PDA

View Full Version : Runaway plane - serious mishap - throttle cable



rcsfca
05-19-2009, 07:55 AM
(This post also appears on the Matronics Kitfox forum)

My Model IV - 1200 Rotax 912UL suffered major wing and prop damage today after striking the corner and face of a concrete hangar due to a throttle cable malfunction.

After a normal start this morning at my own hangar, I taxied across the airport (CCR) to the local avionics FBO for some radio and GPS work. Following a 15 minute inspection of the proposed panel changes, the techs asked me to move the plane to an adjacent hangar. I started the engine, which roared to life at full throttle (6000 rpm?) in a heartbeat, and the plane snapped forward into a circular move even though I was firm on the brakes. (It swung in a circle because I had some left rudder into it, which was lucky because I had other aircraft parked in front of me on the ramp.)

Thinking that somehow the throttle had been pushed in (which it hadn't because I had double-checked it for an idle setting before cranking the starter), I pulled back on the throttle and it came out of its housing and into my hand. Three seconds later, the plane hit the building, caving in the right wing and shearing the IVO composite prop. Turning off the magnetos was too late.

There was no fire and I was not injured, and there was plenty of help around to handle the leaking gasoline from the wing tank. But the wing, flaperon, and prop are finished. The balance of the plane, interestingly enough, was undamaged. The steel frame will have to be checked for squareness but the wind spar and wood absorbed the impact.

The problem with the throttle is as follows:

My plane, built in 1994, has the bellcrank-style throttle control, which divides the forward and reverse motion of the throttle handle between the two carbs. The throttle handle is attached to the bellcrank by a stiff steel 1/16" rod that runs from the backend of the throttle handle to a hole in the bellcrank mechanism where it is secured by a set screw retainer.

What happened in my case was that - unknown to me - the rod was completely straight and ran through the retainer hole with neither a secondary safety retainer on the other side of the hole nor a 90+ degree bend in the rod on the other side of the hole to keep the rod from being pulled out in the event the primary retainer set screw let loose.

So this morning the (single) set screw let go, the throttles on the carbs went to their default full-power position (WOT), and when I pulled back on the handle, the steel rod slid out of its bellcrank retainer and the whole throttle handle ended up in my hand.

An important suggestion to any of the owners out there who have this type of throttle control:

- Immediately check your rod-to-bellcrank retention screw for tightness.

- Get a secondary retainer on the rod.

- Put a bend into the rod after the secondary retainer to allow the rod to hook the bellcrank in case of retainer failure.

- Purchase the Kitfox Aircraft replacement throttle cable ($219) that does not use the bellcrank at all - it is a twin-cable style control of a completely different design.

Hope this has been complete enough - maybe there's been something AD-related already published or a prior thread. It has been a long hard day but I don't want any members of the community to not know about the incident in case their throttle equipment is the same as mine.

Rich Cunningham

DesertFox4
05-19-2009, 08:43 AM
Rich, Sorry about your incident. Hope repairs go smoothly and quickly.
Good advice for others with this throttle setup to make theirs safe.

I was never a big fan of the Rotax engines having springs on the throttle levers to go wide open throttle if the cables break/disconnect . I took mine off but I have the new friction throttle cable set up. The new throttle cable does eliminate most if not all of the possibilities for this to happen. It's a much more positive connection of pilot to engine throttles with less failure points.

Slyfox
05-19-2009, 11:54 AM
I to had this kind of setup for the throttle. I quickly switched out to the new system from John @ kitfox, it has a friction lock, gets rid of all that mess. Just need to open the bracket that holds the cable at the carbs I think to 1/4 inch. Much better system. I would put the new cable as no.1 thing to do before running the engine again.

Bryan
12-29-2010, 09:41 PM
I am replacing the old bellcrank system with the vernier cable, and the hole in the screw at the carb end is to small for the new cable. Steve I believe this is what you were just talking about is everyone just drilling this hole to a larger size? I do not want to make the screw to week as this cable is a fair size larger. After hearing about some of the mishaps I believe I will remove the springs as well. Thanks

HighWing
12-29-2010, 10:02 PM
Rich,

Thanks for the post and sorry for the damage to your airplane. I guess one of the good things, you are home.

A couple of thoughts. I agree with the secondary stops on that wire. I did that on my first Model IV (article in latest Sport Aviation). I also installed an auto parts store throttle spring that counteracted the carb springs almost exactly. My throttle knob was nearly neutral tension wise. All the tension from the carb springs were isolated in the stranded cables and I replaced them each annual. I am still building my second Model IV and plan on using the bell crank set-up. I doubt there is the perfect linkage out there and there is lots of experience with the standard set-up.

Lowell