The C-17 APU: Small Engine, Big Mission Impact

Tucked away in the C-17’s tail is a powerplant most people never think about: the Auxiliary Power Unit. This small turbine engine doesn’t provide thrust, but it’s critical to virtually every C-17 mission. The APU is what lets a C-17 operate independently at remote locations where no ground support exists—a capability that fundamentally changes how and where this aircraft can deploy.

What the APU Does

The APU is a self-contained gas turbine engine that provides:

• Electrical power: 90 kVA generator for aircraft systems
• Bleed air: High-pressure air for engine starting and air conditioning
• Hydraulic power: Backup hydraulic pressure via electric pumps

Think of it as a complete utility plant that fits in the aircraft’s tail section. When the main engines aren’t running, the APU keeps everything else working.

Engine Starting

The most critical APU function is starting the main engines. The four Pratt & Whitney F117-PW-100 turbofans require high-pressure air to spin up the core before ignition. This bleed air comes from the APU during ground starts.

Start Sequence

A typical engine start proceeds like this:

1. APU running and providing bleed air
2. Crew selects engine start
3. APU bleed air spins the engine core
4. At sufficient RPM, fuel and ignition are introduced
5. Engine accelerates to idle on its own power
6. Bleed air automatically transfers to the running engine
7. Next engine start begins

One by one, each engine uses APU bleed air to start, then adds its own bleed air to the system. By the time the fourth engine starts, the APU has done its job and can be shut down if desired—though many crews leave it running as a backup.

Air Start Capability

If an engine fails in flight and won’t restart using bleed air from the remaining engines, the APU can provide backup start capability at altitude. The APU is certified for in-flight starts and operation, giving crews an additional tool for handling engine problems.

Ground Power Independence

The APU’s electrical generator produces 90 kVA of 270 VDC power—enough to run all aircraft systems without external power. This independence is transformative for operations:

Austere Locations

Many airfields where the C-17 operates lack adequate ground power equipment. Some have no ground power at all. The APU lets crews:

• Complete pre-flight checks on aircraft power
• Power cargo systems for loading and unloading
• Maintain cabin environmental control for passengers or patients
• Start engines without ground equipment

This self-sufficiency is essential for the C-17’s tactical mission. The aircraft can land at a remote strip, conduct operations, and depart without requiring any ground support infrastructure.

Quick Turns

Even at well-equipped bases, APU power speeds operations. Instead of waiting for ground power to connect after landing, the crew can keep systems running on APU power while ground crews unload cargo. This saves precious minutes during surge operations or time-critical missions.

Environmental Control

The APU’s bleed air also powers the aircraft’s Environmental Control System (ECS) on the ground. Without ECS, the cargo compartment and cockpit would quickly become uninhabitable in hot weather or dangerously cold in winter conditions.

Medical Evacuation

For aeromedical evacuation missions, APU-powered environmental control is particularly important. Patients may be aboard during ground operations, requiring maintained temperature and pressure conditions. The APU ensures continuous climate control even with main engines shut down.

Personnel Comfort

When troops are aboard as passengers, APU-powered air conditioning makes lengthy ground waits bearable. The cargo compartment of a C-17 sitting on a ramp in summer sun would become dangerously hot without active cooling.

APU Specifications

The C-17’s APU is a Honeywell RE220 gas turbine:

• Power output: 90 kVA electrical, plus bleed air
• Fuel: Uses main aircraft fuel (JP-8)
• Location: Tail section, with exhaust exiting aft
• Start method: Battery-powered electric starter
• Operating altitude: Ground level to 35,000+ feet

The APU’s relatively small size belies its power output. Modern turbine technology packs substantial capability into a compact, reliable package.

In-Flight Operations

Unlike some aircraft where the APU is strictly a ground system, the C-17’s APU is approved for in-flight operation. This provides:

Electrical Backup

If a main engine-driven generator fails, the APU generator can take over that electrical load. This redundancy ensures electrical system failures don’t cascade into larger problems.

Engine Restart

At altitude, if an engine won’t restart using crossbleed from other engines, the APU can provide bleed air for restart attempts. This capability has recovered aircraft from situations that might otherwise have required emergency landings.

Emergency Operations

In the unlikely event of multiple engine failures, the APU provides electrical power and hydraulic capability (through electric motor pumps) to maintain flight control authority. While the APU doesn’t produce thrust, keeping critical systems running buys time for troubleshooting or reaching a suitable airport.

Cockpit Control and Monitoring

APU operation is controlled from the cockpit overhead panel. Pilots monitor:

• Exhaust Gas Temperature: Primary health indicator
• RPM: Operating speed percentage
• Generator output: Load and voltage
• Bleed air status: Pressure and availability

The APU system includes automatic protective shutdowns if parameters exceed limits. Pilots receive EICAS advisories for APU anomalies, allowing informed decisions about continued operation.

Starting the APU

The APU starts from battery power, spinning up its own small turbine until combustion begins. Start times are typically under a minute from switch selection to stable operation. The quick-start capability means crews can have full aircraft power available within minutes of arriving at the aircraft.

Fuel Consumption

While efficient for its power output, the APU does consume fuel—typically 300-400 pounds per hour. For extended ground operations, crews must account for this consumption in fuel planning. At locations where ground power is available, using external power instead of the APU saves fuel for the mission.

Maintenance Access

The APU is accessible through panels in the tail section. While routine operation is highly reliable, maintenance crews regularly inspect the unit, check oil levels, and verify fire detection and suppression systems. The APU has its own fire extinguisher system, automatically triggered by fire detection.

Tactical Significance

The APU transforms the C-17 from an aircraft that needs support infrastructure into one that carries its own. Consider deploying to a remote dirt strip in Africa or a forward operating base under construction:

• No ground power available
• No air conditioning carts
• No start equipment

With its APU, the C-17 lands, operates, and departs with complete independence. This self-sufficiency multiplies the aircraft’s operational flexibility and directly supports its tactical airlift mission.

Why It Matters

The APU doesn’t get the attention that engines or avionics receive, but it’s absolutely essential to C-17 operations. Every mission starts with the APU, and the aircraft’s legendary flexibility would be impossible without it. For crews, maintaining APU awareness—its status, its fuel consumption, its capabilities—is part of comprehensive mission management.

When you see a C-17 operating at an austere location with no visible support equipment, you’re seeing the APU doing its job. It’s the hidden powerplant that makes independent operations possible.