5.4 Partial-Panel Instrument Flight & Systems Failures

Partial-Panel Instrument Flight & Systems Failures

Quick Answer: Partial-panel flight occurs when one or more primary flight instruments fail—most commonly due to a vacuum pump failure, which disables the Attitude Indicator (AI) and the Heading Indicator (HI). In this scenario:

• Primary Bank: Turn Coordinator (electrically powered, independent of vacuum system).
• Primary Pitch (Level Flight): Altimeter (supported by VSI and Airspeed Indicator).
• Primary Power: Tachometer / Manifold Pressure (supported by Airspeed Indicator).
• Turns: Executed via Timed Turns at Standard Rate ($3^\circ/\text{sec} = \text{Degrees} / 3$) or Magnetic Compass Turns using UNOS (Undershoot North, Overshoot South) and ANDS (Accelerate North, Decelerate South).

In conventional general aviation aircraft equipped with analog instruments, the flight panel relies on two independent power sources: the engine-driven vacuum (pneumatic) system and the aircraft electrical system. Understanding the failure modes of these systems and maintaining aircraft control with a degraded instrument panel is a mandatory requirement of the FAA Airman Certification Standards (ACS Area of Operation VI & VII).

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Vacuum System Failure Mechanics & Gyro Degradation

In standard analog aircraft, the engine-driven vacuum pump creates suction (typically 4.5 to 5.5 in. Hg) to spin the gyroscopic rotors of:

1. The Attitude Indicator (AI)
2. The Heading Indicator (Directional Gyro / HI)

Conversely, the Turn Coordinator (or Turn-and-Slip Indicator) is powered by an independent DC electric motor connected to the aircraft's electrical bus.

+-----------------------------------------------------------------------+
|                   ANALOG FLIGHT INSTRUMENT POWER SOURCES              |
|                                                                       |
|   VACUUM PUMP POWERED (Pneumatic)      ELECTRICAL BUS POWERED (DC)    |
|   -------------------------------      ---------------------------    |
|   • Attitude Indicator (AI)            • Turn Coordinator (TC)        |
|   • Heading Indicator (HI)             • Pitot Heat & Avionics        |
|                                                                       |
|   PITOT-STATIC PRESSURE POWERED (Atmospheric)                         |
|   -------------------------------------------                         |
|   • Airspeed Indicator (Pitot + Static)                               |
|   • Altimeter (Static Only)                                           |
|   • Vertical Speed Indicator (Static Only)                            |
+-----------------------------------------------------------------------+

The Deceptive Nature of Vacuum Failures

When an engine-driven vacuum pump shears its drive shaft, the suction gauge immediately drops to zero. However, the heavy mechanical gyroscopes inside the AI and HI do not instantly stop. They spin down slowly over 5 to 15 minutes.

As the rotor RPM decays, rigidity in space degrades and gyroscopic precession accelerates wildly. The Attitude Indicator begins to tilt, wobble, and indicate false banks or dives, while the Heading Indicator begins to precess at rates exceeding $30^\circ$ per minute. Because traditional analog gyros rarely have failure flags, a pilot who fails to monitor the suction gauge can easily be led into a fatal spiral dive by following the misleading AI.

Immediate Pilot Action: The moment a failed vacuum gyro is identified, cover the failed instruments with suction-cup covers, sticky notes, or paper. This eliminates subconscious visual fixation on misleading attitude cues.

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Partial-Panel Instrument Re-Assignment Matrix

When operating partial-panel with the Attitude Indicator and Heading Indicator inoperative, the pilot must mentally reassign primary and supporting roles across the remaining instruments:

+-----------------------------------------------------------------------+
|             PARTIAL-PANEL INSTRUMENT REASSIGNMENT MATRIX              |
|                                                                       |
|   FLIGHT AXIS   PRIMARY INSTRUMENT        SUPPORTING INSTRUMENTS      |
|   -----------   ----------------------    -------------------------   |
|   BANK          Turn Coordinator          Magnetic Compass            |
|   PITCH (Level) Altimeter                 VSI & Airspeed Indicator    |
|   PITCH (Climb) Airspeed Indicator        Altimeter & VSI             |
|   POWER         Tachometer / MAP          Airspeed Indicator          |
+-----------------------------------------------------------------------+

1. Bank Control (Turn Coordinator)

• The Turn Coordinator miniature aircraft indicates roll rate and rate of turn ($3^\circ$/sec when aligned with the standard rate index marks).
• If the miniature aircraft wings are level, the aircraft is maintaining a constant heading (no turning).
• The Inclinometer (ball) indicates aerodynamic coordination. Keep the ball centered with rudder ("step on the ball").

2. Pitch Control (Altimeter, VSI, Airspeed)

• Level Flight: The Altimeter is primary for pitch. If the altimeter needle is frozen on the assigned altitude, pitch is correct.
• VSI: Shows instantaneous trends. If the VSI needle moves off zero, apply immediate, gentle elevator pressure to stop the needle before an altitude deviation develops.
• Airspeed Indicator: Provides pitch trend confirmation. If airspeed decreases without a power change, the pitch attitude is increasing.

3. Power Control (Tachometer / Manifold Pressure)

• Use the tachometer or manifold pressure gauge to set exact, known engine power settings for level cruise, climb, or descent profiles.

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Timed Turns & Magnetic Compass Operations

Without a heading indicator, the pilot must navigate and turn using two methods: Standard Rate Timed Turns and Magnetic Compass Turns.

+-----------------------------------------------------------------------+
|                   STANDARD RATE TIMED TURN FORMULA                    |
|                                                                       |
|          Time (Seconds) = Desired Heading Change (Degrees) / 3        |
|                                                                       |
|   Examples:                                                           |
|   • 30° Heading Change  ===>  30 / 3 = 10 Seconds                     |
|   • 45° Heading Change  ===>  45 / 3 = 15 Seconds                     |
|   • 60° Heading Change  ===>  60 / 3 = 20 Seconds                     |
|   • 90° Heading Change  ===>  90 / 3 = 30 Seconds                     |
|   • 180° Heading Change ===> 180 / 3 = 60 Seconds (1 Minute)          |
+-----------------------------------------------------------------------+

Executing a Standard Rate Timed Turn:

1. Calculate the required time: $\text{Seconds} = \Delta\text{Heading} / 3$.
2. Look at the clock/timer sweep second hand.
3. Roll into a standard rate turn on the Turn Coordinator (deflect miniature aircraft wing to the index line).
4. Start timing as the roll-in begins.
5. Maintain the turn coordinator wing exactly on the standard rate mark with centered ball.
6. At the calculated elapsed time, roll the wings level.
7. Check the magnetic compass once the aircraft is straight, level, and unaccelerated.

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Magnetic Compass Turn & Acceleration Dip Errors

The magnetic compass is the only self-contained directional instrument on board, but it is subject to magnetic dip errors in the Northern Hemisphere that must be compensated for during partial-panel flight.

+-----------------------------------------------------------------------+
|                   MAGNETIC COMPASS DIP CORRECTION RULES               |
|                                                                       |
|   [ U - N - O - S ] =====> Undershoot North, Overshoot South          |
|   • Turning to North: Compass LAGS  -> Roll out BEFORE reaching North |
|   • Turning to South: Compass LEADS -> Roll out PAST reaching South   |
|                                                                       |
|   [ A - N - D - S ] =====> Accelerate North, Decelerate South         |
|   • Accelerate on East/West heading: Compass swings toward North      |
|   • Decelerate on East/West heading: Compass swings toward South      |
+-----------------------------------------------------------------------+

1. Turning Errors (UNOS: Undershoot North, Overshoot South)

Due to magnetic dip, when turning from an easterly or westerly heading:

• Turning Toward North: The compass first indicates a turn in the opposite direction, then lags behind the actual heading — the card shows less turn than the airplane has actually flown. If you wait for the card to read 360°, you have already turned past north, so the pilot must undershoot (roll out prior to) the northern heading. $\text{Rollout Lead Angle} = \text{Latitude} + \frac{\text{Bank Angle}}{2}$ (e.g., At $30^\circ\text{N}$ latitude with a $15^\circ$ bank, lead rollout by $30 + 7.5 = 38^\circ$ before North, rolling out at $038^\circ$ or $322^\circ$).
• Turning Toward South: The compass leads the turn — the card races ahead and shows more turn than the airplane has actually flown. The pilot must therefore overshoot (roll out past) the southern heading by the same latitude correction factor before rolling wings level.
• East and West Headings: There is zero turning error when rolling out directly on headings of East ($090^\circ$) or West ($270^\circ$).

2. Acceleration Errors (ANDS: Accelerate North, Decelerate South)

When flying on an East ($090^\circ$) or West ($270^\circ$) heading:

• Acceleration: The compass card tilts and indicates a false turn toward the North.
• Deceleration: The compass card tilts and indicates a false turn toward the South.
• When airspeed stabilizes, the compass card swings back to the correct heading.

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ATC Assistance & Radar "No-Gyro" Approaches

If a vacuum or primary instrument failure occurs in IMC, the pilot should promptly notify Air Traffic Control:

"Denver Center, Cessna 172SP, N12345, declaring an emergency, experienced vacuum pump failure, operating partial panel, request radar vectors for an ILS or non-precision approach."

ATC No-Gyro Radar Vectors & Approaches

Under FAA Order JO 7110.65, ATC can provide specialized No-Gyro radar vectors:

1. ATC issues turn instructions specifying direction and stop commands:
   • "Cessna 345, turn right."
   • (Pilot immediately enters a standard rate turn).
   • "Cessna 345, stop turn."
   • (Pilot immediately rolls wings level).
2. Turn Rates:
   • During the Vector/En Route segment: Pilot flies Standard Rate Turns ($3^\circ$/second).
   • On the Final Approach segment: Pilot switches to Half-Standard Rate Turns ($1.5^\circ$/second) for fine track alignment.