Hydraulic Systems

A hydraulic system uses a fluid under pressure to drive machinery or move mechanical components.

Virtually all aircraft make use of some hydraulically powered components. In light, general aviation aircraft, this use might be limited to providing pressure to activate the wheel brakes. In larger and more complex aeroplanes, the use of hydraulically powered components is much more common. Depending upon the aircraft concerned, a single hydraulic system, or two or more hydraulic systems working together, might be used to power any or all of the following components:

A hydraulic system consists of the hydraulic fluid plus three major mechanical components. Those components are the “pressure generator” or hydraulic pump, the hydraulically powered “motor” which powers the component concerned and the system “plumbing” which contains and channels the fluid throughout the aircraft as required.

Fluid is the medium via which a hydraulic system transmits its energy and, theoretically, practically any fluid could be utilized. However, given the operating pressure (3000 to 5000 psi) that most aircraft hydraulic systems generate in combination with the environmental conditions and strict safety criteria under which the system must operate, the hydraulic fluid that is used should have the following properties:

Several types of hydraulic pumps driven by a variety of power sources can be found in aviation applications. Pumps include:

The motive power for these pumps can be generated by a wide variety of options inclusive of:

Hydraulic motors and cylinders utilize pressurized fluid to do mechanical work.

Aviation hydraulic systems, in general, are of the "open loop" variety drawing fluid from a reservoir, pressurizing it and making it available to the various user components before returning the fluid to the reservoir. The primary components of the "plumbing" portion of the hydraulic system include the following:

Hydraulic system redundancy is achieved by two primary means - multiple systems and multiple pressure sources within the same system.

Hydraulic systems are subject to several significant threats. These include:

Hydraulic system overheat, loss of pressure or fluid contamination can all result in the loss of the hydraulic system and the loss of function of those components that it powers. Fluid contamination can also result in loss of hydraulic system efficiency, fluid leaks, excessive component wear and premature component failure.

The primary defense against hydraulic fluid contamination lies in robust maintenance practices. Any fluids used to service the system must be as specified in the AOM and fluid types should not be mixed. Care should be taken to ensure that the fluid is not contaminated prior to use and that no contaminants are introduced to the system while topping up the fluid. System filters should be cleaned or replaced as per manufacturer's guidelines.

In the event of a System Overheat or Loss of Pressure, following the Quick Reference Handbook (QRH) or ECAM checklists may result in recovery of the system. If the loss of pressure was as a result a total of loss of hydraulic fluid, the system is not recoverable.

On 7 April 2022, a Boeing 757-200F returning to San Jose after a left side hydraulics failure and MAYDAY declaration suddenly veered off the right hand side of the landing runway there during deceleration and passage over uneven ground led to landing gear collapse and significant fuselage structural damage. This runway excursion immediately followed simultaneous advancement of both thrust levers after their prior asymmetric movement earlier in the landing roll and resulted in high left thrust concurrent with idle thrust on the right. With no airworthiness aspect identified, the excursion was attributed to unintended thrust lever selection by the crew.

On 29 September 2019, an Airbus A330-200 received simultaneous indications of low pressure in two hydraulic systems soon after takeoff. An emergency was declared, and a return to land was followed by a stop on the runway due to a burst main wheel tyre. A manual valve for one of the hydraulic systems located in the left main gear wheel well had completely detached and impact-damaged a pipe in a nearby but separate hydraulic system. Both systems lost their fluid with valve detachment attributed to fatigue failure of the attachment screws, a risk addressed by an un-adopted non-mandatory Service Bulletin. 

On 15 December 2019, an Airbus A330-200 turned back to Sydney shortly after departure when a major hydraulic system leak was annunciated. The return was uneventful until engine shutdown after clearing the runway following which APU use for air conditioning was followed by a gradual build up of hydraulic haze and fumes which eventually prompted an emergency evacuation. The Investigation found that fluid leaking from ruptured rudder servo hose had entered the APU air intake. The resulting evacuation was found to have been somewhat disorganised with this being attributed mainly to a combination of inadequate cabin crew procedures and training.

On 23 July 2011, a Boeing 737-300 being operated by Jet2.com on a passenger flight from Leeds/Bradford to Paris CDG experienced violent vibration from the main landing gear at touch down in normal day visibility on runway 27R at a normal speed off a stabilised approach. This vibration was accompanied by lateral acceleration that made directional control difficult but the aircraft was kept on the runway and at a speed of 75 knots, the vibrations abruptly stopped. Once clear of the runway, the aircraft was stopped and the engines shutdown prior to a tow to the gate. None of the 133 occupants were injured.

On 15 October 2015 a Boeing 747-300 experienced significant vibration from one of the engines almost immediately after take-off from Tehran Mehrabad. After the climb out was continued without reducing the affected engine thrust an uncontained failure followed 3 minutes later. The ejected debris caused the almost simultaneous failure of the No 4 engine, loss of multiple hydraulic systems and all the fuel from one wing tank. The Investigation attributed the vibration to the Operator's continued use of the engine without relevant Airworthiness Directive action and the subsequent failure to continued operation of the engine after its onset.

On 1 November 2011, a Boeing 767-300 landed at Warsaw with its landing gear retracted after declaring an emergency in anticipation of the possible consequences which in this event included an engine fire and a full but successful emergency evacuation. The Investigation attributed inability to achieve successful gear extension using either alternate system or free fall to crew failure to notice that the Battery Busbar CB which controlled power to the uplock release mechanism was tripped. Gear extension using the normal system had been precluded in advance by a partial hydraulic system failure soon after takeoff from New York.

An announcement by the Captain of a fully-boarded Boeing 757-200 about to depart which was intended to initiate a Precautionary Rapid Disembarkation due to smoke from a hydraulic leak was confusing and a partial emergency evacuation followed. The Investigation found that Cabin Crew only knew of this via the announcement and noted subsequent replacement of the applicable procedures by an improved version, although this was still considered to lack resilience in one respect. The event was considered to have illustrated the importance of having cabin crew close to doors when passengers are on board aircraft on the ground.

On 4 October 2014, the fracture of a hydraulic hose during an A330-200 pushback at night at Karachi was followed by dense fumes in the form of hydraulic fluid mist filling the aircraft cabin and flight deck. After some delay, during which a delay in isolating the APU air bleed exacerbated the ingress of fumes, the aircraft was towed back onto stand and an emergency evacuation completed. During the return to stand, a PBE unit malfunctioned and caught fire when one of the cabin crew attempted to use it which prevented use of the exit adjacent to it for evacuation.

On 26 February 2013, the crew of a Boeing 752 temporarily lost full control of their aircraft on a night auto-ILS approach at Keflavik when an un-commanded roll occurred during flap deployment after an earlier partial loss of normal hydraulic system pressure. The origin of the upset was found to have been a latent fatigue failure of a roll spoiler component, the effect of which had only become significant in the absence of normal hydraulic pressure and had been initially masked by autopilot authority until this was exceeded during flap deployment.

On 17 January 2007, a Bombardier CRJ 100 being operated by French airline Brit Air on a scheduled night passenger flight from Paris CDG to Southampton could not be directionally controlled after touchdown on a dry surface in normal visibility and almost calm winds and departed the side of the runway during the landing roll. There were no injuries to any of the 36 occupants and there was no damage to the aircraft.

On 22 June 2009, an Airbus A340-300 being operated by Finnair suffered a single tyre failure during take off on a scheduled passenger flight to Helsinki and malfunction assessed as consequential by the flight crew occurred to the hydraulic system. The flight proceeded to destination and carried out a daylight landing there in normal visibility without any further aircraft damage. Because of a further deterioration in the status of the aircraft hydraulic systems during the landing roll, the aircraft was stopped on the runway and then towed into the gate. No persons were injured in this incident.

On August 12, 1985 a Boeing 747 SR-100 operated by Japan Air Lines experienced a loss of control attributed to loss of the vertical stabiliser. After the declaration of the emergency, the aircraft continued its flight for 30 minutes and subsequently impacted terrain in a mountainous area in Gunma Prefecture, Japan.

On 18 June 1998, the crew of a Swearingen SA226 did not associate directional control difficulty and an extended take off ground run at Montreal with a malfunctioning brake unit. Subsequent evidence of hydraulic problems prompted a decision to return but when evidence of control difficulties and fire in the left engine followed, a single engine diversion to Mirabel was flown where, just before touchdown, the left wing failed upwards. All occupants were killed when the aircraft crashed inverted. The Investigation found that overheated brakes had caused an engine nacelle fire which spread and eventually caused the wing failure.

On 19 July 1989, a GE CF6-6D-powered Douglas DC-10-10 at FL370 suffered a sudden explosive failure of the tail-mounted number 2 engine and a complete loss of hydraulics so that the aircraft could only be controlled by varying thrust on the remaining two engines. With only limited flightpath control, the subsequent Sioux City emergency landing led to the destruction of the aircraft by impact and fire. The Investigation attributed the engine failure to non-identification of a fan disc fatigue crack arising from a manufacturing defect and the loss of hydraulics to debris dispersal which had exceeded the system s certification protection.

On 16 May 1995, an RAF BAe Nimrod on an airworthiness function flight caught fire after an electrical short circuit led indirectly to the No 4 engine starter turbine disc being liberated and breaching the No 2 fuel tank. It was concluded by the Investigation that the leaking fuel had then been ignited by either the electrical arcing or the heat of the adjacent engine. After the fire spread rapidly, the risk of structural break up led the commander to ditch the aircraft whilst it was still controllable. This was successful and all seven occupants were rescued.