The uplock mechanism on the SF260.

George Parker had his SF260TP painted and was returning home. The gear would not come down. For a while he thought he would have to land his freshly painted plane gear up. Then he had a brain storm. Here is his story.

From: "George Parker"   
Subject: Landing Gear actuator 
Date: Sat, 25 Mar 2000 16:31:03 -0500 

Had a little trouble with the actuator....gear would not come down, breaker kept popping. Had to crank it down and was as nervous as a whore in church about the landing. All ended well.

Discovered some very interesting and subtle things about the set up of the actuator. Had the whole thing disassembled on the bench couldn't find anything amiss other than stiff and old grease, re-greased, put it back together, in the airplane, ran the gear up/down a dozen times and found that the limit switches are VERY flat makes a huge diff. The up limit switch was allowing the motor to run too long, jamming the clevis in the housing. If the clevis on the end drives too far into the actuator tube the motor will pop the breaker and the machined end (inside) may jam tight into the base of the housing,causing the thing to overheat and strain on the way out as well. The pin/bolt holding the actuator to the aft mount must be free to turn....the geometry of the up down action requires it. Operative concept here is PIN not BOLT. You probably knew all this but I would have been powerfully unhappy if my first landing back here in Norfolk was a gear up with fresh paint.

There may be a lesson in this for everyone.

Editor's note
When I bought my SF260 the first thing I did was put it up on jacks and learn how to use the emergency gear extension. I have had the actuator out of my plane and had to adjust the push rod actuators to keep the transit light from coming on during flight. I never had any problems as described above. I can only imagine that the paint shop tried to improve the design when they put the plane back together.


Bell Crank Removal

  1. Jack the aircraft on the maintenance stands.
  2. Remove the console covers
  3. Remove the floor panels between sta.X3 and X4. These are the panels that are around the joy sticks.
  4. Partially retract the landing gear with the emergency handcrank.
  5. Disconnect the actuator from the main bellcrank as follows.

  1. Remove bolt (17, fig. 6-9), detail A
  2. Remove bolt (18) with relevant hardware.
  3. Raise the actuator and disconnect the electrical connector (P4); remove the actuator. The actuator must be removed to have enough room to reach the bell cranks.
  4. You can also gain another two inches of space on each side by removing the floor plates.
  5. Disconnect the left and right bellcranks from the main bellcrank. You do this by removing one central bolt.

What we want to inspect is the Swivel joint assembly (parts manual, fig 4/10) part 34 and the UNIBALL SPHERICAL BEARING, part 44.
Remove bolt #19, page 6-15 of maintenance manual fig 6-9. The spacers and bearings need special attention. If you lose one of these tiny spacers you will have to have a replacement made.

The bell crank does not have to be removed, just the swivel joint assembly with the two uniball spherical bearings. This piece cost $800, and there are two, a right and left. We cannot buy them so they must be made. The one on the right is faulty but no one has been able to fix it. There are two bearings in it which need to be reseated and then staked in place. Part 44. I have found the part for $350, but it is made for a lawn mower and can be adapted to the Marchetti if necessary. Because of the liability problems no one will make a part for an airplane.

At first glance the gear is insurmountable. The manual shows the parts but not in a way that lets you integrate the parts into a working concept. It is only with repeated studying and inspection of the gear is it possible to eventually come to grips with the complex system. The maintenance manual does try to break each system down, but it is so basic in its explanations that you really have to know the gear to understand the book. However, my plane had been taken to Fox 51 for gear repair and the center section had been replaced and the extra stiffeners inserted and yet it continued to give the same trouble. Since I could not arrange a trip to Palatka just now I decided to work on the gear just to better understand it when I do take it down.

The problems:

  1. The left down green light does not come on.
  2. Occasionally the transit light stays on and goes off only after pulling 4 G's or negative 2 G's.
  1. Work on the left down microswitch revealed that they cost $152.00 plus shipping and tax making a total of $180. There are six wires that come out of the switch and they are six feet long. In reviewing the picture on the switch I discovered that there are two circuits in each switch with three wires for each. After 20 hours of working and worrying I decided that there was a broken wire in the down light circuit so I swapped to the second circuit in the microswitch.
  2. The picture in Figure 6-21 shows the warning system. Following this are seven sheets showing each of the positions of the microswitches in different gear configurations. They start with gear down and go through retraction stroke, up-locked, then up locked with throttle retarded, flaps lowered to 30 degrees then with horn silenced, and finally the extension stroke with throttle retarded and the flaps lowered. Note: The gear warning horn is supposed to come on at 15 inches but I had to set mine at 12 inches because I use 15 inches to fly formation with the local Cessna 210 and I get tired of hearing the horn blowing. Also, I throttle back to 14 inches to lower the flaps as the plane will not decelerate to 125 knots if I only throttle back to 15 inches. In the past I would climb 500 feet to get the airspeed down but this is poor flying technique.
  3. Knowing how the wiring worked was not enough. I wanted to find the plug that these wires ran into. This was found on page 12-14 fig 12-4 of the Maintanence Manual. This showed plug P5 and I found it under the floor pan in front of the joy stick. Do not use this picture to figure out the wiring diagram. Use the picture in figure 6- 22, sheets 1 to 7 since they are not as cluttered with extra wiring details. I will work through the wiring diagrams later.
  4. Getting the gear up and understanding it took two years of reading and rereading the books. The pictures in Fig. 6-13 show the up lock arms. To see these in action you must disconnect the gear door. You do not need to remove the wheel to see the uplock mechanism in action. The unlatching of the up-lock arm depends upon the play in the control rod end piece. The drag brace presses against the up lock arm and will not unlock until the control rod end presses against the up lock arm bearing. This is described in as follows: This device includes a spring that prevents the up-lock arm from being displaced until the force of the electric actuator is applied to the control rod.
  5. Actuator description is at and reads as follows: The landing gear actuator is installed in the fuselage between the Sta. X4 and X5. "Now to find a picture of this turn to page 6-28 fig 6-18" Sta. X3, X4, X5 are shown. There is a main bell crank that activates the right and left bell crank. These are activated through the spherical uniball bearings and joint swivel assembly attaching parts which can be found in the parts manual in figure 4/10 part 44 and 34. However, Vaughn Armstrong of the Swift Museum fame, who owned two Siai Marchetties several years ago, says the weak point is in the parts manual fig 1/4 part 30 and 31. He said that undue force on the bell crank would rip this part out of the frame of the airplane. This is the part that Fox 51 had fixed on my plane, but it did not fix my LANDING GEAR problem.
  6. To get at part 4/10 #34 it is necessary to take out the actuator assembly LG retracting attaching parts.

On 12-15-97 I fixed the landing gear completely. The down lock microswitch had to be replaced with all of its wiring. This took two days because of all of the wire holders. Next I worked on the landing gear. This required removing the control rods and adjusting them and replacing them. This is a real accomplishment in the marchetti since there is just barely enough room to get the push rod out of the plane. Then, when I started to put it back together I could not remember which holes the control rod went thought. This seems pretty simple, but when you are working under the plane all of the openings look the same. Good luck.

Main gear strut inflation on the SF260.

The SF260 is blessed with a trailing link strut. This makes operation out of an unimproved field very smooth. However, it does have its drawbacks. Proper servicing of the struts is an art that most people are too lazy to undertake. I say lazy because care of the strut is quite simple but very time consuming. It is like cleaning out a plugged drain. You are working in a tight closed space and occasionally hydraulic fluid spills out over everything.

I frequently find a SF-260 sitting on the ramp that looks too level. Closer examination reveals that both main gear struts are over inflated. Pressing on the wing tip will not depress the wing. The major disadvantage is that it has inactivated the squat switch. However, a more subtle problem exists. Although the strut pressure should be 880 lb., once the struts are over inflated the pressure could be 1,000 to 2,000 lb.. This will place a dangerous pressure on the O rings and seals. Proper operation of the landing gear struts require that the struts contain the proper amount of hydraulic fluid and the proper nitrogen or air pressure. Unless you have a hydraulic pump to put fluid in the struts you must use a more time consuming method. This consists of putting the plane on jacks and securing the tail hook on the plane. Then jack the plane to barely get the wheels off the ground. The Shrader valves are a little complex at first. First disconnect the gear doors and secure them out of the way. Next you bleed the air out of one strut. Turn the Shrader valve in a counter clockwise direction for 1/8 of a turn and it suddenly become lose. Then turn until the pressure starts to release. Then release the jack to compress the strut. As the gear collapses fluid will finally start to come out. At this point connect the container with hydraulic fluid in it. The hydraulic fluid will be forced into the container. When the gear is totally collapsed mark the height. Next fill the strut with fluid. This requires putting the container under pressure and jacking up the plane. You will have to repeat this step several times before the strut is completely filled with fluid. Now tighten up the Shrader valve and let the plane sit on the gear for an hour. During this time you can service the other strut. After an hour return to the first strut and let fluid out until the strut is 2 above the bottom position. Close off the Shrader valve and then fill the strut with Nitrogen. Usually 880 lb. will get a 95% extension of the strut. However, you should consult the maintanence manual for exact pressures. The 880 lbs is with the gear on the ground, the figgure given in the maintanence manual is for the plane on jacks. Repeat on the second side and then measure the height of the wing tip tanks from the ground. To even them up we suggest bleeding the high side. Now press on a wing and let it go. If it bangs when it fully extends there is not enough fluid in the struts. Again, if all else fails, read the directions.

Personal experiences. When I first took delivery of my SF260 the right gear was very low. The pressure gauge at out airport only went to 600 lb. so we had to remove the regulator and crack the tank until the strut extended. Unfortunately we compressed the strut without removing the gear door so we cracked the gear door. Then we overinflated the strut. It was only up to 1,300 lb. but pressing on the wing tip was like pressing on the ground. There was absolutely no give to the strut. Then when I finally got the pressure balanced and would taxi out the plane would shift from one side high to the other side high. This was only corrected when enough hydraulic fluid was added to the strut. It has been a long hard road, but the irony is that when I try to enlighten another SF260 owner, my advice falls on deaf ears. People can only understand what they already know and if they have been running around with their gear overinflated they refuse to believe that there is any other way to do it.

When I added fluid to my shock strut assembly grey fluid returned from the shock. I tried to flush it out with no success so we took it apart. Look at the picture below and especially at the retaining ring #7. This must be removed before removing the piston bearing. There is a hole drilled through the cylinder #8 into the piston bearing. If you do not remove the snap ring #7 it will gnarl the threads of #10 and get brass flakes everywhere.

After cleaning out #7 and washing off #13 I reassembled the strut and then replaced it on the plane. When I added hydraulic fluid it returned completely unchanged as a light red fluid.

You may have noticed that I have not mentioned the nose gear strut. That is because it only requires 125 lbs of pressure and I use even less than this. My nose gear strut seals were replaced just before I got the plane and has given little trouble. If the nose gear strut seals have to be replaced the entire nose gear must be removed from the plane.

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The SF260 Landing Gear retraction tutorial.

When I established the SF260 web site my intent was to build a working model of the landing gear and show it in animation. This has been too difficult for me so if anyone has any suggestions as to how to do this please let me know. In the meantime I will relate what I have learned about the complexities of the gear mechanism. A special thanks must go to Dr. Bill Vitale for his inspiration in this field. What we have done is take several drawings and combined them so that you can understand how the gear works without having to spend a year doing trial and error work.

The main gear control rod is the heart of the operation. Note the rod end, #65. This has an elongated slot cut in it. This allows the bolt, #59 that goes through the inboard brace arm assembly to slide inside the rod end. When the gear is retracted the bolt pulls against the proximal side of the rod end as it pulls the brace toward the center of the plane.

What is so confusing is that there is no single picture that shows these critical structures together. Figure 4.1 shows the entire gear but does not show the uplock assembly. One could suspect that it is a closely guarded secret and by no explaining it no one can understand it or reverse engineer it even with the parts in hand. I hope to end this mystery.

Another interesting note.:
I am interested in knowing more about your problem with your gear. As you probably know, I have recently had problems with a "cracked" trunion (or pivot) bolt. (The "porkchop" bolt around which the bellcrank operates.

There have been 5 or 6 Mandatory SB's on the bolt calling for periodic inspections (magnaflux) and replacement with upgraded bolts if found to be cracked. As I recall, one of the SBs addressed the copper bushing as well. Also a note on the proper torque values. The latest SB (just a couple of years ago) addressed a newly designed bolt (a dash 5 bolt) that incorporates a stress reliever flange and is stamped "Reworked...).

No dice!

Therefore, I bit the bullet and ordered a new bolt ($785!) as I did not have a -5 bolt to duplicate. Plus, I am not willing to install a "bogus" part in my Standard Airworthiness airplane. (Potential for liability problems again). Mike Patlin has had my check for more than 2 months now and the bolt has not been delivered as yet. Meantime, I had Don reinstall the "cracked" bolt and reassemble the aircraft. I do not feel uncomfortable with that as a temporary measure. My plans are to write up the entire trunion bolt issue for our Marchetti Web Site.

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