The inability to achieve normal engine power output (e.g., poor static power) can be an insidious, difficult-to-troubleshoot condition that ultimately affects almost every small engine sooner or later. Sometimes, the cause can be pinpointed quickly, especially via multi-probe engine analyzers. But many times it can’t. The best bet is often analyzing all the power instruments that you do have, not just one.
It is more frequently encountered in engines that have not reached TBO but have weeks to months of sitting idle between flights with sometimes as few as 20 hours a year or less of operation. Also, flying past TBO may increase the occurrence of this condition. Static power is relatively easy to check with a fixed-pitch prop at standard atmospheric conditions. It should be done at every annual.
The first and most obvious sign of a problem with any engine is a reduced rate of climb compared to book figures (obviously taking atmospheric conditions into account). This is more noticeable than an airspeed loss and even more obvious to those who have an operating history with their engine.
For example, we recall a Piper PA-32R we flew that went from 1400 FPM lightly loaded to 1000 FPM in identical conditions. Something was not right. It had high time (1640 hours accumulated over 16 years) and weak, essentially original cylinders.
Considerations
What do you do when you know something’s not right, but you don’t know where to start to look for problems? Consider the obvious first.
If it’s a hot day, or you’re visiting a high‑elevation airport, density altitude may be the culprit (and leaning the mixture on the taxiway or runway may be the solution). If the plane recently came from the maintenance shop, ask the mechanics if anything was done that might have affected power output.
Perhaps the mag timing was reset. Maybe pushrods were removed and reinstalled in the wrong locations (swapped), which could easily reduce maximum power. Maybe a shop rag was left in the air scoop or scat tubing left disconnected or loose. (Don’t laugh—these are all based on true‑life occurrences.)
Maybe you don’t really have an engine problem at all: Have you checked your tach and manifold pressure gauges recently? It’s easy to check a manifold pressure gauge: With the engine stopped, the gauge should indicate ambient pressure (29.92 inches at sea level, standard‑day). The mechanical tach is almost as easy to check. The old trick of strobing the prop against a 60‑Hz fluorescent light source at night can work.
At 1800 RPM, your two‑blade prop should appear stopped by a 60‑Hz light (a three‑blade will appear to have six blades), if the RPM is dead‑on. Obviously, if the prop blades are creeping in the forward (normal) direction, your tach indicates low. If they creep in reverse, the tach shows high.
There’s a another way to check tach accuracy, and that’s to use the odometer or hour meter portion of the unit (which is gear driven) as a crosscheck of the needle indication. If you know that your tach gearing is set up to flip the hour-meter over every hour at 2310 RPM, then you can simply time how long it takes for a tenth of an hour (or longer) to roll by on the counter drum, divide the indicated time by the actual time it took, and multiply the result by 2310 or 2566 (or whatever the magic number is for your tach) to arrive at the true RPM.
By far the easiest and most accurate tach check (other than going digital) is to use one of the handheld digital tach checkers. We found that the cheaper units are less accurate and more difficult to use than the higher-end ones on the market. You get what you pay for—use the better unit.
Assuming your tach and MP gauge are not to blame, what’s next? A static runup to check book values is a good idea for a fixed-pitch prop. Bear in mind that variations in day‑to‑day static RPM are inevitable, since static RPM is affected not only by temperature and altitude, but oil viscosity, air filter condition, wind, humidity, etc.
Static RPM will not equal takeoff RPM in any case, since the engine turns faster with air going through the prop disc. But if you’re fairly sure the engine is power deficient, based on rate‑of‑climb or other indications, there are a few things you should check immediately.
First, if it’s a carbureted engine, be sure the carb heat door is rigged properly. This means, among other things, that it should open and close fully before the knob hits the panel stop in both directions. Also, baffle seal material in the carb air box could be missing, allowing for air leaks.
If the panel stop is limiting carb heat, throttle, prop or mixture control travel (rather than the physical stops at the carb, injector servo or prop governor), have a mechanic assist you in rerigging things. You want a quarter‑inch or eighth‑inch of “cushion” at the panel with the power knobs all the way forward. Cables should not slip in their mounting points at all.
Nothing robs power like full-time carb heat, so be sure to check this possibility out. The mere fact that you get an RPM drop on run-up with application of full carb heat doesn’t mean the flapper door isn’t hanging partially open with carb heat off.
Likewise, check your alternate‑air door if you have one—it could be an issue with the air box.
