Electronic Code of Federal Regulations. 14 CFR 91.205, Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements. https://www.ecfr.gov/current/title-14/section-91.205
The six-pack is the six primary flight instruments in a small airplane: the airspeed indicator, attitude indicator, altimeter, turn coordinator, heading indicator and vertical speed indicator. Three run on air pressure from the pitot-static system, and three run on spinning gyroscopes.
This guide shows how each instrument works, what powers it, what happens when a pressure line blocks or a gyro source fails, and which of them federal rules require. The drawing below is original, and every technical claim comes from the FAA Pilot's Handbook of Aeronautical Knowledge or the Code of Federal Regulations.
The six-pack and what drives each instrumentOriginal schematic, based on the FAA Pilot's Handbook of Aeronautical Knowledge
The six-pack in the standard T arrangement, with the pressure, vacuum and electrical sources that drive each dial. Original schematic for PilotPassion, simplified: real aircraft vary in how the lines are routed.
Pitot (ram) pressure compared with static pressure
Attitude indicator
Pitch and bank against the horizon
Gyroscope, usually driven by vacuum
Altimeter
Altitude from air pressure
Static pressure
Turn coordinator
Rate of turn and coordination of the turn
Gyroscope, usually electric
Heading indicator
Heading, steadier than a compass
Gyroscope, usually driven by vacuum
Vertical speed indicator
Rate of climb or descent
Static pressure, changing over time
Pilots also call the set the basic six. In most trainers, the vacuum-driven gyros are the attitude and heading indicators and the turn coordinator is electric, so the airplane has more than one power source for its gyro instruments. The FAA handbook describes that mix as typical and says an aircraft should have at least two power sources.
Why the Panel Is Laid Out as a T
The instruments sit in a fixed pattern. The attitude indicator is top center, the airspeed indicator is on its left, the altimeter is on its right and the direction instrument is below. The pattern is called the basic T. 14 CFR 25.1321(b) requires it for transport category airplanes, and most small airplanes use the same layout so a pilot can scan from one aircraft to another without relearning it.
The attitude indicator sits in the center because it's the instrument pilots rely on most when there's no outside horizon. The others are cross-checked against it.
The Pitot-Static Three
The airspeed indicator, altimeter and vertical speed indicator all work from air pressure. The pitot tube faces forward and catches ram pressure. The static port, a small flush opening on the side of the fuselage, senses the pressure of the still air around the airplane.
Airspeed indicator. It compares pitot and static pressure, so it is the only one of the three that uses both. The colored arcs on the dial mark the limits: the white arc for flap operation, green for normal operation, yellow for caution and a red line for the speed never to exceed.
Altimeter. It uses static pressure only. Pilots set the local pressure in the small window, the Kollsman window, so the instrument reads correctly. A pressure change of one inch of mercury shifts the reading by about 1,000 feet, which is why the setting matters. At and above 18,000 feet in the United States pilots use the standard setting of 29.92 inches (14 CFR 91.121).
Vertical speed indicator. It also uses static pressure only. It measures how fast the static pressure is changing and shows the result in feet per minute. Because it works from a rate of change, it lags behind real changes, so pilots treat it as a trend instrument and cross-check the altimeter and attitude indicator.
The Gyroscopic Three
Gyroscopes work because of two properties. Rigidity in space means a spinning gyro holds its position in space. Precession means a force applied to the spinning wheel acts as if it were applied 90 degrees later in the direction of rotation. The instruments use the first property to hold a reference, and the turn coordinator uses precession to sense a turn.
Attitude indicator. The gyro holds its position in space while the airplane moves around it. The miniature airplane on the face shows the real airplane's pitch and bank against the horizon bar.
Heading indicator. A magnetic compass swings and lags in turns. The heading indicator uses a gyro to give a steady heading, but it doesn't seek north on its own, and gyro drift means the pilot resets it to the magnetic compass periodically.
Turn coordinator. Its gyro is canted, so it senses both roll and yaw. A turn is flown at a standard rate when the little airplane lines up with the marked index, which means 3 degrees per second, a full 360 degree turn in two minutes. The ball underneath is the inclinometer: pilots "step on the ball" by pressing the rudder pedal on the side the ball has moved to.
What Powers Each Instrument
The vacuum pump pulls air through the gyro housings, and the airflow spins the rotors. The FAA handbook gives a typical suction range of 4.5 to 5.5 inches of mercury. A vacuum gauge on the panel lets the pilot monitor it, and when suction drops the attitude and heading indicators become unreliable. The turn coordinator runs from the electrical system, so a vacuum failure leaves it working. That is by design: the pilot can fly the turn coordinator while the other gyros fail.
When Something Blocks or Fails
The handbook describes what each failure does. These are the cases training covers, and they're worth knowing by heart. The table gives the pitot and static ones.
Pitot-static failures and what the instruments do
Failure
Airspeed indicator
Altimeter
Vertical speed indicator
Pitot tube blocked, drain hole open
Drops toward zero
Normal
Normal
Pitot tube and drain blocked
Stops changing with speed, then acts like an altimeter, rising in a climb and falling in a descent
Normal
Normal
Static source blocked
Gives wrong readings
Freezes at the altitude of the blockage
Reads zero
Alternate static source in use
Reads higher than true
Reads slightly high
Shows a momentary climb when selected
Many airplanes have an alternate static source inside the cabin for the case where the outside static port is blocked, by ice for example. Cabin pressure is usually a little lower than outside, which is why the readings shift as shown. In the handbook's words the effects are small, and the pilot should follow the airplane's own flight manual.
On the gyro side, a vacuum failure shows up as a low reading on the suction gauge, then a slowly drifting attitude indicator and heading indicator. A typical scan keeps the turn coordinator as the backup reference.
Which Instruments the Rules Require
Federal rules don't use the word six-pack. They list required equipment by type of flight. For daytime VFR, 14 CFR 91.205(b) includes the airspeed indicator, altimeter and magnetic direction indicator, along with items such as a tachometer and oil pressure gauge. For IFR, 91.205(d) adds a gyroscopic rate-of-turn indicator, a slip-skid indicator, a sensitive altimeter, a clock, a gyroscopic pitch and bank indicator (the attitude indicator) and a gyroscopic direction indicator (the heading indicator).
So an IFR airplane needs essentially the whole six-pack, apart from the vertical speed indicator, which the list does not name even though nearly every IFR panel has one.
Glass Cockpits and the Six-Pack Today
Modern airplanes often show the same information on a primary flight display, with a separate unit sensing the pressure and attitude data. The T arrangement is usually kept: attitude in the middle, airspeed to the left, altitude to the right, heading below. Pilots who learn on the analog six-pack carry the same scan to glass panels. Many airplanes keep a few analog backup instruments, such as an airspeed indicator, altimeter and attitude indicator, in case the screens fail.
Frequently Asked Questions
What is the six-pack in an airplane?
It is the six primary flight instruments of a small airplane: airspeed indicator, attitude indicator, altimeter, turn coordinator, heading indicator and vertical speed indicator.
Which instruments in the six-pack use gyroscopes?
The attitude indicator, heading indicator and turn coordinator. The attitude and heading indicators are usually vacuum driven, and the turn coordinator is usually electric.
Which instruments use the pitot-static system?
The airspeed indicator uses pitot and static pressure. The altimeter and vertical speed indicator use static pressure only.
What is the basic T?
The standard panel layout: attitude indicator top center, airspeed on its left, altimeter on its right and the heading indicator below. 14 CFR 25.1321(b) requires it for transport category airplanes.
What happens if the pitot tube blocks?
With the drain hole open, the airspeed indicator drops toward zero. If both the tube and its drain are blocked, the indicator stops responding to speed and behaves like an altimeter, rising in a climb and falling in a descent.
What happens if the static port blocks?
The altimeter freezes at the altitude of the blockage, the vertical speed indicator reads zero and the airspeed indicator gives wrong readings. Many airplanes have an alternate static source for this.
What is a standard rate turn?
A turn of 3 degrees per second, which is 360 degrees in two minutes. The turn coordinator is marked to show it.
Does a vacuum failure take out the whole panel?
No. It affects the vacuum-driven attitude and heading indicators. The pressure instruments and the electric turn coordinator keep working, which is why the mix of power sources exists.
Do you need all six instruments to fly?
Not for daytime VFR, where 14 CFR 91.205(b) requires an airspeed indicator, altimeter and magnetic direction indicator among other items. IFR flight needs the gyroscopic instruments listed in 91.205(d).
Is the six-pack still used in glass cockpits?
The same information is shown on a primary flight display in the same T arrangement, and many airplanes keep analog backup instruments.
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