airplane and a horizon bar. Attached to a gyroscope is a face with a contrasting horizon line on it. This line represents Earth’s actual horizon. This representation of the horizon moves to show the plane’s position with respect to the real horizon. If the plane is level, the airplane symbol is centered with the horizon bar. If the plane is climbing, descending, or turning, the airplane symbol moves up, down, or tilts.
Use: Helps pilots maintain correct flight of the aircraft, specifically pitch and roll, especially when flying in poor visibility.
Heading Indicator
Function: The heading indicator can take different forms but is basically a gyroscope that shows the plane’s heading. The simplest heading indicators have to be set to match the magnetic compass.
How it works: A dial with headings (0° to 360°) rotates to show the current heading of the aircraft.
Use: Helps pilots navigate in straight lines or maintain turns at specific headings.
Altimeter
Function: The altimeter tells the pilot how high the aircraft is flying. To steer clear of mountains, buildings, and such obstructions as television towers, the pilot must know the altitude at all times. Charts and air traffic rules indicate the minimum heights pilots must maintain. There are also specific altitudes to fly based on the direction of flight, which reduces the risk of collisions when pilots are flying by visual flight rules in good weather. During times of reduced visibility, pilots are assigned altitudes by air traffic control.
How it works: The altimeter is simply a barometer that measures the air pressure and converts that measurement into altitude. Altitude references are generally above sea level, so the pilot has to know the height above sea level of the terrain or obstructions to be sure the plane is at a safe altitude. The altimeter has a knob for adjusting the instrument to take into account changes in barometric pressure at different points of the flight as reported by air traffic control.
Use: Allows pilots to monitor and adjust altitude during climbs, descents, and level flight.
Airspeed Indicator
Function: The airspeed indicator is the airplane equivalent of a car’s speedometer, telling the pilot how fast the plane is traveling through the air.
How it works: Like the altimeter, the airspeed indicator works by measuring air pressure, but the airspeed indicator measures the plane’s impact on the air (ram air pressure). In other words, the airspeed indicator registers the velocity of air molecules striking a sensor as the airplane moves through the air. This is translated into speed in knots, or nautical miles per hour, the standard unit of velocity used in aviation.
Use: Helps pilots stay within safe operating speeds for takeoff, landing, and cruising.
Turn and Bank Indicator
Function: The turn and bank indicator is two instruments in one. It tells the pilot when the plane is turning and how well the turn is being executed whether there is too much or too little bank for the rate of turn. The pilot may also check for balance and coordination in straight and level flight. The turn needle always deflects in the direction of the turn and indicates the rate at which the aircraft is turning about its vertical (yaw) axis. Most modern airplanes have a variation of this instrument called a turn coordinator. It looks a little like an attitude indicator but gives information only about turn, not about pitch attitude.
How it works: The ball part of the turn indicator is simply an agate or steel ball that moves freely inside a curved, sealed glass tube filled with liquid. The lowest point of the glass tube is in the middle of the instrument. In straight and level flight, gravity keeps the ball there, centered between two lines. In a turn, if the aircraft is neither slipping nor skidding, the ball will be kept centered by centrifugal force. If the aircraft were in a slip (the tail sagging into the turn), the ball would fall to the low side of the instrument. In a skid (the tail swinging wide outside the turn), the ball would be to the high side.
Use: Helps the pilot avoid uncoordinated flight, which can lead to a skid or slip.
Vertical Speed Indicator (VSI)
Function: In addition to knowing the airspeed, the pilot must know how rapidly the aircraft is climbing or descending. The vertical speed indicator, or VSI, registers how fast the barometric pressure is changing and converts this information to a speed measured in hundreds of feet per minute. This instrument is important because it is difficult to judge rates of climb or descent using only our human senses. In addition to the flight instruments, the tachometer, the oil pressure gauge, and the temperature gauges also tell the pilot how the plane’s engine is performing.
How it works: Measures changes in static air pressure as the aircraft climbs or descends. The needle moves to show rate of climb (upward) or descent (downward).
Use: Helps pilots monitor and adjust the rate of climb or descent.
Compass
Function: As a Scout, you are familiar with the magnetic compass and probably have used one many times. The compass used in aircraft is not much different, although flight poses special problems in reading a compass. You may know that the magnetic pole is not at the North Pole, or the exact top of Earth. Instead, it is around 800 miles away, which leads to variations in determining true headings. Second, Earth is not uniformly magnetized; in some areas, the compass may vary many degrees from magnetic north. Finally, the metal and electrical equipment within an aircraft can throw off the compass. This variation is known as magnetic deviation. A pilot must consider variation and deviation, as well as wind, when determining what compass reading will get the airplane to its destination.
How it works: Like your Scout compass, it uses Earth’s magnetic field to point to the magnetic north.
Use: Gives pilots a simple, reliable heading reference, especially in turns or when the heading indicator isn’t working.
Tachometer
Function: You may have seen a tachometer on the dashboard of a car. Its purpose is to tell the driver exactly how fast the engine is running. In an airplane with a piston engine and a fixed-pitch propeller, the tachometer has two main purposes: to show whether the propeller is turning at the recommended speed for a particular maneuver and to indicate whether the engine is operating normally. For example, the airplane’s designer might have determined that the best cruise power setting for the engine is 2,300 revolutions per minute (rpm), so the pilot would set the throttle accordingly while cruising. The designer would also have recommended certain rpm settings for climbing and descending.
How it works: Every time a spark plug fires (which happens every time the engine completes a cycle), a small electrical pulse is sent to the tachometer. A tiny computer inside the tachometer counts how many times this happens per second and converts it to RPM. The more sparks per second, the faster the engine is spinning, and the tachometer needle moves up.
Use: Helps pilots monitor and adjust the speed of the engine, to add (or remove) lift.
Oil Pressure Gauge
Function: An airplane’s oil pressure gauge does the same thing as the oil pressure gauge in a car. It shows the pilot the pressure of the oil in the engine, which reveals a great deal about the health of the engine. Dropping oil pressure is a sure sign of engine trouble.
How it works: The engine has an oil pressure sensor (called a sending unit). Inside the sensor is a small diaphragm and a variable resistor (a tiny electrical part that changes resistance based on pressure). When oil pressure increases, the diaphragm presses on the resistor, changing the electrical signal sent to the gauge. The gauge reads this signal and moves the needle accordingly. If oil pressure is too low, the gauge (or a warning light) alerts the pilot that something is wrong with the engine. Oil is critical to the proper functioning of an engine, so this issue must be addressed immediately.
Use: Helps pilots monitor the health of the engine.
Temperature Gauges
Function: The temperature gauges are another indicator of the engine’s health. They measure the temperature of oil and the cylinder heads and show whether the engine is running well, too warm, or too cold. The instruments are generally marked with a green area and a red line. If the needle is “in the green,” that is good. If it passes the red line, there is a problem because that line marks the maximum allowable operating temperature.
How it works: A temperature sensor (called a thermistor) is placed in the engine’s coolant or oil. Inside the sensor is a special resistor that changes its electrical resistance as it gets hotter. The gauge reads this resistance and moves the needle accordingly. If the temperature gets too high, the gauge (or a warning light) alerts the pilot that something is wrong with the engine and must be addressed as soon as possible.
Use: Helps pilots monitor the health of the engine.
Here is a video that walks you through the instrumentation on a Cessna airplane.
What is the difference between traditional “steam gauges” and a modern “glass cockpit”?
Traditional Steam Gauges
- Imagine a bunch of separate clocks on the wall. Each clock tells you something different—one shows how high you’re flying (altimeter), another shows how fast you’re going (airspeed indicator), and another shows which way the airplane is pointed (heading indicator).
- You have to keep looking at all these clocks one by one to understand what’s going on with the airplane. This is how traditional steam gauges work.
Modern Glass Cockpits
- Now, imagine a big, fancy tablet or TV screen that shows all this information in one place. It’s like a super-smart video game interface that combines everything into a single display. It might even show you cool stuff like a map, weather, and what the terrain looks like ahead of you.
- This is how glass cockpits work. They’re digital and show more information in an easier-to-read way.
Here is a helpful video that shows how a glass cockpit works.
Here is a helpful video that shows how a glass cockpit works.
In summary
These instruments are key to flying safely and efficiently, as they provide essential information about the aircraft’s attitude, direction, speed, altitude, and movement. They also help the pilot monitor the overall health of the engine – which is extremely important, especially when they are mid-air!
In summary
These instruments are key to flying safely and efficiently, as they provide essential information about the aircraft’s attitude, direction, speed, altitude, and movement. They also help the pilot monitor the overall health of the engine – which is extremely important, especially when they are mid-air!
Hopefully, Requirement 1 has demonstrated the basic mechanics of how an aircraft flies through the air.
Now, experience the thrill of flight for yourself!
