Gauging Safety

Engine gauges can tell you a lot about whats going on inside your engine - but you have to understand what theyre saying


by Ray Leis

While you are scanning your flight instruments closely as you cruise along, other instruments are recording whats going on inside your engine, and your electrical systems. The instruments over in the other corner of the panel are are worth more than a glance, IFR, or VFR. If you dont include them in your scan, how can you know if your aircraft engine is in good health? True, its running now, but the next question may be how long it will continue running as advertised.

The instrumentation on board the aircraft are designed to monitor engine health and transmit warning information if necessary. The oil pressure, oil temperature, fuel pressure and cylinder head temperature gauges and ammeter can give warning signs, far in advance of serious engine trouble.

Most of these instruments are marked with an operating range (green) and an emergency condition (red) at either end. If there is a fluctuation or a borderline indication, there may be trouble ahead.

The problem comes when the pilot must decide whether an abnormal instrument reading is an indication the gauge itself is faulty or that there is a potential emergency. If one of the gauges is reading abnormally and the others are OK – and everything sounds and feels OK – then the gauge reading may be suspect.

Dont forget about a suspicious reading, though, either in flight or on the ground. The message the instrument is designed to tell is that something is amiss. The decision to land or continue is up to the pilot and depends on important the warning is.

The instrument clusters primarily tell you about engine performance, and include cylinder head temperature, oil pressure, oil temperature, fuel pressure, fuel flow, exhaust gas temperature and turbine inlet temperature. In addition, the ammeter monitors the health of the electrical system.

Cylinder Head Temp
The cylinder head temperature gauge is a primary indicator of the engines health because abnormal temperature is often a first sign of trouble. It takes very little time for high cylinder head temperatures to foul spark plugs or damage valves. You can also foul spark plugs with colder than normal cylinder head temperatures.

However, cylinder head temperatures can vary considerably, even under normal circumstances, so its important to keep track of the operating parameters that generate the temperatures. Temperatures can fluctuate as much as 100 degrees in an injection engine and 200 degrees in a carburetor engine. During rich-of-peak operations, the cylinder getting the least amount of fuel will run at the highest temperature.

One indication of impending trouble is if the CHT gauge moves away from normal when you have not changed mixture or airspeed. The trick is to keep careful records of the cylinder head temperatures and associated conditions. This way, abnormal temperature tendencies can be seen, even while the temperatures are still in the green arc on the instrument.

Important information about whats happening inside the engine comes from the cylinder head temperature readings.

High temps on the CHT gauge could mean the cooling air over the engine is blocked in some way. Assuming youve considered the impact of outside temperature, airspeed and power setting, the high temperature could be an indication of structural ice on the cooling inlets or other physical obstruction.

If the cooling airflow is coming through normally and overheating is indicated at cruise power, the cause is most probably internal. Fuel may be contaminated or of the wrong octane. The magnetos may be timed improperly or the baffling has failed somehow. Land soon.

Oil Pressure
The only way you have of knowing what is taking place in the core of the engine is through the oil pressure and temperature gauges. Watch for either a change in the temperature or a higher than usual temperature reading. On the pressure gauge, and reading besides a normal indication needs to be watched.

Consider the information the two gauges are giving you together, with each giving a hint as to whats going on inside the engine.

The oil pressure gauges function is to tell the pilot that there is oil pressure, and that it is within operating limits. Generally it is a mechanical device that will continue to function as long as the engine is running.

Low oil pressure can be a sign of low oil supply, burned out bearings, a broken pressure relief valve spring or hot oil. High oil pressure may be a sign of a sticking relief valve, an improper pressure relief valve setting or cold oil. If the rise takes place in flight, there may be a possible internal plugging. Fluctuating oil pressure may be an indication of low oil supply or possible oil foaming.

The first thing to do if you notice fluctuating or low oil pressure is to reduce power. Such readings are generally indications that the oil pump is cavitating or that oil exhaustion is imminent.

Check the temperature gauge for a high temp reading. If the temp is normal, the pressure gauge itself is probably erroneous.

The engine will likely continue operating with low oil pressure, showing below the green arc. But the big uncertainty is how long it will continue to do so, and that depends on why the pressure got low in the first place.

If the oil pressure falls into the limbo zone just below the bottom of the green but above, say, 25 psi, and the temperature a bit high but not at redline, it is usually advisable to fly to the nearest airport with a maintenance capability.

Its unusual to get a really high oil pressure reading. The most likely answer is that the oil is of too high a viscosity number. Or there may be a failure of the oil pressure relief valve. This valve is designed to prevent pump overload, in case there is a high load in the lubricating system, for example, the closing of the oil cooler thermostat.

Oil Temperature
Air-cooled engines depend heavily on oil for cooling, with the oil dissipating some of the heat of combustion through the oil cooler. In engines equipped with oil cooler thermostats, you can expect oil temperature fluctuations during takeoff and climb. If the temperature continues to fluctuate during flight, or shows consistently high or low readings, you are probably looking at a thermostat malfunction.

An increase in temperature could stem from a decrease in the quantity, due to worn bearings, burned valves or severe leak, but there are other causes as well. High temp readings can result from the engine being underfilled or overfilled with oil, fuel of too low an octane or contaminated, clogged air passages in the oil cooler or improper power settings.

By themselves, high oil temperature readings may not be cause for alarm as longs as they remain in the green and are not fluctuating, however, consistently low oil temperature readings may be more of a worry.

Contaminants in the oil are either filtered or boiled out, but if temperatures are too low the oil may accumulate water and other contaminants that can cause internal engine damage. This is primarily a problem in engines not equipped with thermostatically controlled oil coolers or in engines with a malfunctioning thermostat operating in cold weather.

A low temperature indication may also be caused by detonation or pre-ignition and can be sign of an upcoming engine failure.

A high oil temperature in flight should prompt you to throttle back and open the cowl flaps, if equipped. If the redline temperature has not been exceeded and the high temperature drops down, the flight can usually be continued. Otherwise, look for the closest airport.

Fuel Pressure Gauge
The fuel-pressure gauge can warn of potential trouble well in advance of the actual emergency.

On carbureted engines, the fuel pressure gauge can diagnose a number of problems in the fuel system, but the situation gets extremely complicated on fuel-injected and turbocharged engines.

With a carburetor, a broken fuel line will show up as a sudden loss of fuel pressure that normalizes when the auxiliary pump is turned on. If this happens, shut off the aux pump, set the fuel selector valve to Off and land immediately. To do otherwise is to risk an engine fire.

A shortage of fuel to the engine-driven fuel pump is shown by fluctuating fuel pressure indications. The reasons can include running a fuel tank dry, or leaks or obstructions in the line from the tank to the pump. If you note a sudden high rise in the fuel pressure and a loss of power, suspect an obstruction in the line from the pump to the carburetor.

If you experience a loss of power and a sudden drop in fuel-pressure, this usually means a leak between the engine-driven pump and the carburetor. If you have these symptoms, look for a gradual drop in oil pressure. This would be due to a rupture in the fuel pump diaphragm that results in fuel leaking into the crankcase and diluting the oil.

If a sudden drop in fuel pressure is accompanied by engine roughness and a loss of power, the float valve in the carburetor has somehow stuck in the open position.

If the drop is gradual and the air temperature is below freezing, you may have ice contamination in the fuel lines or tanks. Get to warmer altitudes or be ready for fuel starvation and a quiet engine.

The fuel pressure gauge on fuel-injected engines presents different information and gets extremely complicated for supercharged or turbocharged engines. To use the fuel pressure gauge to trouble-shoot for injection engines requires a detailed knowledge of the engine manual for each type of engine. Here, your mechanic can steer you to the information you might need.

Fuel-injected engines typically use a fuel flow meter – its actually a pressure gauge- calibrated in gallons per hour. The fuel flow gauge connects to the fuel manifold or the flow divider part of the fuel injector.

What is measured by the fuel flow meter is the pressure inside this flow divider, which is the fuel flow to the engine. It is not accurate enough to use for fine leaning, even though it has per cent power and altitude markings on the dial.

There are also fuel totalizers (or digitizers) that present fuel flow information. Totalizers measure fuel flow directly, and ahead of the firewall, by means of a flow transducer. The information on fuel flow presented by a digital fuel totalizer is much more accurate than what is given by a fuel-pressure gauge. If the transducer fails, thats the end of the information cycle.

Exhaust Gas Temperature
One of the most useful engine instruments is the exhaust gas temperature gauge because it allows you to monitor combustion efficiency and fuel economy. The EGT system consists of a thermocouple probe, or probes, that are mounted in one or more exhaust stacks.

The EGT gauges in most light aircraft use a scale that reads 25 degrees per marking, though many are digital. In either case, however, there is no redline temperature marking.

The standard rules of leaning apply – to ensure the power setting is not high enough to overstress the engine as it approaches peak EGT and peak cylinder pressures.

If you have a single-probe EGT gauge, make certain it is connected to the hottest cylinder. Other than telling you roughly how your engine is leaned, the single-probe EGT provides little information about the combustion process. A multi-probe EGT provides far more information.

For example, a sudden rise in the EGT of one cylinder may mean a spark plug problem or a partially plugged fuel injector. Switch mags. If the plug was the problem one mag setting will show the EGT drop on the bad cylinder and rise on all of the others. If all the EGTs rise, the problem is in the fuel delivery.

You can also detect induction system leaks, valve problems, magneto failure, detonation or preignition with a multi-probe EGT gauge.

Turbocharged airplanes have a turbine inlet temperature gauge that is essentially an EGT connected to a single thermocouple located at the turbochargers inlet. The main thing to note with the turbine inlet temperature gauge is that it does have a redline temperature – 1650 degrees F is typical. If you want to keep your turbocharger intact, dont exceed that TIT redline reading for any reason.

Instrument Problems
If more than one engine instrument shows something is amiss, consider the design of the system before you necessarily conclude the engine has problems. Some installations wire the powerplant instruments in clusters that are wired together into the airplanes electrical bus and may use a common circuit breaker. The wrong kind of failure can effectively knock out the rest of the instruments in the cluster. You could, for example, lose fuel quantity, oil temperature and CHT at the same time.

Despite the improvements in the amount and quality of information about systems, operating practices and techniques, most pilots dont take full advantage of the information available from the airplanes panel. So, while you are cruising across the sky carefully cross-checking the flight instruments, dont forget the other instruments, and what they mean.

Also With This Article
“When Good Gauges Go Bad”

-Ray Leis is an ATP, CFII and FAA Safety Counselor.


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