Modern transport aircraft rely on sophisticated, highly integrated avionics suites to present flight data. The primary flight displays (PFDs) and multi-function displays (MFDs) found on today's glass-panel airliners draw from a web of interconnected sensors, computers, and power buses. That integration, while powerful, creates a vulnerability: a single-point failure — a lost power bus, a failed air data computer, or a software fault — can simultaneously deprive the crew of multiple information sources. To guard against this, regulations and sound airworthiness design require an integrated standby instrument system (ISIS), also called a standby flight display or backup flight display (BFD), that operates completely independently of the primary avionics architecture.
The ISIS is not merely a "small version" of the PFD. It is a self-contained, dedicated instrument that houses its own attitude reference, its own air data sensing, its own power supply, and its own display — all in a single line-replaceable unit (LRU). Understanding how these systems work, when they activate, and how they differ from the primary suite is essential knowledge for the Airline Transport Pilot certificate and for practical airline operations.
Architecture and How the ISIS Works
A traditional round-dial standby package consisted of three separate instruments: a standby attitude indicator (typically an electrically driven gyro), a standby airspeed indicator (connected to the pitot-static system), and a standby altimeter. Each was a discrete unit with its own failure modes. The integrated standby instrument system consolidates these three functions — and sometimes more — into a single self-contained display unit typically measuring four to five inches diagonally.
At the core of the ISIS is an Attitude and Heading Reference System (AHRS) that uses solid-state microelectromechanical systems (MEMS) gyroscopes and accelerometers to derive pitch, roll, and, on some units, magnetic heading. Because solid-state units have no spinning rotors, they are far more reliable than legacy gyroscopic instruments and align to an accurate attitude within a short initialization period that varies by manufacturer and model. The ISIS also contains an integrated Air Data Module (ADM) connected to its own dedicated pitot-static ports (on most designs) or tapped from a separate, redundant pitot-static line specifically reserved for the standby instruments. This means a blockage or failure of the primary pitot-static inputs that feed the air data computers (ADCs) will not necessarily compromise the ISIS air data.
The display itself is a dedicated liquid-crystal display (LCD) or active-matrix display that presents an artificial horizon with pitch and bank scales, a digital or analog airspeed readout, and a digital or analog altitude readout — all on one screen. Some installations also display vertical speed. The symbology is intentionally simplified compared with the PFD: there are no flight director command bars, no autoflight modes, and often no navigation overlays, which forces the crew to revert to the raw-data, hand-flying skills that instrument training emphasizes.
Independent Power Supply
The most operationally significant feature of the ISIS is its independent power supply. The unit is connected to the aircraft's essential bus and, critically, to a dedicated battery that can sustain display operation for a manufacturer-specified period defined by the aircraft type certificate, after a complete electrical failure. On some designs the battery is internal to the LRU itself; on others it is a dedicated battery bus that also powers selected essential equipment. When the ISIS senses a loss of normal bus power, it transitions to battery power automatically without crew action.
This self-contained power architecture means the ISIS continues to function even during a dual-generator failure, battery bus fault, or the very rare complete electrical failure scenario. During abnormal and emergency procedures, when crews are systematically shedding electrical loads to conserve power, the ISIS remains a trusted, reliable reference.
Comparison With Primary Flight Displays
Understanding the distinction between the PFD and the ISIS is important for both the written test and practical operations. The primary flight display receives its attitude data from one or more inertial reference systems (IRS) or AHRS units that are also shared with the flight management system (FMS), the autopilot, and the flight director. Air data for the PFD comes from air data computers (ADCs) that process pitot and static inputs and distribute information over a data bus to multiple systems. Because these sources are shared, a failure upstream — a faulty IRS, a failed ADC — propagates through the bus architecture and can affect multiple displays simultaneously.
The ISIS, by contrast, owns its sensors outright. Its AHRS does not share gyroscope or accelerometer data with any other computer. Its ADM does not share its pitot-static connection with the ADCs. The ISIS is therefore described as an independent, reversionary flight reference: it provides data that is not susceptible to the same failure modes as the primary suite.
Why the ISIS Matters for Safety and Operations
Aviation accident history contains several sobering examples of primary avionics failures that left crews scrambling for reliable attitude information. In cases where ice blocked pitot-static systems, all primary airspeed indications became unreliable simultaneously. The standby instrument — whether a traditional gyro or a modern ISIS — became the primary reference. Crews trained to use the ISIS as a genuine backup, not merely a regulatory box to check, have a significant advantage in these scenarios.
From a regulatory standpoint, 14 CFR Part 25 (Airworthiness Standards: Transport Category Airplanes) requires transport aircraft to be equipped with approved standby attitude, airspeed, and altitude instruments. Section 25.1303 sets out the required flight and navigation instruments generally, and the specific requirement for an independent standby attitude indicator, powered from a source independent of the airplane's normal electrical generating system, is found in 25.1333(b). The ISIS design satisfies these requirements in a single integrated LRU, reducing panel space, weight, and the wiring complexity associated with three separate instruments.
Operationally, Standard Operating Procedures (SOPs) at most major air carriers include cross-checking the ISIS during initial climb, cruise, and approach as part of instrument scan. Crews are also typically required to demonstrate proficiency in flying an approach and performing abnormal procedures using only standby instruments as part of their recurrent simulator training.
Key Numbers and Rules
- Independent power duration: The ISIS battery must sustain operation independently for a minimum period defined by the aircraft type certificate. There is no single FAA-mandated duration applicable to all designs; the actual minimum varies by aircraft and is documented in the type certificate data.
- 14 CFR Part 25.1303 and 25.1333: Section 25.1303 lists the flight and navigation instruments required generally for transport category airplanes, while 25.1333(b) more specifically requires an independent standby attitude indicator powered from a source independent of the normal electrical generating system — together forming the regulatory basis for standby instrument requirements.
- Separate pitot-static source: Many aircraft designs mandate that the standby pitot-static source be physically isolated from the primary pitot-static lines to prevent simultaneous blockage from a single icing event.
- No flight director integration: The ISIS does not receive autopilot or flight director commands; it presents raw data only, requiring the pilot to use fundamental instrument flying skills.
- Initialization time: Solid-state AHRS in modern ISIS units align to a valid attitude reference in a short period on the ground, though the exact time varies by manufacturer and model; some units provide a fast-erect capability in flight after a power interruption.
- Single LRU design: The ISIS is typically one replaceable unit, simplifying maintenance and minimizing the number of components that could individually fail.
Common Test Traps
- Confusing the ISIS power source with the normal essential bus: The ISIS draws from the essential bus in normal operations, but test questions often probe whether students know it has its own dedicated battery for complete electrical failure scenarios — the key differentiator from other essential-bus equipment.
- Assuming the ISIS shares sensors with the PFD: A common misconception is that the ISIS simply receives a signal from the same IRS or ADC that drives the PFDs. It does not — its sensors are wholly independent, which is precisely what makes it a genuine backup rather than a redundant display.
- Overlooking the absence of flight director guidance: Exam scenarios may describe a complete avionics failure and then ask what guidance cues remain available. The ISIS provides raw pitch, bank, airspeed, and altitude data only — no command bars, no glideslope deviation, no localizer deviation on most basic units.
- Misidentifying the regulatory authority: The requirement for independent standby instruments in transport aircraft flows from Part 25 airworthiness standards (25.1303 and 25.1333), not from Part 91 operational rules. ATP-level questions sometimes test this distinction.
- Battery duration specifics: Questions may attempt to anchor students to a single, specific battery duration number. The correct answer is that duration is aircraft- and type-certificate-specific, with no single FAA-mandated figure applying to all designs.