The attitude indicator (AI), called the artificial horizon, gives the pilot an immediate indication of the smallest orientation change by showing the relationship of the aircraft relative to the actual horizon. When the gyro horizon is driven internally or by a vacuum system failure, the instrument can become stuck or act erratically, leaving the flight crew to rely on the standby attitude indicator, an electrically driven unit that will, after a short time, also fail if its electrical power fails.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
Is an attitude indicator required?
Yes, an attitude indicator is required but only for IFR flights. FAR 91.205 states that an attitude indicator is required for IFR day and night flight. An attitude indicator is not required for VFR day and night flight.
I deem an attitude indicator necessary because it provides me with data regarding the aircraft pitch which boosts my situational awareness.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Can you fly when an attitude indicator fails?
Yes, a pilot can continue to fly when the attitude indicator fails. All airplanes have a turn coordinator, and that gyroscope instrument still displays rate-of-roll and rate-of-turn information, so control around the longitudinal axis can be maintained by reference to the miniature airplane and the inclinometer. The same six-pack panel also contains an altimeter, an air-speed indicator, and a heading indicator and when those three are cross-checked they reveal pitch changes through altitude and speed trends, while the heading indicator keeps the nose pointed in the desired azimuth. In VFR conditions, pilots can look outside the cockpit at wings and compare the natural horizon with the airplane's longitudinal axis, so the failed attitude indicator is more an inconvenience than an emergency.
The autopilot is unaffected and remains fully functional in many installations because newer aircraft have glass panels that take their attitude data from independent solid-state gyros or air-data computers, not from the traditional vacuum-driven gyro. If the airplane is so equipped, employing the autopilot allows the pilot to reduce workload while the avionics maintain attitude and heading. When the failure occurs in IMC, the same partial-panel scan taught for instrument training-center the turn - coordinator airplane, hold heading with the heading indicator, and keep altitude with the altimeter - provides a stable platform until clear air or an approach is achieved.
Can you fly with a broken attitude indicator?
Yes, you can fly with a broken attitude indicator. 91.213(e) allows flight with inoperable instruments so long as the aircraft remains airworthy and the failure is placarded. For VFR-day operations the attitude indicator is not on the required list, so the answer is the same as for any item not required for VFR-Day flight: it is inoperative provided the Minimum Equipment List is satisfied and the inop process was followed. A round-dial pilot who sees the vacuum indicator marked inop simply follows the checklist, pulls the breaker if it is pullable - turn coordinator circuit breaker is not pullable - and continues.
Because the turn coordinator is DC-powered, it is still alive after the loss of vacuum pumps and the pilot can use the turn coordinator to keep wings level. A pilot can also use pitch attitude and power settings to control airspeed, and, if the ship is so equipped, the autopilot remains fully functional and can be flown fully coupled to a landing as soon as practical. Thus a broken attitude indicator is not a deal-breaker and the flight legally departs and completes its mission.
I believe that a skilled aviator can pilot the aircraft safely without a working attitude sign, provided the attention shifts into visual meteorological circumstances. My trust in flying without an orientation device is dependent upon keeping those visual meteorological circumstances, because my main emphasis is outside the aircraft, on the true skyline. I construct a full photograph of the aircraft's situation and attitude by cross checking my velocity, measuring device, and reverse coordinator, sticking to the basic fundamentals of relying on external sources. Should I get into instrument meteorological circumstances, the proper exigency process typically includes a 180-degree change of direction to come back to visual contexts, accompanied by a prompt announcement of an exigency.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Why is my attitude indicator not working?
The attitude indicator may not be working due to a vacuum failure; if the pump stops, suction drops below the 4.5-5.2 inch (11.4-13.2 cm) minimum, the gyros slow down, and the instrument can no longer hold itself erect. A leak or crimp anywhere in the lines further reduces airflow, so the gyro case remains tilted and the horizon line creeps off-scale.
Even with good suction, the instrument may misbehave. Sticky pendulous or erecting vanes trap the gyro in a tilted position. When air flows unevenly across the vanes the horizon disc stays canted, giving unreliable bank or pitch indications. If the gyro has tumbled because you exceeded the 60-70 degree pitch or 360 degree roll limits, the vanes jam against the housing. A quick cage and re-erection restores the display, but a mechanical inspection provides a definitive answer.
Worn bearings produce the same symptoms more slowly. When the instrument is not periodically exercised, oil settles to the bottom of the race, leaving bearings not properly lubricated. The gyro then whines, cannot spin at the required RPM, and precession makes the horizon drift or oscillate. In electric units a dead backup battery or failing motor leaves the AI inoperative.
Finally, check the system, not just the instrument. If the heading indicator is also unreliable and both are vacuum powered, your vacuum system has failed. Verify pressure or vacuum pump performance, listen for a loud gyro, and confirm the instrument air gauge reads 4.5-5.2 inches (11.43-13.21 cm) as anything less means the AI will only pretend to work.
Why does the attitude indicator not precess?
The attitude indicator does not precess because it is built to cancel its own precession. A pendulous vane arrangement, actuated by gravity, senses the smallest tilt away from the local vertical and immediately applies a correcting force. The self-erection device keeps the gyro erect when precession tries to send it off-kilter, so the spin axis remains pointed at the centre of the Earth. In straight-and-level, non-accelerated flight this mechanism always works to keep the gyro aligned with the vertical and any slow falling-out or sluggish movement is detected and removed before the pilot notices it.
The rapidly-spinning rotor disk exhibits rigidity in space: the wheel resists any attempt to disturb its orientation, so random friction in the gimbal cannot slowly walk the indication away. During a 360 degree turn the precession induced in the first 180 degree is cancelled by opposite precession in the second 180 degree, leaving the displayed horizon where it started. Newer-design instruments permit full 360-degree rotation about the roll and pitch axes without tumbling, so the gyro never hits the stops and never receives the sharp precessing jolt that follows.
The result is an instrument that re-aligns itself continuously. Errors produced by acceleration or by Earth-rate components are small and erase themselves within a minute after the aircraft returns to straight-and-level flight. Because the attitude gyro levels itself on start-up and then keeps itself level, the pilot sees a horizon that stays fixed and virtually free of precession for the entire flight.
Is flying without an attitude indicator safe?
Flight can be safely accomplished by gyroscopic instrument systems, yet an inoperative attitude indicator introduces measurable risk. When the instrument fails, the pilot is left with a partial panel situation. Scan narrows to the remaining instruments, workload rises, and spatial disorientation becomes more likely. Vacuum failure manifests with the instrument acting erratically: the horizon bar drifts, tumbles, or freezes, giving subtle, conflicting cues. Accelerations cause slight pitch up indication and decelerations cause slight pitch down indication. Anomalies and malfunctions are possible, so you must contemplate carrying backup systems like an electrically driven horizon or an AOA indicator that will provide several benefits. Preflight actions minimize chances of inaccurate information: check suction, flag freedom, and post-flight tumble.
Flying without an attitude indicator is achievable and, in many contexts, harmless. The remaining instruments jointly supply adequate data to determine the aircraft's orientation, so the lack of the attitude device is of small danger under visual conditions. My worry exists only during instrument meteorological circumstances, where the natural view is lost.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What should I do if my attitude indicator fails?
If you suspect a failed instrument, cover the attitude indicator with a piece of paper to remove it from your scan. With the attitude indicator out, you will need to rely on your other instruments for pitch and bank information. Hold the attitude indicator wings level with the nose on the horizon, then announce to yourself which instruments are inop and switch to standby instruments.
In IMC this is an emergency: declare an emergency, aim for VMC, and ask ATC to give you surveillance radar data. Remain calm, stabilize your descent per the IAP, and ignore the inop instruments while you utilize your backup instruments. A proper scan in an AHRS/ADC failure always starts and ends with the standby attitude indicator, or, if that is also lost, the standby airspeed indicator. Verify pressure or vacuum pump performance, and if the aircraft was put in a slow spiral, recognize when you are in an unusual attitude and perform the correct coordinated and smooth flight control application. After landing, have your instrument checked as soon as possible.
My first response during an attitude indicator failure is to move my concentration to my main flight instruments. I depend completely on my continuing gyroscopic devices, especially the turn coordinator, and cross-check the heading indicator, airspeed dial, and measuring device to create a dependable pitch and bank posture. This controlled scan prevents spatial confusion and keeps aircraft command through the rotation coordinator. If circumstances allow, I start a cautious shift to visual flying laws, utilizing the physical skyline as my main source. I perform the proper crisis operations for my particular aircraft and intend a prompt divert to the closest appropriate airfield, meanwhile informing aviation traffic control of my status and intent.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
