I’m working on an article for Military History Quarterly magazine about the famous trans-Himalaya airlift between India and China during World War II, and I keep coming across references to Curtiss C-46s, with P&W R2800 engines, suffering “carburetor icing” while flying the route.
Keeping in mind the fact that cruising altitudes typically were between 15,000 and 20,000 feet for C-46s, and that OAT was almost certainly well below 0 degrees F., it’s my assumption that what is meant by “carburetor icing” was actually impact icing, which would affect the engine’s induction air rather than the carburetor venturi. The remedy would be activating alternate air, not carburetor heat. (I have flown a variety of single-, twin- and four-engine piston aircraft and have never experienced true carb ice in anything but carbureted low-altitude singles.)
Am I right, or is it possible that there actually was such a thing a carburetor icing at so low a temperature? This carburetor-ice canard, if that’s what it is, has been repeated in a wide variety of sources (all of which doubtless feed off each other).
By: Duggy - 7th May 2016 at 20:47
The answer is in the flight training manual, go to page 49,”flying in weather”
LINK – http://www.avialogs.com/index.php/aircraft/usa/curtiss/c-46commando/aaf-50-16-pilot-training-manual-for-the-curtiss-c-46-commando.html
This is also a fantastic site for flight manuals :applause:
By: Robbiesmurf - 7th May 2016 at 18:10
Interesting points Anon, there are other aspects to consider. Fuels are by their nature hygroscopic. I take it the aircraft were being refuelled at lower altitudes where it was hot and humid. The fuel would then attract more water which could then settle and separate in the tank. Plus the hot and cold times of the tanks would form condensation inside, adding to the amount of free water. When being pumped through the carb it’s temperature would also drop when passing through the venturi, that could be a factor. They are still learning about ice formation in aircraft, the crash at Heathrow of the BA 777 has shed some new light on a phenomena that was thought to be well understood for decades.
By: Arabella-Cox - 30th April 2016 at 23:54
I’d suppose that some types or engine installations may have been more prone to carb icing than others, however, the danger zone for carb ice is much warmer and moister air conditions than would have been found at high altitude, even at those latitudes.
Ice accretion might have been the issue, if certain parts of the airframe were not sufficiently protected but any crew, having been made aware of the conditions and (presumably) knowing their aircraft, would have been on their guard for such an eventuality.
The “Hump” aircraft were invariably heavily loaded and as powerful as the R-2800 was, the C-46 was a big aircraft. Add to that the high altitudes and poor weather conditions, any engine problem resulting in power reduction or shutdown would have been serious and probably fatal. Besides that, the C-46 usually had the Curtiss electric props, never a reliable device and on which the prop pitch-change gear often froze.
The carb ice canard, as you call it, is usually peddled by the ignorant, eager to impress that they know what they are talking about. Cold air is low in moisture content and the condition unlikely at the altitudes they were at.
The C-87’s also used on the “Hump”, for all their ills, at least had four engines so could more likely make it across in the event of a major failure. They were powered by the legendary R-1830 – famed for its reliability (at least until the later more highly stressed versions entered service), despite also being cursed with those same Curtiss propellers.
It probably had more to do with mechanical failure(s) and poor handling than ice.
Anon.
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April 29, 2016 at 10:31 pm