Airborne_Again wrote:
I don’t see how that agrees with any explanation I’ve ever read on how carb ice is formed.
Exactly. There is the rub.
I learnt, like everyone else, that carb ice forms best at low engine power. At the same time, the temperature drop vs the outside air temp depends on the venturi effect of the air entering the carburettor. The temperature drop can be as high as 40°C.
So I had 2 instances where my O200 almost died in flight: Once at 10’000 ft in light haze and once in climb at around 7500 ft. In both times the engine was running full out, throttle wide open and mixture regulated to best power. Carb heat immediately restored power, so it was clear what we were dealing with.
I never saw carb heat on the ground or during low power settings such as in approach (even though there carb heat was always full on).
After that I had enough and got myself a carb temp indicator, which I had seen in use to great effect in Finland. My first flights determined something really startling to this 100 hr pilot: With an ambient temp of 20°C or thereabouts, at 1000 RPM the indicator showed me well above the yellow arc. With full power on however, I was right at the lower end of the yellow arc and with 2200 or 2300 RPM right in the middle and at 1500 RPM or thereabouts about out on the top. Adding carb heat would move the dial swiftly upwards, usually some 30-40°C so safely outside the arc.
What I learnt fast is that keeping the temps ouside carb ice range does not always require full carb heat, but particularly at lower power settings, only very few. In Summer with full power on, you’d always be in the range so carb heat would put the temperature above 5° C in almost all conditions, in Winter however, full power ended up often well below the yellow arc. If you add carb heat then, it could happen occasionally that it would heat up the temp just enough to end up with some -2 to -5°, so it would actually put carb temp right into the icing zone.
With the device I never had another carb heat incident. It was kids play to keep temps outside the range whenever visible moisture was seen.
In the Mooney I had planned to install the gauge donated to me by Dan but found that some (censored) had closed off the sensor hole on the carb! Like welded it shut… Next time that carb needs overhaul this will be rectified. The Mooney (O360) however shows a very different icing characteristics than the O200 did. Icing does occurr frequently on the ground, never so far up high.
So my lesson learnt was that different engines have huge differences in behaviour when it comes to carb temp and therefore the power range it most likely occurrs. The other factor of course is the construction of the carburettor, but in my experience, once some ice has formed on the throttle valve, it can under bad conditions increase sufficiently to shut your engine down even at full throttle. So it certainly is good practice to clean the engine just before take off.
Carb ice is affected by the carb temp. It gets some heat from the engine. My Bolkow Junior is even worse than the Jodel DR1050 was. Both O200 engines. Start with carb heat on, engine stops, repeat until exhaust is supplying heat to the air.
Taxi with carb heat on with a cold engine.
Often I apply carb heat when opening throttle fot take-off, closing it as revs reach max.
Even when the temperature is below the carb ice limit, I’ve had it on a Konsin treated runway
I’ve always caught it in time in the air, but have several times had the engine die on the take-off run.
PS FAA, Canadian and UK CAA carb ice info differ slightly.
Mooney_Driver wrote:
After that I had enough and got myself a carb temp indicator, which I had seen in use to great effect in Finland. My first flights determined something really startling to this 100 hr pilot: With an ambient temp of 20°C or thereabouts, at 1000 RPM the indicator showed me well above the yellow arc. With full power on however, I was right at the lower end of the yellow arc and with 2200 or 2300 RPM right in the middle and at 1500 RPM or thereabouts about out on the top. Adding carb heat would move the dial swiftly upwards, usually some 30-40°C so safely outside the arc.
I don’t doubt what you’ve seen, I just don’t understand how to explain it.
Airborne_Again wrote:
I don’t doubt what you’ve seen, I just don’t understand how to explain it.
What I was told is the higher engine power the stronger the flow in the carburettor and therefore the stronger the cooling effect. That sounds logical to me and would explain it.
Mooney_Driver wrote:
What I was told is the higher engine power the stronger the flow in the carburettor and therefore the stronger the cooling effect. That sounds logical to me and would explain it.
It doesn’t seem logical to me as the moisture that forms carb ice comes from the air itself. The cooling effect on the walls of the carburettor certainly is greater, but the cooling effect on the air itself is less due to lower pressure drop. On the other hand a larger amount of moisture will pass through the carburettor at higher power settings.
The aircraft settled into trees shortly after takeoff. The pilot reported that he took action to remain in control of the aircraft until impact and the occupants were rescued uninjured. The pilot suspected that the engine had suffered from carburettor icing and reported that the aircraft encountered turbulence sufficient to cause a loss of control. The aircraft probably exceeded its maximum takeoff weight.
That’s what I keep saying: fly that plane until the end and the probability is quite high that you will climb out on your own (here not true because in the treetops, but unhurt…)
Why weren’t the investigators able to verify whether they were actually above MTOW or not? Was it so close? It’s hard to judge whether it was a factor. Carb icing – if identified – can take a while to vanish and then every single kilogram decides whether the remaining engine power keeps you in the air or not.
To suffer from carb icing sounds like bad airmanship (checklist!), but there have been uncounted forced landings for that reason.
UdoR wrote:
Why weren’t the investigators able to verify whether they were actually above MTOW or not? Was it so close? It’s hard to judge whether it was a factor. Carb icing – if identified – can take a while to vanish and then every single kilogram decides whether the remaining engine power keeps you in the air or not.
AFAIR the weight is measured +5%/-10% (CS-23) and his suspected overwieght was ~3%, so I doubt if the overweight caused that crash, most likely severe carb icing, but could be anything else – I doubt CAA actually investigated the engine that thoroughly – no one has died, case closed.
