In this post I will describe how I figured out the wiring diagram for a real Boeing 737 landing gear lever mechanism, including the solenoid and Korry lights.
I’ve seen (and even owned) several landing gear levers that were designed and marketed for the home simulation market, but they just don’t have the heavy tactile feel of the real thing, and I’ve never seen any that have a working solenoid or override lever. The cockpit I bought fortunately still had the original gear lever still in place, complete with the six Korry annunciators that serve as gear position/transit lights.
The solenoid is a 28 volt device that, when energized, allows the lever to be pulled up to initiate a gear-up cycle. In the real aircraft, a number of logical conditions must be met for the solenoid to be activated, which prevents the gear from being raised while the aircraft is on the ground. The gear handle itself is equipped with an override trigger that allows to pilot to raise the handle in the event that the solenoid fails.
The solenoid wiring was easily determined as there were only two wires, one of which had to be 28 volt and the other a ground. The position switch wires were also easily traced back to the cannon plug. I was surprised on initially removing the assembly that only the gear down position had a switch installed. Although holes were present for mounting a gear up switch, the switch itself was missing. I can only assume that in the real aircraft the up position is read somewhere along the cable that runs to the actuator. For my simulator I just added an identical switch scavenged from another part of my project.
There are several manufacturers of annunciators for the home simulation market, but once again, there’s nothing quite like the real thing which have the press-to-test function available.
The wiring of the model 319 type 1 Korry annunciators have been described by David Allen. These are ground-seeking type circuits with 4 terminal lugs. Terminal 1 is the 28 volt input voltage. Ground terminal 2 to illuminate the indicator. Terminal 3 is grounded for a ‘test all’ function that lights the indicator, but extinguishes the light when pressed. Terminal 4 is grounded for a press-to-test function.
The wiring scheme is as follows:
24-pin cannon plug:
Pin 3: common to all terminal 3’s (test all function)
Pin 4: nose red terminal 2
Pin 5: nose green terminal 2
Pin 6: right red terminal 2
Pin 7: left red terminal 2
Pin 8: right green terminal 2
Pin 9: left green terminal 2
Pin 10: common to all terminal 4’s (press-to-test function)
Now that the floor is level, it’s time to start building up the cockpit structure. The second forward level consists of three sections: left, right and center, with the vertical divisions roughly in line with the inboard rudder pedal linkage on each side.
All three pieces are made of aluminum, but there are thick spars that Boeing designed to protect the pilots, and the sections are bulky and heavy. So I enlisted the help of Elmer and Andy, who came over to help me hoist these pieces into place. The sides were easy enough, but lifting the top onto the structure required some planning.
For some reason I must have been in a hurry on the day that I cut these some three years ago, because I completely neglected to make any indexing brackets. Not really a problem because the windows are structural, and I have an almost complete set of Boeing windows. By mounting these windows and using an awl to line up the bolt holes, I aligned not only the left/right/center sections but also the top to the sides.
At this point in the build I am rediscovering many items that have been in storage for a few years. I had an unwelcome surprise when I pulled out my windows to find what had been advertised as a full set actually consisted of two FO side P1 windows, a matched set of P2 slider windows, and only one of the P3 windows that I needed. Luckily there was a captain side P1 window for sale on eBay at a reasonable price, but the only source of an FO side P3 window is for new old stock at a somewhat less reasonable price. Through one of these window deals I wound up with an extra pair of P2 slider windows, so hopefully these will fetch a good price online and allow me to purchase what I need. For now, I plan to use the captain side P3 as a template to make a plywood insert for the P3 window opening on the FO side. Even the full-motion level-D sims black out these rear windows, so I don’t feel like I’m detracting from the experience by doing this.
Having overcome the major hurdle of implementing dynamic force feedback, I have been busy performing other tasks that will be most easily accomplished while the floor remains in vertical position. These include wiring the brakes and stick shakers, as well as rewiring the throttle quadrant with salvaged AMP cannon plugs that allows the interface cards to easily detach from the mechanical parts of the unit. I used a 55 pin plug for the switches and pots, and a separate plug with larger 16 gauge pins for the power connections.
I spent at least a week going over the underside, testing all the functions and tidying up cable runs. Once this section is put down into its final horizontal position, there will be only 20 inches between the flight deck and the floor of the basement, so I will still be able to get in and work on things, but it will be a lot less convenient and probably a lot more likely to induce neck and shoulder pain. Twenty inches seems like a lot, but that’s the distance from the flight deck to the floor, and there are a lot of mechanical parts in between, such that there won’t even be room for a creeper.
The day finally came, and after one final cleaning, I cut the heavy cable ties holding the floor section to the ceiling joists above, and started sliding the section as far forward as it would go in the room. I called my buddy Adrian, a senior F/O at jetBlue, to come over and help me lower it. I figured it would be pretty easy for two of us to lower it down, as it seemed to me that most of the weight was concentrated in the yoke and rudder mechanisms located fairly far forward. It wasn’t that hard, but the aft end was quite a bit heavier than I thought. Luckily Adrian’s fiancee Kelly, herself a Captain and check airman at Frontier, was there to help place the aft floor supports, which I had made from short sections of 6×6 and 2×6. Once we got it down, a quick check revealed that the flight deck was perfectly level in all directions. Looks like my cuts and indexing were precise enough!
I know, I know…it’s been over a year since my last update. I have been working on implementing dynamic force feedback, a slow but steady process that required designing, and sometimes redesigning, custom made transmissions for driving all three control axes.
I am happy to report that I now have all three axes working after a great deal of calculation, consultation, and trial-and-error experimentation with various transmission designs intended to mate the BFF do-it-yourself force feedback interface cards to the original Boeing flight control structures. There were several examples of such implementations out there on the web, most of which involved expensive custom gearing and none with an original 737 mechanism.
The designer of the BFF cards is extremely helpful with the installation and operation of the cards themselves, but leaves it to the simulator builder to come up with the proper mechanisms for driving the controls. My first step was to consult my pilot buddy Andy Schwartz (of the mechanical engineering firm SSA Engineering) for some badly needed help with the engineering. Armed with some actual Boeing flight control tension specifications from David Allen, Andy confirmed that the specified motors, gears and belts that I had dreamed up would supply the required forces at the man-machine interface.
The BFF card designer specified a particular motor from an Italian manufacturer, which does not accept credit cards, Paypal or Bitcoin and required an international wire transfer for payment. The motors themselves arrived almost a month later, and only then was I able to start designing custom motor mounts to fit onto the existing Boeing structure. One of these was fairly simple but the other two were complex shapes that required some time to fabricate using my old friend, the electric 4.5 inch angle grinder. After cutting the shapes from 3/16 inch steel plate, I cleaned up the sharp edges with a polishing disc attached to the same tool. As the steel was not stainless, I applied several coats of gray primer to prevent corrosion before installing into the floor structure. The motors themselves were mounted through a hole drilled in each plate. A rubber/cork gasket cut from a sheet obtained at my local auto parts store was used to help reduce noise and vibration from the motors. A large number of fasteners was used for each mounting plate with the same goal in mind. The largest plates, the one used for the pitch and roll axes, serve double duty as structural supports since they cross the midline and connect and maintain the proper spatial relationship between the two halves of the divided flight deck floor.
Having finished mounting the motors, I was able to start the task of wiring the cards. Ian provides extensive instructions with the cards, and suggested an inline circuit breaker for each axis to use as a ‘kill’ switch in the event of undesired behavior. I happened to have about ten 28-volt circuit breaker switches lying around as a result of the circuit breaker switch airworthiness directive that became effective for piston Beechcraft aircraft in 2010. The FAA forced operators of these aircraft to replace thousands of these switches with newer models after a few reports of failures and resultant smoke in the cockpit episodes. So I chose three of these with appropriate amperage ratings and installed them in an existing structural support under the FO side left rudder skid. I left enough slack in the wiring to allow them to be repositioned to the underside of the main instrument panel after that component is installed later. These switches are not airworthy per the AD, but perfectly functional for this purpose.
I would have preferred to have mounted all three BFF interface boards somewhere above the flight deck to facilitate their maintenance, but practical concerns prevented me from doing so. Because the motors have integrated position sensors, the signals carried by the small gauge wires back to the interface boards are very low voltage and Ian recommends not extending the shielded wire bundles coming from the motors. The cable bundles are short, no more than three feet. As there are also large heat fins on the cards, they could not be located in an enclosed part of the cockpit without complex cooling systems including ducting. So I had no choice but to mount the cards underneath the floor in areas that would be relatively accessible with a mechanic’s creeper.
The BFF cards can be daisy chained together in order to allow the use of a single USB interface cable. This is advantageous because the boards were designed to use a specialized USB PICAXE cable that uses a 1/8 inch mini stereo jack that costs $30 and must be ordered from the UK. The software identifies the different control axes by the use of jumper pins that are set on each board. Ian recommends that all three interface cards be co-located, so that each length of daisy-chain serial cable between boards are no more than eight inches to eliminate noise. I could have achieved this by mounting one card above the roll axis motor mount plate, but this would have made it very difficult to access from the underside, so I elected to purchase a second PICAXE cable to allow me to locate one interface card about 12 inches away from the other two. As also recommended by Ian, I installed two cooling fans, one for the pitch axis and one that is shared by the roll and yaw axes.
It took a few weekends to cut, crimp and run all the wire between the wall outlet, power supplies, and interface boards. I took advantage of some existing Boeing light fixtures used by mechanics when accessing the forward flight control bay, using another old Beech circuit breaker switch in the process. I will be happy to have these lights operational the first time I have to venture under the flight deck on the creeper.
When I first applied power and fired up the BFF test software, all three axes moved, but not very much. After a few emails exchanges with Ian, I got the pitch and roll axes moving but it was clear the yaw axis, with the belt-driven design that I came up with initially, was never going to work because the motor needed to make at least 120 degrees of revolution in order to achieve initial calibration. So I elected to relocate the yaw axis motor from the midline to a place just outboard and forward of the rudder crossover linkage. I was hoping to build a transmission with spare parts on hand, but a brief experiment with direct drive with a pushrod and the largest gear I had in stock resulted in the same problem: not enough rotation to pass the calibration routine. So I ordered two additional timing belt pulleys, fitting the small one to the motor and the large one to a custom fabricated shaft/plate assembly that attached to the Boeing mechanism. This replaced an original autopilot component made out of plastic, which was apparently designed to shear in the event of a control jam.
The roll axis is controlled by a motor mounted in the midline, fitted with a timing belt pulley and long belt with a number of flat pulleys to correctly set the tension. I was able to find some blocks online that were cut with the timing belt pitch, through which the ends of the belt were sandwiched. I then drilled a hole through the end of each block, passing an existing Boeing cable already connected to the aileron/spoiler controls.
This axis initially had too much slack to calibrate the motor. A call to David Allen revealed that there is a spring inside the FO side aileron hub that allows the Captain and FO side aileron/spoiler controls to decouple in the event of a control jam. Conveniently, there was a rigging pin hole in the mechanism used to immobilize the spring. After testing out what David promised me was true, I simply tapped some threads with a 3/8-16 tap, cut a countersink, and inserted a screw, locking the aileron and spoiler hubs together, resulting in a much smoother mechanism.
The pitch axis motor is fitted with a NEMA 34 planetary gearhead with a 10:1 reduction ratio, a simple, cost effective alternative to building a custom transmission. The planetary gearhead was then fitted with a large pulley which was then attached via a pushrod to a pair of existing tabs on the elevator torque tube.
After reinstalling the control columns and re-linking the rudder pedals and brakes, I started mounting springs and heavy duty drawer slides to implement static control loading like that provided by the excellent gear made by Northern Flight Sim. While searching online for parts I came across Ian Hopper’s force feedback site, where he sells hardware and software for implementing force feedback for mid size (think a heavy duty control yoke, like a CH Products yoke made out of metal parts) and larger cockpit setups.
After discovering this I became obsessed with adding this functionality to my simulator. My basement is fairly big, with nine foot ceilings, but I know I will never have enough room for full motion. Adding force feedback would lend a great deal of realism to my setup. Force feedback adds two characteristics simultaneously: dynamic control loading, so that the pilot feels the aerodynamic effects of turbulence and configuration changes, and autopilot functionality, so that flight controls move appropriately when the autopilot is engaged.
This may turn out to be the most challenging hurdle in the project. Ian has created an excellent, highly cost-effective solution to this problem, but he leaves it up to the buyer of his products to properly construct the mechanisms that connect to this controller cards and specified motors.
I have an advantage in that my flight controls were built by Boeing to withstand years of abuse, so all I really have to do is design three transmissions each using this small motor to create realistic forces on a larger scale. It is this particular problem that has had me stumped for the past three months, trying to specify the proper transmission in the most cost effective way. I know it can definitely be done, the question is how?
After reinstalling the exterior door into the basement and securing the forward floor section to the ceiling, I re-joined the two halves by mounting more of my custom fabricated brackets onto various structural ribs. I had pre-drilled the holes for these brackets using cleco fasteners while the section was still in one piece in the driveway. I discovered early on that the metal fabricator who made the brackets did not punch the holes in identical positions, so each bracket had to be marked in order to match up to a specific location later on.
The throttle quadrant and radio pedestal sit on a large sturdy bracket that covers the big hole in the middle of this section. Luckily for me this bracket also crossed the centerline and was easily removable. Replacing the bracket added a great deal of structural stability to the reassembled floor section.
The next step was to reinstall the flight controls. The rudders and brakes were easily reconnected with control rods and hardware, and the roll axis was re-linked by connecting the threaded control cables.
Reinstalling the control yokes proved a bit more challenging as they are connected to the pitch crossover tube, a large heavy piece with a three inch beveled gear on each end. I had neglected to index the columns in any way, so when reinstalling I used two adjustable sawhorses and a bubble level to make sure that each yoke was indexed to the same position.
Once again it’s been a long, long time since I wrote an update, but work has continued over the past year.
Over the past three years I have had unbelievable luck in a complex cat-and-mouse game with the ninnies at Manassas Regional Airport. I always planned to bring the project home, but my hand was forced one rainy day last summer when I pulled the trailer over to Skyworks for an early morning rearrangement of the disassembled pieces. I finished this task around 7:30 am, and figuring that government employees probably wouldn’t be at the office that early, I drove my pickup with the project in tow up into the semicircular driveway leading to the secure gate onto the ramp. Standing next to the terminal enjoying morning cup of joe was none other than the airport director himself! One glance at the look on his face told me that there was no way to put the project back into the hangar as planned without getting busted, so I just kept driving, and ninety minutes later the trailer was safely parked in my driveway.
The trailer remained there for another two months while I spent days in sweltering heat removing the control columns and yokes, throttle quadrant, and all control linkages crossing over the centerline. All of this was done in preparation for moving the pieces into my basement.
Having finished the prep work, I gave the entire project a good bath with soap and water, then divided the forward floor section along the centerline with an angle grinder and reciprocating saw. This yielded two sections that were not particularly heavy but very bulky and awkward, so I bribed three friends with promises of burgers and beer to come over and help move everything into the basement. This was made easier by temporarily removing the sliding double door.
Having removed almost everything above the waterline, I’m turning my attention to the remaining floor section, which one recent visitor said “looks more like a boat than anything else.”
The strategy with this task is to divide the floor into pieces that are small enough to get through my basement door, taking advantage of the original Boeing structure to leave matched holes that will allow for realignment later.
After some investigation and feasibility experiments with drilling out various rivets, I settled on a line parallel with the lateral axis of the aircraft, about ten inches forward of the cockpit door frame. Underlying this line is a heavy spar that is attached to the aft part of the floor by six lighter spars that run longitudinally. Fortunately these lighter spars were secured to the heavy one with aluminum rivets, thankfully easy to drill out given the awkward spaces in which I had to use the right angle drill. I’m still looking for a more efficient way to drill out the steel huck rivets that continue to slow me down. An online hint to use a masonry drill bit did not seem to be much faster than the standard black oxide.
It’s been way too long since I updated this blog, but I have actually been busy making progress. The cockpit is now cut down to the flight deck floor, also known as the water line. To accomplish this I divided the structure above the floor into seven major pieces, all with the goal of preserving the geometry of the windows and doors. As I discussed in a previous post, I used custom fabricated angle brackets with pre-punched holes that were put in place with cleco fasteners prior to dividing each section. These brackets were labelled according to their location as unfortunately there is some variation in the location of the pre-punched holes. After drilling all the appropriate mounting holes in the original structure, the clecos and brackets were removed and the cutting process began.
The biggest challenge in making many of the cuts in the metal was finding a tool that was small enough to fit in the available spaces. The structure designed by Boeing becomes increasingly more robust as you move forward through the cockpit, and unfortunately the space also narrows simultaneously. I used a combination of a Ridgid 4.5 inch electric angle grinder and a reciprocating saw with various blades to make the cuts. A double cut saw from Harbor Freight also came in handy for some of the thicker pieces.
The resulting segments were awkward to move around but not especially heavy, probably on the order of about 100 pounds each. To make things safer and easier, I took advantage of a steel girder running across the ceiling in the hangar, and used a set of ropes to lift the pieces off the structure and slowly lower them to the floor. Once on the floor, the pieces were easy to slide around.
One major decision involved the roof of the forward part of the flight deck, which is relatively heavy with many thick spars. As I now possess a complete set of real Boeing windows, I wanted to make sure that the window geometry was maintained, especially for the sliding P2 windows. Another goal was to preserve the positions of the mounting brackets for the overhead panels. I determined that the best way to satisfy both requirements was to remove the top as a single piece, cutting about two thirds of the way up the window pillars, with the exception of the center, which was cut at the top. These locations were chosen both to meet design requirements as well as for convenience of cutting.
After removing the top, the next decision was how to divide the remaining portion into manageable pieces.
The removed top fit easily in the bed of my pickup truck, and was moved into my basement with the help of my good friend Miguel. Putting it back into place will require a crew of at least four of us.
The next few weeks were taken up with identifying all the places the instrument panel and CDU bay were connected to the structure. This was removed as one piece, intact but not without some minor damage from the process.
Having exposed the structure, it was time to pick some lines to cut up what remained. It initially seemed appealing to make vertical cuts just under the window pillars between the P1 and P2 windows, but when the structure was fully revealed it became apparent that this would involve cutting some of the thickest parts in the whole setup.
Selecting a different line closer to the longitudinal axis of the aircraft led to a much easier cut that preserved the structure. The hardest part of removing these outer pieces proved to be cutting the spars along the floor line, several of which were in very tight spaces.
Having completed all of those cuts, the sides came off relatively easily and were lowered onto the floor with ropes.
This left the center section, which was cut away from the floor relatively easily. There were only two thick spars to cut in this section, part of the radar mount.
As predicted, the circuit breaker panel on the captain’s side was somewhat easier to remove. I continue to be slowed by tenacious stuck fasteners, some covered with a thick layer of paint courtesy of some past United Airlines refurbishment project. Today after breaking not one but two damaged fastener remover bits, I decided to direct my efforts at something less frustrating: cleaning out the overhead area. The pictures show the results. Hopefully another two days will have the inside metal surface of the airframe completely exposed and ready for cutting into sections.