Partial Panel and Instrument Failure: Identification, Scan, and Recovery

Why does partial panel matter for the IFR checkride?

Partial-panel flight is a mandatory flight test element under the Instrument Rating ACS (FAA-S-ACS-8), appearing primarily in Area of Operation VII (Instrument Approach Procedures) Task C, where the evaluator must simulate instrument failure and observe the applicant's ability to complete an approach without the primary gyroscopic references. The FAA mandates this test element because instrument failure in IMC is one of the highest-risk loss-of-control scenarios in general aviation.

NTSB accident data consistently shows that spatial disorientation following unexpected gyroscopic failure — particularly in IMC — is a leading cause of fatal accidents. The average time to spatial disorientation onset after losing visual reference is well under two minutes, which is why the IFH Chapter 3 treats recognition speed as a critical survival skill. A pilot who can identify a vacuum failure, cover the offending instruments, and transition to a disciplined partial-panel scan within 15 seconds is dramatically safer than one who spends 60 seconds debugging the panel.

Beyond the flight test, partial-panel skill is a direct function of understanding your aircraft's instrument architecture — which instruments are vacuum-driven, which are electric, and what the pitot-static system does or does not affect. That diagnostic knowledge is the foundation of the oral exam questions DPEs ask on this topic.

What is the analog (six-pack) instrument architecture?

A conventional analog IFR panel uses two entirely separate power sources to drive its instruments, which is the key fact DPEs probe at the start of any partial-panel oral discussion. The IFH Chapter 5 classifies the six primary flight instruments into two groups:

Vacuum-driven gyroscopic instruments:

• Attitude Indicator (AI) — gyroscope spun by suction from the vacuum system
• Heading Indicator (HI) / Directional Gyro (DG) — gyroscope spun by suction from the vacuum system

Electrically driven gyroscopic instruments:

• Turn Coordinator (TC) — electrically driven; contains a rate gyro that also provides bank information through the miniature aircraft

Pitot-static instruments (neither vacuum nor electric for the sensing function):

• Airspeed Indicator (ASI) — senses pitot dynamic pressure minus static pressure
• Altimeter — senses absolute static pressure
• Vertical Speed Indicator (VSI) — senses rate of static pressure change

The magnetic compass is self-contained and powered by neither system. This architecture means that a vacuum failure and an electrical failure produce completely different surviving instrument sets — a fact the DPE will exploit during the oral by asking you to diagnose each failure mode from a described symptom set.

How do you recognize a vacuum system failure?

A vacuum failure is recognizable by a combination of attitude indicator tumbling or erecting slowly, a heading indicator that drifts or stops responding, and a vacuum gauge reading at or near zero — all while the turn coordinator, compass, and pitot-static instruments continue to function normally. The IFH Chapter 5 notes that the AI does not fail immediately — it can take several minutes to tumble as the gyro spins down, making early vacuum gauge monitoring critical.

The diagnostic sequence when vacuum failure is suspected:

1. 1
   Check the vacuum gauge — a reading below approximately 4.5 inches Hg (typical minimum per POH; consult your aircraft's AFM) confirms the failure.
2. 2
   Cross-check the AI and TC: if the TC shows a turn but the AI shows wings-level, or if the AI shows a bank but the TC is level, the AI is unreliable.
3. 3
   Cover or placard the AI and HI immediately — a tumbled gyro can display misleading bank and pitch information that induces spatial disorientation.
4. 4
   Transition your scan to the surviving instruments: TC for bank, ASI for pitch trend, altimeter and VSI for pitch confirmation, magnetic compass for heading.
5. 5
   Declare the situation to ATC, request a lower workload environment (radar vectors, extended final), and plan for a non-precision approach.

The most dangerous phase of vacuum failure is the transition period — the gyro spins down slowly and may show plausible but incorrect information. Pilots who catch the failure early on the vacuum gauge are far safer than those who wait for the AI to tumble visibly.

How do you recognize an electrical failure?

An electrical failure produces the mirror-image symptom set: the turn coordinator goes dark or the inclinometer ball becomes the only useful bank reference, while the AI and HI — both vacuum-driven — continue to function normally. The IFH Chapter 5 also notes that many aircraft use an electrically driven backup pump or an electric AI as the standby attitude source, so the specific failure mode depends on your aircraft's electrical architecture.

Secondary electrical failures affect: radios, transponder, GPS/nav receivers, autopilot, pitot heat (creating a secondary pitot-static failure risk in icing conditions), electric fuel pump, and interior lighting. An electrical failure in IMC is therefore a compounding emergency — it degrades both communication and navigation capability simultaneously. 14 CFR 91.205(d) requires an attitude indicator and a turn-and-slip indicator for IFR flight; if the electrical failure disables the turn coordinator and the backup attitude source is also electric, you are operating below IFR equipment minimums and must declare an emergency.

What are pitot-static system blockage failure modes?

Pitot-static failures affect only the three pitot-static instruments. The failure mode depends on whether the pitot tube, the static port, or both are blocked — and each combination produces a distinct symptom pattern. This table is derived from FAA-H-8083-15B Chapter 5:

The alternate static source — required equipment for IFR flight — corrects all three pitot-static instruments when the primary static port is blocked. As noted in IFH Chapter 5, the alternate static source typically draws from cockpit air pressure, which is slightly lower than ambient static pressure. This causes the altimeter to read slightly higher than actual (typically a few feet) after opening the alternate source. Airspeed and VSI also experience small errors but return to functional readings.

How do glass cockpit reversionary modes work?

Glass panel aircraft such as those equipped with the Garmin G1000 or Avidyne Entegra use an Air Data/Attitude and Heading Reference System (ADAHRS) rather than mechanical gyroscopes. The IFH Chapter 7 describes two distinct failure scenarios:

PFD display failure: If the Primary Flight Display fails (red X or blank screen), the pilot activates reversionary mode — typically by pressing a dedicated REVERSIONARY button on the G1000's display bezel. This transfers all primary flight data (attitude, airspeed tape, altitude tape, HSI) to the MFD. The MFD becomes the de facto PFD, though at a smaller size and without the split-screen engine display. All underlying sensor data continues unaffected.

AHRS failure: If the ADAHRS sensor itself fails (rather than just the display), attitude and heading data become unavailable on both screens — a red X will appear across the attitude indicator on the PFD. In this case, the standby attitude indicator (an independent, battery-powered unit mounted separately on the instrument panel) becomes the primary attitude reference. The standby AI is typically a solid-state or battery-backed unit that operates independently of both the ADAHRS and the main electrical bus.

Key oral exam distinction: a failed display is not the same as a failed sensor. A display failure triggers reversionary mode with full data. A sensor failure means you revert to the standby AI for attitude — which changes the partial-panel scan because you now have a physically small, independently positioned attitude source rather than an integrated tape display.

What are the magnetic compass errors — ANDS and UNOS?

The magnetic compass is the only heading reference that survives both vacuum and electrical failures, but it introduces errors that must be corrected for to fly accurate headings. The IFH Chapter 5 describes two classes of compass error relevant to IFR partial-panel operations in the northern hemisphere:

ANDS — Acceleration error (Accelerate North, Decelerate South):

Acceleration and deceleration cause the compass card to swing in the direction of north or south due to the pendulous mounting of the compass. The error is most pronounced on headings near east or west, and is minimal on headings near north or south.

• Accelerating on an easterly or westerly heading → compass swings toward north
• Decelerating on an easterly or westerly heading → compass swings toward south

Practical implication: when executing a partial-panel approach with heading changes, avoid changing airspeed dramatically on east or west headings — or apply the ANDS correction mentally when reading the compass during acceleration.

UNOS — Turning error (Undershoot North, Overshoot South):

When turning using the magnetic compass in the northern hemisphere, the compass leads or lags the aircraft heading because of the dip error produced by the earth's magnetic field. The error is greatest at north and south headings and zero at east and west.

• Rolling out of a turn toward north → lead the rollout (roll out before compass reaches north — undershoot)
• Rolling out of a turn toward south → delay the rollout (roll out after compass passes south — overshoot)

A practical rule: use bank angle divided by 2 as the lead/lag correction in degrees when rolling out on north or south in standard-rate turns. DPEs frequently ask applicants to demonstrate a timed turn to a specific heading using the compass during partial-panel flight — expect this during the flight test.

How do you redistribute your scan without the AI and HI?

The scan substitution is the most cognitively demanding aspect of partial-panel flight. Without the attitude indicator and heading indicator, you must build attitude awareness from a distributed set of instruments that were previously used as supporting (not primary) references. The IFH Chapter 7 describes the control-and-performance method as particularly useful during partial-panel conditions because it relies on pitch and bank control instruments rather than direct attitude display.

New partial-panel scan priorities:

• Bank (primary): Turn coordinator miniature aircraft — zero deflection means wings-level; the standard-rate index marks indicate a 3°/second turn rate.
• Pitch (primary): Airspeed indicator — stable airspeed means stable pitch. Rising airspeed indicates nose-low; decaying airspeed indicates nose-high.
• Pitch (confirmation): Altimeter and VSI — confirms pitch trend. A stable altimeter and a VSI near zero confirm level pitch.
• Heading: Magnetic compass — usable only when in straight, unaccelerated flight; apply ANDS and UNOS corrections for turns and speed changes.
• Power: RPM and manifold pressure tapes (or analog gauges) — used to set and confirm cruise and approach power settings as a pitch surrogate.

The critical habit change is dwell time. In full-panel flying, the AI gets the most eye time. Partial-panel, the turn coordinator and airspeed indicator share that attention. Many pilots under partial-panel conditions fix on the turn coordinator and neglect airspeed — leading to pitch excursions. Practice deliberately includes the ASI in every scan cycle.

How do you fly an approach partial-panel?

A partial-panel non-precision approach — typically a VOR or localizer approach — is the standard checkride scenario under ACS Area VII Task C. The technique differs from full-panel in three ways: heading control uses the compass and TC instead of the HI and AI; descent management relies more heavily on airspeed and VSI; and workload is higher, requiring simplified configuration changes.

1. 1
   Request a long, straight-in final from ATC when possible — eliminating a procedure turn removes a high-workload phase and reduces the compass-error exposure from turning maneuvers.
2. 2
   Brief the approach plate thoroughly before the IAF: note the inbound course, MDA, timing from FAF to MAP (for timed approaches), and the missed approach procedure. Reduce workload once established inbound.
3. 3
   Establish approach configuration (gear, flaps per POH) before the FAF so that no configuration changes are needed on final. Partial-panel approaches reward simplicity.
4. 4
   Use standard-rate turns only — verify with the turn coordinator. Time the turn using the 3°/second rule: a 90° turn takes 30 seconds at standard rate. Apply UNOS corrections when rolling out on cardinal headings.
5. 5
   At the FAF, set approach power and note time. Monitor CDI for course guidance and ASI + VSI for descent rate. Target a descent rate that achieves MDA with a reasonable time buffer above the MAP.
6. 6
   Level at MDA using airspeed and VSI as primary references. Maintain MDA with small power adjustments — hold until the MAP then execute the published missed approach.

One technique DPEs note as a differentiator: the strongest partial-panel applicants mentally pre-calculate their required descent rate (altitude to lose ÷ time from FAF to MAP × 60 = feet per minute) and set that rate from the FAF rather than chasing the VDI or glidepath. This stabilizes the approach and reduces the scan workload on final.

What are the ACS skill tolerances under partial panel?

The Instrument Rating ACS (FAA-S-ACS-8) does not publish separate, numerically wider tolerances for partial-panel operations — the standards remain heading ±10°, altitude ±100 feet, airspeed ±10 knots. However, the ACS explicitly directs the evaluator to provide adequate time to complete the task and adjust scenario complexity based on the conditions presented.

What this means operationally: the DPE will not fail you for a momentary heading excursion of 12° that you catch and correct while simultaneously identifying the failure. What will cause an unsatisfactory is continued exceedance after identification, failure to cover or disregard failed instruments, failure to apply compass corrections when turning, or a loss of positive aircraft control at any point. The 14 CFR 91.213 inoperative equipment framework also informs the oral portion — a DPE may ask whether the flight is legal to continue given a specific instrument failure and the applicable MEL or KOEL.

What oral and flight test questions do DPEs ask on partial panel?

DPEs approach partial-panel questioning in two phases: an oral diagnosis phase (can you identify which instruments have failed and why?) and a flight test phase (can you actually fly partial-panel to ACS standards?). The most common oral question set follows a pattern of "I tell you a symptom — you diagnose the failure":

• "Your attitude indicator shows a 20° right bank. Your turn coordinator shows wings-level. What is wrong and what do you do?" — Vacuum failure; cover the AI, transition to TC-primary scan.
• "Your turn coordinator is dark. Your attitude indicator shows wings-level. What failed?" — Electrical failure; AI and HI survive, TC is gone. Use AI as primary, use standby AI if available, manage electrical load.
• "Your altimeter has been frozen for the last 5 minutes. Airspeed is normal. VSI reads zero. What happened?" — Static port blocked. Open the alternate static source.
• "You are on a heading of 090°. You accelerate from 90 to 120 knots. What does your magnetic compass read during the acceleration?" — Compass swings toward north (ANDS: Accelerate North).
• "You need to turn to a heading of 360°. Using only the magnetic compass, how do you know when to roll out?" — Roll out before the compass reaches 360° — undershoot north (UNOS). Lead by approximately half the bank angle.

The flight test scenario is typically announced with the DPE covering the AI and HI with suction cups or a foggles-style cover after you are established in cruise. You may be given vectors and then cleared for a VOR or LOC approach. The evaluator is watching for: immediate recognition that the covered instruments are unavailable, a deliberate scan transition, composed compass use during any turns, a stabilized descent on final, and a level-off at MDA with no further descent without visual references.

This article was researched from FAA primary sources (FAA-H-8083-15B Chapters 5–7, Instrument Rating ACS FAA-S-ACS-8, AIM, 14 CFR Part 91) and citing current regulations — drafted by MockDPE. Last updated: May 2026. If you spot an inaccuracy, email corrections@mockdpe.org.