A jet engine does not rotate at one constant speed from engine start to landing. Yet conventional large-aircraft electrical systems need a stable alternating-current frequency even while engine speed changes continuously. The Integrated Drive Generator, or IDG, solves that problem by combining an electrical generator with a constant-speed drive so that changing engine accessory-gearbox speed is converted into nearly constant generator speed. EASA flightcrew knowledge standards specifically identify Constant Speed Drive and Integrated Drive Generator systems as a core part of transport-aircraft AC generation, while engine type-certificate data confirm that modern turbofans provide dedicated accessory-drive pads for IDGs. [1] [2]
The familiar result on many conventional airliners is 115/200-volt, 400 Hz three-phase AC power. The exact voltage and architecture vary by aircraft generation, and newer more-electric designs can use variable-frequency generation for some systems. But on aircraft built around constant-frequency AC, the IDG is the mechanical-electrical link that lets a variable-speed turbofan supply stable aircraft power.
Why generator frequency depends on rotational speed
An AC generator creates electrical frequency from the speed at which its magnetic field rotates relative to the stator windings and from the number of magnetic poles in the machine. If generator shaft speed changes while the pole count remains fixed, output frequency changes too. EASA theoretical-knowledge standards explicitly require pilots to understand the relationship between AC-generator output frequency and rpm. [1]
That creates a problem on a turbine engine. The high-pressure spool speed is not fixed. It rises during start, changes with thrust and varies with altitude and operating condition. A generator directly geared to that spool would therefore naturally produce a frequency that changes with engine rpm unless another device or electronic conversion stage intervenes.
Why aircraft historically wanted 400 Hz
Aircraft electrical systems traditionally use a much higher AC frequency than the 50 or 60 Hz found in domestic power grids. Higher frequency allows transformers, motors and some magnetic components to be made smaller and lighter for a given power rating. Weight reduction is especially valuable in aviation, where every kilogram must be carried for the entire flight.
The trade-off is that higher frequency can increase some losses and places different requirements on electrical equipment and wiring. The 400 Hz standard became deeply embedded in transport-aircraft generators, motors, transformers and ground-power equipment, which is why parked airliners commonly accept 400 Hz external AC power.
The older Constant Speed Drive
Before the generator and drive were commonly integrated into one assembly, aircraft used a separate Constant Speed Drive, or CSD, between the engine accessory gearbox and the AC generator. The CSD accepted a variable mechanical input speed and produced a much more constant output speed for the generator.
Hydromechanical CSDs use internal hydraulic pumps, motors and control mechanisms to vary the effective transmission ratio. As engine speed changes, the unit adjusts its internal hydraulic relationship so the output shaft remains close to the required generator speed.
What “integrated” means
An IDG combines the constant-speed-drive function and AC generator in a single integrated assembly. This reduces separate interfaces and allows the drive, generator, lubrication and monitoring functions to be packaged as one engine-mounted component.
The unit remains an airframe accessory even when mounted on the engine. EASA type-certificate data for engines such as the PW1500G explicitly list an IDG accessory drive while noting that the generator itself is part of the airframe installation. [2]
Where the IDG gets its mechanical power
The engine’s high-pressure spool drives an accessory gearbox through tower shafts and gearing according to engine design. That gearbox provides drive pads for equipment such as hydraulic pumps, fuel pumps and the IDG. The IDG therefore takes a small portion of the engine’s shaft power and converts it into electrical power for the aircraft.
EASA engine data show this relationship directly. The PW1500G type certificate lists an IDG drive-pad speed ratio relative to N2, while Rolls-Royce Trent data identify accessory-gearbox provision for an Integrated Drive Generator supplying aircraft electrical power. [2] [3]
The engine does pay for the electricity
Electrical power is not free energy. When the generator supplies aircraft loads, mechanical torque is required at the IDG input. The engine must ultimately produce that shaft power, which slightly increases fuel consumption compared with an unloaded condition.
The exact fuel penalty changes with electrical load, engine efficiency and flight condition. This is why load management matters and why replacing pneumatic systems with electrical systems does not eliminate energy cost; it changes where and how the engine delivers that energy.
Hydromechanical speed regulation
A classic CSD/IDG uses a variable hydraulic transmission. One hydraulic element acts as a pump and another as a motor. By changing displacement or swashplate geometry, the unit changes the relationship between input and output speed.
At some engine speeds, the hydraulic section effectively adds speed to the mechanical path; at others it subtracts or recirculates power. The control mechanism continuously adjusts so the generator section turns at the required speed despite large changes in input rpm.
Why generator speed must stay tightly controlled
Aircraft AC loads are designed for a defined frequency range. Motors change speed with frequency, transformers have frequency-dependent characteristics and protective devices expect the electrical system to remain within specified limits. A generator whose frequency wandered directly with engine thrust would create unacceptable electrical behaviour.
The IDG therefore acts as a frequency-stabilising mechanical converter before electrical regulation even begins. Generator voltage control then manages excitation to keep output voltage within limits while the CSD portion manages shaft speed and therefore frequency.
Voltage regulation and frequency regulation are different
The IDG’s constant-speed mechanism primarily solves the frequency problem. Voltage is controlled through the generator’s excitation system and Generator Control Unit or equivalent electronics. As electrical load changes, the regulator adjusts excitation to keep voltage within the required range.
This separation matters because a generator can have correct shaft speed but incorrect voltage, or correct voltage but abnormal frequency if the drive is malfunctioning. Aircraft monitoring therefore supervises several parameters rather than treating “generator on” as a single binary condition.
Three-phase AC
Large-aircraft generators commonly produce three-phase AC. Three sinusoidal phases are separated electrically by 120 degrees. This arrangement delivers power efficiently and supports compact motors and other high-power equipment.
The conventional 115/200-volt notation reflects approximately 115 volts from each phase to neutral and about 200 volts phase-to-phase in a wye-connected system. Exact nominal values and tolerances are defined by the aircraft electrical specification.
Why there is usually one main IDG per engine
A twin-engine airliner commonly has one engine-driven generator on each engine, giving two independent main sources. Electrical buses can be arranged so either source supplies its normal side while bus-tie logic allows reconfiguration after a failure.
A four-engine aircraft can have four engine-driven generators. The broader safety principle is source redundancy: losing one engine or generator should not automatically remove all normal AC power. The APU generator and emergency electrical sources can add further layers.
The APU generator is related but not always identical
The APU also drives an AC generator on many aircraft, but its mechanical architecture does not necessarily require the same type of CSD because an APU itself is designed to run close to a governed constant speed while producing electrical power.
The main engine has a much wider useful rpm range, so the constant-speed-drive problem is more prominent there. Aircraft-specific documentation determines whether the APU generator uses a conventional generator, variable-speed design or another integrated architecture.
Generator Control Units
The IDG does not operate alone. Generator Control Units monitor voltage, frequency, phase relationship and protection conditions. They control excitation and the electrical contactors that connect a generator to aircraft buses.
If output moves outside safe limits, the system can disconnect the generator rather than expose the aircraft network to abnormal power. The crew then receives an electrical-system indication or warning appropriate to the aircraft.
Paralleling and bus ties
Some aircraft electrical systems allow generators to operate in parallel; others normally keep sources on separate buses and use automatic transfer logic. When AC sources are paralleled, their voltage, frequency and phase must be compatible before connection.
Electrical-protection systems prevent an unsynchronised generator from simply being connected to a live bus. A large phase mismatch could create severe currents and torque. Exact source-paralleling philosophy varies by aircraft generation.
Load sharing
If generators operate in parallel, the system needs to share electrical load appropriately. One generator should not carry nearly all aircraft demand while another contributes almost nothing unless the system is specifically designed that way. Control electronics adjust excitation or system configuration to maintain acceptable sharing.
In split-bus architectures, load sharing can instead mean automatic reconfiguration after one source is lost. The surviving generator may then feed additional buses, subject to its rated capacity and automatic load shedding.
Why some loads are shed after a generator failure
One remaining generator may be capable of supporting all essential systems but not every galley, heater or convenience load at once. Aircraft electrical systems therefore classify loads and can disconnect less essential equipment after a source failure.
This allows the available generating capacity to be reserved for avionics, flight controls, fuel systems, anti-ice or other required functions. Load shedding is therefore part of electrical redundancy, not evidence that the remaining generator is defective.
IDG oil and cooling
A hydromechanical IDG contains bearings, gears and hydraulic elements handling substantial power. It needs lubrication and heat rejection. IDG oil circulates through the unit and commonly passes through a cooler using fuel or another aircraft heat sink depending on design.
Temperature and oil condition are therefore important. An overheated or mechanically failing IDG can be disconnected to prevent further damage. The aircraft maintenance system records relevant faults so the unit can be inspected or replaced.
The IDG disconnect
Many aircraft provide an IDG disconnect function allowing the flight crew to mechanically separate a malfunctioning IDG from the engine accessory gearbox. This protects the gearbox and engine from a generator or drive that has seized or developed a serious internal fault.
On many designs, an IDG that has been mechanically disconnected cannot be reconnected in flight and requires maintenance action on the ground. This makes the disconnect a protective last-resort action rather than an ordinary electrical reset switch. Exact procedures vary by type.
Electrical disconnect versus mechanical disconnect
Switching a generator off electrically is not the same as disconnecting the IDG mechanically. Opening the generator breaker removes its electrical output from the bus, while the IDG may continue rotating with the engine. A mechanical disconnect physically separates the drive.
This distinction matters in fault handling. An electrical fault may require only isolation of the generator output, while a severe drive or oil problem can require mechanical disconnection. Approved checklists determine the appropriate action.
Why IDGs appear in airworthiness directives
IDGs and their feeder cables carry large amounts of power, so failures can have significant consequences. Regulatory airworthiness directives have addressed issues involving IDG controls, wiring and feeder-cable chafing on various transport aircraft. For example, FAA/EASA material documents inspections and modifications where IDG power cables could arc after chafing. [4]
These directives do not imply the basic IDG concept is unsafe. They show that high-power generation hardware is treated as critical continued-airworthiness equipment and that specific in-service defects are corrected through mandatory action when necessary.
Variable-frequency generators
A constant-speed drive adds weight, complexity, hydraulic mechanisms and maintenance. Modern power-electronics and aircraft-system architectures can instead accept variable-frequency AC or electronically convert power. This allows some aircraft to eliminate the traditional CSD function.
EASA training standards explicitly teach both constant-speed-drive systems and the concept of a “frequency wild” generator, reflecting the fact that modern aircraft can use different generation philosophies. [1]
Why the Boeing 787 is different
The Boeing 787 uses a more-electric architecture with high-capacity variable-frequency generation rather than the conventional 400 Hz IDG arrangement used on many earlier transports. This removes the need to mechanically hold every main generator at constant speed and shifts more conditioning into the electrical architecture.
That example is important because “all airliners generate 400 Hz through IDGs” is no longer universally correct. The IDG article describes a major and still widely used architecture, not every modern commercial aircraft.
Ground power has to match the aircraft system
When a conventional 400 Hz aircraft connects to external power, the ground unit has to supply voltage, frequency and phase quality within the aircraft’s acceptance limits. The electrical system verifies the source before closing the external-power contactor.
This means the IDG and the airport ground-power unit are alternative sources feeding the same aircraft network. One creates constant-frequency AC from engine shaft power; the other creates compatible AC outside the aircraft.
What happens during engine start
At low engine speed the main IDG may not yet be capable of supplying stable rated power. The aircraft remains powered from the APU generator or external power while the engine accelerates. Once generator parameters are within the accepted range, the electrical system can connect the engine-driven source and transfer loads.
The transition is controlled by contactors and generator logic so the cabin does not experience an uncontrolled collision between two unsynchronised sources. Depending on aircraft design, the transfer can produce a brief change in lighting or system sound but should remain within certificated electrical behaviour.
What happens when engine thrust changes?
After connection, the engine may move from idle to take-off thrust, climb, cruise and descent. The accessory gearbox input speed changes throughout those phases. Inside a traditional IDG, the constant-speed-drive mechanism continuously adjusts its ratio so the generator section remains close to its required rpm.
The pilot does not manually adjust that ratio. The hydromechanical control does it automatically. The flight crew generally sees only the result: generator frequency and voltage remain stable while the engine changes speed.
Why IDG rating matters
Every generator has a maximum continuous electrical load and thermal capability. Aircraft designers calculate normal and failure loads so the remaining sources can support essential operation after defined failures. Engine accessory-drive limits also define the mechanical torque that can be extracted.
EASA engine type-certificate data publish IDG drive-pad speed ratios and torque limits because the engine gearbox has to tolerate the accessory power demand. [2]
The generator is part electrical machine, part transmission
An IDG therefore solves two engineering problems at once. The generator section converts mechanical shaft power into three-phase AC. The integrated drive section converts variable input rpm into the near-constant generator rpm needed for constant frequency.
Voltage-regulation electronics, bus contactors and protection systems then make that output usable as part of the aircraft network. What appears to the crew as one “GEN” indication is actually the final result of mechanical speed regulation, electromagnetic generation and electrical control.
Why 400 Hz can stay constant while N2 changes
This is the central trick. The engine accessory drive can speed up or slow down, but the IDG changes its internal transmission ratio in the opposite sense. Higher input speed requires less effective speed multiplication; lower input speed requires more. The generator shaft therefore remains near its design speed.
Because generator frequency is tied to shaft speed and pole count, holding that shaft speed steady keeps the electrical frequency steady. The aircraft can move from idle to high thrust without every AC motor and transformer seeing an equivalent frequency swing.
A hidden mechanical converter behind stable aircraft power
Passengers see stable lights and flight crews see a steady 115/200 V, 400 Hz electrical system, but the engine driving that system is continually changing rpm. On a traditional IDG-equipped transport, a hydromechanical constant-speed drive inside the generator assembly is quietly compensating for those changes. [1]
That makes the IDG a classic piece of aviation engineering: a compact transmission and generator mounted on a high-power engine, handling significant mechanical torque, producing tightly controlled three-phase electrical power and supporting an aircraft network that cannot simply speed up and slow down with the turbofan. Newer architectures can solve the problem electronically, but thousands of airliners still rely on the elegant mechanical principle of turning variable engine speed into constant-frequency electrical power.
Verified Sources / References
- EASA Easy Access Rules for Aircrew — Aircraft Electrical Systems syllabus. EASA material covering AC generation, frequency/rpm relationships, Constant Speed Drives, IDGs and variable-frequency generation.
- EASA Type Certificate Data Sheet IM.E.090 — Pratt & Whitney PW1500G/PW1900G Series. Current manufacturer/regulatory data identifying the IDG accessory drive and its speed/torque limits.
- EASA Type Certificate Data Sheet E.042 — Rolls-Royce Trent 700 Series. Regulatory engine data confirming provision for an Integrated Drive Generator supplying aircraft electrical power.
- FAA/EASA Airworthiness Directive material — IDG power feeder cable inspection/modification. Example of continued-airworthiness controls for high-power IDG electrical installations.
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