Weird Engine Failure

[Hedley]

Didn't happen to me. A cherokee on final today - pretty cold, at least -20C - experienced a loss of engine power on very short final. Prop didn't stop immediately, the engine kept windmilling for a bit. He flared and landed and the prop stopped and the aircraft coasted to a stop halfway down the runway. We had to tow it off.

I talked to the pilot afterwards. He said he used power and flaps during base and final to keep the engine warm, "Just like you taught me".

Not sure what the cause of this was. Did the engine cool excessively on approach, despite the use of power? Was there water suspended in the fuel which froze and plugged up some bottleneck in the fuel system?

I'm not sure what we can learn from this, except please please please don't fly unnecessarily large circuits when it's very cold. Try to fly your base and final so that at no point are you dependent upon power to make the runway.

[Hedley]

Thinking about this ....

The prop didn't stop, so likely the magnetos continued to provide juice to the spark plugs, which were not fouled excessively because it later started, after sitting for a while.

I don't think there were any air restrictions, either going in or out of the engine. Not a likely any carb ice - too cold for it, no moisture in the air - and the exhaust baffles didn't break off and plug up or anything like that.

So. Either

1) the engine got cold enough that the 100LL didn't evaporate well enough to burn - remember, liquid gasoline will not burn, only an atomized mixture will burn, or

2) there was a temporary restriction to the fuel supply, possibly caused by suspended water freezing in the gas, forming ice, and plugging up in a bottleneck in the fuel system somewhere. See the LHR 777 and the canned pilot that got his job back.

WRT to #2 above, you would logically expect such a restriction to be most noticeable at WOT, not at idle. Don't need much fuel to idle - it must have plugged up pretty badly, if that indeed was the cause.


I'm all ears, guys. What do you think? The definition of stupidity is making the same mistake over and over again, and I want to learn from this, so it never happens to me.

[Grantmac]

Did you take a tester of fuel from the aircraft tanks and let it settle out in a sealed jar somewhere warm?
That might give you an idea of the water content of that exact batch of fuel.

Did the aircraft start after sitting in a warm hangar or somewhere cold?
Any chance the tank vents could have had any ice in them?

-Grant

[Hedley]

The engine re-started after being towed in. It was NOT in a heated hangar. Fuel venting is certainly a possibility - it was responsible for my one and only mayday in 35 years. Generally when a fuel vent plugs up a tank will collapse, at least partly, and will make loud BUNG noises as it pops back out into shape, as the suction leaks off. And, switching tanks will generally cure the fuel flow problem, unless that tank's fuel vent is plugged up, too.

The engine had some heat in it, and perhaps with the lack of cold air blowing over it, the crucial bits warmed up enough to run again.

[Cat Driver]

Did it have an inline fuel filter?

[cgzro]

idle mixture too lean (cold air aggrevates it) or throttle idle stop screw moved.
id do an idle mixture and idle speed check and verify the idle stop etc.

[cgzro]

another long shot. stuck bendix could drag down engine enough perhaps it retracted after it stopped. also perhaps an accessory is dragging too much. vacuum alternator.. perhaps alt belt is super tight. ... bunch of guesses..

[Pratt X 3]

Was carb heat used all the way to landing or was it just checked on downwind then returned to cold? If the latter was used, it might relate to the non-vapourinzing of fuel idea you had.

[Hedley]

I asked about that - Carb heat was only cycled on downwind, Piper style.

It was NOT left on all the way down, Cessna style.

Did it have an inline fuel filter?

I'm sure it must, somewhere. I am unfamliar with the Cherokee fuel system.

A friend of mine had a problem with slowly decreasing fuel pressure
in a Pitts S-2B in Florida. Had to run the electric boost pump to get
it to an airport. Turned out the fuel filter was plugged with black gorp.
The cause was a disintegrating flop tube in the fuel tank - the rubber
in it was breaking up and flowing into the fuel filter.

[Big Pistons Forever]

idle mixture too lean (cold air aggrevates it) or throttle idle stop screw moved.
id do an idle mixture and idle speed check and verify the idle stop etc.

My guess too.....

[Hedley]

I hadn't thought of that ... I do know that I calculated the
density altitude here today, and it was -5,000 feet! That's
a mile below sea level.

That's a lot of oxygen going into the engine, and if the idle
mixture was a bit on the lean side, perhaps it was lean enough
to quit.

I was talking about mixture to a friend of mine today with
a Stinson - he claimed he still saw an RPM rise at 1700 RPM
during the runup so he was still on the rich side.

Dunno about the Cherokee, though. Remember the engine
wasn't at idle - he had a bit of power on.

[shitdisturber]

idle mixture too lean (cold air aggrevates it) or throttle idle stop screw moved.
id do an idle mixture and idle speed check and verify the idle stop etc.

My guess too.....

Make that three; had it happen to me once in the middle of a stall of all things, the startled look on the face of the renter I was checking out when the prop stopped was almost comical. I should probably have let him sort it out on his own but I was busy looking after my own skin.

[cgzro]

likewise but mine stopped on a spin. Had just had fuel injection system overhaul and mixture was too rich.

I think the idle mixture is not just at idle it takes affect at lower power settings sort of progressively.

Anyway its the most obvious.

[gaamin]

Two months ago I had an engine failure just after touchdown with a student in his own C150 when practicing circuits.
It was not the first circuit, power had been idle for most of the final leg because he was too high. Temperature was around 5°C that day.
The aircraft was able to roll out on the taxiway past the hold short line, it restarted easily, no backfire or roughness, and my subsequent attempts to reproduce it were unsuccessful.

He hadn't flown in the previous month. I was with him during the pre-flight inspection, we drained a very small amount of water from each tank. The run-up showed nothing unexpected, mixture was working fine, idle was smooth. Any ideas about the cause for this one?

[Hedley]

engine failure just after touchdown ... C150 ... power had been idle for most of the final leg ... temperature was around 5°C that day

It could have been carburetor icing. The TCM O-200's with their carburetors hung out there make ice like a refrigerator. I had one ice up on me in a full power climb!

After the engine quit, the heat from the engine would warm the carburetor and melt the ice, and presto, problem gone.

[Bede]

I'm with GZRO and BPF. That happened to the wifey last week, except her engine came back to life after she dicked around with the throttle and mixture.

[Hedley]

Interesting. Does anyone here use "summer" and "winter" idle mixture settings?

Thinking about it, the density altitude here ranges from -5000 in the winter, to +4000 in the summer. That's a nearly 10,000 foot range. You wouldn't climb from sea level to 10,000 feet on an ISA day and not adjust the mixture, would you?

Our engines develop rated power at sea level, +15C, 29.92, yada yada. The -5000 foot density altitude we experienced today might be equivalent to a dry squirt of nitrous in the summertime. That much more oxygen is being sucked into the engine, and it's going lean with the "one size fits all" idle mixture setting.

Most people would raise an eyebrow if you put dry nitrous on your certified aircraft engine. But that's what you're doing in cold winter temperatures - even if it's "natural".

Next time you go flying in -20C or colder wx, pay careful attention to your mixture leaning check during the runup. Are you still seeing a slight increase in RPM before it falls off? If not, what does that say about your mixture?

[gaamin]

Thanks for your answer Hedley. The C150 IS a carb ice machine indeed. This time the carb heat was on and it didn't have any of the symptoms I usually experience with carb ice (we also have a C150), but I would not know for sure.

As for the mixture setting, I also wondered about it. Does an aircraft maintenance manual specify a particular adjustment for the mixture? How much movement of the mixture control in the cabin does how much change in the mixture ratio?

[Pirate Pilot]

I don't know about a 150 specifically but I had a very similar experience with our 182 with the 470. I suspected leaning and I was right!! The induction tube in front of the 470 has those two rubber (?) rad hose looking pieces on each side of the engine. It was very cold out like your -20 and the engine just sucked in too much air. We took a vacuum cleaner and reversed the air and pushed air into the airbox and squirted soapy water around the fittings. They leaked bubbles everywhere!!! So, long and short of it...the engine thought it needed to be shut down, leaned right out. ALMOST quit...full rich, full throttle (no carb heat though?) and off we went JUST TO MAKE THE RUNWAY! I was not doing circuits, I was off on an adventure. I got one.

[Apache64_]

Flew a flying club c-172 once that developed a hole in the carb float, ran fine during run up, take off, downwind to final, shut down when the student pulled the throttle to idle on final to adjust altitude, and fired up soon as he added power again.

[Strega]

Does anyone know what speed a windmilling prop will stop on a 172/150 Pa-28 ?

[into the blue]

As for the mixture setting, I also wondered about it. Does an aircraft maintenance manual specify a particular adjustment for the mixture? How much movement of the mixture control in the cabin does how much change in the mixture ratio?

In float-type carburetors, idle mixture is adjusted by tweaking a single screw, which controls the size of the primary idle jet opening (bigger opening = richer mixture, smaller opening = leaner mixture).

http://www.micro-tools.net/pdf/Cessna/150_sm_69.pdf
You'll find the exact procedure and numbers for checking the idle mixture on page 155 of the document (paragragh 11-37). The check itself is very easy and can be done during any routine post-flight engine shutdown.

[Hedley]

172 ... developed a hole in the carb float

There was a string of AD's on carb floats a few years back - the hollow ones developed holes and sank as you described, then we went to the solid ones, then they sank, then back to the hollow ones again?!

Anyways, a sinking float would pull the pins up and allow more fuel to exit the float bowl through the jets. That would result in an excessively rich mixture, not an excessively lean "winter" mixture as discussed above. You can often see clear evidence of an excessively rich mixture as black smoke coming out of the exhaust.

what speed a windmilling prop will stop on a 172

For grins, I have stopped the prop a couple times in a 172 - overhead a quiet airport - and you wouldn't believe how ridiculously slow you have to get. Very close to the stall.

The Cherokee pilot in this instance felt the loss of thrust - remember he had a little power on - which transitioned to windmilling drag.

http://www.micro-tools.net/pdf/Cessna/150_sm_69.pdf

Good Lord, that mother is 31MB! (sound of thumbs twiddlng).

Ok, acrobat reader is fired up. Para 11-37 suggests an idle of 600 RPM, and a rise of 25 rpm when the mixture is leaned at 1000 RPM. That's pretty much what I was recommending above - check the mixture leaning, and if you don't see a slight RPM rise, your mixture is too lean, which you can't do anything about in the cockpit, except perhaps pump the primer If you see a rise more than 25 rpm, at 1000 RPM, then the idle mixture is too rich, which you can actually fix in the cockpit simply by leaning the mixture at idle.

The important numbers here for the 150 are 1000/25, sort of like a mag check.

Next time you go flying in the extreme cold, check the RPM rise at 1000 RPM with lean mixture!

[Grantmac]

The important numbers here for the 150 are 1000/25, sort of like a mag check.

Next time you go flying in the extreme cold, check the RPM rise at 1000 RPM with lean mixture!

Also a very critical step in a high altitude run-up when you want all the HP you can get that motor to develop.

Interesting side note on the NO2 system: its a common modification in the competition STOL aircraft world. Of course those are all experimental so they can get away with it.

-Grant

[Hedley]

Yes, but I expect they use a "wet" system (which adds fuel as well as nitrous oxide gas) as opposed to a "dry" system which does not add gasoline, and thus runs the risk of going excessively lean at high power settings, which causes detonation, which causes you to go out and buy new rings and pistons.