Speaker A
Alright, this week we have a new topic: the aircraft's hydraulic system. And then we'll cover the landing gear.
Detailed overview of the DA42 aircraft hydraulic system and landing gear operation, maintenance, and components.
Key Takeaways
- The DA42 features a fully-fledged hydraulic system primarily for retractable landing gear operation.
- Hydraulic components are compactly located in the rear baggage compartment, making maintenance challenging.
- The hydraulic system operates on a normally closed cycle with fluid pressure maintained by an accumulator.
- Safety valves on the hydraulic tank protect against pressure changes caused by altitude variations.
- The system’s design and components mirror larger aircraft hydraulic systems but scaled down.
What the video covers
- Introduction to the DA42 hydraulic system and its difference from the DA40, emphasizing the presence of a full hydraulic system due to retractable landing gear.
- Explanation of the hydraulic system components including pump, reservoir, accumulator, control unit, and their location in the rear baggage compartment.
- Description of the hydraulic system operation: normally closed system with fluid cycling from tank through pump, accumulator, control unit, to landing gear cylinders and back.
- Details on the hydraulic tank design, including safety valves to equalize pressure changes due to altitude variations and a sight glass for fluid level monitoring.
- Maintenance challenges due to cramped location of hydraulic components and infrequent need for major hydraulic work.
- Overview of the landing gear system powered by hydraulic cylinders, with three cylinders controlling each landing gear leg.
- Explanation of the hydraulic accumulator’s role in maintaining system pressure and providing elasticity to the incompressible fluid.
- Mention of the electric motor-driven plunger pump and its function in pressurizing hydraulic fluid.
- Discussion of safety features such as valves that prevent pressure build-up and ensure system reliability.
- Brief coverage of related components like autopilot servo mounting and ground-air limit switches for landing gear.
Chapters
- 00:00Introduction to DA42 Hydraulic System and Landing Gear
- 04:00Hydraulic System Components Overview
- 08:05Autopilot Servo and Instrument Equipment Location
- 13:00Hydraulic Tank Design and Safety Valves
- 17:08Hydraulic Fluid Maintenance and Replacement
- 21:17Hydraulic Pump and Electric Motor Details
- 25:53Hydraulic System Operation and Safety Features
- 30:31Hydraulic Control Block and Emergency Lines
- 35:14Landing Gear Design and Retraction Mechanism
- 40:38Shock Absorber and Limit Switches on Landing Gear
Full Transcript — Download SRT & Markdown
Speaker A
These two topics are interconnected. It is important that we examine this topic carefully, as it is new compared to the aircraft you are already familiar with, the 40. Meaning, on the 40, we consider that we didn't have a hydraulic system.
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You might say that there were brakes that seemed to have hydraulic control, yes, but that doesn't constitute a system. A fully-fledged one. There are no pressure sources, no energy conversion; you simply press the pedals, using hydraulic fluid as an
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intermediary, and that's it. Therefore, a full-fledged system is only on this aircraft. In this regard, a special section has appeared in the Maintenance Manual. I will open it for you. Look at the table of contents. Here, there is
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subsection 29, Hydraulic Power. There is no such section on the 40. Meaning, hydraulic power, hydraulic equipment.
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Well, it's all very brief here, though. So, why was a hydraulic system installed on this aircraft? The fact is that, unlike the previous one again, this aircraft has retractable landing gear. Landing gear can be retracted using various, uh, energy sources. For
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instance, there is pneumatic retraction, like it was on the Yak-18. Nowadays, on some small aircraft, electric actuators are sometimes used for retraction, but that's very rare.
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Mostly, aircraft use the energy of a hydraulic system. So, the sole purpose of this aircraft's hydraulic system is to handle the extension and retraction process. And that's it. Nevertheless, the hydraulic system is fully-fledged and resembles those you will find on
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large mainline aircraft, just all in miniature. Here, the knowledge you may have acquired in the lecture course when I told you about hydraulics will be useful. We talked about what types of hydraulic systems exist, the principles of their operation, normally
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closed, short-circuited, what pumps are, and what varieties of pumps are used in hydraulic systems. Yes. And what are hydraulic accumulators, uh, filters?
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All of this. Meaning, we talked about some theoretical points. We will encounter all of this here in one form or another. And when we start studying the landing gear design and operation, all of this will come in handy. So,
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what is included in the hydraulic system? Well, we have a slide for this. The computer is running slowly now. See, uh, the hydraulic unit contains all the components necessary to generate hydraulic power on the DA42 aircraft and includes these elements: hydraulic
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pump, reservoir, accumulator, control unit. All of this is located in one unit. In one place on the aircraft.
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This place is located on the left side. A so-called mounting shelf is installed there. Where exactly is it? It is in the area of the rear baggage compartment. Well, we know that the rear baggage compartment is basically a
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molded plastic part. Attached to the frame. So when we look into the back of the aircraft, we see nothing there but the baggage compartment. And you have to remove this baggage compartment part, and then we will discover this unit
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on the left side. For quick access, there is a removable hatch in the floor of the baggage compartment. You can see some things there, but generally speaking, you won't see this equipment unless you get on an aircraft that is
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undergoing, well, major hydraulic work, where the baggage compartment is removed and all these internals are exposed. So, in operation, you know you have a hydraulic system, but you don't really see anything. Flight operations.
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There is also no access to this unit from the outside; that is, there are no hatches on the fuselage. Therefore, maintaining the hydraulic system is quite difficult. That is, as I said, you have to remove the baggage compartment, and then we can fit in
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here. Well, the baggage compartment itself and the tail section of the fuselage are very cramped, so it's not spacious even for one person. So there are certain difficulties for maintenance, but this concerns, firstly,
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the ground crew, and secondly, it is done quite rarely according to the procedures, fortunately. And so you don't really have to dismantle all this and climb in here that often. Here is what the inside looks like. See? This is where the baggage compartment is removed, opening a view of the rear
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fuselage. And this here is our left side. There is the mounting shelf, you see, it's visible. The hydraulic equipment is assembled here. This is, you see, the yellowish bulkhead of the rear baggage compartment frame. And these are the ring frames; remember,
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there are some equipment units in the fuselage like that. You see the rod going to the elevator. That very long rod in the roller guides. There they are, visible, the roller ones. Let's zoom in now. You see, the roller guides.
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And these, look, are the mounting clamps for the autopilot servo. This is where the autopilot actuator is located for the elevator channel, that is, the servo. Well, that's just for completeness. For completeness. And some instrument equipment units are
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located here too. So, all this gear is hidden by the baggage compartment itself. Well, here is a view, see, from the other side, larger. Well, the location is somewhere here on the left side. Right. Uh, what do we find here?
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Here, uh, elements of the hydraulic system are mounted. Look, this is the hydraulic tank, which holds the supply of hydraulic fluid. This is the pump, which uses the fluid to create the necessary pressure. This is the hydraulic accumulator to maintain the
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necessary pressure and prevent fluctuations. This is the filter, and this is the control unit, the hydraulic control unit. So, in a nutshell. Now, as we know, what is the operating principle of the hydraulic system?
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Normally closed. Hydraulic fluid is drawn from the tank to the pump, which builds pressure and charges the hydraulic accumulator. Then, the filtered fluid is distributed through the control unit to the consumer. As I said, we only have one consumer. This
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is the landing gear extension and retraction system. That is, pressurized fluid goes into the landing gear extension and retraction hydraulic cylinders, and there are three of them, one for each leg; it acts within these cylinders and then drains back into the
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tank, and the cycle repeats. So, the fluid cyclically moves from the tank through the consumers and back into the tank. And what do the elements that make up the system represent? Well, the first thing we'll name is the hydraulic
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tank. Here it is. The hydraulic tank is a welded structure, secured, as you can see, by two tension straps to a vertical wall. On the top part of the tank, there are a hydraulic tank filling port where fluid can be added,
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although it is not usually done that way. Then there are two safety valves. One valve protects against vacuum differential, the other against positive differential. What does that mean? This entire system is exposed to external pressure. That is, when the
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aircraft is flying, it changes altitude and the pressure changes. This variable pressure acts on the tank. So, we climb, the external pressure drops, but the pressure inside remains. After all, the tank is airtight. Consequently, the tank could swell. We descend, the
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pressure increases, but the pressure inside remains low. The tank could collapse. So, to prevent this, there are these two valves. Through this valve, air enters from outside into the tank, equalizing the pressure. And through this valve, by pushing against
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its spring, air exits outward, venting excess pressure from inside the tank. Well, the air intake through this valve passes through an air filter for purification. That is the system. You see, the tank is protected. On the side surface of the tank is a sight glass. A
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glazed opening. On this glass, there is a mark in the middle. So, the fluid level must not be lower than this mark.
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By the way, you can see the top of the tank. This is the filler cap, this is th
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This is the positive pressure relief valve. Well, there is also a hose attached here for some maintenance purposes. You see, the fluid level is visible in this window. What kind of fluid is filled into this tank? Well, uh, hydraulic oil of the AMG-10 type is
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filled. This is our domestic variant, AMG-10. Aviation hydraulic oil. For convenience of monitoring and, uh, detecting leaks, a dye is added to this composition. Therefore, the color, as you can see, is red. The level is clearly visible, very easy to see. Next
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to it, you see a plate attached, which says hydraulic fluid, either AHEL or G- 10. Mixing is strictly prohibited.
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There should be a checkmark here, made with a felt-tip pen, indicating exactly which fluid is currently filled. The level is monitored. The level is monitored periodically when the hydraulic system is in its initial position, meaning the landing gear is
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extended, the system is pressurized, that is, it is charged, the hydraulic accumulator is charged, and only then the level should be no lower than the middle. marks of this indicator. And who does this? This is done by the
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technical staff. You are not required to do this, and you won't be able to anyway, because as I said, all of this is closed off and hidden by the luggage compartment. Therefore, you rely on the fact that maintenance has been carried
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out properly. The system is quite airtight, and the need to top up or replace the fluid is extremely rare.
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Extremely rare. Well, there is information that the volume of this tank is actually a small reservoir, 1.5 liters, with a normal level of 0.9 liters. There. Or even a little less, because 0.9 is when the glass is completely filled. So, look, you are
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required to change the fluid in the hydraulic system every 2 years. Regardless of the level, a complete fluid replacement must be performed once every 2 years. Otherwise, it is only monitored. You see, there is a plate that shows to verify the correct
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hydraulic fluid level. You don't even need to do it at 100 hours. Every 200 flight hours, the technical staff needs to check the fluid level in this tank.
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Right. What else is interesting here? On the bottom surface of the tank, there is a drain hole, closed with a plug. That is, if you need to drain the fluid completely from the system and the tank, say, to dismantle it, that
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can be done. Also, on the bottom surface of the tank, there are fittings connected to the fluid suction lines to the pump and the fluid return line from the consumers back to the tank. This is a typical design. By the way, there is
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a more detailed diagram of the hydraulic system. Here it is. It is in your manuals and in the technical operation manual. For the Flight Manual , this is considered an excessively detailed diagram, but it is clear, so you can use it during training. It is
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not all that complicated. Look, here, represented schematically, is the tank. The tank, you see, the hydraulic one.
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Here is the drain plug. There. The protection, you see, against reverse pressure differential, which I mentioned, with an air filter, and against direct differential with a spring. You can even see the operating pressure indicated as 14. Here is the
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level indicator. Here is the filler neck. And this, you see, is the fluid drain, and this is the suction. The only thing is, the layout here is a bit off. It should be at the bottom, but for the convenience of the diagram, it
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was drawn at the top. Like this. The next element on the fluid's path from the tank is the pump. The hydraulic pump. This is how it is displayed here.
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As we recall, pumps can vary in their operating principle and have different types of drive. Here we have a plunger-type pump driven by an electric motor. The motor is powered by 24V DC.
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You see, this pump here. This pump is installed underneath the tank. There it is. The pump itself has two main parts.
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This is the electric motor, see the cooling fins, and this is the pumping unit, that is, the plunger-type pump itself. Hydraulic fluid is supplied to it from the tank and is then discharged under pressure. This means logically it
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follows from what has been said that to start the pump, you need to apply voltage to it. There's a buzzing outside the window. Those idiots are clearing leaves with their gas-powered blowers. So anyway, how exactly is the pump turned on and off? To control the
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pump, there is a component on this control block that is a special element . Here it is, which serves as an electro-hydraulic switch. Or, it is also called this is the pump pressure switch in our schematics. A pressure
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switch. What does this pressure switch represent? Inside the housing, there is a membrane and a microswitch installed.
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Pressure is applied to the membrane, and the force bends it in one direction or the other, resulting in the microswitch being triggered to either turn the pump on or off. You see, this element is located on the control block
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. Here it is, by the way, in real life. For the pump control circuit to work, it is necessary to have a power supply.
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That means we need to turn on the Electric Master. Upon switching on the Electric Master, what happens is, let me find it, here is what happens. If if the system pressure is below 96.5 bar, the pump turns on. What does "turns on"
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mean? It means the electric motor starts rotating and the plunger pump increases the system pressure. The pressure rises. And when the pressure reaches 113.8 bar, the pump turns off automatically via this membrane switch, or pressure switch, as it is called in
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the diagram. When the pump is off, the pressure gradually begins to drop. Well , that's natural, even if we aren't doing anything with the hydraulic system. Internal leaks are inevitable.
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Through seals, through various valves. There is always internal leakage, so the pressure slowly drops. And if we are operating, say, the landing gear, retracting or extending it, then the pressure drops quickly. Either way, the pressure falls, and when it drops below
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113, nothing happens. But when it reaches the lower threshold of 96.5, the pump turns on, raises the pressure back to 113, and turns off again. So, this is a periodic pump switching process. It maintains pressure within this range. We don't receive
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information about this during flight. Nothing lights up, nothing goes out. Well, if it's quiet enough on the plane , though that's unlikely in flight, we would hear the pump humming behind us.
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On the ground, it's quite audible. So, this is the only acoustic signal of the pump operating. The system is fully automated, which is why pilots sometimes tend to forget about all these, um, that they can't even remember later how the pump is actually
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controlled. It just works. But in reality, you really need to know this. Yeah. Moreover, you are required to check the pump’s functionality. And how do we check that? It happens before the flight. When you arrive at the aircraft and turn on the Electric
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Master, then the pump might turn on, or it might not. What does that depend on?
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It depends on how long the aircraft has been parked. So, for instance, if the aircraft was just powered down and we turned it on immediately, the pressure hasn't had time to drop below 96, so nothing happens when you flip the
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Electric Master. But if the aircraft has been sitting, especially, say, overnight, then the pump will certainly turn on. And we will hear the sound of the pump working. So, we should estimate its running time. I’ve made an excerpt. Let's see, an excerpt from
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the QRH. Look, the pre-engine start stage. When the Electric Master is switched on, the pump may start to restore system pressure. Its operating time is 5 to 20 seconds, meaning the time it makes noise. This duration depends on the time that has passed
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since the last time the Electric Master was switched on. Ah, but in any case, it shouldn't exceed 20 seconds. If the pressure is obviously running for more than 20 seconds—that is, the pump is running even though the time should be
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up—we consider it a malfunction. Or if the pump shut off within those seconds but then immediately turned back on. Shut off again, turned back on again. This indicates that the pump itself is fine, but there is a leak in
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the system. And it’s not necessarily an external leak; it could be an internal system leak. Here on the control unit, on the control unit, there is this safety-wired valve. Do you see it? This is called the bleed valve. The bleed valve. It is used—or
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unloading valve. It is used for maintenance, if you need to quickly bleed pressure from the system. For that, you break the safety wire here, unscrew the nut, and press the valve to vent the pressure so you can work on
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the hydraulics. Then this valve must be closed and safety-wired. So, if someone forgot to close it, pressure will be constantly bleeding through it. Meaning the system might be perfectly fine.
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It’s just that during maintenance, they missed this detail and left the valve open. Because of this, the pump runs but can't maintain pressure, cycling on and off. In any case, for a pilot, this is a sign that something is
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wrong. You need to stop further flight preparations and report this malfunction so it can be fixed. That is how the pump operation is monitored.
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And in flight, too, according to the manual, you will see there is a point where in flight, you can hear the pump running and not shutting off. This is quite dangerous, because the pump can overheat while constantly energized. it
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heats up. And this can cause a fire, among other things. Therefore, we see that actions are required, up to terminating the flight and making an emergency landing. That is how we manage and monitor the pump. Basically, everything is automatic, but we listen
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to and watch its behavior. There. What is after the pump? Next, after the pump , we have a filter. A fine hydraulic filter. And here it is, filtering of the hydraulic fluid takes place. And after that, the fluid goes to charge
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the hydraulic accumulator. There it goes. This is the hydraulic accumulator , which is charged through certain valves that ensure smooth charging. And the hydraulic accumulator is located like this. You see, it is also secured with tension straps in this compartment
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. Here is the cylindrical hydraulic accumulator. So, we stopped at the hydraulic accumulator. Who remembers what we need a hydraulic accumulator for in the system? From the general course. We discussed it there, to always maintain pressure. No. Well,
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the pump is there for that. The pump maintains it. Isn't it? Well, I think there are also some pulsations that occur when the pump is working.
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And so, to dampen them, we need this, yes, we talked about it correctly, yes, the fluid is incompressible, we fear pulsations, so pulsations are smoothed out by an element like an accumulator.
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But that is not its main function. Sometimes people also say it's an emergency reserve of energy. Right? I mean, let's say your pump has failed, but the hydraulic accumulator is charged, and for a while it should theoretically be enough. And that is
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also correct, right? Maybe this reserve is just not enough to lock the landing gear. That is true. But still, that is not the main function of the accumulator. Its main function is to give the system elastic properties, elastic properties to an incompressible
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system. The thing is, the fluid is, as I already said, incompressible. And so, if we imagine that there is no hydraulic accumulator, well, we removed it and connected the line directly, what would happen then? The pump turns on, creates pressure, and turns off.
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And what happens to that pressure? It would drop almost instantly. Why? Because in the system, as I have already said, internal leaks are inevitable; they are small, but with an incompressible fluid, even very minor leaks or internal flows through valves
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and seals are enough for the pressure to drop to zero. Well, think about it: in hydraulics, an incompressible medium really doesn't have the concept of more or less. Either there is pressure, or there is none. Therefore, after shutting off, the pump will immediately
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turn back on due to the pressure drop. Again, it will restore the pressure very sharply with a surge. It will turn off and after just a few seconds turn on again, and so on. That is, the pump will constantly cycle in an
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incompressible medium. And how long will it last? Unlikely for very long. Plus, there will be constant hydraulic shocks in the system, meaning pressure will drop and rise in surges. That is the point. So, what is a hydraulic accumulator? Inside this cylinder,
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there is a floating piston. That is, one chamber is hydraulic, where fluid is supplied, and the other is nitrogen, which is charged with nitrogen at a pressure of about 80 atmospheres. And so, when there is no pressure, when
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there is no pressure, the nitrogen pressure forces the piston to the extreme position. The system is considered discharged. When the pump turns on, the pressure rises. And when this pressure exceeds the nitrogen pressure, that is, 80 atmospheres, the
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piston starts to move and compress the nitrogen. This means the system reaches a balanced state where the piston occupies a middle position. And the fluid pressure equals the nitrogen pressure. So, we reach our value of 113 , when the pump turns off. At this
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point, the nitrogen pressure and fluid pressure are equal. And as the pressure drops, the nitrogen expands and slowly pushes the fluid out of its chamber, from the accumulator cavity into the system, thereby preventing a sharp collapse of pressure. That is, the
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pressure drops slowly, which is what we want. Therefore, the pump activation characteristic becomes a stretched-out sine wave. That is, the pressure now falls very slowly. And how slowly? Well , practice shows that when the landing gear is not in operation, the pump
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turns on no more than once an hour, or even less. Can you imagine? This is a completely satisfactory characteristic.
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There. Right. So, the pump turns on very rarely in flight. This is all thanks to this accumulator. By the way, many are not aware of this purpose.
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People often name dampening pulsations or something else as the task of the hydraulic accumulator, but that is not the main reason; the main one is what I just said. And, by the way, when you turn on the pump, when you flip the
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Electric Master, what are those 5 to 20 seconds for? Exactly for charging the accumulator. That is, the longer the time, the more discharged the hydraulic accumulator was at the moment of activation. Even if it is completely discharged, the charging time should
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not exceed 20 seconds, which we verify before every flight. Thus, when switching on the Electric Master, we monitor not just the pump by sound, but also the health of the accumulator. If it is, say, discharged or improperly charged, then this time will also fall
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outside the tolerance. That’s one such element. We actually had problems with an accumulator on one aircraft. We even had to replace it. Its internal seal was compromised, which caused the pump operating parameters to deviate from the requirements. So, that is how
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we figured that out. The charging of the accumulator happens along this line , see, through throttles, so that the accumulator charges more smoothly. And the discharge of the accumulator happens along this same line. But there is also a valve added here that ensures
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a faster release of energy into the system when needed. This is the hydraulic chamber, and this is the nitrogen one. And here is the charging port for nitrogen from a ground-based compressed nitrogen source. There. And after the accumulator is charged, the
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fluid’s path continues to these valves. They are called, as you can see , solenoids. One is "retract," the other is "extend." These are solenoid valves for landing gear control. They are located on the control block. There they are. One and the second one, both
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black in color. Or, as you can see, the upper solenoid valve and the lower solenoid valve. One is responsible for gear retraction, the other for gear extension. These are the exact valves that you will control with the retract/
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extend switch. There they are, see, an electrical signal goes to them. That is , when you set the gear switch on your panel to the extension or retraction position, either this valve or that one will actuate. As a result, the fluid
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standing by here will flow either into the extension line or the retraction line. There. You see, this is the control block, this is the fluid supply to this block, and this is the fluid drain, the fluid drain to the gear
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cylinders. These valves, they have special elements on the top and bottom here for checking. That is, during maintenance, they check them for clear actuation. And how are they checked?
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They send a signal to the valve and apply a screwdriver. If the screwdriver is pulled, meaning the valve magnet is energized, then the valve has actuated.
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Such an ingenious way to check functionality. There. Those are the valves. And after these valves, the fluid goes to the hydraulic cylinders.
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Installed along the fluid path are these throttle blocks, which ensure a smooth flow of fluid. That is, one of the features of the hydraulic system is pressure pulsations, which are inevitable during pump operation and when consumers are activated. So, to
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smooth out these pulsations, they try in every way to stabilize the fluid flow, including by using these hydraulic resistances, or throttles.
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And one more thing, we have looked at the normal operation of the hydraulic system, but if, let's say, a pump control failure occurs and it doesn't shut off in time, then this could lead to a pressure increase, rupture of the
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lines, and fluid spraying out in the hydraulic bay. We are concerned about that. For that, we have protection, and it is a multi-stage protection. And there are these two valves installed.
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One is a regulating valve, and the second one is a relief valve. And they are also located on the block. One of them is set to this pressure in PSI units, and the other to this one. Now, I had a translation somewhere. Here it
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is. You see, the regulating valve limits the system's operating pressure to within 124 bar. The relief valve limits the pressure to 179 bar. Our normal pressure, as we already established, is in the range of 96.5 to 113.8. If this diaphragm switch, let's
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say, fails to trigger and the pressure rises higher, reaching this value. The valve spring will compress, it will open, and the pressure you see, it will open, and the pressure will start bleeding off into the tank. It will be
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elevated, but not yet critical. If this valve fails to work, then this one will also protect, using a spring as well, at a pressure of 179 and will also bleed excess pressure into the tank. So , look, let's return again to that
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situation in flight where the pump does not shut off. That is, we hear, for example, the hum of the running pump.
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The pump is running, but it cannot restore pressure. In this case, it could be either an internal leak, meaning fluid is spraying out somewhere , and that's why the pump is running but the pressure isn't rising. Or the
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pressure is rising, but the pump doesn't shut off due to a control failure, and these safety valves are working, bleeding pressure into the tank, which is also not good, since the pump is constantly running, heating up, and could fail or simply become a fire
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hazard, a cause of fire. So, um, listen to the behavior of the pump. So, well, um, that is essentially all. We have discussed the features of the hydraulic system. Once again. You see, fluid goes from the tank to the pump, then travels
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to the filter. Then it goes to the control block, from there it goes to charge the hydraulic accumulator and stands by near these solenoid electromagnetic valves. these ones here . If the valves open, then the fluid goes into the retraction and extension
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system. Here, you see, are four braided hoses. Three hoses lead each to its own landing gear leg, nose and main, and one hose is for the return. Fluid flowing from the cylinders into this block and then into the tank goes
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through it. And this control block, as you can see, is packed with various elements. We called this a pressure relay or a diaphragm switch. These right here are the two safety valves, remember, at 179 and how much was it?
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124. This right here is the relief valve, also called the Dump Valve, which must remain closed for control.
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This is where the so-called Mini connector is located. This is for maintenance. You can connect a pressure gauge here to monitor the hydraulic system's operation. Again, this is not our task. There. And this middle part, you see, between the solenoid valves
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and the return lines. This right here is the emergency landing gear extension unit. We will take a look at this system soon. And you can see the cable coming in, which goes from the emergency release handle. There is a
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small block here that distributes fluid to the emergency control line. So, we will return to this unit, but as something we are already familiar with.
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So, there is quite a lot written in the maintenance manual about this hydraulic system, but as I said, luckily, it doesn't need to be done very often. And this is handled by the technical staff.
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It is not necessary for you to use this diagram. I don't require you to trace all the lines here and analyze it in detail. This is a diagram, in fact, for the engineering and technical staff, but it is clear and, in general, you
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can use it. As we can see, there is nothing complicated here. We have figured it out. So, the main thing we need to know is the pump control. Of course, it turns on when switched on.
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For some reason, there are many inaccuracies and problems regarding control during the exam. When the pump turns on and what its state depends on are common sources of errors. Well, we will return to this hydraulic system when we reach the extension and
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retraction system. That is all for this topic. If there are no questions, then in the remaining time, let's start the next topic. Landing gear, we need to hurry a little bit. We will study the landing gear of this aircraft in two
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parts. In the first part, we must examine the design features of the landing gear. It is important for us to see what is interesting here from the perspective of a pre-flight inspection.
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That is, while walking around the aircraft, you must inspect the landing gear legs and know what to look for.
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Well, and the manual also points this out. Landing gear check points in section four. And in the second part, we will examine the functional landing gear systems. What systems? On the DA40 , you only had one system, which was
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the braking system. Here you have the same system, but in addition, there is a landing gear extension and retraction system and a nose wheel steering system . nose wheel. Although it is simple, it exists, so there is a bit more to talk
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about. So, what we already know about the landing gear design is that the layout is tricycle with a nose support leg; the landing gear is retractable, as we now know, using hydraulics. The nose wheel is steerable. In the
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retracted position, the landing gear is covered by doors. All of this is already known. This is what it looks like, you see, the landing gear. The nose gear and two main ones. After retraction, the nose bay is fully
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closed by its doors. For the main gears , the doors partially close the bay.
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The wheel section remains exposed. Since the left and right main gears are identical, we only need to study two, one main and one nose gear. You see, the hydraulic block is also shown here, implying it is inextricably linked to
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this structure. So, let's start with the main landing gear. And what elements does it consist of? Well, each landing gear consists of basic elements like the strut, ah, the wheel. the wheel, doors, and retraction and extension components. Oh yes, this also
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includes the shock-absorbing device; so , the strut, wheel, shock-absorbing device, retraction/extension components , and doors. This is what the strut looks like. It is this welded structure . Here it is. It is shown here separately. At the top of the strut,
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there is a crossbeam with two trunnions attached. The crossbeam is a structural element that provides both mounting and the ability for the gear to move, specifically rotation about this axis passing through the trunnions.
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Trunnions are gear mounting elements, representing these special surfaces polished to a mirror finish, which fit into the corresponding landing gear attachment points, located in our case on the center section. You see, these brackets are attached with bolts. And
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there are pressed-in bushings here, either copper or bronze, which are lubricated and form sliding bearings with the trunnions. That is, the main gear rotates relative to these mounting points in the transverse plane. The rear trunnion has, as you can see, a
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threaded part onto which a nut is screwed and locked with a cotter pin. This design allows for the gear to be removed. If you remove all this and unscrew it, the gear can be shifted and taken out of the bay. In the upper part
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, there is also an eye on the crossbeam to which the retraction/extension hydraulic cylinder is attached. In the middle section, a powerful element is mounted, like a pin. We can call it a pin. The folding brace is attached to
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it. So, here we form a third fixation point. Look, we have two points located on one axis, front and rear. And the third fixation point is right here for the folding brace. Thus, we get a truss structure. To hold the gear, we need at
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least three fixation points. And there they are. One, two, and three. At the bottom of the strut, there is an eye welded on. Here is its own weld seam for connecting the next element, which will be the drive lever. For a long
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time, we had no issues at all with the design of either the nose or the main gears of this aircraft. Unlike, by the way, the 40, which as you've already heard, we struggled with, especially the nose gear. Everything here seemed
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solid and problem-free. However, recently, perhaps due to service life limits, cracks have started to be discovered right here on some aircraft.
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Right here along the weld seam of the lower gear strut lug. Yes. And there have even been through-cracks on some aircraft, so there is now increased attention paid to this area. Well, you probably won't notice it, unless it's a
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very obvious crack. But the technical staff must carefully inspect this area. Now, moving on, as I already said, this trailing arm is attached. Trailing arm, which is connected to the strut by this bolt and bushing. On the lower part of
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the arm, there are four holes for mounting the wheel axle. The axle is bolted to these holes. And a brake plate is installed between the axle flange and the arm. There it is. This plate, secured by the same bolts,
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serves to mount the brake caliper. This means the braking torque will be transmitted here. And on the axle, we see the bearing seats and the thread for installing the wheel retaining nut.
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In the middle part of the arm, we see a lug where the shock-absorbing device, or as they call it here, the damper, is attached. This damper limits the movement of the arm. In other words, the arm, as we can see, rotates
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relative to this point with its travel limited by the damper. The damper is attached to the strut on one side. The attachment point is not visible here, but on the other side, it connects to the lug via an intermediate universal
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joint. This results in a hinged mounting design, which provides— recalling landing gear theory—this type of wheel installation. Well, I won't wait for an answer. This is a trailing-link wheel installation.
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Trailing-link. You see, the wheel is on the arm. It rotates relative to this upper arm point. The damper, by changing its length, allows the arm to rotate, but with resistance, which absorbs the energy received during landing, takeoff, and taxiing. This
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damper is, in essence, a single-chamber hydraulic shock absorber, or more precisely, a hydro-pneumatic shock absorber. One more point that we discussed in the general lecture course and which you should recall. We previously analyzed the operating principle of a single-chamber
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hydro-pneumatic shock absorber, how it is charged, and how it absorbs and dissipates energy. Recalling that when an impact or shock load occurs, the first phase of the shock absorber or damper operation takes place: energy absorption. And in the second phase,
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when the external impact has ceased and the nitrogen inside the damper expands, energy dissipation occurs—that is, the rod extends with increased resistance so that the strut and damper action is not too bouncy. In other words, so that after an impact, the
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aircraft doesn't bounce like a ball, and its vertical oscillatory movements are damped, quickly damped. For this damper to function properly, it must be charged. Hydraulic fluid is poured into it. You see, or AMG, just like in a hydraulic system. And then it is
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charged with nitrogen. Here is the valve at the top, which we will, which we will keep an eye on during inspection. If you unscrew it, an opening will appear through which fluid is poured inside. And after filling, this valve is screwed back in, and a
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source of compressed nitrogen is connected. and it is charged with nitrogen to the required pressure. This is done while the aircraft is hoisted on jacks. We write here that, you see, the nitrogen pressure is 19 bar in the
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hoisted state. Every 200 hours, the technical staff must connect a device here and check this pressure. We will not connect this device, but you need to monitor the charge and condition of this damper based on a criterion such
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as shock absorber shrinkage or damper shrinkage. When the aircraft is on the ground, depending on its weight and center of gravity, this part changes, the so-called visible part of the rod.
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That is, the greater the weight, the smaller this value, the greater the shrinkage. And this value is measured.
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In the hoisted state, this visible part of the rod is 15 cm, but that's hoisted . But you inspect it during a walk-around in the compressed state, and the flight manual gives you a figure of no less than 4 cm. This part
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of the rod must be at least 4 cm under the heaviest conditions, that is, at the aircraft's maximum weight. This figure must be remembered. This is what the damper looks like. You see, this strut, the welds are visible here. This
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is that very lug. There is the weld. By the way, on, do you see? this lug is attached a limit switch, covered by a cap. This is the ground-air limit switch. And here is the damper, this is the rod, the one we mentioned. Alright,
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let's stop here.
Topics:DA42 hydraulic systemaircraft landing gearhydraulic pumphydraulic accumulatorhydraulic maintenanceretractable landing gearaircraft hydraulicshydraulic tank safety valveshydraulic control unitsmall aircraft systems
