On a business or small jet the APU is a convenience item, not a necessity for flight, so the aircraft can be dispatched without it when ground support is available. If it fails on a small aeroplane, the aircraft may remain fit for flight, provided operators accept the weight and payload saving that comes from omitting the unit. The APU can be used during flight to supply electrical or hydraulic power if an engine or its generator fails, yet such backup is optional on smaller types. History shows that the 707's APU was added only to reduce ground time at stations lacking carts, illustrating that jets were designed to fly long before the APU became a common, still optional, accessory.
Can a plane fly without an APU?
Aircraft can operate without an APU if the Minimum Equipment List, known as the MEL, allows such dispatch and its stated requirements are met. Airliners can operate without an APU if both engine generators are operating normally and the airplane can be ferried or flown on regular routes while the unit is removed for repairs. Flight with no APU is, however, restricted to routes over land where ground support is readily available.
Because the APU does not provide thrust, its absence does not prevent flight; the engines supply all propulsion. Historically, the Boeing 707 was delivered without an APU, and several modern business jets LIKE One Aviation's Eclipse and the Cessna Citation CJ fly without one. When an APU is present but failed, the same MEL-driven rules apply: aircraft can operate without an APU if MEL requirements are in place, using external conditioned air at the gate and in-flight bleed air from the engines for environmental control.
Does a plane need an APU?
A plane does not necessarily need an APU. Large airliners carry an Auxiliary Power Unit because it can supply electricity to cockpit avionics, power the environmental control system, and turn the electric starters for the engines. On the ground it removes the need to start a main engine while passengers board, and in flight it can be started to back up engine generators or to feed wing anti-icing, galley electrics, lighting, cabin heat and cabin cooling. However, the device is not universally mandatory: a functional APU is not needed provided the flight crew can start at least one engine by other means.
The FAA requires that the APU be present - and able to start in flight - only when the aircraft type certification or the planned ETOPS operation lists it as necessary for compliance. On a short domestic leg the dispatcher can legally release the airplane with the APU deferred, while on a long over-water leg under ETOPS rules the unit must be installed and operative. Smaller business jets like the Cessna Citation CJ do not carry an APU because the extra weight impacts payload, relying instead on a battery, external air-conditioning unit or ground cart. A jet can fly perfectly well without an APU when the route, performance calculations and backup systems allow it. The device is retained on most transports because it adds flexibility, not because every take-off is impossible without it.
An aircraft is capable of flight without an operative APU, yet the unit remains a vital infrastructure that enables a self-sufficient aircraft. The APU's main purpose is to supply electrical power and air conditioning while the primary generators are idle. By equipping the aircraft in this way, the APU reduces dependence on extrinsic ground power components, boosting functional preparedness and total security. The lack of an APU would require reliance on external ground assistance apparatus, presenting possible disruptions and weaknesses, particularly at distant or badly fitted airports.
How to start a plane without an APU
To start a plane without an APU, an ASU is used. The ASU delivers high-pressure bleed air through hoses to the left engine (or Engine 1/2, sequence chosen from FCOM); once that engine is running, its load compressor supplies bleed air to start the remaining engine. Big airlines prefer using this ground-hookup method even when the APU is serviceable, so at least one engine is always started before pushback.
What must be done if the plane's auxiliary power unit is not working?
If the APU is inoperative, write it up and coordinate with maintenance and dispatch; the MEL then dictates the resulting restrictions. An inoperative APU is not a safety issue; the airplane goes dark when both engines are turned off and neither ground power nor the APU is available. Ground staff can prevent this by connecting a ground power unit, which provides electricity and compressed air at the gate, and can also supply air conditioning if the APU is unavailable.
If the APU fails in flight, engine generators continue to power all systems. The APU does not output thrust and is deemed a convenience item. On arrival, crews will reignite the APU after landing or continue to use ground power. The exhaust at the tail and the fire-extinguishing system, which operates automatically, remain unchanged whether the unit is working or broken.
What causes airplane APU failure?
APU failure can be caused due to different reasons. Due to frequent cycling, stress, and high heat, APUs are susceptible to common maintenance issues. Typical faults include compressor and hot-section degradation. Bearings and carbon seals are common sites of failure. Oil leaks and excessive oil consumption are most often caused by worn seals or bearings. Starter-motor failures and mechanical breakdowns account for many no-start and in-flight shutdowns. Overheating pushes exhaust-gas temperature above limits and warps or cracks turbine blades and rotors damaged by stress, overall fatigue, or FOD. Low utilization aggravates corrosion because of inadequate preservation, while excessive electrical load or the presence of both ground power and a tripped APU points to aircraft-side electrical problems. Fuel-control-unit malfunctions cause shutdown, and leaks in the shrouded fuel line sometimes let fuel reach the drain mast. Inappropriate crew action creates a safety hazard, while inappropriate maintenance action leads to a malfunction which endangers the aircraft.
How long does it take to fix an APU on a plane? Repair duration varies by APU type and failure mode: carbon-seal or bearing replacement during a line-maintenance night stop takes three-to-five work-hours, while a hot-section tear-down or a unit overhaul during a scheduled check requires several shifts and extensive service-bay time.
What happens to an aircraft if the APU fails?
If the APU fails while the aircraft is on the ground, the event will cause the flight to be grounded completely until a replacement source of power and air is found. Mechanics must tow a ground power unit and a ground air cart to the parking stand; without these ground assets the crew cannot start the engines, run the air-conditioning, or complete the pre-flight electrical checks, so the schedule collapses.
APU failure in flight is not a major issue because the two engine-driven generators already supply all the electricity the jet needs. When one of those engine generators fails, the failed-generator checklist instructs the pilots simply to start the APU, allowing it to provide electrical power in place of the failed generator and restoring a full set of generators. Even with the APU already inoperative, the aircraft flies perfectly well on the remaining generator, although the crew must now treat any further loss as an emergency that leaves only vital power and requires landing at the nearest suitable airport.
When an engine shuts down the aircraft loses two generators, yet starting the APU can make up the deficit and keep every bus alive. Without the APU, vital power alone must suffice until the jet reaches the runway. Thus, while an APU fire or shutdown complicates departure, it never endangers the airplane in the air but merely obliges the crew to carry a working generator and, on the ground, to summon a GPU when the little turbine will not light.
What was the first plane to have an APU?
The first jetliner to carry a gas-turbine APU was the Boeing 727, which entered service in 1964. Boeing installed the unit so the tri-jet could operate from small airports that lacked ground power or air-start carts. APUs had existed earlier in military form; the Supermarine Nighthawk of World War I and later the B-29 Superfortress carried small piston engines for electrical power on the ground, but these were not gas-turbine units. Garrett AiResearch created the first turbine-powered auxiliary power unit, the GTCP85, and this design became the heart of the 727's system, mounted in the main landing-gear bay with exhaust louvers in the top of the right wing. Once the 727 proved the concept, the device spread rapidly. By the late 1960s most new transports were delivered with an APU, and many earlier jets like the Boeing 707 were retro-fitted. Today the vast majority of passenger jets carry an APU, although a few narrow-body and regional types still rely on ground carts or batteries for electrical and pneumatic power.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
