Flight Instruments

Use the panel without losing the outside picture

During VFR flight, the outside view supplies attitude references, traffic, terrain, and weather information. Instruments supplement that picture with measured airspeed, altitude, heading, and system status. Looking at a moving map or attitude display for a long period can leave traffic and changing conditions unnoticed.

Make short, purposeful instrument checks between an effective outside scan. Look at the indication that answers the present question, then return outside. Reconcile the outside attitude with airspeed and altitude trends rather than staring at one pointer. Prepare frequencies, map pages, and unfamiliar avionics functions before flight to reduce time spent looking down.

A good visual scan uses deliberate, regularly spaced eye movements rather than a continuous unfocused sweep. Allow your eyes to refocus when moving between the panel and distant objects. This lesson develops VFR instrument awareness; it does not qualify a pilot to continue into cloud or replace instructor-led basic instrument training.

FAA AIM 8-1-6: vision and scanning; 8-1-8: cockpit management and collision avoidance

Three instruments share outside pressure

A forward-facing pitot inlet senses total pressure. A static port senses the surrounding static pressure. Conventional instruments or an electronic air data computer use these sources to determine airspeed, altitude, and vertical speed.

Indicated airspeed (IAS) is the instrument reading, not groundspeed. Groundspeed describes movement over the ground and changes with wind. True airspeed is speed relative to the surrounding air, with pressure and temperature effects accounted for. A GPS groundspeed cannot simply replace a failed IAS indication.

Set and verify the altimeter according to the aircraft checklist and current setting. With an appropriate local setting, indicated altitude references mean sea level; it is not a direct measurement of height above terrain. Incorrect settings and nonstandard atmospheric conditions can introduce error. On the ground, compare with known airport elevation as the applicable check requires.

A conventional VSI needs time to settle after a change. Avoid chasing its initial movement as though it were an immediate pitch command. Some instruments reduce that delay electronically or mechanically. Always identify the installed type and use other evidence, including the altimeter trend.

PHAK Chapter 8, pp. 8-1–8-10 and 8-14: pitot-static instruments and air data

A blocked opening can produce a plausible wrong reading

Pressure instruments depend on unobstructed sources. Check that covers are removed and inlets are clear using the aircraft’s approved preflight method. Do not insert objects into ports or assume a clean display proves that the pressure system is clear.

• Pitot inlet blocked, drain open, static source clear: the trapped pitot pressure vents, so indicated airspeed falls toward zero. The altimeter and VSI still receive static pressure.
• Pitot inlet and drain both blocked, static source clear: pitot pressure is trapped. The ASI can act like an altimeter, increasing during a climb and decreasing during a descent even without the corresponding speed change.
• Static source blocked, pitot source clear: the altimeter remains at the blockage altitude, the conventional VSI settles toward zero, and airspeed becomes inaccurate. At unchanged actual speed, it underreads above the blockage altitude and overreads below it.

These are simplified pressure-system cases, not a complete diagnostic procedure. Partial blockages, leaks, multiple failures, and electronic fault logic can produce other behavior. Compare the indications with attitude, the outside picture, and any independent backup. Use pitot heat or an alternate static source only according to the applicable procedure. An alternate static source may change readings and require published corrections.

PHAK Chapter 8, pp. 8-10–8-11: pitot/static blockages and alternate static source

Attitude and heading answer different questions

The attitude indicator shows pitch and bank relative to its sensed horizon. Pitch attitude is not the same as flight path: the airplane can have its nose above the horizon while losing altitude. Cross-check airspeed, altitude trend, and the outside horizon rather than assuming the attitude display proves a climb.

A heading indicator shows nose direction. Ground track is the path over the ground; wind can make it differ from heading. A conventional, unslaved gyro heading indicator must be aligned and periodically compared with the magnetic compass because it can drift. Make the comparison in steady, unaccelerated flight as the installed procedure specifies. A slaved electronic heading system may use a magnetometer rather than manual alignment.

The magnetic compass responds to Earth’s magnetic field, but it has errors. Variation separates true and magnetic directions; deviation comes from magnetic influences in the airplane and is addressed with its correction card. Turning and acceleration can temporarily disturb a conventional compass. Let the appropriate steady-flight conditions return before using it as a heading-check reference. Geographic hemisphere and compass design matter; do not apply a memorized correction without understanding its scope.

Traditional attitude and heading gyros may use vacuum, pressure, or electrical power. If two instruments depend on the same failed source, their agreement is not independent confirmation. Identify the power arrangement and its warning or suction indication before flight.

PHAK Chapter 8, pp. 8-16–8-20 and 8-23–8-27: power, attitude, heading, and compass errors

Turn rate is not bank angle

A turn coordinator responds initially to roll and, once the roll stabilizes, indicates turn direction and rate. Its miniature airplane is not an attitude indicator: it supplies neither pitch attitude nor a direct bank-angle measurement. The bank needed for a given turn rate varies with airspeed.

The inclinometer ball or electronic slip/skid indication shows coordination. A centered indication supports coordinated flight under normal conditions; displacement calls for attention to coordinated control use. It is not an airspeed, heading, or stall-margin indicator. Review Turning and Coordination for the relationship between bank, lift, and control inputs.

Learn which source powers the installed turn instrument and how a loss of that source is identified. A panel with several round instruments can still have shared dependencies; independence must be established from the installation, not from the number of faces.

PHAK Chapter 8, pp. 8-16–8-18: turn indicators, turn coordinator, and inclinometer

A glass display combines information, not independent evidence

A primary flight display (PFD) groups attitude, airspeed, altitude, vertical speed, and heading. A multifunction display (MFD) can show maps, engine information, and other pages. Layouts and functions vary; learn the actual display, alerts, and backup or reversionary mode before using it.

The screen is only the visible end of the system. An air data computer (ADC) processes pressure information for airspeed and altitude. An attitude and heading reference system (AHRS) supplies attitude information and works with magnetic-heading inputs as installed. A failed sensor, failed processing unit, lost power supply, or failed screen affects different parts of that chain.

Two screens may use the same ADC, AHRS, or electrical bus. Two matching attitude pictures therefore do not necessarily confirm a healthy attitude source. Likewise, a backup airspeed indicator connected to the same blocked static source is not independent of that pressure fault. Determine which backups have genuinely separate sources and power.

If an indication becomes flagged or inconsistent, keep control using reliable references, follow the applicable failure procedure, and reassess the flight. A moving map, terrain picture, or synthetic horizon does not make continued VFR flight into cloud safe or authorized. Do not turn a display problem into prolonged fixation on menus while traffic and weather go unobserved.

PHAK Chapter 8, pp. 8-12–8-15 and 8-20: electronic displays, ADC, and AHRS · PHAK Chapter 9, p. 9-5: installed-equipment supplements

Brief expected indications and a usable backup

Before departure, perform the actual aircraft’s instrument and system checks. Confirm pressure settings, expected startup behavior, cleared warning flags, and required power or suction indications. During the approved taxi checks, compare instrument movement with the airplane’s actual movement while retaining control and an outside lookout. A powered screen is not the same as valid sensor data.

For each main indication, be able to answer: What does it measure? What supplies its data and power? What other indication can check it? Ask your instructor to review the airplane’s backups, alerts, and relevant failure checklist. Practice this on the ground before a distraction occurs aloft.

In daylight visual conditions, an attitude display that disagrees with a clear natural horizon is a reason to cross-check the installation, maintain aircraft control with reliable references, and reassess the flight. Use the published procedure and assistance as appropriate. If the natural horizon or other needed visual reference is deteriorating, that adds urgency to preserving a safe visual route and landing option; it does not justify relying on a suspect display to enter cloud.

PHAK Chapter 8: instrument-check subsections on pp. 8-6, 8-8, 8-10, 8-18, and 8-22 · PHAK Chapter 9, pp. 9-4–9-5: procedures and supplements

FLIGHT SCENARIO

What would change your plan?

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Connect this lesson with related concepts:

• Understand instrument power and dependencies
• Understand bank, coordination, and load factor