Carbs aren’t heated per se. “carb heat” is a bypass of normal cold air to a warm air source such as from a shroud around the exhaust. You could run it all the time but it takes away hp from the engine.
Under certain meteorological conditions the cold air can ice up in the throat of the carb. If the engine starts to lose hp the first thing to do is add carb heat to eliminate that possibility.
When doing a run-up before flight you would add “carb heat” to see if the engine RPM’s drop. If they don’t then the cable to the valve may have come loose.
Frozen/blocked pitot on a Pitts Special. It was frozen during the take-off roll. I’d never been great at doing an airspeed check on take-off so only noticed once airborne. I continued up to altitude, did my aerobatic routine using the sound of the air through the wires between the wings as a guide to speed (it makes a very distinctive whistle with the pitch/volume rising with increasing airspeed). Once I’d finished and it was time to land the pitot had cleared up and the airspeed indicator was working again.
Twice I had the tailwheel break off during landing (same plane as above). Flying out of Queenstown in New Zealand often on the grass strip that crosses the sealed main runway. The transition from the grass to the seal wasn’t very smooth and it took its toll on the poor tailwheel. No big deal really, it just meant I was stuck on the runway while someone came and helped move the plane.
Had two fuel starvation events in a Tiger Moth. One of them was caused by getting a little bit of negative G during aerobatics without enough airspeed to keep the prop windmilling. No electric starter fitted and not enough height to comfortably attempt to get it windmilling so I landed “dead stick”. It happened directly above the runway so no hassles there. The other was when a passenger pulled on the fuel on/off lever, turning it off. I worked out what had happened quickly enough and turned it back on. The prop was comfortably windmilling so it was a bit of a non-event.
I had a magneto failure in a Shrike Commander during take-off. The magneto check had gone fine prior to take-off but on during the roll, one of the engines lost some power and sounded rough so I stopped on the runway.
An interesting one was a fuel pump failure on the Aero Commander. The fuel pump on/off switches also act as circuit breakers. It took me a couple of times turning one of them on, looking away then back again to find it off, before I realised that I wasn’t going mad, the circuit breaker / switch was tripping to the off position shortly after I was turning it on.
Edit: That wasn’t intended to be specifically addressed to @Magiver.
It’s not that simple. First, you don’t want carb heat if you can avoid it, because warmer air is less dense and provides less power. So it has to be used only when needed.
Second, you can’t just measure the air trmperature, because the tempersture drops dramatically in the throat of the carb - as much as twenty degrees due to the Bernoulli effect. So you can get carb icing on a warm day. And humidity matters as well.
Could you build an automatic system to do it? Maybe. But it would be a certification nightmare. These planes are flying behind 1930’s era engine designs with magnetos instead of plugs and carbs instead of fuel injection for a reason: simplicity. The O-235 in our Grumman and Cessna 152’s entered the market in 1940 and is still being produced today. It’s a simple 4-cylinder, 115 hp engine - and it costs $40,000! Everything complex on an airplane is hella expensive, due to the need for it to work perfectly all the time and the associated costs of getting certification.
Porsche tried to build a ‘modern’ aviation engine with their air cooled boxer engine from the 911. It had altitude-compensating fuel injection, single-lever control (no mixture), produced almost twice the horsepower as a similarly-sized lycoming, They ran at higher RPM, using gear reduction for the prop. This made them very smooth as well, but increased weight and complexity.
They only produced 80 of them before abandoning the market after having spent $75 million or so on development. They were just too complex, too expensive, and too hard to retrofit to make them available as replacement engines for existing aircraft, and new aircraft weren’t selling because they had become too expensive for the mass market…
A small Lycoming or Continental will run without battery, due to the magnetos. Carb heat, throttle and mixture are simple levers connected mechanically to the engine bits. Once you add things like electronic ignition, fuel injection and other complexities the potential failure modes go up dramatically, leading to higher production, certification, and maintenance costs.
Are we telling equipment failure stories?
Second solo cross-country as a student pilot, leg 1 (Houston County, MN [CHU] to Waterloo, IA [ALO]) Cessna 150
It had one radio with a mechanical frequency display (turn the freq nob, and display wheel rotates)
Turned knob – display did not change. Turned knob again, – display may not have changed (I don’t remember). Put my finger on display and noted I could manipulate it.
At this point I did not know what frequency I was on. I found a frequency where I head people talk and asked. Once I knew, I just counted knob turns (IIRC it was only one digit that was broken)
Flew into Waterloo OK, and went to the FBO. They were able to fix the radio, but it took a while so I just flew back to CHU rather than continue my planned course (from Waterloo to Dubuque [DBQ] then to Platteville [PVB] and back to CHU)
I rented a Robinson R22 one day. First, I lost a screw out of my glasses. I found a piece of wire on the ground and made a temporary repair. Next, I couldn’t get the oil dipstick out. They’re only supposed to be ‘finger tight’, and the previous renter really cranked it down. I had to go find a pair of pliers to get the dipstick out. It was a cool day, and raining. Great conditions for carb icing. Everything was normal as I began my takeoff run. Climbing out, I saw the needle on the carburettor air temperature gauge bouncing wildly. Hm. It’s never done that before! I decided three incidents on one flight was enough, so I made a pattern, landed, and called it a day.
I would like to think we could do better than a 1950’s design. Carb icing to be a common enough issue. @LSLGuy had it as a fist guess. By your thinking GA planes will forever be stuck with 1950s engine tech which seems nuts.
In the video I linked the host noted that car engines tend to be a lot more reliable. Maybe some of that new tech works.
Airplane engines v. automobile engines. If my memory serves me, the Cessna 150 I learned to fly 55 years ago took 80 octane. Do they still make that? Now that they no longer make leaded auto gasoline, what’s the difference between 100 octane auto gas v. avgas? If two spark plugs per cylinder are better than one, why don’t they use that in cars? I’ve owned several diesel cars and once drove a car with a 2-stroke engine. Anyone ever put these in a production airplane?
Airplane manufacturers don’t do anything unless there’s money in it. Heck a brand new Cessna 172 Skyhawk isn’t that different from a 1974 172N – which aside from a rear window and a slanted tail wasn’t that different from the first ones that came out in 1957. With the cost of developing – and more importantly certifying – a new engine, not to mention the enormous liability issues that will ensue, yeah; we’re ‘forever stuck’ with 1950s technology.
For one thing, 100LL (100 octane low-lead) has more lead in it than regular leaded automotive fuel did.
There are lots of modern aircraft engines - they just aren’t certified under normal certification rules. The FAA finally realized that they were killing general aviation with excessive regulation, and created the Light Sport Aircraft category, which is much less regulated. And the homebuilt industry is almost completely unregulated, and that’s where all the innovation is happening.
An example of a modern aviation engine would be the Rotax 912. It’s fuel injected, has liquid cooled cylinder heads, and like Porsche PFM runs at a higher RPM and uses a reduction gearbox for the prop.
Let’s compare it to a similar certified engine:
Continental O-200
Price: $40,000
Hp: 100
Fuel burn typical at cruise: 4.3 gph
Weight: 190 lbs
Fuel: 100LL Avgas
Time between overhaul: 2000 hrs
Rotax 912S
Price: $17,600-$29,000 (depends on acc, plus discount when bought with a kit)
Hp: 100
Fuel burn at cruise: 3.2 gph
Fuel: unleaded auto fuel, avgas
Weight: 125 lbs
TBO: 2000 hrs
So the Rotax is 65 lbs lighter, just as reliable, can run on much cheaper autogas, Has much better fuel efficiency, and costs half to two thirds as much. You can also get a turbo version, the 914, and turbos are awesome for airplanes.
As for aircraft, let’s compare a Grumman AA-1, which is already more efficient than your average GA light plane, and compare it to a similar modern homebuilt/LSA, the Van’s RV-12.
Both are two seat sport planes with a low wing, bubble canopy and almost identical size.
1971 AA1-A
Horsepower: 108
Empty Weight: 1018 lbs
Max takeoff wt: 1500 lbs
Cruise Speed: 125 mph
Stall speed: 60 mph
Fuel: 22.5 gal
Range: 488 miles
Max rate of climb: 765 ft/min
Service ceiling: 13,750 ft
Now the Van’s:
Horsepower: 100
Empty Weight: 775 lbs
Max takeoff wt: 1320 lbs
Cruise Speed: 135 mph
Stall speed: 47 mph
Range: 660 miles
Fuel: 20 gal
Max rate of climb: 1000 ft/min
Service ceiling: 17,500
These two aircraft have almost exactly the same dimensions, same cockpit size, etc. The Van’s airplane stalls 13 mph slower, cruises 10 mph faster on 8 less horsepower, burns 1.1 gal/hr less at cruise, climbs 235 fpm faster, has 132 miles more range, and can cruise 4,000 ft higher.
The Grumman has a 482 lb useful load (later versions had slightly more). With 135 lbs of fuel, only 347 lbs are left for pilot, passenger, and baggage. The Van’s has 545 lbs, and max fuel is only about 120 lbs, so you have 425 lbs of useful load. That’s a huge difference. My wife and I would have 17 lbs left for baggage in the Grumman, but could carry 78 lbs of baggage in the Van’s (Max 75 lbs in the baggage compartment).
The RV-12 has short field takeoff and landing capability. It can take off in 700 ft and land in 500. The Grumman is more like 1200 ft and 1000 ft. The Grumman is a bit of a pig in climb, and because performance drops of rapidly with altitude due to the older wing and airfoil design, needs a lot more runway, especially at high density altitudes. The RV12 with a slightly longer wing and lower weight is much less sensitive tomdensity altitude.
The RV-12 also has easily removable wings so you can trailer it home rather than pay for amhanger or tiedown.
As light aircraft go, these are dramatic differences.
The Rv-12 kit is between $75,000-$89,000, depending on options. It takes about 700 hours to build. The Grumman isn’t made anymore, but a Cessna Skyhawk starts at $369,000 and goes to $438,000. As a guess, a new Grumman AA1 today would probably set you back somewhere in the range of $200,000.
Because of all the above, in 2017 Van’s saw it’s 10,000th aircraft finished. In comparison, all certified piston plane sales in the U.S. are now under 1,000 per year. That includes a whole range of sport, training, and business planes. In terms of recreational flying, homebuilts and LSAs are where the action is.
There are engines with automatic carb heat. I hesitate to say “all Tiger Moths” because there were lots of different variations built by different countries, but the Tiger Moths I’m familiar with had automatic carb heat that was a simple mechanical linkage that applied heat at low throttle settings.
The problem with carb heat is how do you know when it’s needed? Carburetor icing isn’t based on temperature and unless it’s icing up the process robs HP from the engine. I’d be interested in knowing how de Havilland worked this out. Cars had it for years with a simple valve in the air cleaner snorkel. Actually it was the only reason for the snorkel.
I’ve personally never experienced icing in a plane although I would engage carb heat when landing if I thought it might be prudent. I’d add carb heat when power was reduced for descent.
And to be clear, when I say cars had carb heat it wasn’t for icing problems. It was because car engines were in a constant state of rpm change and required help in the warm-up process. It was something of an art form to get the choke and carb heat to work well. There was a time when pulling out in traffic was a roll of the dice when the engine was cold. Fuel injection was a godsend for that reason alone.
As you note it is mostly needed at low power settings (landing, descent). The Tiger Moth system had it come on when the throttle was close to idle. I doubt there was a lot of science involved. It had obvious flaws, such as needing to reduce power if you wanted to get the carb heat on “manually”, which would just make the situation worse, initially at least, but I suspect it was “good enough” for most circumstances.
I’ve found some Canadian Tiger Moth pilot notes which include 'Carb Heat OFF" in the pre-take off checklist. That would suggest that at least some of them had a manual carb heat control. The same checklist mentions securing the canopy! Canadian Tiger Moths had many luxuries.
