This article provides guidance to pilots on the recognition and consequences of hydraulic problems, and also some of the factors they should consider during their decision process and subsequent operation of the aircraft to help ensure a safe outcome.
Hydraulic systems of some description are present on virtually all aircraft types. In a light or general aviation aircraft, the use of hydraulic power may be limited to the application of wheel brakes only. In larger and more complex aircraft multiple systems may be used to provide the ‘muscle’ to operate a wide variety of components and systems. For example, these could include primary and secondary flight controls, the landing gear, nosewheel steering, wheel brakes, thrust reversers and cargo doors.
As the dependency upon hydraulic power increases, the integrity of the hydraulic systems becomes ever more critical to the safety of flight. Based on this hydraulic system criticality, many design features are incorporated to ensure reliability, redundancy and the ability to maintain control of the aircraft in the event of one or more failures. Often two or more hydraulic systems are built into the design of an aircraft.
Each system is provided with different sources of fluid pressurization and power generation. This power is transmitted by the hydraulic fluid through system specific hydraulic lines and used to drive the motors and actuators associated with that system. While hydraulic systems may be designed to exchange power under controlled conditions via a Power Transfer Unit (PTU), there are very rarely provisions for any exchange of fluids incorporated into system designs.
Depending on the specific failure or the extent of damage to the hydraulic system(s), the following effects could result:
- Loss of control
- Partial or complete loss of control over specific control surfaces
- Loss of autopilot
- Reversion to a degraded flight control law
- Impact on/collateral damage to other systems (e.g. due to a ruptured hydraulic pipe)
- Possible loss of ETOPS and/or RVSM capability
- Loss of low visibility landing capability due to degraded autopilot or flight controls
- Difficulties with normal landing gear extension
- Inability to retract landing gear
- Inability to extend/retract high lift devices such as flaps or slats
- Reduced braking capability upon landing
- Loss of anti-skid systems
- Inability to actuate thrust reversers
- Loss of nosewheel steering
Hydraulic failures can be subtle (as would be the case with a slow fluid leak) or immediate (as the result of a pump failure, an actuator failure or the rupture of a hydraulic line). Depending upon the sophistication level of the aircraft warning systems in question, the failure could be presented to the crew by means of an EICAS or ECAM message, a Master Caution or Master Warning light, an annunciator panel fail light, a system warning light or by means of a pressure or quantity gauge indication. An aural warning may also be associated with the failure.
Pilots should acknowledge the failure annunciation by cancelling any aural warnings and, where possible, confirm the failure in accordance with the aircraft operator's Standard Operating Procedures and with the manufacturer's guidance prior to carrying out the immediate corrective actions.
Manufacturers provide aircrew with comprehensive abnormal and emergency procedures for use in the event of a single or multiple failure. These procedures and the associated protocols will include the immediate actions required to secure the emergency, limitations and system losses resulting from the failure and, when applicable, the appropriate configuration and performance penalties to be utilized for continuing the flight and subsequent landing. The consequences of multiple failures are taken into consideration where applicable.
As is the case with any unusual or emergency situation, pilots should complete the required memory items and then comply with the manufacturer’s procedures, checklists and protocols and with any applicable company policies or directives. They should then consider the consequences of the failure and the associated impact on the continuation of the flight.
Pilots should consider all available information, inclusive of but not limited to, aircraft controllability, range, operating altitudes, runway requirements and weather, and exercise their best judgement when making diversion decisions. In all cases, the primary pilot responsibility is to FLY THE AIRCRAFT.
- FLY THE AIRCRAFT. A hydraulic failure may or may not result in loss of some primary or secondary control surfaces. It may also result in the loss of the autopilot. Therefore, it is critical that the pilot flying (PF) maintain focus on the continued safe control of the aircraft. With multiple hydraulic system or component failures, control of the aircraft may be difficult. The extreme, but highly unlikely, case of a total loss of aircraft hydraulics could necessitate the non-standard use of engine thrust to maintain aircraft control (e.g. DC-10, Sioux City, 1989).
- Perform Memory Actions. If there are memory drills associated with the failure, they should be completed in a timely fashion. In a multi-crew aircraft, memory items and the follow-on checklists and procedures will be completed by the pilot monitoring (PM) with confirmation of critical actions, when appropriate, from the PF, using challenge and response.
- Complete Associated Checklists. Perform QRH, checklist or ECAM procedures as appropriate to the aircraft type.
The type of aircraft, the complexity of the failure, previous and/or subsequent failures, and the circumstances under which the malfunction has occurred will determine what secondary actions should be taken. Actions that could be applicable to the situation include:
- Advise ATS. Depending upon the specifics of the failure, it may be prudent to declare an emergency using the appropriate Emergency Communications (MAYDAY or PAN) format. While the crew may not have yet formulated their plan of action, advising ATC of the hydraulic problem(s) will permit them to provide early assistance which may include:
- separation of the aircraft from other traffic
- providing diversion clearance or facilitating delaying tactics by providing vectors or a holding clearance
- prioritisation of the emergency aircraft for landing allowing for a long final if requested
- assigning a discrete radio frequency when possible
- advising the airport emergency services and all concerned parties in accordance with local procedures/protocols
- providing any information that might be requested by the crew such as weather, type of approaches available, runway length and other pertinent aerodrome details
- Confirm Aircraft Systems Status. The crew should ascertain the status of not only the hydraulic system(s), but also any other aircraft system(s) that may have been affected by the failure.
- Calculate Approach Speeds and Landing Distance. In many cases, a hydraulic failure will have an impact on approach and landing speeds, cross-wind limits and landing distance required. Higher approach and landing speeds will be required if flight controls are degraded or if high lift devices cannot be extended due to the failure. Higher approach speeds will result in significantly longer than normal landing distances as landing distance is a function of mv2. Landing distances will also be increased should the failure result in degradation of braking capability, loss of ground spoilers or the inability to deploy thrust reversers.
- Determine Landing Weather Requirements. Some hydraulic failures can result in the loss of the aircraft all weather capability due to loss of the autopilot, the resultant landing flap position or to degradation in flight control function.
- Confirm Range and Endurance. If the hydraulic failure has resulted in the inability to retract the undercarriage, flaps or slats, a higher than normal rate of fuel consumption will result. It may also be necessary to operate at a lower than normal altitude, in which case minimum safe altitudes must be checked. In these cases, it is critical that flight crew fuel management takes the abnormal configuration or operating level into consideration. During any hydraulic emergency, the time required before the crew is ready to land may be extensive due to the requirement for a diversion and/or to otherwise prepare for the landing. The crew must remain aware of the fuel state at all times.
- Gather Any Other Pertinent Information. Use onboard information sources such as approach charts and the MEL plus external resources (via ATS, ACARS or Company communications) to compile airfield data, weather reports, runway condition and other information to be considered in the decision process. Request technical support as required.
- Consider the Implications of the Failure. Develop a clear picture of the impact of the failure on the approach, landing and post landing operations. For example:
- will the failure require an alternate landing gear extension procedure?
- if an alternate extension is required, when would be the most prudent time to carry out the procedure?
- if an alternate extension is required, will the inability to retract the gear compromise missed approach obstacle clearance in the event a go around is required?
- on a fly by wire aircraft, will the failure result in a change of control law when the landing gear is extended?
- will the failure allow for normal braking or will it require alternate braking techniques due to the loss of anti-skid or to sole reliance on accumulator pressure?
- are thrust reversers available?
- is the nosewheel steering functional?
- Should the landing procedure consider leaking hydraulic fluid might come into contact with hot brakes on landing?
- can the aircraft be taxied clear of the active runway?
Formulate, Communicate and Execute Plan of Action
Once the applicable secondary actions have been completed, pilots can determine their best course of action. The salient details of the plan should be transmitted to ATC and to Company Operations to enable co-ordination of any required support. ATC will need to know:
- diversion aerodrome, requested runway and intended approach.
- distance at which the crew would like to join the final approach course.
- If the aircraft will need to configure earlier than normal during approach and require non-standard speed control/vertical profile.
- approach speed if it will be significantly higher than normal.
- if the aircraft will be stopped on the runway.
- if the aircraft will be able to vacate the runway under its own power.
Company Operations should also be advised so maintenance co-ordination (such as arrangements to tow the aircraft off of the runway) and commercial considerations (customs, passenger handling etc.) can be arranged.
Accidents & Incidents
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 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.
A30B, initial climb, Baghdad Iraq, 2003
On 22 November 2003 an A300B4 operating a cargo flight from Baghdad to Bahrain suffered a surface-to-air missile strike to the left wing whilst passing 8,000ft in the climb, causing serious damage to the wing. Two hydraulic circuits were lost immediately, followed shortly by the third hydraulic system. This was accompanied by a significant fuel leak from the left wing. Due to the total loss of hydraulics, both primary and secondary flight controls were lost; however, both engines were still running. The crew successfully used engine thrust to return the aircraft back to the departure airfield where a controlled landing was accomplished 25 minutes after the missile strike.
B747, Haneda Japan, 1985
On 12 August 1985, the aft bulkhead of a JAL B747 tore open due to a pre-existing defect. The resultant explosive decompression severed the hydraulic lines and the aircraft progressively became uncontrollable.