General System Layout
The hydraulic system of the VC10 is not as complicated as those systems found on contemporary aircraft. The main reason for this has been Vickers' decision to base the flight controls system on electrical power, using individual (hydraulic) powered flying control units (PFCU) to power the moving surfaces. This resulted in far fewer hydraulically powered components. The system is composed of separate A and B systems, each fed from two hydraulic pumps, one on each engine. The systems are normally split, creating redundancy in case of leaks or other failures, but can be connected for the specific purpose of a three-engine ferry flight. In this configuration the loss of the remaining engine on one side would result in the complete loss of one system, while with both systems connected there will still be hydraulic power on both systems, provided by the two hydraulic pumps on one side.
Components using hydraulic power are:
As noted above, the flight controls are the 'missing link' in the list above. In any other large airliner these are hydraulically powered, and therefore the main hydraulic system is crucial in maintaining the controllability of the aircraft. In the VC10 this is not so. Each PFCU is an individual small hydraulic system with an electrical pump and small reservoir to feed the control jack. As long as these are all provided with electric power the controls can be moved as demanded by the servo valves which are connected to the flying controls. When the RAF ferried the Super VC10s that had been stored for several years the main hydraulic system was almost not used at all. The gear was locked down, the flaps and slats were set and locked, the tailplane was set and locked, only the brakes and spoilers were used during the short flight.
The distribution of hydraulic power
The system is based on four engine driven pumps (EDP), these being variable delivery pumps. This means that the quantity of fluid delivered is dependant on hydraulic pressure. A mechanism on the side of each EDP senses hydraulic pressure and will adjust the pump output (flow) to maintain 3000 psi. When system demand is low or non-existant the pump will still be delivering a certain amount of fluid to cater for internal leakage in system components. The EDPs are cooled by their own (designed) internal leakage. For this purpose fluid passes out of the case drain line to the cooler which is located in the stub wing and back to the reservoir.
In the case of a hydraulic leak a solenoid is energised which releases a blocking valve, which closes the pump system delivery line. Because of the blocked line there will be no output, and the sensing mechanism will sense 3000 psi and position the pump swashplate to minimum delivery. In this state the pump will produce 300 psi with a limited flow through the case drain line to keep the pump lubricated.
An additional hydraulic source in the form of an AC powered electric pump is provided for use on the ground, to recharge the brake accumulators and enable brake operation at a reduced rate.
Landing gear operation is split between A and B system, with A system powering the nose gear and left main gear and B system powering the right main gear. The sequence during gear retraction is as follows: doors unlocked and opened, main bogies angled, brakes applied, down-locks released and gear retracted, doors closed and wheel brakes released. This sequence will take 9.5 seconds, and this period will remain the same with one engine inoperative.
Left main gear of a Super VC10 (Standard is similar)
Flaps and Slats
The hydraulic circuits for the flaps and slats are similar and each system uses two hydraulic motors. The drive unit for the flaps is mounted on the aft spar in the left main gear bay, and the slat drive unit is positioned in the centre section, mounted on the front bulkhead of the center wing tank. One motor of each drive unit is powered by A system, and the other by system B. The motors drive a gear box through a differential gear and run simultaneously. In the event of failure of one system, the service is reduced to half speed, the motor on the non-functioning system being locked be a hydraulic valve. In the event of an overrun or limit switch failure, isolating valves will remove hydraulic power from the drive units. The flaps and slats operate together, the slats moving from IN to OUT as the flaps move from UP to 20 degrees and vice versa. For maintenance purposes both the flaps and slats can be manually extended and retracted, for this task a pneumatic winding tool is available and is preferred over a completely manual operation.
This drawing shows the location of the slat drive unit on the front spar
There are three spoiler sections on each wing, with the centre section being powered by A system and the two outer sections by system B. In the event of a servo valve failure when any section is in the UP position a let-down valve isolates that particular section from the pressure supply and allows the spoiler section to be closed by the airflow load on the section. The let-down valves are paired left and right to ensure symmetry in operation. A mixer unit allows differential operation in conjunction with aileron application, and symmetrical operation when the surfaces are used as spoilers or lift-dumping devices.
In the event of either a spoiler system or aileron system failure (with associated control wheel jamming) a lever on the flight deck disconnects a mechanical link between the captain's and copilot's steering wheel, whereupon one steering wheel controls the ailerons and the other the differential spoilers. In this situation the remaining system can be used for roll control. A side effect of this system is the now possible strange sight of one steering wheel turned to the left with the other one turned to the right (best demonstrated on the ground).
Similar to the flaps and slats, the tailplane incidence is powered using two hydraulic motors, one powered by each system. The hydraulic motors drive a screw jack - through a differential gearbox - to vary the incidence of the tailplane. The control system is split so that the pilot controls tailplane incidence through system B while the second pilot controls the tailplane incidence through system A. The levers for this system are mounted on the pedestal outboard of the throttle levers. On the captain's side two sets of levers are mounted, the second set of levers being mechanically linked to the copilot's levers, thereby providing the captain with a secondary trimming ability in case of B system failure. Of the two hydraulic motors in the system, one is locked while the other is operating. Each circuit contains an arming valve that isolates the circuit from the pressure supply except when a trim selection is made or the autopilot is in operation. With the autopilot in operation, separate selector valves bypassing the manual trimming control valves are used which provide tailplane movements at a fixed rate for the duration of the autopilot signal. Each tailplane circuit is connected to a separate autopilot.
The tailplane screwjack system
Normal main-wheel braking is operated by system B, incorporating anti-skid units. System A will, on selection, operate the circuits but without the anti-skid units. Differential and equal braking can be applied through the foot pedals when the aircraft is moving, and equal braking is applied when the parking brake handle is used. This hand lever is mechanically latched in the ON position to apply the parking brake, and is of course only used when the aircraft is stationary as the parking brake applies the full 3000 psi to the brakes without any moderation. The anti-skid units operated with the normal system are fitted for each wheel and are operated by the foot pedals only. An accumulator provided in each system has sufficient capacity for at least six full brake applications.
Two opposing jacks are mounted on the nose-wheel strut, connected to a steering head. System A normally powers the system, in the event of failure an alternative supply is fed in from system B. Steering inputs can be provided by the rudder pedals, limiting the steering to 20 degrees either side, or by handwheels which provide 70 degrees of movement to either side. For ground maneuvering the nose-wheel may be rotated through 240 degrees without disconnecting the steering gear. When the landing gear is selected UP, the RAISE supply centralizes the nose-wheel.
The VC10 nosegear, the steering jacks are mounted on top of the strut