The Auxiliary Power Unit — universally abbreviated APU — is a self-contained, small-turbine powerplant installed in most transport-category aircraft, almost always in the tail cone. Unlike the two or four propulsion engines that move the aircraft through the air, the APU's core role is to deliver independent electrical power and high-pressure bleed air whenever the main engines are not running or cannot carry the full utility load, and its specific functions — such as bleed air for engine starting or supplemental in-flight power — vary by aircraft type. For ATP candidates and working airline pilots alike, a thorough understanding of what the APU does, how it is limited, and how it fails is not merely exam knowledge — it is the foundation of sound abnormal and emergency decision-making at the front of a 150-seat jet.
What the APU Actually Does
The APU is, in simplified terms, a gas turbine engine that does not produce thrust. Its turbine section drives two primary outputs: a load compressor and an AC generator (alternator). The load compressor draws ambient air through an intake door — typically a scoop or louvered door on the fuselage — compresses it, and delivers it to the aircraft's pneumatic (bleed air) manifold. The AC generator connects to an APU Generator Bus and, through bus tie contactors, can power the entire main AC electrical distribution system.
Bleed Air Functions
On the ground, bleed air from the APU serves two purposes that would otherwise require running the main engines or connecting ground equipment. First, it provides the pneumatic pressure needed to drive the air cycle machines (ACMs) in the environmental control system, conditioning and pressurizing the cabin before departure. Second, and critically for normal operations, it drives the air turbine starters on the main engines. During a standard engine start, the flight crew opens the APU bleed air valve, routes high-pressure air to the starter, and the starter spools the engine's N2 compressor to the ignition threshold — typically in the range specified by the AFM — at which point fuel and ignition are introduced. Once combustion is self-sustaining and the starter cuts out, the main engine takes over its own bleed air and electrical generation, and the APU is normally shut down before taxi.
Electrical Functions
The APU generator is rated in kilovolt-amperes (kVA) for a specific load, and this rating varies significantly by aircraft type — pilots should consult the specific AFM rather than assume a standard figure. This capacity is sufficient to power avionics, lighting, galleys, and passenger systems during ground operations, but may require load shedding — automatic or manual deselection of non-essential buses — if demand approaches the generator's limit. Crews should not assume that every system operable on two main engine generators is simultaneously operable on the APU generator alone.
APU Operating Envelope and Altitude Limitations
Every APU has a certified operating envelope published in the Aircraft Flight Manual (AFM) and often summarized in the Quick Reference Handbook (QRH). The envelope defines three distinct altitude thresholds that pilots must internalize:
- Maximum altitude for APU operation: The highest altitude at which the APU, once running, can continue to supply bleed air and/or electrical power. This ceiling varies widely by aircraft type and must be verified in the specific AFM rather than assumed from a general figure.
- Maximum altitude for in-flight APU start: The highest altitude at which an in-flight relight can be attempted. This ceiling is always lower than the operation ceiling and is aircraft-specific, published in the AFM. Above this altitude, if the APU is shut down or flames out, the crew cannot restart it.
- Maximum altitude for bleed air use: Some APUs can supply electrical power at altitudes where their load compressor cannot deliver useful bleed pressure. The AFM will specify separate bleed-air and electrical ceilings if applicable.
The practical implication is clear: if a crew shuts the APU down for any reason during cruise above the in-flight start ceiling, they cannot restore APU-sourced power or bleed air until descending below that threshold. This is why many airline SOPs direct crews to leave the APU running through a certain flight phase if an electrical abnormality has been detected.
Cross-Bleed Engine Starts
A cross-bleed start is the procedure used when one main engine is already running and its bleed air is routed across the pneumatic manifold to start the opposite engine — bypassing the APU entirely. However, if the first engine also fails to start, or if the aircraft is at a remote airport with no GPU available and the APU is inoperative, the crew has no bleed air source for engine starting. This scenario underscores why APU serviceability is a dispatch consideration under the Minimum Equipment List (MEL). Most MELs permit dispatch with an inoperative APU under specific conditions, but the flight planning implications — needing a GPU at every stop, for example — are significant and must be coordinated.
APU Fire Detection and Extinguishing
The APU has its own dedicated fire detection loop and fire extinguisher bottle, completely independent of the main engine fire suppression system. When an APU fire warning illuminates — typically a red warning light and associated master warning — the QRH directs the crew to immediately shut down the APU and discharge the fire agent. On many transport aircraft there is only one APU fire extinguisher bottle, meaning the crew may get only one shot at extinguishment, though this varies by aircraft type and must be confirmed against the specific AFM. If the fire does not go out after bottle discharge, the remaining options are limited to assuring APU fuel shutoff is confirmed and preparing for a possible diversion or precautionary landing.
A critical distinction: do not apply main-engine fire procedures to an APU fire. The APU fire handle (where fitted) controls its own dedicated fuel shutoff valve and fire agent squib. On aircraft where the APU fire control is integrated into the overhead panel rather than a separate fire handle, the QRH sequence must be followed precisely. Confusing the two systems in a high-workload environment is a well-documented error mode.
Fuel Considerations
The APU draws fuel from the aircraft's own tanks, typically from the center tank or a designated collector cell, depending on aircraft type. Extended APU operation — for example, two hours of ground air conditioning at a hot destination — can consume a meaningful quantity of fuel. For ATP exam purposes and real-world dispatch, crews must account for APU burn when calculating total fuel required, particularly on maximum-range flights or when operating near minimum fuel limits. An APU that is run continuously from block-in to block-out on a turnaround can consume fuel that was planned for the next flight segment.
Key Numbers and Rules
- APU start ceiling (in-flight) is always published in the AFM and is aircraft-specific — always verify the specific type's limitation rather than assuming a general figure.
- APU operation ceiling is higher than the start ceiling and varies by aircraft type — verify in the AFM.
- APU generator capacity is limited — load shedding may be required on a single APU generator.
- A single fire extinguisher bottle is common on many designs; verify bottle count and any second-discharge capability against the specific AFM.
- APU fuel consumption must be included in fuel planning for extended ground operations.
- MEL provisions govern dispatch with inoperative APU — always verify before departure.
Common Test Traps
- Assuming the APU can always be restarted in flight: Above the AFM-specified in-flight start ceiling, an APU that has been shut down cannot be restarted. This limitation reflects a real operational constraint that candidates should understand thoroughly.
- Confusing the APU generator with the Ram Air Turbine (RAT): The RAT is a completely separate emergency device that deploys into the airstream during a total power loss; depending on aircraft type, it may generate hydraulic pressure, limited electrical power, or both. The APU and the RAT are not interchangeable and do not back each other up in the same failure scenario.
- Overlooking bleed-air versus electrical-only capability at altitude: Some APUs can generate electricity at altitudes where they cannot produce usable bleed air. The AFM separates these ceilings; treating them as identical is an error.
- Applying engine fire procedures to an APU fire: The APU has its own checklist, its own fuel shutoff, and its own extinguisher agent path. Using the wrong procedure wastes the limited fire agent available.
- Ignoring APU fuel burn in dispatch planning: For MEL dispatches requiring APU use throughout the flight, the added fuel consumption must be reflected in the fuel load. Omitting it can result in a fuel planning violation under 14 CFR Part 121 fuel requirements.