Question regarding a possible carb icing incident, C150L

[parksie]

Hi all,

I'm posting this in hopes that someone might be able to help shed some light on an incident I had while flying last Friday (14 January). I was flying a rented Cessna 150L from CYOO.

The incident in question occurred while I was conducting a simulated forced approach. I started down from 4000' ASL, with power set to idle and carb heat applied. I flew the approach at about 65 KIAS. Through the approach, everything appeared normal; I warmed the engine at every 500 feet of descent and it responded as it should each time. At around 500' AGL, I prepared to overshoot and recover. As is my usual practice, I applied full power, and using my extended thumb, shut off the carb heat as I pushed the throttle in. However, the engine did not respond this time, and instead continued to idle. In the few seconds while I assessed what was going on, I believe the engine quit firing, though I am not sure on this point.

About the only thing I could think of doing was to re-apply carb heat; when I did so, the engine immediately ran up to full power and I was able to climb out and return to altitude. I should note that there was no rough running or other signs of melting carb ice; in fact I left the heat applied for only a few seconds before shutting it off again and the engine behaved as it normally would - gaining 100 RPM or so and continuing at full power.

If it will help any, the temperature at the time was between -5 and -10 Celsius. Sky conditions where I was flying were clear, though there was a 2500 foot ceiling and light snowfall within 10 miles. Prior to starting the forced approach, I had done the usual engine check: the primer was in and locked, both mags were on, mixture was set to full rich and as previously stated, the carb heat was turned on.

I have since discussed this incident with 3 different instructors at the Oshawa airport, as well as a friend of mine who is a military pilot. There has been no clear consensus as to the cause of this incident. Opinions have ranged to the engine having been shock cooled when I turned off the carb heat (while simultaneously applying full power) to a stuck butterfly valve that was jarred loose by a change in pressure when I re-applied the carb heat. It is worth noting that one of the instructors I spoke to experienced a nearly identical incident a few years ago while performing a simulated forced approach in a 150 (not sure if it was the same aircraft). The school staff told me it is now their practice during winter weather to do forced approach training at 1200 RPM with 20 degrees of flap in order to generate more engine heat, in case this incident was in fact due to engine icing.

If you have any suggestions as to a possible cause for this incident, I would be most interested in hearing your thoughts. I have two primary concerns - first that I might have done something I shouldn't have that caused the engine to perform this way, and secondly that there might be some mechanical issue with this plane that hasn't been caught by the AME at Oshawa (if I'm not mistaken, this plane only had 1 hour of flight since its last inspection). I probably don't need to tell you that this incident scared the bejazus out of me and I have no particular interest in it ever happening again! Thanks in advance for your time in reading this, and for any advice you might be able to offer.

Cheers and Happy Friday!

[Vortex_driver]

you ended up in a situation too lean all of a sudden for your engine... the carburator didn't have enough time to vaporise enough fuel, plus you increased the density of the air at the same time... your engine probably stalled...

that's sounds normal to me... happened to me a couple time...

Apply power slowly, and then remove the carb heat... not the opposite, or both at the same time...

anyway that's how I remember we used to do it...

[T-roper]

agreed with above. Adding power and carb heat cold at the same time leads to a lean mixture. I had class 1 that always said 2 things...
1. Power is not a switch (don't go full throttle instantly)
2. Never add power and turn off the carb heat with hand and thumb as you described
I have also found some 152's when overshooting take an extra half second to react... good learning experience

[crazy_aviator]

Im going to agree with vortex on this. Also, the carb on the 0-200 is not rocket science and it also is NOT perfect. The problem lies in the fact that there have been venturi/float/atomization problems over the years and EVERY carb on the 0-200 engine will be slightly different in the atomization, leanness and acceleration properties!!! Best thing to do, go to ALTITUDE and FIGURE it out!!! Simulate your emerg/prec approaches there and try to fail the engine in NORMAL operation, if it isnt predictable or doesnt conform to the standard, pull the carb and "repair" it till it works properly, make sure all other variables are ok, valve sticking, etc. There is still ignorance in the industry concerning the problems with this carb!!!!

[parksie]

All duly noted, Vortex.

However, I had throttled the engine up in the exact same way recovering from a stall and from slow flight a few minutes before, with no issue. Same for having warmed the engine on the way down, except in that case I had left carb heat on.

I know what you mean about being gentle on the throttle... though I move it in briskly, I don't jam it in as fast as I can.. that's just asking for trouble!

[parksie]

Best thing to do, go to ALTITUDE and FIGURE it out!!! Simulate your emerg/prec approaches there and try to fail the engine in NORMAL operation, if it isnt predictable or doesnt conform to the standard, pull the carb and "repair" it till it works properly, make sure all other variables are ok, valve sticking, etc.

I think I'll leave this one to the AME at the airport. I noted that this was a rental plane - what I didn't mention is that I only have 50 hours total flight time and hold a recreational permit... I don't think I'll be intentionally trying to fail the engine in any sort of operation!!

But if I understand you correctly, the 0-200 has something of a history of losing power under these sorts of conditions? Can you elaborate on that at all?

[Redneck_pilot86]

However, I had throttled the engine up in the exact same way recovering from a stall and from slow flight a few minutes before, with no issue. Same for having warmed the engine on the way down, except in that case I had left carb heat on.

Guessing here, but you were probably recovering from the stall/slow flight at a higher altitude, where the difference in air density was enough to avoid the over-leaning you most likely experienced, as the carb would have to atomize less fuel to come up with a mixture that will burn.

[Cranium]

"I know what you mean about being gentle on the throttle... though I move it in briskly, I don't jam it in as fast as I can.. that's just asking for trouble!"

If it takes you less than two seconds to advance your throttle from idle to full, you're asking for trouble.

[Old Dog Flying]

Having instructed for many years under some hellish cold conditions, I found that your procedure on overshoot is the wrong way to go in cold weather. Advance the throttle slowly and only when you hit the stops do you push in the carb heat control. What little heat is being generated is needed to vapourize the fuel and the 0-200 is a real pig for quiting. Use this procedure even when doing t/gs to prevent a loss of power on the "go".

[iflyforpie]

Here's Aircraft Carb 101 from an AME. (I wish I still had the chance to teach this is in ground school, was so much fun).




Your typical aircraft carb is just a venturi with a nozzle right in the middle of it and a throttle plate to regulate the airflow to the engine. Air flows from bottom to top (updraft).

Fuel goes into the nozzle from a float chamber (who's level is regulated by the float and main metering jet) and is held just below the holes in the nozzle so as not to spill out if the engine is not running (like pre-start).

Air goes through the venturi, creating low pressure in proportion to the speed of the air going through (which is regulated by the throttle plate or butterfly valve). So as we open the throttle, more air goes into the engine, which means by Bernoulli's Theorem it must go faster through the venturi, which creates lower pressure, which sucks proportionally more fuel out of the nozzle. Great system right.

Well... it's great for constant throttle settings, but when we try to change them, we start having issues.



The problem is we have two completely different types of matter we are trying to mix. The air (a gas) and the fuel (a liquid). If we open the throttle up rapidly, the air goes in right away but the fuel lags. So...

We have an accelerator pump. Basically just a plunger and an extra nozzle that squirts fuel into the engine to match the airflow until the main nozzle can catch up. Each time you open the throttle either partially or fully, you depress the plunger partially or fully.



Now, this system is far from perfect. We are used to modern electronic fuel injection systems on cars where complex fuel-air maps are combined with throttle position sensors, mass-airflow sensors, and O2 sensors to get the mixture perfect to each cylinder no matter what we throw at it. My car has an old British Constant Depression carb on it and just like the 150, if you gun it when the engine is cold, it will sputter.

Some theories I have of why aircraft carbs do this. The accelerator pump, just like a primer, is prone to leakage and blowby. The amount of fuel squirted doesn't really change for density (mixture) or throttle position (the same is squirted at low throttle as high throttle). Also the fuel comes out of the accelerator pump as a stream, not atomized. So it is up to the engine to swirl the fuel air mixture so it is somewhat even.


I have left out things like the economizer valve, idle metering system, and mixture control, but hopefully this will give you a bit better understanding of why carbs behave the way they do.




So, going to full throttle, we should leave carb heat on since it will make for a richer mixture (less air due to lower density) and we should apply the throttle smoothly (at least a three-count from idle to full, more is better). If the aircraft stutters during normal throttle applications (run up, going to full power, regular power changes) then the carb needs to be checked and/or replaced.

[KK7]

I don't know what the ambient temperatures were in your area at the time, but this sounds pretty standard for cold weather ops. I know of an accident that occured very similar to this, except the engine did not produce any more power and the forced approach was continued into the planned field. The aircraft (C-150M) was written off, however the student and instructor were unhurt. However the instructor got frost bite in the walk to the farm house after the incident.

Generally I would do a cold weather simulated forced approach whenever temperatures on the ground were below roughly -5 or -10 depending on how well that particular plane handled the cold air. This procedure involves leaving some power on during the simulated forced approach, but using flap to counteract the effect of power. I can't remember the exact numbers, but I seem to recall something along the lines of 1500 RPM and 20 degrees of flap in the 150, but I might be thinking of the 172. It's not ideal to practice since it reduces the amount of flap available to you to plan your approach, however it is far better than risking the engine quitting when you suddenly apply power at 500' AGL.

[parksie]

Thanks everyone for your responses. That's a lot better than the guys at the flight school were able to offer me.