CFIs - Explaining retract. landing gear

[cookmg]

I am working on my commerical and cfi somewhat simultaneously. My istructor has assigned me the task of being able to draw and explain the retractable landing gear system on the 172rg. So, I am curious about what you CFIs would include in this discussion. The system is new to me . . this is my first complex. Do I need to draw the schematic for the hydralic system? From memory? How do the actuators and down locks work? I'm finding Kirchner, Jeppeson, and the FAA manuals very helpful for the prop, but not enough detail for me on the landing gear system. Any help will be much appreciated.

 

[bigD]

I was given a pretty good drawing of the landing gear system of the 172RG by the school where I did my complex/HP signoff. I think one of the instructors sat down and drew it. It's pretty good for understanding the verbal or written description that's always given. I'm at work right now, but once I get home I'll try and dig it up for you. I think the POH does a reasonable job of explaining it, but I don't recall there being a diagram, really. It's been awhile since I've looked at a 172RG POH, though.

The school where I did my single engine commercial didn't require any drawing, just an explanation. But the place where I did my multi-engine commercial required me to draw every system of the airplane - mainly because that's what the DE of the area expected during the oral. So I guess it depends on where you are and what the DE will expect.

 

[cookmg]

Thanks!

 

[jdog78]

Most pilots aren't engineers or A&Ps so most reasonable instructors and examiners will not expect you to draw a perfect replica of a systems schematic. I can't speak for cessnas, but a Piper POH gives a pretty good schematic drawing. In pipers, gravity and about 400 psi of hydraulic pressure bring the gear down and J-locks hold the gear in place. To retract, the j-locks disengage and 1800 psi of hydraulic pressure in the piston in the reverse direction causes the wheels to retract. If you can take phrases from the POH and use those in your description, that would be ideal.
Just know the basics like the pistons, hydraulic lines, psi numbers, etc., and be able to explain your drawing. Also, be sure to talk about micro-switches, gear unsafe indications and emergency gear extension.

-j

 

[Timebuilder]

For your drawing, you can make a block diagram or a cutaway of the airplane. For the cutaway, you can draw the main gear, the box that contains the actuating mechanism (hopefully with the bushing AD complete) the hydraulic "pack", which contains the motor/pump assembly, the emergency hand pump, the gear handle, CB (if I recall, there is more than one CB for the gear system), and the nose mechanism.

You might also note the placement of the carburetor on the RG. It's out of the normal position to make room for the nose wheel mechanism.

 

[Iceman21]

The carburator in a 172RG is a side draft carb.

For the gear. When you are going to explain it as a CFI, you really will not need to get too in depth with it or you will confuse the student, you won't confuse your instructor because he/she knows it already.

Now for the system. (I am learning it now too!) It's a hydro-electic system. Hydraulically operated with a power pack to give the system a kick in the pants when it needs it. The power pack has a two lines that flow through it, a pressure line and a return line. When the lever is moved into the up or down selector an electric switch is thrown and opens the hyraulic line, which must maintain 1000-1500 psi or the power pack will kick in to supply the correct pressure.

Safety features of the gear system include: a gear horn that operates whenever the manifold pressure drops below 14 psi with the gear up, flaps down at 20 degrees without the gear, and finally you will hear the horn went the squat switch is closed (meaning the nose gear strut has pressure on it). There are two electric relays per wheel. One set for each landing gear indicator light.

I hope this helps a bit, but once you can make youself a diagram it will help immensly

(sorry I forgot how to spell today)

 

[Timebuilder]

a gear horn that operates whenever the manifold pressure drops below 14 psi with the gear up,

It's easy to confuse PSI with InHg, or inches of mercury. Cessna says that the setting for the warning horn should approximate 12 inches.

 

[avbug]

The easiest way to illustrate the landing gear on the 172RG is to draw a picture of a frog with it's legs being broken.

Then show a picture of an airplane that's landed gear up.

Show them where the circuit breaker is, and the resorvoir fill panel, and you're done. If you look out the window and it aint' there, and the switch is down, then you have a problem. If you look out the window and it's eye level with the window, you have a problem. If it's shaking and shimmying and seems all screwed up, looks goofy and makes you think it's about to fail, it's doing okay.

 

[Iceman21]

It's easy to confuse PSI with InHg, or inches of mercury. Cessna says that the setting for the warning horn should approximate 12 inches.

Whoops, I meant inches of InHg. And the POH backs up Timebuilders number and shoots mine down like a clay pigeon.

 

[Timebuilder]

My setting often varied from that "ideal" 12 inches, and I spent a LOT of time in that airplane.

Does yours idle as rough as most of the RG's? We started leaning the mixture for the taxi, and saved some runup time that way, but it never ran as smooth as a conventional 172.

One day, it was not responding to my usual ministrations to get it to pass the mag drop test. The boss was sure I was doing something wrong.

When I returned to the ramp with the disappointed student, the boss went out and went through a series of idle changes, mixture adjustments, and mag switching like a monkey on crack, certain that I, his intrepid instructor was the problem.

Can you say "shorted plug wire"? I knew you could.

After the repair, it returned to its usual rough idle.

 

[cookmg]

Thanks for the help!

What happens differently in the hydraulic system when bringing the gear up vs. putting the gear down? Are different lines put under hydraulic pressure in the different situations?

How do actuators operate?

How do the mechanical gear down locks work?

What's the function of the hydraulic returns?

Thanks again!

 

[avbug]

Cook,

The heart of the Cessna single gear system is the power pack, located in the pedestle. It supplies the hydraulic pressure to retract and lower the gear. Actuators all function essentially the same. Imagine a tube with a plug in the middle, and a ram running from that plug through one end of the tube. Flood one end of the tube, the ram moves to the other end and in so doing, retracts the ram (rod,whatever you wish to call it). Now flood the opposite end, and the ram moves the other way.

The first line you used to do the flooding, call line A. When the ram is being retracted, it's supplying pressure. When it's being extended, line A serves as a return line. Line B, attached to the other end of the acutator, is now the pressure line. Whent he ram moves to retract, it's the return line.

To tell the hydraulic fluid which line to enter, a valve is used: the selector valve. It's controlled by the electrical switch we call the gear handle. All you are doing when you move that switch is repositioning the selector valve.

The easiest way to learn how to draw out the system is to practice drawing it from a pilot handbook, or better yet, the maintenance handbook.

Have you had a chance to watch the airplane do a retract test on jacks? You'll be interested to see exactly what the gear is doing. It's something to watch. You'll wonder how something so goofy looking could find it's way onto an airplane. Yet, it works well. You'll be especially interested to see just how far that gear drops down before going aft.

You'll also have a much clearer idea of how the other mechanisms work, such as uplatches. If you do this, disconnect the nosegear doors to watch the mechanism in there, and you'll see the function of the uplatch clearly (it's been a while since I worked on or flew one of these, so forgive any errors here. I also have few functional brain cells remaining, if that helps).

Downlocks work in several different ways, and the Cessna single gear is not typical. Most downlocks are either an overcenter lever, or a sliding physical lock that's actuated in sequence hydraulically. The Cessna single system is unique.

Timebuilder, was it the plug lead, or the plug? Most commonly, it's the plug. The lead is very rarely the problem. If it's the plug, then it probably was the instructor who was the problem: fouled plugs are usually pilot error.

 

[mattfish42]

The RG's maintenance manual has a pretty clear diagram of the hydraulic system. You might try asking your friendly neighborhood mechanic to let you photocopy figure 5-1. The electrical diagram on that same page is probably more detailed than you're interested in, but the plumbing is actually pretty simple. It might give you a starting point for your own diagram, anyway.

To tell the hydraulic fluid which line to enter, a valve is used: the selector valve. It's controlled by the electrical switch we call the gear handle. All you are doing when you move that switch is repositioning the selector valve.

I have to point out that the gear handle on this plane is not an electrical switch. The handle is connected directly to the valve.

If anybody ever tries to tell you that the squat switch makes it safe to place the selector in the up position while the aircraft is on the ground, it is very very important not to believe them. If the switch is defective and/or the nose strut is extended far enough, the nose gear will retract and the plane will suffer about $14,000 worth of damage.

It does look kinda like a broke-legged frog, doesn't it?

 

[avbug]

Well, like I said...it's been a while. Few brain cells. Not much hair. Keep forgetting where I put my glasses, then remember I don't wear any. That sort of thing.

 

[Timebuilder]

Timebuilder, was it the plug lead, or the plug?

Plug lead shorted to ground.

 

[cookmg]

Avbug -- Very helpful thank you . . especially with regard to the actuators and the selector valve.

Can you or someone else help me understand a few more things about where hydraulic fluid is located at different times?

If the gear is extending does the fluid flow into the actuator on one side and remain there until the gear is retracted? Does it return through the same line that it flowed into the actuator from? Or, immediately after the action of the gear is completed does the fluid return to the power pack through the other line? When the gear is in a given position is all of the fluid at the selector valve waiting to be disributed, or is the fluid right up against the actuator and the selector valve will just determine which line will be the pressure line?

Finally, I understand that squat switch disables the pressure switch, but if the system has adequate pressure (1000-1500 PSI) can't it still operate the gear without the pressure switch coming into play? My understanding is that the pressure switch regulates when the power pack must increase the pressure in the system. If the system has adequate pressure, is anything electronic needed for the gear handle to work? Especially considering what MattFish said about the selector valve being a direct connection to the valve . . . How does the squat switch prevent retraction. I'd appreciate anyone's help clearing up my misconceptions regarding this process.

By the way . . this board rocks. It's really cool that so many people will take time to help out.
Thanks again.

Mike

 

[avbug]

The idea of pressure remaining in the system is misleading. Pressure remains in the line because it was applied to move something, and it's the pressure that remains after that something has been moved. However, move the switch the other way won't provide adequate pressure to move anything. It will bleed to zero almost instantaneously.

Storage of pressure in a hydraulic system is accomplished with an accumulator. You can think of that like the magazine spring in a gun. Push more rounds into the magazine, and it holds them under spring tension (spring pressure), ready to push them back out of the magazine. A hydraulic accumulator works the same way. Internally, it acts like a one-way hydraulic acutator. You push pressure in one end, and the other end is filled with nitrogen pressure. You push a lot of hydraulic fluid in there under pressure, and when you need that fluid under pressure, you can open a and release it into the system.

Accumulators are necessary to "store" pressure. Accumulators are used to effect changes in gear, emergency braking and extention systems, unfeather propellers, etc. Accumulators also serve as dampeners in hydraulic systems, to absorb the jarring that sometimes takes place with the opening and closing of valves, or the sudden stopping of hydralic actuators (such as landing gear reaching full extention or full retraction).

Hydraulic fluid flowing to an actuator stays there. It stays there often under pressure, until the actuator is reversed. Again, fluid is squirted in one end of the tube, moving the plug to the opposite end. This fluid stays put until fluid is introduced on the other side of the plug at the other end of the tube...forcing the plug back. Valves are used to accomplish this.

Some valves are sequenced mechanically or electrically by other components. Sometimes limit switches are used to shut off pumps, or open and close valves. A common useage of both is to sequence landing gear doors up and down during retraction.

The pressure switch to which you referred serves to cycle the pump on and off. As pressure bleeds down, not enough pressure is available at the switch. This applies power to the pump, and the pump runs. Once the gear is where you want it, there is no need to keep running the pump. (On the ground, for example) The point of the squat switch, or uplimit switch cutting out the pump, is to stop the pump from running once the gear is in place.

Pumps have limited times, both in terms of life, and in terms of duty cycle. Too much running, not enough pump. That's bad.

Anytime you have the system sitting for any period, the pressure is eventually going to bleed off. Few systems are tight enough that they stay put forever. Pressure bleeds off through seals, etc. Therefore you have a pressure switch to cycle the pump on and off to keep the pressure up.

On the 172RG, the mains likely won't collapse if the gear switch is raised on the ground, as they must drop down a considerable distance, and can't raise the airplane up to do it. But the nose gear is another matter. It operates on an overcenter principle, and can collapse.

Other airplanes can completely retract on the ground. I've never witnessed it, but worked on a P2V that had that happen.

 

[mattfish42]

Glad we can help! I have so little flight experience I usually lurk around soaking up info instead of posting.

Each actuator has fluid on both sides at all times. Move the selector valve one way and fluid flows from the power pack to the actuator through a pressure line. Simultaneously, fluid flows from the actuator to the power pack through a return line. Move the selector valve to the other position, and fluid flows through the same lines as before, just in the opposite direction. What was a pressure line is now a return line; the old return line is now pressurized.

Be sure to know whether your particular plane has a "gear up" light or a "gear in transition" light. They don't work the same way.

Hydraulic accumulators are very useful things, but the afore-mentioned broke-legged frog doesn't have one, so don't worry about it.

If it's shaking and shimmying and seems all screwed up, looks goofy and makes you think it's about to fail, it's doing okay.

Yep! Gotta agree with that!

Matt

 

[hydroflyer]

I have two real simple suggestions for you, one of which was briefly brought up in an earlier reply. I've found that A&P manuals are a good place to go for diagrams and cutaways, especially of the vacuum pump for example. The Jeppesen A&P book has a good color gear diagram, however it better explains a Piper Arrow's gear system rather than an RG.
The other suggestion is to talk with your mechanics who do the maintenance on the airplane. Some of these guys may seem hard to approach (there are some in every shop), but they will likely be more than happy to help you out and explain how things work. Think about it, if a mechanic takes the time to explain how a system works to a pilot, he may very well save himself a headache later on if a pilot has to write up a problem on that system. The better you know your airplanes systems, the better you will be at not only trouble shooting the problem, but also isolating a possible cause which can save your company a lot of money and the mechanic a lot of time. Don't be shy to your mechanics.

 

[mattfish42]

Some of these guys may seem hard to approach (there are some in every shop), but they will likely be more than happy to help you out and explain how things work.

Bring food.
And caffeine.