A ram air turbine, or RAT, is one of the most distinctive backup systems fitted to some transport aircraft. During normal operation it remains stowed inside the airframe. If the aircraft loses the normal sources of electrical or hydraulic power for which the RAT is designed to provide backup, it can be deployed into the airflow. The relative wind turns the turbine, allowing it to drive an emergency power system while the aircraft remains airborne. EASA guidance explicitly recognises a ram air turbine as an alternate electrical power source on large aeroplanes. [1]
The important detail is that a RAT is not intended to recreate the entire normal electrical network. Certification guidance focuses on keeping the services needed for continued safe flight available after failures of normal generated power. EASA’s acceptable means of compliance uses a typical twin-engined transport-aircraft example in which the two engine-driven generator channels are backed by a passive emergency power channel driven by a RAT. [2]
Why airliners need independent backup power
Modern transport aircraft depend on electrical power for flight instruments, avionics, communications, navigation, flight-control electronics and many other functions. EASA CS 25.1351 requires the generating system to be designed so that a failure or malfunction of one power source cannot create a hazard or prevent remaining sources from supplying essential loads. The required generating capacity and types of sources are established through an electrical load analysis and the broader system-safety requirements of CS 25.1309. [1]
A twin-engine airliner commonly has electrical generation associated with each engine, while additional sources may include an auxiliary power unit and batteries. EASA’s certification examples show that a RAT can form another independent layer in that architecture. The exact arrangement varies by aircraft type, so it is not accurate to say that every airliner uses the same sequence or that every RAT performs the same function. [2]
How airflow becomes usable power
Once deployed, the RAT places a small turbine in the airstream. The aircraft’s forward motion creates relative airflow through the turbine, causing it to rotate. That mechanical rotation can then be converted into electrical power, hydraulic power or both depending on the aircraft’s design. An FAA technical report describes modern transport-category RAT arrangements as wind-powered turbines that may provide 28-volt DC, 115-volt three-phase AC or hydraulic output, depending on the installation. [3]
The RAT is therefore an energy-conversion device rather than an energy-storage device. A battery carries a finite stored electrical charge; a RAT extracts energy continuously from the airflow for as long as the flight conditions remain within the range in which it can operate effectively. EASA consequently describes a RAT as a potentially non-time-limited alternate source, while also requiring designers to account for limitations imposed by aircraft speed, attitude and altitude. [1]
Why airspeed matters
The power available from a turbine in the airstream depends on the airflow reaching it. EASA’s guidance specifically requires consideration of speed, altitude, attitude and other conditions that may affect an alternate power source. The FAA technical report likewise notes that RAT effectiveness can diminish at low airspeeds and that batteries may assume functions that the RAT cannot support during very low-speed portions of flight, depending on aircraft design. [1] [3]
This is why it would be wrong to describe a RAT as producing a fixed amount of power regardless of conditions. Its certified operating capability is defined as part of the aircraft design, and EASA explicitly instructs manufacturers to consider the flight-envelope limitations that affect the source. The RAT is sized and integrated to support defined essential functions, not to behave like an unlimited replacement for all normal generators. [1]
Automatic and manual deployment
EASA’s acceptable means of compliance describes typical RAT emergency-power systems as remaining stowed during normal operation and being deployed automatically and/or manually when the normal generating channels are lost. The regulator also stresses that the means of bringing the RAT into service must not itself depend solely on the power source that has already failed. Independent deployment capability is therefore part of the system-safety problem. [1] [2]
That design requirement matters because an emergency source is useful only if it can be activated after the initiating failure. EASA says the RAT and its deployment mechanism must satisfy the overall reliability requirements, and it specifically discusses the need for independent or duplicate deployment means. The system is therefore designed as part of the aircraft’s failure-tolerant architecture rather than as an accessory that relies on normal power being available. [1]
What stays powered when the RAT is operating
The exact list is aircraft-specific, but the certification objective is to supply the essential loads required for continued safe flight. The FAA technical report describes emergency systems as supporting essential loads such as minimum instrumentation and flight-control capability. EASA requires designers to identify essential electrical loads and ensure the remaining sources can support them under the relevant failure conditions. [3] [1]
This explains why a cockpit can look different when an aircraft is in an emergency electrical configuration. Non-essential services can be removed from the powered network so that limited generation is reserved for higher-priority equipment. Airbus describes an A320 Family emergency electrical configuration in which a reduced set of displays and systems remained powered after the RAT deployed. That manufacturer example illustrates load prioritisation without implying that every Airbus or every other airliner behaves identically. [4]
Why batteries still matter
A RAT does not make batteries unnecessary. EASA notes that continuity of electrical power may require battery support during the interval before the emergency source is supplying power, unless deployment and power availability occur automatically within a time that does not jeopardise continued safe flight. Batteries can therefore bridge transitions and support loads that require uninterrupted power. [1]
The FAA report also notes that RAT output can become less effective at low speed on some designs, making batteries important during later phases of flight. The actual power-transfer sequence depends on the aircraft, which is why emergency electrical procedures are type-specific. The source-backed principle is that batteries and RATs can perform complementary roles rather than one simply replacing the other. [3]
Electrical RATs and hydraulic RATs
Not every RAT drives the same downstream system. The FAA describes designs that can provide electrical output, hydraulic output or both. Some aircraft use the turbine to drive a hydraulic pump, which can support flight-control hydraulics; others use it to drive an electrical generator, and some architectures combine functions. The important common feature is that the energy source is the airflow rather than a normally powered engine accessory. [3]
EASA’s guidance is similarly technology-neutral. It recognises a RAT as one possible alternate electrical source alongside an APU or pneumatic/hydraulic motor-driven generator. Certification focuses on whether the source can reliably supply the required services under the relevant flight conditions, rather than requiring every manufacturer to use an identical hardware architecture. [1]
Why emergency power is deliberately limited
Supplying every normal cabin, galley, utility and aircraft system would require much more generation capacity than supplying essential flight functions. Emergency electrical architecture is therefore built around prioritisation. EASA’s CS 25.1351 framework requires an electrical load analysis and specifically protects essential loads when other sources fail. A RAT can consequently be sized around the emergency task rather than the aircraft’s maximum normal electrical demand. [1]
This prioritisation also improves independence. A small deployable turbine can remain stowed and isolated from much of the normal generation system until it is required. EASA’s example of a passive-backup RAT channel shows how that separate source can be used as an additional layer behind engine-driven generators and, in some configurations, an APU-driven generator. [2]
Why RAT reliability is tested and maintained
A backup system that spends nearly all of its life stowed still has to work when demanded. EASA therefore states that both the RAT and its deployment system must meet the applicable reliability requirements. Aircraft maintenance programmes include inspections and functional tests defined for the specific type and installation so that the emergency source remains serviceable even though it is not used during normal flights. [1]
Airbus’ technical material on A320 Family electrical generation provides a practical illustration of how the RAT sits within a real aircraft network. Airbus explains that when the aircraft enters its emergency electrical configuration following loss of both normal engine generators, the RAT can deploy and power the emergency network. That is a manufacturer-specific example of the broader certification principle described by EASA. [4]
The RAT is part of a layered system
It is misleading to think of the RAT as the aircraft’s single “last-resort generator” in isolation. Transport-aircraft electrical systems are designed around multiple sources, distribution buses, protection devices, batteries and procedures. EASA’s examples show engine-driven channels, optional APU generation and RAT backup working as layers whose independence and failure behaviour are analysed as a complete system. [2] [1]
The RAT’s value comes from its independence and simplicity of energy source: as long as the aircraft is moving through suitable airflow and the system is within its certified operating conditions, the turbine can extract mechanical energy from that airflow. It therefore provides a different failure path from engine-driven generation and stored battery energy. [3]
Why the system is such an elegant piece of engineering
A RAT takes a fundamental property of an aircraft that is still flying—airflow over the airframe—and converts it into a source of emergency power. It remains out of the airstream during normal operation, avoiding unnecessary drag, and is deployed only when the emergency architecture requires it. EASA recognises the concept as a legitimate alternate source provided its deployment, performance and reliability are properly addressed in the aircraft design. [1]
The key fact is therefore not that a small propeller can suddenly power an entire airliner. It generally does not. The engineering achievement is that a relatively compact deployable turbine can support a carefully selected set of essential electrical or hydraulic functions after normal power sources have been lost, giving the aircraft another independent layer of resilience. [3] [2]
Verified Sources / References
- EASA — CS-25 Electrical Systems and Alternate Power Guidance
- EASA — AMC-20 Transport Aircraft RAT Electrical-System Examples
- FAA — Emergency Power System Technical Report
- Airbus Safety First — Emergency Electrical Configuration and RAT Operation
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