Landing Gear - ATA 32-32 and 32-33 - Part 7.
The year 2023 is already ending and we are also going to end here this series of articles dedicated to landing gears.
In this part we will present two important landing gear lowering commands:
- Standby lowering
- Emergency lowering
To ensure a safe return to the ground, these two functions can be activated depending on a possible breakdown or the situation.
STANDBY LOWERING ATA 32-32
Standby lowering used in the event of loss of the GREEN circuit is ensured by the hydraulic pressure of the YELLOW circuit.
Use of the YELLOW system automatically cuts-off the pressure in the normal system by the return and pressure release valves.
The yellow standby system can only lower the landing gear and tail bumper. A safety device attached to frame 25 on the 'standby' control automatic cable tensioner, automatically locks the control if one of the cables break. This device avoids any accidental operation of the landing gear doors in flight.
Controlled by a 3-position emergency switch located on the rear part of the centre console. In this case there are only 2 phases: opening of the doors and descent of the gears.
The «standby control» is purely mechanical landing gear lowering operations being carried out manually by a three position lever located on the rear part of the central console. In this case there are only 2 phases: opening of the hatches and lowering of the gears In the « standby mode » the landing gear doors are not closed.
A sealed cover allows the selector lever to be freed from its NORMAL position.
A stop at the NORMAL and GEARS DOWN positions can be over ridden by means of a spring loaded catch on the selector lever. In the intermediate DOORS OPEN position the lever movement is interrupted by a stop. The GEARS DOWN position can only be selected after manually removing the stop.
The three positions of the selector lever are:
NORMAL,
DOORS OPEN and
GEARS DOWN
Before moving on to the rest of the presentation, a quick digression on how we discovered the two commands that we will talk about below.
When replacing the carpet which was offered to us by the Tarkett Company, we had to remove the carpet initially glued in 1976 and on this occasion the hatches which correspond to access to the emergency controls appeared in the two cabins.
Aft cabin:
This hatch is located in the centre of the aft cabin corridor between the seats, it is accessible to the crew if necessary by lifting the carpet.
Four locks allow the total or partial opening of the hatch to access the emergency descent controls of the main landing gear.
Forward cabin:
In the floor at the level of the flight test engineers' console is the hatch which encloses the control wheel which activates the emergency lowering of the nose gear.
Now that the scene is set, we can move on to the presentation of the emergency landing gear lowering controls.
EMERGENCY LOWERING ATA 32-33
If the normal and stand-by lowering systems are both defective, the pilot has a third and final mechanical means of lowering the landing gear by mechanical control.
A control wheel for the nose gear and a lever for the main gear provide these mechanical means for unlocking the doors and the landing gear.
The doors open under the thrust of the landing gear.
After the landing gear and doors have been unlocked, lowering is achieved by gravity and aerodynamic forces.
Main Landing Gear
Four locks allow the total or partial opening of the hatch to access the emergency descent controls of the main landing gear.
A lever, set in two sockets and attached to two cable runs, located beneath a hatchlet in to the floor at frame 57 and linked to the right hand main landing gear, can be used to perform the following operations through a system of cables, rods and pulleys:
Opening the automatic pressure relief valve, venting the main gear triple valves to atmosphere, and
mechanically unlocking the doors and main landing gear.
Unlocking and lowering the main landing gear.
The air vent valves allow the landing gear to fall freely by gravity and by aerodynamic pressure, until it mechanically locks down. The gear doors remain open.
On aircraft 02, a bottle of compressed nitrogen is installed beneath the floor near the mechanical controls, if difficulties are experienced in locking the gear down, then the pressure of nitrogen feeding the shocks truths facilitates the locking of same.
In the photo above you can see the clear tubes where the nitrogen bottle was connected (which are not present in SA nowadays).
In case of doubt on the part of the crew, by simply opening the front part of the hatch in the floor, we accessed the two optical sights which made it possible to check the opening and lower locking of the two main gears.
Nose Landing Gear
A control wheel, located beneath a hatch let into the floor between frames 22 and 23 simultaneously operates, through a system of rods, universal joints and bellcranks.
The opening of an automatic pressure relief valve and nose gear double vent valve
The mechanical unlocking of the nose gear and doors. Pressure relief and venting to atmosphere door unlocking.
This wheel is well known by our friend Maxime…
We can say that the system was really effective…
40 years after the last flight, the removal of the pin and a simple rotation of the wheel freed the nose gear hatches which had been closed since 1976.
Now we will start preparing a new series of articles which will be devoted to the powertrain. ATA 71 and 72
See you in 2024.
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