Low-cost Third Redundancy in Hydraulic Power System

Category: Flight Safety
System: Hydraulic Power
a) Combined ‘controls’ and ‘utility
b) Landing Gear Lowering
The hydraulic power system is designed as a combined ‘controls’ and ‘utility’ system for efficient management .
The ‘utility’ pressure displayed in cockpit drops to zero when there is heavy fluid loss in system which warrants reserving available pressure for ‘controls’, as designed by redundancy system .
This is the result of Isolation Valve closing
This is done by a fluid low level sensor in the bootstrap reservoir which in turn supplies 28V-DC to solenoid of ‘Isolation Valve’ to energise the valve to close.

Abstract:
The utility system pressure being zero now , it disables Landing Gear lowering. Pilot cannot undo this hence he now has to plan alternate modes of recovery viz: Belly Landing, Ditching in Water or Ejection, in case of Military aircraft.
Devising a method to lower the landing gear by apportioning the reserved pressure in ‘controls’ is the design change envisaged.
The knowledge whether a solenoid valve is ‘energised close’ or ‘energised open’ is crucial, when dealing with system failure which results in an emergency.

Design Change:
A separate switch in cockpit console to selectively ‘de-energise open’ an ‘Isolation Valve’ in the hydraulic system, is the design change envisaged by the undersigned.
The scheme requires minimum wiring changes and an additional guarded toggle switch . The safety advantages of the scheme which facilitates landing gear lowering are:
1) The claw lock built into the actuators will lock the landing gear down
2) Belly landing and consequent hull loss/costly structural repairs is avoided.
3) Ejection and consequent hull loss is again avoided where belly landing is not possible.
a) If weapons are loaded under wing
b) Non-availability of suitable terrain
c) Belly landing is not allowed by design
When ‘Up-locks’ are unlocked the main landing gears will swing down and lock down if ‘g’ manoeuvres are employed by pilot. Nose landing gear that swing forward may not locking down. A ‘two wheel landing’ can be effected as given in Aircrew Manual

Proving Test
Jack up aircraft, layout safety cordon around, and remove ground locks from landing gears
Connect electric power, check three greens and then connect hydraulic power trolley.
After ensuring cockpit safety checks, start hydraulic power and ensure 206 bars (3000psi)
Indication in ‘controls’ and ‘utility’
Deflate back pressure (60-100psi) of hydraulic power trolley and observe ‘utility’ pressure indicated in cockpit drops to zero Select the new toggle switch, connected in series to solenoid of isolation valve.
Select the new toggle switch, connected in series to solenoid of isolation valve.
Observe landing gears come down and lockdown. Hold primary controls in equilibrium, simulating straight and level flight.
The generic schematic , can be adapted to any other aircraft.