ACS Task IR.II.B — Aircraft Flight Instruments and Navigation Equipment

What is ACS Task IR.II.B and what will the DPE ask?

ACS Task IR.II.B is the preflight knowledge evaluation covering every flight instrument and navigation system in your cockpit. Under the Instrument Rating ACS (FAA-S-ACS-8C) , you must exhibit knowledge of how each instrument works, what errors it produces, and what its practical limitations are during IFR flight.

The DPE will typically frame this around your specific aircraft. Expect scenario questions like: "Your vacuum pump fails shortly after entering IMC — which instruments are now unreliable and how do you identify them?" or "Walk me through the magnetic compass errors you'd expect turning from east to a northerly heading." Candidates who can only recite definitions will struggle; candidates who can connect instrument failure modes to real-time cockpit management will satisfy the DPE quickly.

This task pairs closely with

, which covers the underlying systems (vacuum pump, pitot heat, electrical bus) that power these instruments.

How do pitot-static instruments work and what errors affect them?

The ASI, altimeter, and VSI all receive their inputs from the pitot-static system, as described in FAA-H-8083-15B Chapter 5 . Understanding the source of each reading tells you exactly how each instrument fails.

Pitot tube blockage with drain hole open causes the ASI to read zero — the drain hole equalizes both ports. Pitot tube blockage with drain hole also blocked traps ram pressure: the ASI becomes a crude altimeter, rising in a climb and falling in a descent. The alternate static source, required equipment under 14 CFR 91.205(d) , restores static inputs from inside the cabin. Cabin air is slightly lower pressure than outside, so altimeter reads slightly high and ASI reads slightly fast on alternate static — consult the POH for the specific correction.

How do gyroscopic instruments work and what errors affect them?

The attitude indicator (AI), heading indicator (HI), and turn coordinator are all gyroscopic instruments, though they sense different axes and use different gyro orientations, per FAA-H-8083-15B Chapter 5 .

Attitude Indicator (AI): Uses a gyro spinning in the horizontal plane, kept erect by an air-driven or electric erection system. The AI is the primary pitch and bank reference during IFR flight. Errors include:

• Precession during a prolonged coordinated turn — the AI may indicate a slight bank after rolling out due to gyro drift
• Tumbling if the gyro gimbal limits are exceeded (typically ±60–70° pitch, ±100° bank on most vacuum-driven indicators)
• Power failure — vacuum AI becomes unreliable immediately; electric AI may have a brief spin-down period
• Erection lag — after a cold start, full erection takes 3–5 minutes; never attempt IFR departure before the AI is erected

Heading Indicator (HI / Directional Gyro): Senses rotation about the vertical axis. The HI has no magnetic north reference and must be aligned with the magnetic compass in straight, level, unaccelerated flight. Because bearing friction causes slow gyro drift, the HI requires realignment every 10–15 minutes in flight. Many modern HSIs with flux gates or magnetometers slaved to the compass eliminate manual realignment.

Turn Coordinator: Contains a gyro canted 30° from horizontal, sensing both rate of turn and roll rate. The ball (slip/skid indicator) is a separate inclinometer — it shows coordination, not turn rate. The turn coordinator confirms gyro function and is the primary partial-panel turn reference when the AI fails.

What are the magnetic compass errors every IFR pilot must know?

The magnetic compass is the only instrument that senses magnetic north directly, making it the primary backup heading reference if the HI fails. It also has the most idiosyncratic error behavior of any instrument in the cockpit. The Instrument Flying Handbook (FAA-H-8083-15B, Chapter 5) identifies two categories of error that every IFR pilot must be able to explain and compensate for.

Oscillation errors occur during turbulence or abrupt maneuvers — the compass card swings and is difficult to read accurately. The fix is to read the compass in smooth air.

Acceleration / Deceleration Errors — ANDS:

In the northern hemisphere, the compass float assembly is slightly bottom-heavy, which causes the compass card to dip toward the north magnetic pole when the aircraft accelerates or decelerates. The mnemonic is:

Accelerate — North (false turn toward north) Decelerate — South (false turn toward south)

ANDS errors are greatest on east and west headings and are absent on north and south headings. On an east heading, accelerating causes the compass to swing toward north; decelerating swings it toward south — even though the aircraft is flying straight.

Turning Errors — UNOS:

When turning through northerly headings, the dip of the compass float causes the compass to lag behind the actual heading. When turning through southerly headings, it leads. The mnemonic:

Undershoot North (roll out early — compass reads less than actual heading) Overshoot South (roll out late — compass reads more than actual heading)

In the northern hemisphere: to turn from east to north, start rolling out when the compass reads approximately 30° (bank angle added to the lag). To turn from east to south, continue the turn until the compass reads approximately 150–180° before rolling out, depending on bank angle and latitude. Turning errors are greatest on north and south headings and absent on east and west.

What navigation systems does IR.II.B cover and what do you need to know about each?

IR.II.B covers every navigation system installed in your checkride aircraft. The DPE expects you to know how each system works, what it displays, what its failure modes are, and what regulatory requirements govern its use, per FAA-H-8083-15B Chapter 9 .

VOR currency under 14 CFR 91.171: Every VOR receiver used for IFR flight must be checked within the preceding 30 days. Acceptable check methods, per 14 CFR 91.171(a) , include: a VOT (VOR test facility, ±4°), a designated ground checkpoint (±4°), a designated airborne checkpoint (±6°), or a dual VOR cross-check (both receivers tuned same VOR, within 4° of each other). The check must be logged with date, location, bearing error, and pilot signature.

GPS RAIM: RAIM (Receiver Autonomous Integrity Monitoring) uses redundant satellite geometry to detect errors exceeding the protection level for the operation. Before any GPS approach, confirm RAIM availability at your ETA. Many avionics suites display RAIM prediction on the approach page. A predicted or active RAIM alert means you cannot conduct that GPS approach — brief an alternate approach procedure.

ILS false glideslope: ILS glideslopes transmit on two lobes. The false glideslope intersects the ground at approximately 3× the true glideslope angle. Never intercept the glideslope from above without a verified altitude cross-check — you may capture the false course and descend at 9° or more.

What are the risk management elements for IR.II.B?

The ACS requires you to assess and mitigate risk related to instrument and navigation equipment. The DPE is listening for whether you treat these as abstract technical facts or as real-time decision inputs.

• Partial-panel operations — identify which instruments are unreliable before departure and brief your failure response; do not wait until IMC to discover a failed vacuum pump
• Gyro erection lag — a cold gyro that has not fully erected before departure will indicate false attitude and heading data early in the flight, greatest risk during the departure climb
• Compass deviation — all compasses have a calibration card; know where it is and apply corrections when using the compass as the primary heading reference
• GPS signal outages and RAIM failures — always file alternate navigation capability; never depart IFR with GPS as the sole means of navigation unless alternates are briefed
• Database currency — an expired nav database used for an approach is a regulatory violation and a safety risk; check expiry date during preflight
• Over-reliance on autopilot and flight director — these tools use the same sensors; a failed AI or degraded GPS feeds bad data to the automation without an obvious annunciator

What does the DPE look for when evaluating IR.II.B?

The DPE's standard is not recitation — it is applied understanding. According to the Instrument Rating ACS , you must exhibit the ability to explain each instrument's function, recognize failure indications, and describe the operational impact on IFR flight.

Specifically, the DPE looks for:

1. 1
   Correct and concise explanations of pitot-static instrument operation without confusing the port sources (e.g., stating the altimeter reads pitot pressure is a critical error)
2. 2
   Accurate description of both ANDS and UNOS compass errors — not just the mnemonic, but the physical cause and the corrective technique for the specific maneuver
3. 3
   Knowledge of which instruments are vacuum-driven vs. electrically powered in your aircraft, and the failure mode of each
4. 4
   Citation of the 30-day VOR check requirement under 14 CFR 91.171 with the acceptable check methods and logging requirements
5. 5
   Explanation of RAIM — what it monitors, when it can fail, and what you do when it fails before or during an approach
6. 6
   Database currency requirement for GPS approaches and the distinction between approaches (current required) and enroute/terminal (expired may be acceptable with verification)

What are the most common errors on IR.II.B?

• Confusing pitot and static sources — stating the altimeter uses pitot pressure or that the ASI uses only one port instead of a differential between pitot and static
• Mixing up ANDS and UNOS — candidates often reverse the mnemonics under pressure; if you learn the physical cause (dip toward the pole), you can reconstruct the mnemonic from first principles
• Not knowing the VOR check tolerance for each method — VOT and ground checkpoint are ±4°, airborne is ±6°, dual VOR is ±4° between receivers
• Stating GPS RAIM is a ground station — RAIM is an onboard receiver function that uses satellite geometry mathematics; WAAS is the augmentation that uses ground stations
• Not knowing which instruments in your specific aircraft are vacuum vs. electric — the DPE will ask about your airplane, not a generic aircraft
• Forgetting the logging requirement for VOR checks — date, place, bearing error, and signature are all required under 14 CFR 91.171(b)