FYI, the 150 club has an informative article on the fuel burn being much higher than book states. It’s a good read.
https://www.cessna150152.com/ubbthreads/attachments/6587-May-Jun2003.pdf
Good article, but there are details left out. The cruise chart for the '77 150 says this:
The author says that the POH says "lean mixture." It doesn't; it says "recommended lean mixture.' To define that, we go to another part of that same POH and see this:
Now, how many 150 drivers lean that aggressively? Most don't. Many won't. Too scared. They'll lean until it roughens a bit and then enrich it until smooth. Roughness? Several causes for that. Poorly maintained magnetos, ignition harness and plugs are responsible much of the time. And a weak spark or mistimed mags will eat a lot more fuel. The POH numbers were obtained in a new airplane, not some 1500-hour engine with almost everything needing replacement. Then there's the MA-3 carb used on that engine, a carb that has had numerous ADs against it. There were ADs for the venturis and fuel nozzle that hurt performance, and I would think that fuel efficiency suffered somewhat with those mods. Many of the engines ran poorly after the venturis and nozzles were replaced. The MA-3 carb was never too good to start with. Then there was an AD against certain O-200 cylinders that were suffereing head separation; the fix was to reduce the timing from 28° BTDC to 24°. The AD should have mandated a fuel consumption chart revision. It didn't.
https://rgl.faa.gov/Regulatory_and_...5FD3727F609E1E0686256848004B6075?OpenDocument
On top of that, how many propellers have been maintained to the manufacturer's specifications? I have seen props badly dinged up. Props dressed until they're well below minimum width and thickness specifications. Props improperly dressed, with blunt leading edges. Yet we expect original performance from them?
Fuel consumption figures might also include wheelpants. How many 150s still sporting those?
Tachometers. They age. They use a spinning magnet to drag an aluminum cup around, using the phenomenon known as eddy current, to make the needle move. The magnets weaken with age, and the tach starts to under-read. So the pilot sets the throttle to get, say, 2500 RPM, but could easily be getting 2600 or more. That eats more fuel. In Canada we have a regulation that requires annual checking of magnetic-drag tachometers, and if it's off by more than 4% it must be replaced. Any error within 4% must be placarded on the tach so the pilot can make adjustments. 4% of 2500 RPM is 100 RPM.
The point here? Don't blame Cessna. They weren't cheating on the POH numbers. These are old, often poorly-maintained airplanes, yet we still expect factory-new performance? That 1977 150 is 45 years old. Even the newest 150 will be 36 years old, at least.
