-signal the doors to close
-maintain the L/G retract signal to keep the L/G up-line pressurized.
(d)
As the last door closes and locks, the operations that follow will occur:
-
the L/G retract signal will cancel
-
the L/G selector valve will de-energize
-
the L/G retract lines will de-pressurize.
When the calculated airspeed increases to 264 kts, a signal from the Air-Data Intertial-Reference Units (ADIRU) causes the safety valve to close. This isolates the Green hydraulic system from the L/G hydraulic system. When the pressure has decreased sufficiently, a spring in the door selector valve will adjust the valve to center. This causes the door close line to depressurize.
(2) L/G Extension Sequence
When the calculated airspeed decreases to 260 kt or less, the ADIRU's will function. This lets a "valve open" signal to the safety valve when you make a DOWN selection. The safety valve then opens and connects the L/G system to the Green system hydraulic pressure.
With the L/Gs up and locked and the doors closed and locked, on selection of L/G DOWN, the LGCIU will:
(a)
Signal the doors to open.
(b)
When all the doors are fully open:
-signal the L/G to extend
-hold the door open signal to keep the door open-line pressurized.
(c)
When all the L/Gs are down and locked:
-signal the doors to close and pressurize the lockstay actuating cylinder.
(d)
As the first door moves from the fully open position:
-cancel the L/G extend signal.
(e)
Maintain the doors closed signal and the L/G lockstay actuating-cylinder pressurized.
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Hydraulic Schematic - L/G UP Selection, Door Movement to Open R Figure 056
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Hydraulic Schematic - L/G UP Selection, Gear Movement to Up R Figure 057
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Hydraulic Schematic - L/G UP Selection, Door Movement to Close R Figure 058
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Hydraulic Schematic - L/G DOWN Selection, Door Movement to Open R Figure 059
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Hydraulic Schematic - L/G DOWN Selection, Gear Movement to Down R Figure 060
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Hydraulic Schematic - L/G DOWN Selection, Door Movement to Close
Figure 061
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1
Landing Gear System - Sequence of Operations Figure 062
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(3) Sequence Control
The sequence control is such that:
-
it is not possible to move the L/G unless all the doors are fully open
-
it is not possible to close the doors unless all the L/Gs are locked in their selected position.
With the L/Gs locked up and the doors locked closed, the doors will not be signalled to open if a door becomes unlocked. This is only if the control lever is selected to the UP position.
(4) LGCIU Control
Only one LGCIU controls the operation of the system at any one time. The LGCIU in control is the "Active" unit and the unit not in control is the "Standby" unit. Each LGCIU has a non-volatile random access memory (NOVOL RAM) which makes a record if it was "Active" or "Standby" at the last retraction and extension sequence. Control of the L/G system automatically changes from one LGCIU to the other when:
-the L/G control lever is moved away from the DOWN position
-a failure occurs in the "Active" LGCIU.
Each LGCIU sends a status signal to the other whether it is "Active" or "Standby". It also sends position and status data to other aircraft systems. If one LGCIU fails, it sends a "Standby " status signal to the other LGCIU. The serviceable LGCIU becomes "Active", if it was not already, and stays "Active" for all the subsequent control sequences.
If a failure occurs in an LGCIU that does not affect the control of the system, the LGCIU can still be "Active". The failure is stored in the NOVOL RAM.
B. Indicating
(Ref. 32-61-00)
The System Display shows the position of the L/G and related doors. This is shown as part of the WHEEL page.
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