Aircraft Flight Instruments and Navigation Equipment

Three instruments, two pressures:

• Airspeed indicator — the only one using both pitot (ram) and static pressure; it displays the difference
• Altimeter — static only
• VSI — static only, displaying rate of change

The pitot tube (or the air data computer it feeds in glass installations) senses ram air; static pressure comes from flush ports positioned in undisturbed airflow (IFH ch. 5).

• Ram opening blocked, drain hole open — static pressure equalizes on both sides of the ASI diaphragm and the indicated airspeed slowly drops to zero
• Ram opening and drain hole both blocked — pressure is trapped, and the ASI acts like an altimeter: reading rises in a climb, falls in a descent — dangerously intuitive backwards

Only the ASI is affected; altimeter and VSI keep working. Prevention is the preflight (cover off, opening clear) plus pitot heat in visible moisture (IFH ch. 5).

• Altimeter — freezes at the altitude where the blockage occurred
• VSI — continuous zero indication
• ASI — keeps working but lies: above the blockage altitude it reads lower than actual (trapped static pressure is too high); below it reads faster than actual

The fix is the alternate static source, normally plumbed to the flight deck (IFH ch. 5).

Cabin pressure is slightly lower than outside static due to the venturi effect of air flowing around the fuselage, so:

• Altimeter indicates slightly higher than actual
• ASI indicates greater than actual airspeed
• VSI shows a momentary climb, then stabilizes

Check the POH/AFM for correction values, and verify on preflight that the valve opens and closes fully (IFH ch. 5).

Deliberate redundancy: manufacturers power the gyros from separate electrical and pneumatic sources so no single failure takes out your whole attitude reference. The classic arrangement is attitude and heading indicators on the vacuum system with an electric turn coordinator — or the reverse. The attitude instruments need roughly 4 inches Hg of suction; a turn-and-slip needs only about 2. Some instruments label their power source on the dial, but the POH/AFM is the authority — know which instrument survives which failure before the failure (IFH ch. 5).

Recognition: the gyros don't fail flag — the suction gauge is the direct clue, but the AI and heading indicator spin down gradually and drift, inviting you to follow them. Catch it with a disciplined cross-check: the AI disagrees with the turn coordinator, altimeter, and ASI trends.

What's left: the electric turn coordinator, pitot-static instruments, and compass — partial panel.

This is why the instrument flight deck check verifies the suction reading and why glass aircraft carry standby instruments (IFH ch. 5).

The compass card's center of gravity sits below its pivot, so magnetic dip drags it during turns:

• Turning through north — the card leads in the turn's direction, showing a false turn; roll out early (undershoot)
• Turning through south — the card lags; let it pass the desired heading (overshoot)

Rule of thumb: stop the turn 15° plus half your latitude before (north) or after (south) the target — at 40° latitude, 35° of anticipation. Errors grow with proximity to the poles (IFH ch. 5).

ANDS — accelerate north, decelerate south. On easterly or westerly headings, the pendulous card tilts with speed changes and magnetic dip converts the tilt into a false indication: acceleration shows an apparent turn toward north, deceleration toward south. Add oscillation error — the card swinging around the heading — and the lesson is to read the compass only in straight, level, unaccelerated flight, using the average of the swings when setting the heading indicator (IFH ch. 5).

• AHRS (attitude and heading reference system) — solid-state replacement for the attitude, heading, and rate gyros; it feeds attitude and heading to the primary flight display (PFD)
• ADC (air data computer) — takes pitot, static, and temperature inputs and computes altitude, IAS, TAS, and air temperature, output digitally to systems like the EFIS, autopilot, pressurization, and FMS

One box's failure takes out its whole family of indications at once — which is why you learn the failure annunciations and the standby instruments (IFH ch. 5).

Not a navigation system itself — a computer that manages the onboard navigation systems (IFH ch. 9). It carries a database of NAVAIDs, airways, intersections, DPs, STARs, and approaches plus aircraft performance data; you build and edit the flight plan through a control display unit (CDU) instead of managing each sensor separately. The FMS blends all available sensors (GPS, VOR, DME, etc.) into a best computed position — auto-tuning DME stations, it can fix position nearly as accurately as GPS — and modern units add VNAV to the lateral flight plan, providing guidance through departures, arrivals, and approaches (IFH ch. 5, ch. 9).

The system reverts to a composite (reversionary) mode — the surviving display (typically the multi-function display, MFD) presents the primary flight instruments. You lose real estate (the moving map page, airport data access), not the data itself. If the failure is deeper — AHRS, ADC, or total display loss — you're on the standby attitude indicator, altimeter, ASI, and magnetic compass installed for exactly this case. Practice the reversionary switch and the standby scan before you need them in IMC (IFH ch. 11).

It's an emergency — unlike round-dial aircraft, where alternator failure was just an abnormality, a technically advanced airplane's AHRS, ADC, and displays all feed on electrons (IFH ch. 11). Expect roughly 30–40 minutes of main battery — an approximation, never a guarantee — and land at the nearest suitable airport; at no time should you consider continuing the flight once the charging system has failed. The load-shed drill, standby battery, and essential bus are covered under Task VII.D.

Broadly the same airspace (91.215, 91.225):

• Class A, B, and C
• Within 30 NM of a listed Class B primary airport (the Mode C veil), surface to 10,000 ft MSL
• Above the ceiling and within the lateral limits of Class B or C, up to 10,000 ft MSL
• At and above 10,000 ft MSL, excluding at and below 2,500 ft AGL (for ADS-B this is Class E airspace)

Two differences: a transponder is also required from the surface to 10,000 ft MSL within 10 NM of certain listed (appendix D, section 2) airports (91.215(b)(5)(ii)); ADS-B is also required in Class E at and above 3,000 ft MSL over the Gulf of Mexico out to 12 NM from the coastline (91.225(d)(5)).

The transponder replies with Mode A codes and Mode C pressure altitude in 100-ft increments; ADS-B Out must be in transmit at all times once installed, and the transponder must be on in all controlled airspace (91.215(c), 91.225(f)).

Three signal families feed the panel (IFH ch. 9):

• Localizer — lateral guidance on 108.10–111.95 MHz, odd tenths; identified by a three-letter ident prefixed with "I" (e.g., I-SGF)
• Glideslope — vertical guidance, radiated on the front course only
• Marker beacons on 75 MHz — or DME/compass locators substituting — for range along the approach

Course width, glideslope geometry, and the marker light/tone indications are covered in depth under Task VI.B.

IFR GPS must:

• Meet TSO-C129 (or later equivalent standards) — WAAS units under TSO-C145a/C146a
• Be installed per airworthiness requirements and approved for that IFR operation
• Be operated per the POH/AFM or flight manual supplement
• Carry a current, updatable database supporting the operation (en route, terminal, approach)

Equipment approved under TSO-C115a, VFR GPS, and hand-held units meet none of that: they are not authorized for IFR navigation or approaches and count only as situational-awareness aids (IFH ch. 9).

Receiver autonomous integrity monitoring — the receiver's self-check that no satellite is feeding it corrupted data. Without RAIM you have no assurance the GPS position is accurate, which is why integrity monitoring is part of what makes an installation IFR-approved (IFH ch. 9). Satellite counts, RAIM predictions, and the in-flight RAIM messages are covered under Task V.A.

Surveyed wide-area ground reference stations monitor GPS signals; a master station computes corrections that a geostationary satellite broadcasts on the GPS frequency. Results:

• Accuracy — differential corrections shrink position error
• Integrity — real-time monitoring of the constellation
• Availability — the GEO acts as an additional ranging satellite

The payoff for you: an electronic glidepath independent of ground equipment or baro-aiding, immune to cold-temperature and altimeter-setting errors — the basis of LPV-style vertically guided approaches. Avionics are certified under TSO-C145a (sensor) or TSO-C146a (stand-alone) (IFH ch. 9).

Yes — GPS certified for IFR en route and terminal operations may substitute for ADF and DME, and it's also how the DME requirement at and above FL 240 gets met (91.205(e); IFH ch. 9). The conditions — current database, integrity monitoring, CDI sensitivity, and the final-approach limitation — are covered under Task V.A.

Electronic navigation databases update on a 28-day cycle; en route charts revise every 56 days (IFH ch. 1). An updatable database that supports the operation is required for IFR GPS, and GPS-for-DME/ADF substitution explicitly requires the database be current (IFH ch. 9). The confirmation step lives in your flight deck check (Task II.C) — and the risk item is quietly flying procedures that changed at the last cycle: moved waypoints, amended courses, changed minimums.

The governing guidance is AC 91-78 (per the ACS references). Treat the tablet like any other single point of failure:

• Currency — charts and databases downloaded and up to date before the flight
• Power — charged, with a backup power source or second device (or paper) reachable in flight
• Reliability — a locked-up or overheated device in IMC is a lost chart source; know how you'd continue
• Heads-down risk — program and brief on the ground

The IFH's broader warning applies: when the electronic source becomes your only source, its failure compromises your decision-making — plan the degraded case (IFH ch. 11).