Flight Instruments

Six flight instruments sit in front of the pilot, and they come from three sources. Three of them run on air pressure. The handbook says the pitot-static system "utilizes the static air pressure and the dynamic pressure due to the motion of the aircraft through the air," and that "these combined pressures are utilized for the operation of the airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI)" 1. Three run on gyroscopes: the attitude indicator, the heading indicator, and the turn coordinator 2. The magnetic compass runs on the Earth's magnetic field and needs neither air pressure nor a gyro.

Pitot and static

The static ports on the side of the fuselage sense the pressure of the still air around the airplane. The pitot tube under the left wing faces forward. It senses static pressure plus the impact pressure of the air that enters it as the airplane moves. The altimeter and the vertical speed indicator use static pressure only. The airspeed indicator uses both.

The airspeed indicator

The handbook says "the ASI is a sensitive, differential pressure gauge that measures and promptly indicates the difference between pitot (impact/dynamic pressure) and static pressure" 3. The two pressures "are equal when the aircraft is parked on the ground in calm air." As the airplane moves, the pitot pressure rises above the static pressure, and the difference moves the needle. The airspeed indicator is the only instrument on the panel that needs the pitot tube. A cover left on the pitot tube disables one instrument, the airspeed indicator.

Three airspeeds

Indicated airspeed is the number on the dial. The handbook defines it as "the direct instrument reading obtained from the ASI, uncorrected for variations in atmospheric density, installation error, or instrument error," and adds that "takeoff, landing, and stall speeds listed in the AFM/POH are IAS and do not normally vary with altitude or temperature" 4. Calibrated airspeed is "IAS corrected for installation error and instrument error," an error that "is generally greatest at low airspeeds." True airspeed is "CAS corrected for altitude and nonstandard temperature." True airspeed is the speed through the air mass. A pilot files a flight plan with true airspeed.

Two percent per thousand feet

True airspeed rises above calibrated airspeed with altitude. The air is thinner, so the same dynamic pressure, and the same indicated airspeed, means a faster airplane. The handbook gives an approximation: "Simply add 2 percent to the CAS for each 1,000 feet of altitude" 4. A Warrior at 100 mph calibrated at 5,000 feet moves through the air at about 110 mph. The indicated airspeed does not change as the airplane climbs. The speed through the air does.

The Warrior's book is in calibrated mph

The Warrior's manual quotes its speeds in calibrated miles per hour. Page 7-9 says "all airspeeds quoted in this manual are calibrated unless otherwise noted. Calibrated airspeed is indicated airspeed corrected for instrument and position errors" 5. The pilot flies indicated airspeed. The page carries a correction table from indicated to calibrated, flaps up and flaps down. The table is "valid only when flown at maximum gross weight in level flight."

The stall speed of 64.5 mph in the limitations section is a calibrated number. The dial reads a number close to it. The table gives the difference.

The arcs

The airspeed indicator's face is color-coded, and the handbook defines each mark 6:

• the white arc is "the flap operating range," from the full-flap stall speed, VSO, at its bottom to the maximum flap speed, VFE, at its top
• the green arc is "the normal operating range," from the clean stall speed, VS1, at its bottom to the maximum structural cruising speed, VNO, at its top
• the yellow arc is the "caution range," to be flown "only in smooth air and then only with caution"
• the red line is VNE, "never exceed speed."

The Warrior's arcs, in calibrated mph, are:

• white, 58 to 125 or 115, by serial number
• green, 64.5 to 140
• yellow, 140 to 176
• red line, 176.

The flight of N62WR

On June 18, 2021, a 38-year-old private pilot took off alone from Conway Regional Airport in Arkansas in a Cirrus SR20. The pitot tube cover was still on. His airspeed indicator did not work. Within minutes he announced an emergency landing on a runway the airport does not have. He flew over the runway "crazy fast" and rolled into a steep left bank at low altitude. The airplane spun one and a half turns and struck the ground, and he died 7.

He held a private pilot certificate with an instrument rating, flew under BasicMed, had a flight review three months earlier, and had 211 hours. The airplane was a 2006 SR20, an airplane with a glass cockpit and a whole-airframe parachute. The sky was clear with 6 miles of visibility in haze. The wind was from 120 degrees at 5 knots gusting to 12. The temperature was 32 degrees Celsius. The runway was 5,500 feet long.

He took off from runway 22. A witness heard him transmit on the UNICOM frequency: "six-two whiskey romeo emergency landing runway 2." Conway has runways 4 and 22. The witness then watched the airplane turn left and fly over runway 4, and said it looked "crazy fast." Past the runway the airplane entered a steep left bank, rotated one and a half times, and struck the ground nose-low and inverted.

The recovered GPS data showed the highest altitude of the flight was 593 feet above the ground, on the crosswind leg. From there the airplane descended continuously to impact. He flew the downwind leg about 0.1 nautical mile south of the runway, a tenth of the normal spacing. The parachute deployed in the impact sequence, too late to open.

Investigators found "the pitot tube cover remained secured on the pitot tube with a 'REMOVE BEFORE FLIGHT' banner attached." Toxicology found an antidepressant and a mood stabilizer, and the NTSB could not determine whether the medications or the conditions behind them were factors. The probable cause is "the pilot's failure to maintain adequate airspeed, which resulted in an aerodynamic stall and spin. Contributing to the accident was the failure of the pilot to perform an adequate preflight inspection, to include removal of the pitot tube cover" 7.

Where the accident chain could have been broken

The accident was four decisions. Each is listed below with its PAVE category and the decision that would have ended the flight safely at that point. The first was on the ramp. The pilot made the last three in about two minutes, with one instrument failed and five working.

A stack of wafers

The altimeter measures static pressure and displays it as feet. The handbook says "a stack of sealed aneroid wafers comprise the main component of the altimeter. An aneroid wafer is a sealed wafer that is evacuated to an internal pressure of 29.92 inches of mercury" 8. Higher static pressure compresses the wafers, and lower pressure lets them expand. A linkage turns the movement into needle motion. The instrument measures pressure. The pilot reads altitude.

The Kollsman window

The altimeter is calibrated to a standard atmosphere, in which sea-level pressure is 29.92 inches of mercury and the sea-level temperature is 15 degrees Celsius. Real pressure is different on each day. The pilot dials the current pressure into a small window on the face, the barometric scale, and the handbook says that "the barometric pressure window is sometimes referred to as the Kollsman window; only after the altimeter is set does it indicate the correct altitude" 9.

14 CFR 91.121(a) requires an altimeter set, below 18,000 feet, to "the current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft" 10. At and above 18,000 feet the setting is 29.92. A pilot updates the setting in flight as the airplane moves from one reporting station's area to the next. The setting comes from the ATIS or AWOS, from a controller, or from Flight Service.

One inch, one thousand feet

One inch of altimeter setting equals about 1,000 feet of indicated altitude. The handbook works an example between two Texas airports whose settings differ by 0.25 inch: "Since 1 inch of pressure is equal to approximately 1,000 feet of altitude, 0.25 × 1,000 feet = 250 feet" 11. A tenth of an inch is 100 feet. A pilot who sets 29.92 when the station reports 30.12 reads 200 feet wrong.

High to low, look out below

An altimeter with an old setting is wrong in a known direction. The handbook says that "when the actual pressure is lower than what is set in the altimeter window, the actual altitude of the aircraft is lower than what is indicated on the altimeter" 11. A pilot who flies from high pressure toward low pressure without resetting reads an altitude higher than the airplane's actual altitude. The mnemonic is "high to low, look out below." Cold air has the same effect as low pressure. From hot to cold, the altimeter reads high.

Five altitudes

Five altitude terms apply to one dial 12:

• indicated altitude, "read directly from the altimeter (uncorrected) when it is set to the current altimeter setting"
• true altitude, "the vertical distance of the aircraft above sea level," the altitude on the chart for terrain and obstacles
• absolute altitude, "the vertical distance of an aircraft above the terrain, or above ground level (AGL)"
• pressure altitude, "the altitude indicated when the altimeter setting window (barometric scale) is adjusted to 29.92"
• density altitude, "pressure altitude corrected for variations from standard temperature."

Indicated altitude is the altitude the pilot flies. True altitude is the altitude of the terrain and obstacles on the chart. Pressure altitude and density altitude are the inputs to the performance charts.

Cold days

A correct altimeter setting corrects for pressure and not for temperature. The handbook says "adjustments to compensate for nonstandard pressure do not compensate for nonstandard temperature. Since cold air is denser than warm air, when operating in temperatures that are colder than standard, the altitude is lower than the altimeter indication" 13. The AIM gives the standard: 15 degrees Celsius at sea level, falling 2 degrees per 1,000 feet, and "when the ambient (at altitude) temperature is colder than standard, the aircraft's true altitude is lower than the indicated barometric altitude" 14. The AIM's table 7-3-1 gives the size of the error. On a cold day the terrain clearance a pilot planned is smaller than the altimeter shows, even with the right setting in the window.

The VSI lags

The vertical speed indicator measures the rate of change of static pressure. Inside it, static pressure reaches a diaphragm directly and reaches the case around it through a calibrated leak. While the airplane climbs or descends the two pressures differ, and the difference moves the needle. The handbook says the instrument "displays two different types of information": trend, "an immediate indication of an increase or decrease in the aircraft's rate of climb or descent," and rate, "a stabilized rate of change in altitude" 15.

The trend is instant. The rate takes several seconds to settle, because the case pressure takes that long to equalize through the leak. A pilot who corrects pitch from the VSI rate corrects to a number that is several seconds old.

The attitude indicator

The attitude indicator is the only instrument that shows pitch and bank directly. The handbook says it "displays a picture of the attitude of the aircraft" through a miniature airplane and a horizon bar, and that "the relationship of the miniature aircraft to the horizon bar is the same as the relationship of the real aircraft to the actual horizon" 16. Inside, "the gyro in the attitude indicator is mounted in a horizontal plane and depends upon rigidity in space for its operation." The gyro holds still while the airplane, and the instrument case, move around it.

The attitude indicator's limits

Older attitude indicators have limits and small errors. The handbook says "the pitch and bank limits of an attitude indicator depend upon the make and model of the instrument. Some attitude indicators have limits in the banking plane from 100° to 110°, and the pitch limits can be from 60° to 70°" 17. Past those limits the instrument can "tumble" and show wrong pitch and bank until it re-erects. A vacuum-driven gyro also precesses slowly out of true and re-erects in level flight. During the takeoff acceleration an older instrument shows a slight climb that the airplane is not making. A pilot who knows these errors allows for them, and a pilot who does not know them follows a wrong indication.

The heading indicator drifts

The heading indicator is a gyro too, and a gyro that is not slaved to a compass drifts. The handbook says "it is important to check the indications frequently (approximately every 15 minutes) and reset the heading indicator to align it with the magnetic compass when required. Adjust the heading indicator to the magnetic compass heading when the aircraft is straight and level at a constant speed to avoid compass errors" 18. The drift comes from precession and from the Earth turning under the gyro. The realignment is a checklist item at each cruise check, done in straight, level, unaccelerated flight, because the compass is only correct then.

The turn coordinator

The turn coordinator's gyro is tilted. The handbook says "the turn coordinator is mounted at an angle, or canted, so it can initially show roll rate. When the roll stabilizes, it indicates rate of turn" 19. The miniature airplane on its face tips as the airplane rolls and then holds a position that means rate of turn. It "does not display a specific angle of bank," and it shows no pitch at all. A turn coordinator with its wings on the index shows a standard-rate turn, at whatever bank angle produces that rate.

Standard rate

The handbook says "a standard-rate turn is defined as a turn rate of 3° per second" 19. Three degrees per second is 180 degrees in one minute and a full circle in two. The index marks on the turn coordinator's face are the standard-rate marks. The bank angle that gives standard rate rises with airspeed. In a Warrior at pattern speeds that bank angle is about 15 degrees.

Slip and skid

The inclinometer is the curved glass tube under the turn coordinator with a ball in it. The ball shows whether the turn is coordinated. The handbook says "in a skid, the rate of turn is too great for the angle of bank, and the ball moves to the outside of the turn," and in a slip the ball moves to the inside 20. The rule is three words: "step on the ball." Rudder on the side the ball has moved to returns the ball to the center.

The skid on base to final

A left turn with the ball out to the right is a skid. The airplane yaws left faster than the bank rate requires, because the pilot holds too much left rudder. That indication is not dangerous by itself at altitude and at speed. The same indication on the turn from base to final, at low speed, is the setup for a cross-control stall. The pilot presses inside rudder to turn the nose toward the runway, the inside wing stalls first, and the airplane spins toward the low wing. A student who sees that indication on base steps on the ball, lowers the nose, and goes around.

Variation and deviation

The compass points at magnetic north, and charts are drawn to true north. The handbook says "in aerial navigation, the difference between true and magnetic directions is called variation" 21. The variation for a place is printed on the sectional chart as an isogonic line.

Deviation is a different error. The airplane's own magnetic fields, from "flowing electrical current, magnetized parts," deflect the compass, and "deviation, unlike variation, depends on the aircraft heading." A mechanic swings the compass and records the remaining error on a compass correction card mounted beside it. The pilot applies variation to get from true to magnetic, and the card to get from magnetic to compass.

Dip errors

The compass card floats on a pivot, and the Earth's magnetic field pulls down on it as well as sideways. The handbook says "the compass only indicates correctly if the card is horizontal. Once tilted out of the horizontal plane, it will be affected by the vertical component of the Earth's field" 22. Acceleration and turns tilt the card. The errors that follow are predictable, and two mnemonics name them.

ANDS

On an east or west heading, a change of speed reads as a turn. The handbook says "when accelerating on either an easterly or westerly heading, the error appears as a turn indication toward north. When decelerating on either of these headings, the compass indicates a turn toward south," and gives the mnemonic "ANDS" for Accelerate North, Decelerate South 23. The airplane has not turned. The card has tilted.

UNOS

In a turn from a northerly heading the compass lags, and at first shows a turn the other way. The handbook says "the result is a false northerly turn indication," and that "a northerly turn should be stopped prior to arrival at the desired heading" 24. From a southerly heading the compass leads, and the airplane "should be allowed to pass the desired heading prior to stopping the turn." The mnemonic is UNOS: Undershoot North, Overshoot South.

How much

The lead and lag depend on latitude. The handbook's approximation is to stop the turn "15 degrees plus half of the latitude" before or after the heading: at 40 degrees north, 35 degrees 24. The Instrument Flying Handbook gives the same rule 25. At 40 degrees north the number is 35 degrees. The error is largest turning through north and south and zero turning through east and west.

When the compass is correct

The magnetic compass is correct in straight, level, unaccelerated flight, and at no other time. In a turn it leads or lags. In a change of speed it swings toward north or south. In turbulence it oscillates, and the handbook says to "use the average indication between the swings." A pilot who resets the heading indicator does it wings level, at a steady speed, on a steady heading. That condition is the only one in which the compass reads correctly.

The scan

The Instrument Flying Handbook defines the first skill of instrument flying: "Cross-checking is the continuous and logical observation of instruments for attitude and performance information" 26. The eyes move through the panel with the attitude indicator at the center, out to a performance instrument and back. The handbook names three errors 27. Fixation is "staring at a single instrument." Omission is leaving one out of the scan, often from "failure to anticipate significant instrument indications following attitude changes." Emphasis is "reliance on a single instrument," which is "poor technique" even when the instrument is correct, and fatal when the instrument is wrong.

A second instrument confirms the first

A pilot believes no single instrument until a second instrument shows the same condition. An airspeed indicator that reads low with the nose level and cruise power set is a failed airspeed indicator. The attitude indicator and the tachometer show cruise attitude and cruise power, which produce cruise speed. An altimeter that unwinds with the VSI at zero and the attitude level is a static port problem. The pilot of N62WR had one instrument reading nothing and five instruments reading correctly. The cross-check finds a failed instrument by comparison with the others before the pilot acts on it.

Control and performance

Attitude flying works the same way on instruments as it does with the horizon out the window. The Instrument Flying Handbook says "aircraft performance is achieved by controlling the aircraft attitude and power. Aircraft attitude is the relationship of both the aircraft's pitch and roll axes in relation to the Earth's horizon" 28. The order is set the attitude, set the power, trim, then cross-check the performance instruments to confirm the result. A known pitch and a known power produce a known airspeed and a known rate of climb. A pilot who flies attitude and power has an airspeed even when the airspeed indicator has failed.