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From CP70, Page 5, January, 1992
This is a report of a stuck throttle "near-miss"
incident. In hindsight, it is quite similar to the Dan Patch report in CP 65 and the
Charles Hewison report in CP 66. 1 consider I was lucky to not have pranged the airplane.
I have just converted my Long-EZ from an MA-3A (non-accelerator pump carb) to an MA-4SPA (accelerator pump carb) as part of a change from an 0-235 C2C to an 0-320 E2G. After about seven hours of uneventful flying, I sent the MA-4SPA away for an overhaul, including a new throttle shaft and a rebuilt accelerator pump. This greatly improved the smoothness and mixture control of the engine but the rebuilt carburetor requires about two pound of force to operate, when applied to the throttle bellcrank arm at the inner most hole, using the plans carburetor cable bracket. The force to operate the throttle belicrank is about the same whether or not the engine is running (two pounds). The MA-3A carburetor springs itself to full throttle, since it had no accelerator pump; the non-rebuilt MA4SPA was much looser than the rebuilt one. The problem is that the throttle quadrant is not able to supply this much force at idle without help from a spring. With a spring, the throttle sticking problem never occurred with the engine shutdown, only with the engine running after the throttle had been pulled to hard idle, and then slowly advanced.
I found this out over several days of troubleshooting when I noticed the throttle response of the engine was occasionally delayed when coming off a slow idle. I investigated by cycling the throttle and visually inspecting the system, but could not reproduce the problem or find a cause for it on the ground. Being foolish and thinking the problem had fixed itself, I went flying, landed, and when I tried to apply some power to taxi, I could not get any power response, only a very spongy throttle movement to about half throttle position (2 inches of throttle knob motion). My first stealth forced landing! After engine shutdown, throttle response was normal! (Good thing he did not have to go around! -ED)
I then verified visually (top cowl removed) that engine movement was not binding the cable somehow. I increased the throttle spring tension, and slightly relocated the throttle cable clamp to perfectly position the cable at the throttle cable end bushing. These changes appeared to eliminate the slow response. I flew again, and on landing, still had some reduced amount of sluggish response off of idle. Suspecting a damaged cable, I made the force measurements on the carburetor and the cable using some string, a volunteer to make the measurements with the engine running, and a 1-10 pound fish scale. These measurements confirmed that the system could not operate the carburetor without a spring assist. Suspecting damage to the cable, I then removed the cable from the airplane for inspection (yes, it was floxed in every foot or so: no, my consoles were not removable: yes, hell of a mess and lots of swearing). The cable was not damaged, nor was the cable sheath. Interestingly, however, if you pull on the cable shroud from opposite ends, even as little as 2 pounds of force will stretch it some.
I really didn't want to put a spam can-sort of throttle system in, but it appears that something with greater push authority than the original design is needed. I don't want to just increase the throttle spring force since spring failure will mean possible throttle failure. Do you have any thoughts or suggestions?
Five years ago, Mike Melvill went to an aircraft push-pull throttle cable and has been pleased with the result. -ED