1 Flight Instruments To give pilots an accurate indication of their current flight condition, planes come equipped with numerous flight instruments. Similar to the gauges in a car, pilots can use their instruments to determine such things as: how fast they are going, how high they are, the direction they are heading, et cetera. If you have ever seen the inside of an airplane, you’d know there can be a lot of instruments, and, at first glance, it can look a bit overwhelming. We’ll organize the chaos into smaller, manageable groups of instruments and what they’re showing, and how they work. We’ll start with the old analog round dial instruments, and then move over to the newer advanced avionics and glass cockpit systems like the G1000. This lesson will cover: • Pitot-Static Instruments • Gyroscopic Instruments • The Magnetic Compass • And the Garmin G1000 The Pitot-Static System The pitot-static system is connected to the airplane’s altimeter, airspeed indicator, and the vertical speed indicator. These instruments tell the pilot how high they are, how fast they are going, and how fast they climb or descend. These are determined by measuring the pressure of the atmosphere. Before we go any further, let us become familiar with the atmosphere. If we take a column of air, starting at the ground, and traveling up all the way into space, you’d notice that the molecules of air at earth’s surface are much more compacted together than the molecules up in space. The reason for this is gravity. Since air is matter, it has weight. So, all the air molecules at the top of the column are pushing down on the molecules below them, which compresses them. This compression results in higher pressure at sea level compared to the pressure on top of a mountain. How much of a pressure change are we talking about? Well, if we say that the atmosphere goes up to about 375 miles above the surface of the earth, 50% of those air molecules would be found in the first 18,000 feet of the surface. So, because the pressure of the atmosphere decreases the higher up we go, our Pitot-Static Instruments are able to measure that pressure and calculate our altitude and speed. The pitot-static system gathers its pressure information from two sources: the pitot tube, and the static port…hence the name Pitot-Static. The pitot tube is designed to measure the pressure of the air as the airplane flies through it. In most smaller aircraft, this tube is located under the wing so that it can measure the flow of air without any interference. Sometimes aircraft manufacturers will use a pitot mast instead of the tube shape, but it still functions exactly the same way. On the back side of the tube, there is a drain hole, which allows any rain or water that is collected while flying, to drain out and not go into the system. Finally, the pitot tube can also be heated. This is used to prevent ice from forming on the tube, which could potentially block the hole, and prevent the system from functioning correctly. The counter-part 1 2 to the pitot tube is the static port. The location of this port will vary with different aircraft designs, but should be in a location where it can measure the static pressure of the air, unaffected by the dynamic airflow around the airplane. On the Cessna 172, the static port is located on the left side of the forward fuselage. The pitot tube and static port openings are connected to tubes that join into the pitot-static instruments. The pressure inside the instruments match the pressure of the outside atmosphere. All three pitot-static instruments connect to the static port, but only the airspeed indicator connects to the pitot tube. Sounds simple right? Let’s discuss how each pitot-static instrument works. Altimeter Perhaps the most basic of all the pitot-static instruments is the altimeter, which displays the airplane’s altitude. The instrument contains a set of aneroid wafers, which expand and contract based on the pressure. The air inside the wafers is trapped, but the air in the rest of the case is able to change to match the pressure from the static port. As we increase altitude, the static pressure goes down. This means that the air inside the case will escape out the back, and result in there being less air pressure in the case compared to the wafers. Because of this, the wafers will expand until both pressures are equal. Getting the wafers to result in an altitude readout is done through a series of gears, pinions, arms, and levers, also known as the mechanical linkages. These linkages will rotate the hands on the face of the instrument and show the airplane’s altitude. Now, when the airplane descends the opposite happens. Descending to a lower altitude results in a higher static pressure. Air from the static port will now enter the case of the instrument and squeezes the aneroid wafers until both the case pressure and the wafer pressure are equalized. The mechanical linkages will then rotate the hands on the face, to show a lower altitude. The face of the altimeter contains three hands: the 10,000 foot, 1,000 foot, and 100 foot hands. These hands move clockwise and counterclockwise to display the appropriate altitude. Most altimeters in smaller aircraft will only work up to around 20,000 feet, but those airplanes usually can’t get that high anyway. Here are some examples of altitudes. • 3,000ft • 8,400ft • 12,000ft • 5,280ft This altimeter is actually called a sensitive altimeter, not because you have to hug it every once in a while, but because it can be adjusted for the current atmospheric pressure. Because the pressure at any given point on earth never stays the same, altimeters would always read incorrectly. Fortunately, pilots can correct this issue. Once the pilot knows the current atmospheric pressure, also known as the altimeter setting, all they have to do is rotate the dial on the lower left side of the instrument until the current pressure is selected in the little window on the face, called the Kollsman Window. This then realigns the gears inside, and the instrument reads accurately. The realignment is accomplished by rotating the entire inside mechanics of the instrument. 2 3 There are five types of altitudes that pilots interact with on a daily basis; but an altimeter typically is only going to display one: the height above Mean Sea Level, or in other words, the height above the average sea level. An indication of 3,000 on the altimeter is read off as 3,000 feet MSL. The other types of altitude include: • AGL, or Above Ground Level • Pressure Altitude • Density Altitude • And, finally, True Altitude As the name implies, an AGL altitude is the vertical distance between the aircraft and the ground below. This is simply just the MSL altitude minus the terrain elevation. Pressure and Density altitudes are theoretical altitudes used to calculate the performance of the airplane. Simply put, it’s the altitude that the airplane performs like it’s flying at. This is based on the pressure and temperature of the atmosphere. More of this will be explained in the Performance lesson. Finally, there is True Altitude, which, similar to Pressure and Density altitudes, takes both the pressure and temperature of the air into account to give a more accurate reading. Altimeter Errors Although the altimeter is very reliable, it is susceptible to a few errors. Mechanical Error – Every time you begin a flight you will check the accuracy of your instruments. When checking the altimeter, it should not be more than 75 feet off the airport’s elevation, after setting in the correct altimeter setting. If you notice that it is off by more than 75 feet, the instrument should be checked by an appropriate maintenance technician. The altimeter can read incorrectly as a result of a pressure change. If the altimeter setting is not corrected when flying to an area of lower pressure, the airplane will then be lower than what the altimeter is reading. This could also pose a threat for mountain or obstacle clearance. There is a saying to remember: When flying from an area of High to Low, look out below. Similar errors can also happen based on the temperature of the air. When the temperature is warmer than standard, the air is less dense and the pressure levels are farther apart. What this means is that when the airplane’s altimeter is reading 5,000 feet on a warm day, that airplane’s true altitude is actually higher than 5,000 feet. The opposite is also appropriate on a cold day. Since cold air is more dense, the pressure levels are closer together, so an altimeter reading 5,000 will be at a true altitude that is lower than 5,000 feet. This could be a bad thing when the airplane is flying in an area with mountains or obstacles. So, also remember the saying: When flying from HOT to COLD, look out below. 3 4 Vertical Speed Indicator Another instrument that uses only the information from the static port is the vertical speed indicator, more commonly called the VSI. The VSI measures the vertical speed of the aircraft in terms of feet per minute. This is accomplished by comparing the current pressure of the air with the pressure of the air from a few seconds ago.