The airline transport pilot flew his experimental, amateur-built biplane on a cross-country flight with multiple legs and refueling stops. About one hour, 15 minutes into the flight and 15 miles from his destination, he initiated a descent from 3,500′ to 2,500′.
When he added power to level off, the Acro Sport II’s engine began to run rough and experienced a partial loss of power.
The pilot said he immediately applied carburetor heat, but observed no improvement in engine operation. The engine continued to lose power and “sputter.”
During the subsequent forced landing near Loudonville, Ohio, the airplane touched down on unsuitable terrain at the edge of a soybean field, decelerated rapidly, and nosed over.
A post-accident examination of the wreckage revealed no pre-impact anomalies that would have precluded normal operation of the engine. Nearby weather was conducive to serious icing at descent power about the time of the engine power loss.
Although the pilot reported that he applied carburetor heat, it is likely that, at the time of application, the ice had already accumulated in the carburetor to the extent that the carburetor heat was insufficient to melt the ice and restore full engine power.
Probable cause: A partial loss of engine power due to carburetor icing.
NTSB Identification: CEN17LA295
This July 2017 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Years of training !!!
I’m not a pilot but I am very interested in aviation, however, I admit I’m not very knowledgeable so please excuse if this is a silly question. I’m trying to learn,
Did the type of fuel being used contribute in any way to this accident? As I’ve said, I’m just learning about aviation and am wondering if automobile gasoline was used without butane being added and caused a vapor lock that contributed to the icing ?
Good question Sandra. The answer is no, the type of gasoline has little to do with the potential for carb ice. My understanding is that MOGAS is laced with butane only during winter months and mainly to aid in starting the cold soaked engine. Modern automotive engines don’t require that help. Carb ice is caused by the temperature drop between the inlet (high pressure) side of the venturi and the outlet (low pressure) side. The moisture in the air condenses as the temperature drops and because of the volatility of the fuel it can drop below freezing even with a temperature delta of more than 30 degrees so it doesn’t matter what kind of fuel you use, the moisture isn’t in the fuel it’s in the air.
Reduce power for a descent? Apply carb heat, EVERY TIME! Don’t wait for the engine to tell you somethings wrong. Simple, no gauge or automatic device required. Carb icing at cruise power? Carb intake air temperature gauge will be useful.
The engine is a vital component in any powered aircraft so why not have such a simple and low cost instrument as a carburetor air temp gauge to ensure that the induction system will remain free of ice? It is cheap insurance against this type of incident and a lot less expensive then rebuilding a damaged aircraft not to mention the risk of death and injury. Also considering this was an Experimental Amateur Built aircraft it could have been easily incorporated into the original build along with the other engine instrumentation. There is NO reason why a pilot should have to to try to outguess the atmospheric and operational conditions to prevent induction icing when such a simple instrument would show exactly when the carb heat needs to be applied, we do have fuel gauges so we do not have to guess at fuel state and land every hour for refueling just to be safe. We see far too many of these type of incidents in this column when it can be so easily avoided.
Carb icing is not just about outside air temp, it’s also about humidity. The greatest risk falls roughly between 50 and 70 degrees F and the relative humidity is greater than 60 percent. When these conditions are present one should have carb heat on at all times. If a sensor were possible for carb ice we would have them…the pilot is responsible for planning and judging. The same applies to other situations like shock cooling, excessive EGT temps, proper setting of the constant speed prop, proper setting of flaps, trim and power and avoiding stall due to extreme angle of attack and load factors.
There is a sensor available to read temp in the carb intake throat. Not all carbs have the necessary port to install it however. Carb ice from atmospheric conditions which don’t include snow/ice accumulation will seldom occur at high power settings so you don’t need carb heat anytime temp/humidity are conducive to ice formation only at low power settings for an extended period of time. However anytime a carbureted engine starts running rough the first thing to do is apply carb heat but don’t turn it off till the engine is running smooth again. If that doesn’t happen you dont have carb ice but another problem. Keep in mind that when the ice is melting the roughness will get worse before it gets better.
The sensor I was taking about, carb. air temperature, measures the air temp downstream of the venturi and tells the pilot when it has dropped into the freezing range. It is nothing new and it is effective, they just do not seem to bother with putting them in aircraft in spite of their usefulness. It is a bit like having an EGT gauge to get a good reference for leaning other then pulling the mixture back until the engine runs rough. That too would very useful but was not at all common in aircraft back in the round dial days, and even today.
Sarah, you’re right, I misspoke. The carb air temp isn’t on the inlet side of the carb but downstream of the Venturi. The problem is not all carbs have the port to install it. Also carb ice is a problem relatively easy to deal with even without a gauge to read temp in the intake stream. Simply know that low power operations are conducive to carb ice and apply carb heat, it doesn’t matter whether the ice is actually forming or not. There is no downside to using carb heat in that condition. As for those rare situations in particular engines where carb ice at cruise power or above occurs, again operational understanding will suffice without the gauge. If the engine starts running rough apply carb heat and dont turn it off till its running smoothly again.