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Ground School · PHAK Chapter 8
Flight Instruments Explained: The Six Pack, The VSI And Glass Cockpits
Six flight instruments tell a pilot everything about the airplane’s attitude and performance. Three run on air pressure from the pitot-static system: the airspeed indicator, the altimeter and the vertical speed indicator. Three run on gyroscopes: the attitude indicator, the heading indicator and the turn coordinator. A primary flight display shows the same six on one screen. Here is how each one works, how each one fails, and how we teach them at Van Nuys.
Six Instruments, Two SystemsPitot-Static And GyroGarmin G300 GlassRedbird MCX Practice
The Redbird MCX flight deck at Van Nuys, where students practice the instrument scan before and between flights.
6flight instruments in the classic six pack, in two rows of three
6–9 stypical VSI lag before it shows a stable rate, per the PHAK
15 minhow often to check the heading indicator against the compass
75 ftaltimeter error at which accuracy is questionable on the ground
The Short Answer
What Are The Six-Pack Instruments?
The aircraft six pack is the standard layout of six flight instruments: airspeed, attitude and altimeter on the top row, turn coordinator, heading and vertical speed on the bottom row.
The layout is called the basic T, plus two. Attitude sits in the center of the top row because it is the instrument you look at most. Airspeed is to its left, altitude to its right, and heading directly below. The turn coordinator and the vertical speed indicator fill the corners of the bottom row. The same arrangement survives on every primary flight display, with tapes in place of dials.
The six divide by the system that drives them, and that split is the most useful thing to learn first. When something fails, the system tells you which instruments to stop trusting. The Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 8, is the FAA’s reference for all of it, and the Instrument Flying Handbook (FAA-H-8083-15B) covers how to fly by them.
| Instrument | Driven By | What It Tells You | Classic Failure Sign |
|---|---|---|---|
| Airspeed Indicator | Pitot and static pressure | Indicated airspeed, with colored arcs for flap, normal and caution ranges | Reads zero, or acts like an altimeter, when the pitot tube blocks |
| Attitude Indicator | Gyro (usually vacuum in older trainers) | Pitch and bank against an artificial horizon | Slow tilt or tumble as the gyro spins down |
| Altimeter | Static pressure | Height above mean sea level, corrected by the Kollsman setting | Freezes at one altitude when the static port blocks |
| Turn Coordinator | Gyro (usually electric) | Rate of roll and turn; the ball shows coordination | Warning flag, or no response in a turn |
| Heading Indicator | Gyro (usually vacuum in older trainers) | Magnetic heading, without compass swing | Slow drift from the compass, called precession |
| Vertical Speed Indicator | Static pressure | Climb or descent rate in feet per minute, and the trend | Stuck at zero when the static port blocks |
Pitot-Static System
How The Airspeed Indicator, Altimeter And VSI Work
Two sources of air drive three instruments. The pitot tube faces into the wind and measures ram pressure. The static port sits flush on the side of the airplane and measures still air pressure.
The airspeed indicator is the only instrument that uses both. Inside the case a diaphragm fills with pitot pressure while static pressure surrounds it. The difference, which is dynamic pressure, moves the needle. The colored arcs come from the aircraft’s limitations: white for the flap operating range, green for normal operation, yellow for the caution range in smooth air only, and a red line at the never-exceed speed.
The altimeter uses static pressure alone. Sealed aneroid wafers expand as the airplane climbs into lower pressure, and gearing turns that into feet. The small window on the face, the Kollsman window, takes the local altimeter setting. Set it wrong by one inch of mercury and you are about 1,000 feet off. Under 14 CFR 91.121, below 18,000 feet you set a station within 100 nautical miles; Van Nuys gives it on the ATIS.
The vertical speed indicator also uses static pressure alone, in a way the next section explains.
What Each Blockage Does
| Blockage | Airspeed | Altimeter | VSI |
|---|---|---|---|
| Pitot tube blocked, drain hole open | Drops toward zero | Normal | Normal |
| Pitot tube and drain hole blocked | Acts like an altimeter: rises in a climb, falls in a descent | Normal | Normal |
| Static port blocked | Reads low in a climb and high in a descent | Frozen at the altitude where it blocked | Frozen at zero |
| Alternate static source selected (unpressurized cabin) | Reads slightly high | Reads slightly high | Shows a brief climb, then settles |
The dangerous one is the fully blocked pitot tube, because the airspeed looks plausible. In a climb it shows more speed than you have, and a pilot who believes it can pitch up toward a stall. The PHAK walks through each case. The practical defense is pitot heat in visible moisture and a cross-check: if attitude and power have not changed, airspeed should not have changed either. For a blocked static port, use the alternate static source if the airplane has one.
Vertical Speed Indicator
How The Vertical Speed Indicator Works, And Why It Lags
The vertical speed indicator shows the rate of climb or descent in feet per minute. It is a trend instrument first and a rate instrument second, and learning the difference saves a lot of chasing.
Inside the VSI is a diaphragm connected directly to the static line, sitting inside a sealed case that is also connected to static pressure, but through a tiny restriction called a calibrated leak. When the airplane climbs, pressure inside the diaphragm drops at once while pressure in the case drops slowly through the leak. The diaphragm contracts, and the needle points up. Level off, and the two pressures equalize and the needle returns to zero.
That design produces the lag. The PHAK notes that the VSI shows the trend immediately but takes roughly 6 to 9 seconds to settle on an accurate rate. Pilots who try to fly the needle end up chasing it, overcorrecting each time. The Instrument Flying Handbook classes the VSI as a pitch instrument: the first movement tells you the nose has moved, and you correct with the attitude indicator, not with the VSI.
Some aircraft carry an instantaneous VSI, or IVSI, which adds small accelerometer-driven pumps to remove most of the lag. On a primary flight display the vertical speed is computed by the air data computer and shown as a tape or a numeric box beside the altitude tape, and it responds much faster.
Two practical uses come up on every lesson. On approach, 500 feet per minute is a common descent rate on a stabilized final, and the VSI is where you check it. On the Private Pilot Checkride, holding altitude within 100 feet in steep turns is far easier if you catch the trend on the VSI.
Gyroscopic Instruments
The Attitude Indicator, Heading Indicator And Turn Coordinator
A gyroscope is a spinning wheel that resists being tilted. Two properties make it useful: rigidity in space, and precession.
01
Attitude Indicator
A gyro spinning on a vertical axis holds its position in space while the airplane pitches and rolls around it. The horizon card stays level and the miniature airplane moves with you. It is the only instrument that shows pitch and bank directly, which is why it is the center of the scan.
02
Heading Indicator
A gyro on a horizontal axis holds a heading steady while the magnetic compass swings in turbulence and turns. Friction makes it drift, called precession, so the PHAK says to check it against the compass about every 15 minutes, in straight and level, unaccelerated flight.
03
Turn Coordinator
A gyro canted about 30 degrees senses both roll and yaw, so it responds as soon as you start a roll. The marks show a standard-rate turn of 3 degrees per second, or 360 degrees in two minutes. Below it, the inclinometer ball shows whether the turn is coordinated: step on the ball.
The power source matters. In many older trainers the attitude and heading indicators are vacuum-driven by an engine pump, while the turn coordinator is electric. That split is deliberate: if the vacuum pump fails, the turn coordinator still works, and you can keep the wings level on it. A failing vacuum attitude indicator tips slowly and looks believable, so watch the suction gauge and cross-check.
The older turn-and-slip indicator looks similar but has its gyro mounted flat, so it shows rate of turn only, not rate of roll. Either one satisfies the gyroscopic rate-of-turn requirement for IFR in 14 CFR 91.205(d).
Glass Cockpit
What Is A Primary Flight Display?
A primary flight display, or PFD, is a screen that shows all six flight instruments in one picture: attitude across the middle, airspeed as a tape on the left, altitude and vertical speed on the right, and a heading arc at the bottom.
The PFD does not replace the physics, only the hardware. An attitude and heading reference system, or AHRS, uses solid-state rate sensors, accelerometers and a magnetometer in place of spinning gyros. An air data computer reads the same pitot and static pressures as the round gauges and turns them into airspeed, altitude and vertical speed. The pitot-static blockages in the table above affect a PFD exactly as they affect steam gauges.
What glass adds is information. Airspeed and altitude trend vectors show where you will be in six seconds. The heading arc carries a course needle. Most glass cockpits pair the PFD with a multi-function display, or MFD, for the moving map, engine data and traffic. Our Cessna 162 Skycatchers fly behind a Garmin G300 display, and the Cirrus SR22 Turbo has a full glass cockpit with two large screens.
Every glass panel carries backup instruments, either standby gauges or a separate electronic standby unit, because a screen can fail. Learning to fly the backup without help is part of training. The argument for starting on glass versus round gauges is on our Glass Cockpit Vs Steam Gauge Training page.
Before Every Flight
How To Check The Flight Instruments During Taxi
Most instrument faults show up on the ground if you look for them. This is the check our instructors teach, and it fits any trainer; your POH checklist comes first.
STEP ONE
Set And Check The Altimeter
Set the current altimeter setting from the ATIS and compare the reading with field elevation. The Instrument Flying Handbook says an error over 75 feet makes the altimeter’s accuracy questionable.
STEP TWO
Confirm The VSI Reads Zero
Sitting still on the ramp, the vertical speed indicator should read zero. If it does not, note the error and treat that reading as zero in flight.
STEP THREE
Let The Gyros Spin Up
Give vacuum gyros about five minutes to reach speed. The attitude indicator should settle level within a few degrees of pitch and bank and stay there during taxi turns.
STEP FOUR
Turn Left, Then Right
In a taxi turn, the turn coordinator shows a turn in the correct direction, the ball swings to the outside, the heading indicator turns the right way and the compass swings freely.
STEP FIVE
Align The Heading Indicator
Set the heading indicator to the magnetic compass, or check the PFD heading against the compass and the runway heading at the hold short line.
STEP SIX
Watch Airspeed On The Roll
On the takeoff roll, check that the airspeed is alive early. If it is not moving by the speed your instructor briefs, abort while you have runway.
The Regulation
Which Flight Instruments The Regulations Require
14 CFR 91.205 lists the minimum instruments for each kind of flight. Many trainers carry more, but none may fly with less.
| Flight | Flight Instruments Required By 14 CFR 91.205 | Paragraph |
|---|---|---|
| VFR By Day | Airspeed indicator, altimeter, magnetic direction indicator, plus engine instruments such as tachometer, oil pressure and temperature, and fuel gauges | 91.205(b) |
| VFR At Night | Everything for day VFR, plus position lights, anticollision lights, a landing light if for hire, and an adequate source of electricity | 91.205(c) |
| IFR | Everything for VFR, plus two-way radio and navigation equipment, a gyroscopic rate-of-turn indicator, a slip-skid indicator, a sensitive altimeter adjustable for pressure, a clock showing hours, minutes and seconds, a generator or alternator, a gyroscopic pitch and bank indicator and a gyroscopic direction indicator | 91.205(d) |
Notice what is not on the day VFR list: the attitude indicator, heading indicator, turn coordinator and VSI. You could legally fly day VFR with none of the gyros working, and you learn to do exactly that in training. The IFR list is the reason Instrument Rating students spend so long on partial-panel work, and it is part of what separates Instrument Flight Rules (IFR) from Visual Flight Rules (VFR).
The instruments are the second half of the panel. The first half is the controls they respond to, covered in Airplane Controls Explained. Expect questions on both in the FAA Written Exam, and expect the examiner to ask what you would do with a failed attitude indicator in the Private Pilot Oral Exam.
Van Nuys Airport
See The Six Pack Move With Your Hands On The Controls
On a $229 discovery flight you fly the airplane out of Van Nuys while an instructor points out each instrument as it responds: the VSI as you climb, the turn coordinator as you bank, the altimeter as you level off. It is the fastest way to make this page make sense.
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FAQ
Flight Instrument Questions
The six basic flight instruments are the airspeed indicator, attitude indicator, altimeter, turn coordinator, heading indicator and vertical speed indicator. The airspeed indicator, altimeter and VSI are pitot-static instruments that measure air pressure. The attitude indicator, heading indicator and turn coordinator are gyroscopic. Together they are called the six pack, arranged in two rows of three with the attitude indicator top center. The magnetic compass is required but is not usually counted as one of the six.
A vertical speed indicator shows how fast the airplane is climbing or descending, in feet per minute, and the direction of the trend. It works from static pressure alone, comparing the pressure in a diaphragm with the pressure in its case, which changes slowly through a calibrated leak. Because of that design it shows a trend at once but takes about 6 to 9 seconds to give an accurate rate, according to the FAA’s Pilot’s Handbook of Aeronautical Knowledge.
A primary flight display shows the flight instruments: attitude, airspeed, altitude, vertical speed and heading, plus navigation needles. A multi-function display shows everything else, usually a moving map, engine instruments, traffic, weather and checklists. In many systems either screen can take over the other’s job if one fails, a mode called reversion. Some smaller systems show both pictures side by side on a single screen.
If the pitot tube is blocked but its drain hole stays open, airspeed falls toward zero. If both the tube and the drain hole are blocked, the airspeed indicator traps the pressure and behaves like an altimeter, showing an increase in a climb and a decrease in a descent. The altimeter and VSI are unaffected because they use static pressure only. Pitot heat, used in visible moisture, prevents the icing that causes most blockages.
The heading indicator drifts because of gyroscopic precession, caused by friction in its bearings and by the earth’s rotation. The FAA’s handbook says to check it against the magnetic compass about every 15 minutes, in straight and level, unaccelerated flight, when the compass is reading accurately. Some heading indicators are slaved to a magnetic sensor and correct themselves, and an AHRS on a glass panel uses a magnetometer for the same reason.
Both routes lead to the same certificate, and the FAA tests the same instrument knowledge either way. At Van Nuys our Cessna 162 Skycatchers have a Garmin G300 glass display and the Cirrus SR22 Turbo has a full glass cockpit, so you can start on glass from the first lesson. Whichever panel you learn on, expect a checkout before flying an unfamiliar one, and on glass you must also learn its reversionary and backup modes.
14 CFR 91.205(b) requires an airspeed indicator, an altimeter and a magnetic direction indicator for day VFR, along with engine instruments such as a tachometer, oil pressure and temperature gauges and fuel quantity gauges. The gyroscopic instruments are not required for VFR. For IFR, 91.205(d) adds a gyroscopic rate-of-turn indicator, slip-skid indicator, sensitive altimeter, clock, alternator or generator, attitude indicator and directional gyro, plus suitable radios.
The instrument scan is the habit of moving your eyes between instruments in a pattern so you catch changes early. The Instrument Flying Handbook teaches a radial scan that keeps returning to the attitude indicator, the control instrument, and checks the performance instruments around it. Under VFR the scan includes the outside horizon most of the time, with short glances inside. Practicing it in a simulator before flight saves expensive airplane time.
Keep Reading
Where To Go Next
Sources
Sources And Regulations Cited
Every rule on this page traces to Title 14 of the Code of Federal Regulations, the Aeronautical Information Manual or an FAA handbook or advisory circular. These links open the primary source.
FAA Handbooks
Pilot’S Handbook Of Aeronautical Knowledge, FAA-H-8083-25CInstrument Flying Handbook, FAA-H-8083-15BAirplane Flying Handbook, FAA-H-8083-3CAeronautical Information Manual
AIM Chapter 4 — Air Traffic ControlAIM 4-1 — Services Available To PilotsInstrument principles are from the Pilot’s Handbook of Aeronautical Knowledge, Chapter 8, and the Instrument Flying Handbook. Equipment requirements are from 14 CFR 91.205. Your aircraft’s POH or AFM governs its own instrument checks and limitations.
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Flying is my husband's hobby and I was incredibly anxious and apprehensive about the discovery flight. However, I felt completely safe and at ease once we...
SAFETY FIRST
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Very safety and detail oriented. Pre-flight briefing explained everything there was. To my knowledge, this flight school is exceptional in their ability.
DISCOVERY FLIGHT
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I recently had the chance to take my first-ever discovery flight, and it was nothing short of amazing! From the moment I stepped into the cockpit, the...
SUNSET CIRRUS
★★★★★
It was a great flight! I did a sunset flight and Alex let us fly an awesome. Website was easy to book, and the plane itself was really fun to fly...
FLYING LESSON
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We had a great flying lesson with Ralph and came away with a much better understanding of both the aircraft and the principles of flight. Ralph is a...
KIDS LUXURY FLIGHT
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I got 2 luxury discovery flights for my kids. I wanted them to have the experience of flying. Our instructor Alex was amazing he made sure they feel...
HONEYMOON SURPRISE
★★★★★
Last minute booking to surprise my wife on our honeymoon. She had no experience with flying and was scared of flying, but Alex was very easygoing and knew...
FULL-MOTION SIM
★★★★★
Amazing flight school in van nuys airport. I have been a student here for the past few months and i could not be happier. I started doing a lot of...
DISCOVERY FOR TWO
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I booked a Discovery Flight for Two for my girlfriend's birthday and it was the perfect gift for her. She couldn't have been more excited to get to...
SIMULATORS
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Very professional and great airplanes. I love the flight simulators. The pricing is very fair compared to any other flight school
TEEN FIRST LESSON
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We gifted our teenage son his first flying lesson. He was so excited! Alex greeted us and gave a reassuring brief that made us all feel comfortable and at...
FIRST-TIME FLYER
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Flying is my husband's hobby and I was incredibly anxious and apprehensive about the discovery flight. However, I felt completely safe and at ease once we...
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