The engine won't stop - now what?
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The engine won't stop - now what?
Last Friday, a friend drove me over to High River airport so that I could pick up my club's Pawnee towplane, which had been there for maintenance. After a careful pre-flight and an even more careful downwind taxy and backtrack - the wind was 25G30, luckily straight down the runway - I took off for the 20 minute upwind flight to Black Diamond. I was cruising at a leisurely 70 kts and being overtaken by westbound cars below me! There was a spectacular lee-wave Chinook Arch to the west:
After landing and taxying into the lee of the hangar, I did the post-landing checks and pulled the mixture to Idle Cutoff. The engine kept running!
I cycled the mixture control several times with the same result. So my shutdown option was Mags Off. Nothing happened - the engine was still running! I had to pull the Fuel Shut-off valve to stop the engine. The engine continued to run for nearly a minute.
When my friend arrived back from High River in his car, I explained what had happened and he hopped in and started the engine. The mixture control behaviour was still the same, but this time, he was able to stop the engine by turning off the mags.
Inspection under the cowling revealed that the mixture cable had fatigue-fractured where it joined the mixture-control lever on the carburettor. Here is a view of the underside of the engine (O-540) looking forward - the gold-coloured mixture-control lever is in the centre and you can see the fractured end of the wire:
So the reason why the mixture control failed to shut down the engine was obvious, but the intermittent mag switch problem was initially a mystery. However, when I talked to our AME to tell him what had happened, he said that the P-leads were showing their age (it's a 1962 Pawnee) and he was planning to replace them this winter during the off-season.
I learned some interesting lessons from this experience.
1. Shutting off the fuel doesn't stop the engine immediately.
2. When one P-lead has an open circuit, you can't tell which one it is by doing a L-R mag check.
3. I shall continue an old habit of always treating a prop as live, even if the switches are off.
4. Early in my flying career, I was told that carburetor controls were spring-loaded to "fail safe". I now know this is not the case.
I don't know when the cable broke, but if it was in flight, the mixture-control lever could have easily vibrated to a position that would have shut down the engine. I might have had the conversation with our engineer while standing in a field! However he did say that this was the best time of year to have an engine failure - lots of half-mile-long flat stubble fields, as is obvious from my picture.
After landing and taxying into the lee of the hangar, I did the post-landing checks and pulled the mixture to Idle Cutoff. The engine kept running!
I cycled the mixture control several times with the same result. So my shutdown option was Mags Off. Nothing happened - the engine was still running! I had to pull the Fuel Shut-off valve to stop the engine. The engine continued to run for nearly a minute.
When my friend arrived back from High River in his car, I explained what had happened and he hopped in and started the engine. The mixture control behaviour was still the same, but this time, he was able to stop the engine by turning off the mags.
Inspection under the cowling revealed that the mixture cable had fatigue-fractured where it joined the mixture-control lever on the carburettor. Here is a view of the underside of the engine (O-540) looking forward - the gold-coloured mixture-control lever is in the centre and you can see the fractured end of the wire:
So the reason why the mixture control failed to shut down the engine was obvious, but the intermittent mag switch problem was initially a mystery. However, when I talked to our AME to tell him what had happened, he said that the P-leads were showing their age (it's a 1962 Pawnee) and he was planning to replace them this winter during the off-season.
I learned some interesting lessons from this experience.
1. Shutting off the fuel doesn't stop the engine immediately.
2. When one P-lead has an open circuit, you can't tell which one it is by doing a L-R mag check.
3. I shall continue an old habit of always treating a prop as live, even if the switches are off.
4. Early in my flying career, I was told that carburetor controls were spring-loaded to "fail safe". I now know this is not the case.
I don't know when the cable broke, but if it was in flight, the mixture-control lever could have easily vibrated to a position that would have shut down the engine. I might have had the conversation with our engineer while standing in a field! However he did say that this was the best time of year to have an engine failure - lots of half-mile-long flat stubble fields, as is obvious from my picture.
Avoid imitations
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It’s many years since I flew and taught on piston engined aircraft but the magneto check I was taught (with a combined rotary switch) was:
(From BOTH). L = drop no stop, BOTH. R = drop no stop, BOTH. OFF = drop and stop, BOTH.
If you don’t see a drop, or an RPM recovery at BOTH, the check is a failure.
I have known switches go “erratic” when the internal contacts were worn.
The engine was always shut down using the mixture control knob pulled to “cutoff”. That proves both systems. You were either doubly unlucky, or someone hadn’t been doing this.
As for the maintenance.....
(From BOTH). L = drop no stop, BOTH. R = drop no stop, BOTH. OFF = drop and stop, BOTH.
If you don’t see a drop, or an RPM recovery at BOTH, the check is a failure.
I have known switches go “erratic” when the internal contacts were worn.
The engine was always shut down using the mixture control knob pulled to “cutoff”. That proves both systems. You were either doubly unlucky, or someone hadn’t been doing this.
As for the maintenance.....
An engineer should look for frayed or damaged cables during maintenance but looking at that failure it might be worth reporting it to the manufacturer for them to investigate.
Last edited by N707ZS; 29th Sep 2020 at 15:17.
Years ago I went to do some circuits in a Cherokee 235 (similar engine). The standard procedure was to leave the fuel tanks selected on, or to the fuller tank. I omitted to check the fuel selector as part of the normal start checklist, because I had become so used to this. Unknown to me, the tank selector was awaiting repair, as it had developed a leak, which manifested as a steady drip under the fuse, hence the temporary procedure of leaving it turned off after use.
Started, taxied out, did a run up, then when backtracking for takeoff the engine coughed a few times and died. Worked out the problem (fuel pressure gauge gave it away) and restarted normally, double checked the stuff I should have checked earlier, and proceeded with flight.
If I hadn't done a run up, it would probably have failed half way through the takeoff run, the point being that the carb float bowl on the 0_540 holds rather a lot of fuel. Lesson learned!
Started, taxied out, did a run up, then when backtracking for takeoff the engine coughed a few times and died. Worked out the problem (fuel pressure gauge gave it away) and restarted normally, double checked the stuff I should have checked earlier, and proceeded with flight.
If I hadn't done a run up, it would probably have failed half way through the takeoff run, the point being that the carb float bowl on the 0_540 holds rather a lot of fuel. Lesson learned!
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Seems to me that always leaving the fuel ON was setting a trap, simply because pilots might then get out of the habit of checking the selector's position.
I know of one company (rotary wing) who for their own reasons decided on a policy (contrary to the RFM) to have the electric LP fuel cock switches permanently left at the ON position. Probably because on the aircraft type involved, the number two / right hand switch is hidden under the collective lever and there is no warning, other than the position of the switch itself, to indicate that the airframe fuel system is OFF. The fuel pump pressure check is done after start.
It's not unheard of for a pilot to get an engine going only for it to stop when it runs out of fuel after a minute or so. The company in question then had the switches modified by fitting a copper "tell tale" wire. That would certainly prevent an engine from failing after start up.
However, a pilot later had cause to switch the fuel OFF but one of the cocks wouldn't motor closed; probably due to lack of regular movement of the valve mechanism. Not good because one of the immediate actions for an engine failure or engine fire is to switch off / close the valve....
I know of one company (rotary wing) who for their own reasons decided on a policy (contrary to the RFM) to have the electric LP fuel cock switches permanently left at the ON position. Probably because on the aircraft type involved, the number two / right hand switch is hidden under the collective lever and there is no warning, other than the position of the switch itself, to indicate that the airframe fuel system is OFF. The fuel pump pressure check is done after start.
It's not unheard of for a pilot to get an engine going only for it to stop when it runs out of fuel after a minute or so. The company in question then had the switches modified by fitting a copper "tell tale" wire. That would certainly prevent an engine from failing after start up.
However, a pilot later had cause to switch the fuel OFF but one of the cocks wouldn't motor closed; probably due to lack of regular movement of the valve mechanism. Not good because one of the immediate actions for an engine failure or engine fire is to switch off / close the valve....
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4. Early in my flying career, I was told that carburetor controls were spring-loaded to "fail safe". I now know this is not the case.
Some years back the company I worked for had a tug Pawnee in for an annual. When I went to turn the fuel selector off, as part of the scheduled inspection for full and free movement, it was solid. The operators knew about it and weren't concerned but we told them it had to be done, They weren't happy about the delay in sourcing a new one and the down time but the Chief Engineer was rightly adamant. An un-extinguishable engine fire could have been interesting, especially considering its airframe fabric covering.
I've seen the same problem with PA 28 fuel selectors. Once they're forced, they leak fuel into the cabin.
I'd suggest that they're exercised on a regular basis.
I've seen the same problem with PA 28 fuel selectors. Once they're forced, they leak fuel into the cabin.
I'd suggest that they're exercised on a regular basis.
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I was told that carburetor controls were spring-loaded to "fail safe". I now know this is not the case
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Yeah, at idle the fuel can take quite a while to run itself dry after the shutoff. Thread title got me to thinking, what might be left to try if that too didn't get it to quit. I don't know if that engine has an accelerator pump in the carb, but you could try to instantaneously slam the throttle from idle to full to see if it'll quit.
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The operation of the fuel selector to stop the fuel flow is something I check during post maintenance testing - for exactly this reason. If the fuel cut off will not cut off (and I had one once), you also could not starve an engine fire. Yes, for carburetted engines, the engine must burn out the contents of the carburettor fuel bowl. If you need to shut down the engine with the fuel valve, run the engine at a higher RPM, it'll use up the fuel more quickly. If you're flying a plane which has the failure of a mixture control, the failure of both mag switches, and a fuel valve which will not turn off the fuel, well.... it's not airworthy. Sure, a mixture cable can break in a moment. But you should have done a live mag check preflight, and not taken off if you cannot mag switch off the engine (plus it's unsafe when it's parked). Few carburetted engines do not have an accelerator pump. If it lacks one, yeah, jamming the throttle will make it quit. Try it, nothing to lose.
So, mixture has not control, mag switches are failed, fuel valve will not stop the fuel flow.... Turn the tail into the wind, Carb heat on ('cause it reduces the RPM a little), open the throttle somewhat, and try to select the CS prop to full coarse (depending upon the prop design, it might go coarse and stay there), turn on everything electrical (the alternator creates torque proportional to draw), and select throttle to idle (reduce torque to a minimum). Maybe the engine [at idle] torque will be so low that the engine will stop under the added load.
If it's an injected engine, operating the electric fuel pump, particularly at high speed, while the engine is at slow idle, 'might make it quit by flooding it (it did on the C 310 I used to fly
).
If not, you're either going to have to sit in it until it runs out of gas. If you can't wait, and are willing to damage the engine to stop it, spray lots of water into the air intake. Not a good ideas, but it sounds like you have an emergency - you gotta do something!
So, mixture has not control, mag switches are failed, fuel valve will not stop the fuel flow.... Turn the tail into the wind, Carb heat on ('cause it reduces the RPM a little), open the throttle somewhat, and try to select the CS prop to full coarse (depending upon the prop design, it might go coarse and stay there), turn on everything electrical (the alternator creates torque proportional to draw), and select throttle to idle (reduce torque to a minimum). Maybe the engine [at idle] torque will be so low that the engine will stop under the added load.
If it's an injected engine, operating the electric fuel pump, particularly at high speed, while the engine is at slow idle, 'might make it quit by flooding it (it did on the C 310 I used to fly
).If not, you're either going to have to sit in it until it runs out of gas. If you can't wait, and are willing to damage the engine to stop it, spray lots of water into the air intake. Not a good ideas, but it sounds like you have an emergency - you gotta do something!
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This thread reminds me of an incident that occurred many years ago when I was flying a very new three-axis microlight on a cold day.
After flying for about 30 minutes, I noticed that the throttle was jammed at cruise revs. No problem, I thought- I will simply fly back to above the airfield and switch off the mags and glide to a landing. However, when I tried this, one of the mag switches literally disintegrated leaving the engine running on one mag. Still not a problem, I thought- I will switch off the fuel and glide to a landing. This was when I discovered how long the engine would run after switching off the fuel, particularly at lowish revs. It ran so long (even though my passenger/student was applying full choke) that I overran the main landing area and touched down in a rough area, collapsing the nose-wheel, fortunately with no injuries. It turned out the throttle Bowden cable had some moisture in it and had frozen.
I have always made a point since of running the engine in each aircraft I have owned at idle revs on the ground with the fuel switched off to see how long it will keep going- it is usually 1-2 mins. Incidentally, it is a top tip (by Bob Grimstead, I think) to switch the fuel off before you try to hand start solo. You normally have more than enough time to walk round and switch the fuel on again and if it does start up too fast and jump the chocks then at least it will not get too far! (Although this will not happen either if you apply his other top tip- tie the tail-wheel to something really solid with a link you can release from the cockpit).
After flying for about 30 minutes, I noticed that the throttle was jammed at cruise revs. No problem, I thought- I will simply fly back to above the airfield and switch off the mags and glide to a landing. However, when I tried this, one of the mag switches literally disintegrated leaving the engine running on one mag. Still not a problem, I thought- I will switch off the fuel and glide to a landing. This was when I discovered how long the engine would run after switching off the fuel, particularly at lowish revs. It ran so long (even though my passenger/student was applying full choke) that I overran the main landing area and touched down in a rough area, collapsing the nose-wheel, fortunately with no injuries. It turned out the throttle Bowden cable had some moisture in it and had frozen.
I have always made a point since of running the engine in each aircraft I have owned at idle revs on the ground with the fuel switched off to see how long it will keep going- it is usually 1-2 mins. Incidentally, it is a top tip (by Bob Grimstead, I think) to switch the fuel off before you try to hand start solo. You normally have more than enough time to walk round and switch the fuel on again and if it does start up too fast and jump the chocks then at least it will not get too far! (Although this will not happen either if you apply his other top tip- tie the tail-wheel to something really solid with a link you can release from the cockpit).
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there is a spring on the other end of the butterfly shaft were it connects to the jet pump that should turn it to full open.
I think you mean "closed".
I think you mean "closed".
A known failure point has been a worn spherical rod end from the throttle cable to the throttle arm. If worn enough, the chassis of the rod end can pop off the ball, and you just lost throttle control. This would nearly never be a part of a pilot daily inspection for lack of easy access. So it is very common that a large diameter "penny washer" be added to the rod end bolt beyond the spherical rod end, so if it does fail, the chassis of the rod end is still captured somewhere around the ball, of not over it. You'd still have control of the throttle. Unfortunately, I have seen carelessness in reassembly, and this is missed. A carburetted Continental 550 engine was replaced in a C 182 amphibian I used to fly. I was visiting, and the owner mentioned this to me. He asked if I'd help with the oil change (during my social visit, but okay). I watched, they got dirty hands. As I looked around, I saw that the engine installer had indeed placed the penny washer on the wring side, and failure would have been possible. This was among several errors under the cowl.
So if the cowls are off your plane, it's worth a look around, and ask questions if there's something which does not look right!
Had a throttle linkage fail in a Cessna 310 many years ago, was an interesting approach, fancy footwork required to have either full power or a windmilling prop. Seem to remember taxying in was also fun.
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NutLoose wrote:
"No I mean open, as in full power, better to have all than none."
Except, perhaps, on the ground!
I often have my hand near the mag switches when manoeuvering near hangars or other aircraft because the Rotax 912 series engines also have springs that fully open the throttle if a cable breaks.
"No I mean open, as in full power, better to have all than none."
Except, perhaps, on the ground!
I often have my hand near the mag switches when manoeuvering near hangars or other aircraft because the Rotax 912 series engines also have springs that fully open the throttle if a cable breaks.
On multi-engine aircraft, it's been pretty much dictated by the regulators that 'fail-safe' is idle (or shutdown), not full power. On the ground, an engine runaway is considered catastrophic - unless the crew shuts down the engine almost immediately, you're going off the runway/taxiway or running into something and the feds consider that potentially catastrophic. A controlled shutdown or fail to idle on a single engine is seldom considered worse than "major" (and never supposed to be worse than 'hazardous') so all newer designs fail-safe to idle or off.
Granted a single engine aircraft is a different story...
Granted a single engine aircraft is a different story...
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It may have been that there was no swivel in the attachment to the carb mixture arm. The broken cable looks like a fatigue failure. Have had it happen on a solid rudder cable on takeoff.