Acknowledgements: AOPA Air Safety Foundation     

Do you know what to do if the engine burps and coughs during the runup, or runs rough during cruise? In-depth systems knowledge can give you the tools needed to assess the engine’s actual condition. Aircraft engines are extremely reliable when properly cared for, and can deliver years of safe flight. That being said, not all pilots know as much as they should about the proper care and maintenance of engines, or that mechanical failure accounts for 15 to 20 percent of all accidents. Knowing how to manage a powerplant helps you fly more safely and can minimise the cost of flying. 

First and foremost – try to fly your engine at least an hour a week. Far more engines rust out than wear out. They rust because the oil drains off the cylinder walls and the moisture in the air then reacts with the iron in the engine. The rust creates roughness, which increases wear. Piston aircraft engines are made mostly of steel and aluminium, which expand and contract at different rates, depending on temperature. When flying at varying altitudes and from one climatic zone to another, temperature changes can be extreme. By keeping large engine temperature changes over a short period of time to a minimum, and within prescribed limits, the safety, reliability and longevity of the engine are significantly enhanced. 

 Avoiding rapid descents at idle power near your destination airport will help avoid “shock cooling,” which is the too-rapid cooling of hot engine metals. Shock cooling causes stress that can lead to cylinder head cracks. Good descent planning takes a little work, but your engine and passengers will thank you. This may take some negotiation with ATC, if IFR, or you may have to increase drag such as lowering the landing gear or flaps to keep airspeed and resultant engine cooling in check. 

 Fuel Leaning: Pilots should lean appropriately anytime they are below 75% power, regardless of altitude. For most airplanes, correct mixture settings are detailed in the POH. As you gain experience with leaning, you’ll find that it saves gallons of fuel and helps the engine run better. Follow the POH mixture settings carefully; this is not the time to experiment on your expensive engine! 

Common Engine Problems (and solutions): 

  • What do I do if the engine runs rough during runup? Engine roughness while checking the magnetos during runup could indicate a fouled spark plug or other ignition system problem. Accelerate the engine to runup rpm and lean the mixture until the engine runs rough. Let the engine run for about 30 seconds. Enrich the mixture then check the mags again. If this doesn’t clear the roughness, have the ignition system checkedby a mechanic before flying. 
  • What if the mag drop is more than 200 rpm? A larger than normal mag drop is not as critical as a rough mag. A smooth drop up to 200 rpm is fine. A drop greater than 200 rpmcould indicate a mag-timing problem that should be checked. A mis-timed magneto can rob some power from the engine and also cause engine damage. 
  • Can I fly if the carb heat drop is 300 rpm or more during runup? No. A large carb heat drop during runup, more than the typical 50 to 100 rpm, is caused by an exhaust leak inside the shroud where hot air is diverted to the carburettor. All exhaust leaks are dangerous and must be fixed, because firewall air leaks can allow exhaust fumes and possibly carbon monoxide from the engine compartment into the cockpit. A leak can also direct hot exhaust onto vulnerable components such as fuel lines and possibly cause a fire in the engine compartment. 
  • Is it possible for the carburettor to ice up during ground operations? Yes. Under certain conditions carb icing can occur while taxying. If you don’t leave the carb heat on for at least 10 seconds during the runup check, the ice might not melt and could cause lower power output during take-off and possibly engine failure. If the carburettor is iced up during runup, carb heat application will result in an initial small rpm drop, then a rise higher than the runup rpm. 
  • How do I know if the engine is developing full power during take-off? The engine must reach the specified static rpm range (before releasing the brakes) at full rpm. Check the POH for these numbers. If the aircraft can’t reach this rpm range on the ground there may be a problem with the tachometer indication or something wrong with the engine. Possible problems include a worn propeller (fixed-pitch), improperly set propeller governor (constant-speed), mis-timed magnetos, fouled spark plugs, clogged fuel injector nozzle, or a blocked muffler. 
  • What is a hot magneto and how can I troubleshoot this? A “hot” magneto is a magneto that can’t be turned off. If someone manually turns the prop with a hot mag, it could begin turning even though the magneto switch is in the OFF position. You can easily check for a hot magneto:
    • During runup. If no rpm drop is noticed during the mag check on runup, you may have a hot mag. 
    • During engine shutdown. check for a hot mag by running the engine at idle and turning the ignition to Off. If the engine continues running with the ignition in the OFF position, the mag is hot. 
  • Can I take off if the oil temperature isn’t in the green? Yes, but check to make sure the engine picks up smoothly as the throttle is advanced. Throttle advancement should take several seconds from idle to full power. Cold oil doesn’t lubricate as well, and damage could occur if the oil isn’t warm enough. While engines can be started at very low temperatures, it is generally safer to preheat below -7C. Preheating improves oil lubrication, the fuel vaporizes for easier starting, and engine parts expand uniformly. 
  • My engine is so hard to start, especially when hot; what can I do about it? There are many causes of hard starting, including a weak battery, fouled spark plugs, worn magnetos, worn impulse couplings, fuel vapour lock, and improper technique. Fuel-injected engines can be difficult to start when the engine is hot because fuel can turn into vapor in fuel lines near the hot engine. With air bubbles/vapour in the fuel lines the engine will not start or will not run after starting. Follow POH instructions for hot starting, but be sure that the mechanical items mentioned above aren’t making the problem worse. 
  • My engine runs very rough while starting then smooths out as it warms up. Is there something wrong? Yes, there is a strong likelihood that you have a stuck valve. The valve sticks inside the cylinder head when the engine is cold and the metal parts are contracted. As the engine warms up, the valve eventually loosens and the engine runs smoother. A stuck valve is dangerous because the sticking can occur during normal operations, and it can cause catastrophic engine failure. Have this symptom checked thoroughly before flying. 
  • Can I hurt my engine by leaning too much? Yes, at higher power settings you can hurt the engine by over leaning. Follow the POH leaning instructions to avoid damage. There is one time that over leaning isn’t a problem and that is when running at just above idle power during ground operations. During a long taxy or a lengthy wait for take-off clearance, you can lean the engine aggressively without the risk of damage. Leaning on the ground helps prevent spark plug fouling. Just don’t forget to enrich the mixture before take-off. 
  • Is it okay to lean below 3,000 feet? Yes, you can lean the engine at any altitude. There is no reason not to lean during cruise; it saves gas and is better for the engine. While you will still see recommendations not to lean until reaching 3,000 or 5,000 feet, this advice is to keep pilots from forgetting to enrich the mixture before descending, and it is not relatedto any potential engine problems! 
  • I learned what to do if the engine fails, but what do I do if there is just a partial power loss? This is more likely to occur than a complete engine failure. The key is to determine if there is enough power to remain aloft to troubleshoot the problem. If the engine is losing power steadily, you’ll need to find a place to land quickly. An example might be a gradual loss of oil pressure; the end result is still total engine failure. A forced landing is in the very near future. A fuel line or muffler blockage could cause a partial power failure but leave enough power to stay level. In this case, you may be able to nurse the airplane to a nearby airport, but this will depend on terrain and weather. The bottom line for partial power is to treat it like a full engine failure. Troubleshoot as needed but plan to land at the nearest suitable airport. 


Tony Birth