We typically see two new 210 customers per month and about half of the planes have sheared steering pins. This is a fairly old topic, but there are a lot of new 210 owners out there that are not aware of this pin or its importance. The steering limit pin is a steel dowel pin, part number MS16555-56, mounted in an aluminum casting called the lower trunnion. The pin is designed to limit the nose gear steering travel. This protects the rest of the steering and rudder control system from exceeding travel limits, putting undue stress on the systems’ components. This event is not going to be caused using a hand tow bar, but the FBO tug will do it in a heart beat without ever knowing it. Every preflight should include a quick look for this pin. There are a few things that need to be done if the pin is sheared.
Cessna issued SEB94-19 in July of 1994 requiring inspection or replacement of the nose gear lower trunnion due to possible cracks. You can view the service bulletin from the members’ only section of the Cessna Pilots Association website. One of the stated contributing factors for trunnion failures is over-steering of the nose gear, which is evidenced by a sheared steering limit pin. Inspection and/or replacement of the lower trunnion make for an entire article in itself but I’d like to limit the topic here to the importance of the limit pin and its replacement.
What to do
If the pin is damaged, an inspection of the trunnion per the service bulletin should be seriously considered. Eddy Current nondestructive testing (NDT), as called for in the bulletin, is difficult to come by at most of our small shops. As a minimum course of action, consider a die penetrant inspection of the lower trunnion. The bulletin details the primary areas of concern, and the slightest hint of a crack found is cause for replacement. The time between the beginning of the crack and total failure is uncertain, but the costly results are not. Next, a thorough inspection of the rudder pedal linkages, steering arms, and rudder system is a must. Anytime the nose gear travels past its limits, the rest of the steering and rudder system are exposed to possible damaging stresses. Once the nose gear turns past the limit, the rudder contacts its travel stop, and the rudder cable on that side rapidly exceeds the design tension. At some point the stress will relieve itself in the form of a bent rudder horn, a damaged pulley mount, or a damaged rudder pedal torque tube. Hopefully, the pressure didn’t go that far, but a sheared pin is the obvious evidence of the possibility. No matter what else is found, the pin must be replaced.
More harm than good
We’ve tried several pin removal techniques and found problems with most. The typical first reaction is to drive the remainder of the pin out of the trunnion with a large hammer and a punch. At best, it is difficult to get a decent hammer swing from the top due to the confines of the nose wheel and the gear itself. In addition, a huge amount of stress is applied during each hammer strike to an already compromised trunnion.
When driving from the bottom, the punch won’t lay parallel to the pin because of interference with the nose gear yoke. Proceeding with the punch at this angle can gouge the side walls of the hole in the trunnion. Also consider that one edge of the sheared face of the failed pin now exceeds the diameter of the hole and will cause damage to the trunnion if driven from below. This makes for a progressively looser pin fit each time a replacement is made.
Installation of the new pin is critical as well. If the pin is not properly aligned from the beginning, it will drag metal from the trunnion walls. Of course, the hammer blows to drive the new pin in place don’t help the condition of the trunnion any more than did driving out the old pin.
It is a sure sign that several pins have been removed from the bottom up if the old pin comes out with just a couple of light taps. Depending on how tight the old pin is, removing it using these methods may take anywhere from 10 minutes to 3 hours.
Personal time in the fab shop
Tired of fighting these problems, I decided the best way to remove the pin is in a press as called out in the bulletin. That of course requires removing the trunnion from the aircraft. Removing and replacing the trunnion is an expensive job at about a 16 man hours, so I began to think about ways to bring a press to the trunnion.
I really get a kick out of designing and fabricating tools. No special calculations are required to make sure it is strong enough but not too rigid, no FAA Form 337 and no log entries. I can just play away on the milling machine, lathe, and tig welder, using the TLAR system (That Looks About Right). Most of my tool designs begin with an inventory of the scrap steel pile. For the press, I found a steel bar 1.5” by 1.0” and cut 3 pieces each 3.0” long. Of course the prototype is made quick and dirty with the understanding that it will probably be tossed in the “suicide tool” drawer, never to be seen again.
To make everything fit in place required a lot of trial fits and grinding but nothing terribly precise. I’m not a machinist so this is mostly about ‘arts and crafts’ stuff. The final version of the press is made in three main pieces. The top piece houses a press screw to extract the old pin. The center piece serves as the backup for the trunnion when the press screw pushes on the pin. The bottom section includes a larger press screw whose threaded hole also acts as an alignment chamber to press the new pin in place.
Out with the old
For pin extraction, the top and middle sections of the press are set on the ‘ear’ of the trunnion that make up the limit pin mount. As the press screw is tightened, the middle section of the tool makes contact with the bottom side of the steel plate under the trunnion. The pin moves out of the trunnion and into the holding chamber of the middle section of the press as more pressure is applied.
In with the new
For installation of a new pin, the bottom section of the press is added. The new pin with a little Loctite 222 applied slides in through the lower press screw hole which holds the pin true to the trunnion hole. As the bottom screw applies pressure to the pin, the top press screw is backed out to allow the new pin access to the hole. Now the lower press screw turns in, pushing the new pin in place, and the top of the press tool provides back-up for stress relief on the trunnion. The screw gets backed off a bit and the press lifts off the trunnion once the pin is in place.
Armed with my new tool, I’ve been practicing on speed. From the time I walk up to a plane until I leave it with a newly installed pin can take as little as three minutes. I love it when a plan comes together. Of course, someone has to pay for the nine hours fabricating the prototype and eight more hours on the finished version. I’m thinking a flat rate of one hour for pin replacement is a fair price for a process that won’t increase the chances for sudden retraction of the nose gear. No charge for the hours I spent day dreaming about how to make this thing work. One hour versus one month of down time, new propeller $, engine tear down $, gear doors $, wheel well repair $, lower cowl skins $, paint $, etcetera $$$. We can all do that math.
Copyright Paul New 2007 all rights reserved