The Six Pack of Flight Instruments It seems as if though cockpits of modern jets are becoming more and more intricate and complex. The have added greatly to the primary flight instruments and have filled vast amounts of space in cockpits. It is still very relevant to the aircraft for which it is designed, but lets get back to basics. Lets cut out all of the excess and focus on a few primary instruments, instruments that one would be guaranteed to find no matter the plane. These are the instruments that make up the “Six Pack” (Learn to Fly, 2013). The “Six Pack” is comprised of the six basic flight instruments that are used by pilots. These instruments are typically found in two rows of three. Going from left to right the instruments that make up the “Six Pack” are the air speed indicator, attitude indicator, altimeter, vertical speed indicator, heading indicator, and turn coordinator. These instruments are the main source of information for pilots. These instruments can be divided into two categories based off of how they work, which are static or pitot-static and gyroscopic instruments. The instruments that fall into the static category are the airspeed indicator, altimeter, and vertical speed indicator. The airspeed indicator shows the aircraft’s airspeed in knots relative to the surrounding air. This is known as true airspeed and can be very valuable information for flight planning. It is also usually just a few knots different than indicated airspeed in small aircraft. As most aircraft instruments airspeed indicators are color coded, enabling a pilot to look at them and know instantly if they are in a normal operating condition or even an extreme condition to where changes must be made. This instrument has a white arc for it’s flap operating range, a green arc for normal operating ranges, a yellow arc for a caution range, and a red line for a never exceed speed, which could result in damage or structural failure if exceeded. There are also markings for important speeds known as V-speeds as well. The pneumatic version of this instrument basically works by comparing ram air pressure to static air pressure. When the aircraft is on the ground these two air pressures are equal. As the aircraft moves through the air the pressure in the pitot tubes become greater than that of the pressure in the static lines. This difference is then registered by the needle on the face of the instrument and is calibrated in knots. An example of redundancy can be that some systems have multiple lines that can direct ram air pressure to the static air pressure, just in case one of them were to get clogged. The altimeter shows the pilot height above mean sea level. This instrument actually measures changes in barometric pressure. It is important to calibrate this instrument because it is affected by the current weather conditions. This system works by comparing the static pressure inside of a sealed aneroid capsule to the changing pressure around it. As the aircraft makes changes in elevation the outside air pressure is constantly being compared to the static air pressure that is contained in this aneroid capsule. This change is then transmitted though a means of a calibrated mechanical linkage and pointer and is shown on the face of the instrument. Redundancy for this instrument is to have two altimeters, a main one and a backup. Usually if there is a 300ft difference between the main and the backup that means that there is a problem somewhere not necessarily that the altimeter is broken. Altimeters have to undergo testing to ensure that they are in an airworthy condition every 24 calendar months according to FAR Part 91 Section 411. The last basic instrument that is part of the pitot static category is the vertical speed indicator. This instrument measures the aircraft’s rate of climb or descent. It usually is depicted in feet per minute. During level flight the vertical speed indicator needle will point to “0” feet. The vertical speed indicator is a differential pressure instrument. It has two lines where it receives static pressure through the aircraft’s pitot-static system. These lines are routed to the diaphragm and case of the instrument. The case of the instrument is connected through a restricted orifice, which is a calibrated leak. When the aircraft climbs or descends, the pressure inside the diaphragm changes immediately, but due to the metering action of the restricted passage, the case pressure remains higher or lower for a short time, causing the diaphragm to contract or expand. This pressure differential is indicated on the face of the instrument through the instrument needle and is shown as a climb or descent. The instruments that fall into the gyroscopic category are the attitude indicator, heading indicator, and turn coordinator. The attitude indicator is located in the center of the top row of the “Six Pack” and can be considered the most important instrument for pilots. At a quick glance a pilot can tell exactly where there aircraft is going, whether it be straight and level, climbing, descending, or turning. The face of this instrument has a small aircraft and an artificial horizon that can show the attitude of the aircraft. It shows even the smallest changes in attitude. How it works is that the artificial horizon senses movement from the gyroscope and remains suspended in relation to a self-erecting gyroscope, which holds its position in reference to the actual horizon. The gyroscope itself can be vacuum-driven or electric. There are a few forms of redundancy for attitude indicators. Most aircraft come with a second attitude indicator, others have the backup electrically powered, and some have air pump redundancy with an electric or engine driven pump. Next there is the heading indicator. The heading indicator provides directional information to the pilot in a way similar to a magnetic compass. The face of the instrument depicts a small airplane and as the aircraft moves left or right the gyroscopic instrument around the small depicted airplane moves while the small airplane moves with the actual aircraft. In a left turn the small airplane will rotate slightly to the left while the heading indicator (the gyroscopic instrument) will move towards the right showing the new heading. The heading indicator will show a number between zero and 359 degrees on a compass card. The gyroscopic instrument can be either pneumatically or electrically driven and the operation of the heading indicator depends upon the principle of rigidity in space. This system can also have air pump redundancy with an electric or air driven pump. Lastly there is the turn coordinator. This instrument provides the pilot with information of the direction and rate of a turn. In addition to that it tells the pilot if the aircraft is slipping or skidding during a turn. The face of the instrument has a picture of a small airplane that looks like its heading towards you. Underneath it is an inclinometer, which looks like a ball in a tube. There are two lines for the ball to stay in which would indicate during a turn that the aircraft is going through a coordinated turn. The picture of the aircraft will also rotate to whichever direction it is turning and the ball will go to the left or the right depending if there is slipping or skidding occurring. The gyroscopic instrument behind this can either be pneumatically or electrically driven and once again goes off of the principle of rigidity in space. It maintains it’s position and as the aircraft moves around it, this movement is translated to the face of the instrument. The “Six Pack” are the typical flight instruments that are used by pilots. Even the newer more modern planes have these instruments one way or another. Some are electronic and are projected through a monitor and others can be a newer version of these instruments. They are either pitot-static or gyroscopically driven. These are the instruments that are necessary for a pilot to be able to fly an aircraft. References
Air Sea Tac Virtual. (2013). Basic Flight Instrument. Retrieved from http://www.airseatac.com/academy/academy.instruments.basic.php
Federal Aviation Administration. (2008). Pilot's Handbook of Aeronautical Knowledge. Oklahoma City, OK: United States Department of Transportation, Federal Aviation Administration, Airman Testing Standards Branch.
Learn to Fly. (2013). Six Pack-The Primary Flight Instruments. Retrieved from http://www.learntofly.ca/six-pack-primary-flight-instruments/