Had my first excitement in the cockpit

[MGXsport]

Powering down to idle before base and the engine didn't follow what the throttle was doing. Realatively uneventful, landed with mixture control and coasted off the runway to a nice tug driver that took me back to my FBO.

Now my question is how is the linkage connected to the handle in a PA28-151? Because it worked fine moving the power on but obviously it couldn't pull the throttle shut. Was I lucky that it was made this way or did is it a purposeful design?

 

[rumorhasit]

Powering down to idle before base and the engine didn't follow what the throttle was doing. Realatively uneventful, landed with mixture control and coasted off the runway to a nice tug driver that took me back to my FBO.

Now my question is how is the linkage connected to the handle in a PA28-151? Because it worked fine moving the power on but obviously it couldn't pull the throttle shut. Was I lucky that it was made this way or did is it a purposeful design?

okay how did you land with the mixture control? that must have been a fun approach...

 

[flyifrvfr]

throttle cables are spring loaded to go to full throttle in case the cale breaks. You did the only thing you could do which was reduce the mixture. This is how it was done in the past because engines would run at full throttle and the mixture was used to land.

 

[MGXsport]

Interesting thanks. We looked at the engine and couldn't find where it was spring loaded. I thought it was probably like how it is in a car (obviously reversed) but as I said before we couldn't find the spring.

Next time I preflight I am going to look more closely for that spring.

 

[avbug]

As a practicing mechanic for a fair few years now, including repair station work, inspector, and director of maintenance, I've never worked on, nor seen a throttle system that is "spring loaded" to full throttle. Or maximum thrust, RPM, or torque.

In fact, the only system I can think of off hand that might compare actually does just the opposite; autofeather. Snatch-back cables that retard the thrust lever to idle on certain Citation series during reverser deployment may also qualify as an automatic system...but only to do the opposite of "spring loaded to full throttle."

Can you identify one?

Sounds as though the linkage broke. As to how, why, or what...not nearly enough information is provided.

 

[flyifrvfr]

Spring loaded is the term numerous mechanics have used to describe what happens when a throttle cable breaks. I'am not a mechanic,but I did stay in a Holiday Express last night.It would seem logical that if the throttle cable breaks, tension between the throttle body arm and cable is lost. The throttle body arm on the carburator goes to full open when the tension is lost, hence the term spring loaded.

When the throttle cable breaks, you can move the throttle in and out until you are blue in the face, the carb will go to full open. If it wasn't designed that way, imagine what would happen if the throttle body arm went to close because of a broken cable. I hope this helps you understand the operation of a carb.

Remember to ask your mechanic to explain it or show it to you.

 

[avbug]

The throttle body arm on the carburator goes to full open when the tension is lost, hence the term spring loaded.

It does? On what aircraft would that be, exactly? On what carburetor? Have you ever seen a carburetor or throttle assembly in an aircraft in which this occurs?? I haven't.

Unless of course you've been flying something with a LeRhone, lately...

Spring loaded on a car, perhaps.

When the throttle cable breaks, you can move the throttle in and out until you are blue in the face, the carb will go to full open.

It will?? Perhaps you had better go to that holiday in express and either stay another night, or get your money back.

Even on automotive carburetors, the carburetor doesn't go open during a "cable" failure. In automotive applications, the throttle plate is spring loaded closed. In aircraft applications, it isn't spring loaded at all; if linkage fails, it fails...but the throttle plate in an aircraft carburetor doesn't fail open or shut. Not in present small carburetors, or older pressure carburetors.

If it wasn't designed that way, imagine what would happen if the throttle body arm went to close because of a broken cable.

I don't need to imagine it, as I've experienced it...but wouldn't need to have experienced it as I work on the equipment. Most "cables" are not cables as in an automotive application, but a flexible push-pull assembly that functions more like a pushrod than a cable. Seldom do these assemblies fail by breaking; more frequently they jam. When failures in the form of breakage occur it's more typical a linkage failure. If the failure occurs at the control arm end of the assembly, the throttle setting will typically remain unchanged; control will be by pulling the mixture to shut off fuel flow. If the failure occurs at the carburetor or fuel control, typically the power setting may remain unchanged, or may vary with engine vibration.

In such a case you're usually better to shut it down and execute a forced landing.

I hope this helps you understand the operation of a carb.

Aaah, no.

Remember to ask your mechanic to explain it or show it to you.

I'll be sure to ask myself to show myself real soon now, y'hear?

 

[flyifrvfr]

It does? On what aircraft would that be, exactly? On what carburetor? Have you ever seen a carburetor or throttle assembly in an aircraft in which this occurs?? I haven't.

Unless of course you've been flying something with a LeRhone, lately...

Spring loaded on a car, perhaps.


It will?? Perhaps you had better go to that holiday in express and either stay another night, or get your money back.

Even on automotive carburetors, the carburetor doesn't go open during a "cable" failure. In automotive applications, the throttle plate is spring loaded closed. In aircraft applications, it isn't spring loaded at all; if linkage fails, it fails...but the throttle plate in an aircraft carburetor doesn't fail open or shut. Not in present small carburetors, or older pressure carburetors.


I don't need to imagine it, as I've experienced it...but wouldn't need to have experienced it as I work on the equipment. Most "cables" are not cables as in an automotive application, but a flexible push-pull assembly that functions more like a pushrod than a cable. Seldom do these assemblies fail by breaking; more frequently they jam. When failures in the form of breakage occur it's more typical a linkage failure. If the failure occurs at the control arm end of the assembly, the throttle setting will typically remain unchanged; control will be by pulling the mixture to shut off fuel flow. If the failure occurs at the carburetor or fuel control, typically the power setting may remain unchanged, or may vary with engine vibration.

In such a case you're usually better to shut it down and execute a forced landing.


Aaah, no.


I'll be sure to ask myself to show myself real soon now, y'hear?

Avbug, I won't get into a pissing match with you, but I can say that I have had more than one A&P explain it to me the way I described above. I will say that I have run into several pilots who have said that when the cable or rod if you prefer breaks, the rpms went to full as if the throttle was shoved full forward. The mixture control was used in each case to land the airplane. I can't think of a reason why incorrect information would be given unless these pilots and A&P didn't like me and wanted to give me incorrect information. Yall have a good day now y'ear

 

[avbug]

Well, for one thing, chances are that few, if any, of those mechanics were pilots. Mechanics fix things, but may not be prepared to address what will or will not happen in an aircraft emergency, or how to deal with it. What will occur with a throttle plate largely depends on the system, how the failure occurs, where it occurs, etc.

Marvel Schebler carbs do have spring assist, but weather or not the carburetor goes wide open on the throttle plate depends on a number of factors. To suggest that the engine will go to full throttle during a throttle control failue is fanciful at best, and borderline criminal when informing an inexperienced pilot of what to expect.

An aircraft equipped with an ellison throttle body, such as in certain injection systems, will generally experience a closed throttle system when a control failure occurs. The engine suction tends to pull the throttle plate closed, with the force to do so considerably stronger at lower throttle settings (plate tending toward closed, lower power setting). In fact to quote the Ellison service manual:

2-5 Throttle Linkage

During engine operation at less than full throttle, a substantial pressure difference exists between the two ends of the throttle slide. This pressure gradient causes a strong buoyancy force acting to close the throttle. This force is greatest at idle and diminishes at increased throttle openings.

CautionIn the event of any throttle linkage failure allowing unrestrained throttle movement, the engine will immediately and without hesitation, return to idle.
Because of the higher throttle friction associated with the TBI, linkage installations utilizing a pull cable in only one direction with spring return in the opposite direction are not satisfactory.

Other systems vary specifically within the aircraft. In a given experimental airplane, for example, a rotax 582 engine may fail closed, while the installation on a rotax 912 may fail open.

In the aforementioned marvel schebler installation, the spring assembly will generally not give full throttle, even with a completely disconnected and free throttle arm. If the arm is attached to anything (eg, the break in the throttle assembly is at the control in the cockpit, chances are that it's not moving at all.

The bottom line is that while it's good to dispense accurate advice, don't get in the habit of painting a rosier picture than reality might permit. If a pilot experiences a power change that does not correspond with power lever positoning, or which can't be controlled, or that does not follow the action of the thrust lever, it's time to get on the ground. Yes, the engine might fail to full power, but i might not. Don't bank on it going to full throttle.

In dual-cable installations where one cable pulls the throttle plate open and the other closed, failure of one cable means that the throttle can be moved in the opposite direction, but not back again. That means that if the open-cable breaks, it may remain there, but moving it to closed may mean a permenant change. In a push-pull type arrangement utilizing a wire, failure may often result in the throttle plate staying right where it is when the failure occurs.

 

[MGXsport]

As I was described like avbug said it is not a cable put a rod inside of a greased tube. At the begining of the flight about an hour and a half earlier we were having trouble moving the throttle and thought something was wrong with the friction. We losened it some and it felt better. As we continued to fly it got easier and easier to move it. Then on our last T&G we couldn't reduce the throttle on downwind and then we just extended our final a little and made it in nicely.