Those Other Instruments

Whether VFR or IFR, dont forget to scan the gauges that tell how the airplane feels today


While you are scanning your flight instruments closely, over in some other corner of the panel are some other instruments that are worth more than a glance, IFR or VFR: the engine performance instruments.

If you dont include them in your scan, you cant really tell if your aircraft engine is in good health. Theyll help you decide when to draw the line and say that the engine is not safe enough to continue on the flight. It may be running now, but your instruments may help you determine how long it will continue to run as advertised.

There are symptoms of failing engine health that an aware pilot should recognize. In some cases they may pinpoint a problem before it becomes trouble. In other cases they will allow the pilot to get on the ground while the engine is still capable of producing power. In the extreme they will let you cope with an emergency rather than letting it escalate to a crisis, as in a fuel problem that becomes an in-flight fire.

Several instruments that often are ignored or just barely noticed are there to help diagnose trouble. The oil pressure, oil temperature, fuel pressure and cylinder-head temperature gauges can give warning signs far in advance of serious engine trouble.

The engine instruments in general aviation aircraft are usually marked with an operating range in green and a hazardous or inoperative condition in red at each end. If the indicator needle is steady and in the green, the function is normal. If there is a fluctuation or a borderline indication, there may be trouble ahead.

The pilot needs to be able to decide whether the trouble is an erroneous gauge reading or a potential emergency. If one of the gauges is reading abnormally but the others are reading normally and the engine sounds and feels OK, the gauge may be suspect.

It isnt a good idea to forget about a suspicious reading, though – either in flight or later on the ground. The message the instrument is designed to communicate is that something in the engine may have malfunctioned.

The decision to land or continue is for the pilot to make, depending on how important the warning is. If any of the other instruments confirm unhealthy engine operation, no matter how slight, then it may be best to assume the instrument is giving a true indication and take the necessary action.

Oil Pressure
The oil pressure gauges function is to tell the pilot that the oil pressure is within operating limits, ensuring the engine is getting the lubrication it needs. Most oil pressure gauges are purely mechanical and do not require electricity to operate. As long as the engine is running, youll know whether the oil pump is working and if theres oil in the crankcase. There are a few electric oil gauges that use pressure on a transducer.

A sudden and extremely low oil pressure reading can mean low oil quantity. An abnormal rise in oil temperature usually goes along with low oil pressure readings. Total loss of oil pressure, because of oil pump failure, oil starvation or a bad leak somewhere, can have serious results. At some point, the bearings may seize and the engine simply stops cold.

Fluctuating readings may mean oil pump cavitation or imminent oil exhaustion due to a leak.

The first thing to do when you notice fluctuating or low oil pressure is reduce power. The engine may continue operating with low oil pressure, but how long it will crank out power depends on what kind of problem is causing the low reading.

When the oil pressure falls into the limbo zone just below the bottom of the green arc and oil temperatures are high but not at redline, it is usually advisable to fly to the nearest airport with a maintenance capability. More extreme readings may call for a precautionary off-airport landing.

Its unusual to get a really high oil pressure reading except at startup on a cold day. The most likely answer is that the oil viscosity is too high, but there may be a failure of the oil pressure relief valve. This valve is designed to prevent pump overload if there is a high load in the lubricating system – the closing of the oil cooler thermostat, for example.

Remember that the type of engine has some bearing on the symptoms the oil pressure gauge will display. In Lycoming engines, the pressure is read just downstream of the oil pump, which makes for a higher oil pressure reading than youd find in most Continentals. The oil needs to travel a longer distance in the Continentals from the connection point to the instrument, so the normal indication is somewhat delayed.

The Lycoming setup results in quicker indications of fluctuations in oil pressure or pump cavitation than the Continental type. In Continentals, the design of the system and routing of the oil means the pressure is read after the oil has traveled to most of the engine.

Oil Temperature
The oil temperature sensor is generally located just downstream of the oil cooler, if one is installed. In most older airplanes, the temperature was measured by connecting a Bourdon tube bulb with an electrical relay to the cockpit instrument. It worked fine when it worked. Newer airplanes use a thermocouple that does not need external power.

Oil temperature is closely related to oil pressure. In engines equipped with oil cooler thermostats, you can expect oil temperature fluctuations. They are most noticeable shortly after takeoff and during climb. If the temperature continues to fluctuate during cruise or shows consistently high or low readings, a thermostat malfunction is the most likely culprit. If the temperature rises very rapidly in extremely cold weather, the oil cooler may be blocked with congealed oil.

There are numerous reasons for high oil temperature readings. If you underfill or overfill the engine with oil, if you run your engine on fuel with too low an octane, if the oil cooler air passages are clogged with structural ice or insects, youll get high oil temp readings. Other causes can be running at low airspeeds with high power settings, operating too far oversquare, or having clogged or dirty oil filters or screens.

Oil temperatures that run consistently low, at the bottom of the green, are more of a worry. Dirt in the oil can cause some contamination, but filters usually remove most of it. The filters wont sort out all contaminants, though, some of which only boil off at high temperatures. The contaminants left in the oil will then corrode internal engine parts.

If the oil temperature is not high and you have a low pressure indication, suspect a bad gauge rather than a bona fide oil pressure problem. If the oil is leaking onto the floor underneath the gauge, of course, you can be pretty sure the problem is the gauge.

Whatever the case, a high oil temperature in flight is cause for an immediate reduction in power. If you have cowl flaps, open them. 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.

Cylinder Head Temperature
Oil temperature gauges are excellent for reporting some of the thermal balance of an engine, but they dont cover all of the heat changes that can occur rapidly – such as at the tops of the cylinders. To get this information, a temperature-sensing probe is installed in one of the rear cylinders of the engine. In some installations each cylinder is fitted with a probe.

Usually a high CHT reading is an indication that the flow of engine cooling air is blocked in some way. The high reading can also result from a combination of warm air, slow airspeed and high power.

In cold weather, structural ice can block the cooling inlets. Birds may build nests on top of the engine. Incorrectly installed, broken or missing baffles may prevent efficient cooling. Even an ingested scrap of paper can make a difference.

If the cooling airflow is coming through normally and overheating is indicated at cruise power, the cause is most probably internal. If the problem arises just after an annual inspection or other engine maintenance, look for improper magneto timing. Operating too far oversquare or experiencing detonation because of an excessively lean mixture will send up the temperature as well.

A low cylinder head temp probably means the aircraft has not been sufficiently winterized, the cowl flaps are too far open or the power setting is too low.

The mechanical tachometer in a single-engine airplane works very much like the speedometer in a car. A long, flexible-drive cable is connected from a place on the back of the engine case, to the instrument on the panel. Most larger airplanes and twins use electric tachs because routing the flexible cable a long distance isnt practical. The electric tach can handle the loss of aircraft electrical power because it uses its own AC power source.

Fluctuations in the engine rpm can point out several problems. In airplanes with a constant-speed prop, fluctuations in the tach may indicate problems in the prop governor. Generally you will hear or feel engine rpm changes as the tach indicates them.

One of the most valuable diagnoses the tach helps you make comes during the pre-takeoff engine runup. Excessive magneto drops can result from a number of causes. Some are errors in operating the engine. Others are signs of impending disaster – or at least repair bills.

When a spark plug operates normally, it is self-cleaning. An excessive mag drop indicates one or more of the plugs is fouled. Pulling the plugs may help pinpoint where the problem is.

A gray metallic deposit on the plugs means lead fouling. It is almost always caused by improper engine operation or use of fuel with too high an octane rating. Lead fouling is not an indicator of poor engine health.

Fuel fouling appears as a dark black soot deposit on the plugs. It means the engine has been operated with an excessively rich mixture. It can be caused by not enough leaning during high altitude flights, very rich idle mixtures, long power-off descents from altitude and more than normal ground operation.

If the soot deposits show up on only one bank of cylinders, there is a fuel distribution problem in the engine. Its not an indication of poor engine health, but a malfunction of the fuel metering equipment. Excessively lean mixtures are indicated by the top plugs being very clean, with a deposit of white powder on them. If the bottom plugs are the same, the engine is being operated with too lean a mixture. Your engine may be close to a top overhaul.

Oil fouling leaves a grayish, wet deposit. If it appears on the top and bottom plugs, its time for an overhaul. If it shows up only on one cylinder, it shows cylinder trouble, possibly broken rings. On a turbocharged engine, plugs that are wet with oil can also be a result of oil moving from the turbocharger into the compressor, due to bearing or seal problems in the turbocharger.

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

There are several kinds of fuel pressure gauges. The Bourdon type is very similar to the oil pressure gauge. There is also a bellows type in which pressure causes the bellows to expand and contract, like an accordion, which causes the needles to move. A break in the line behind the instrument panel may cause raw gas to flow into the cockpit.

The fuel pressure gauge can give an indication of trouble before takeoff. It tells if the auxiliary pump is working (engine off, boost pump on), whether the engine-driven pump is working (engine running, boost pump off), and whether there is vapor in the fuel lines.

On carburetor-equipped engines, a sudden loss of fuel pressure that returns to normal with the boost pump on usually indicates a broken fuel line. If this happens, shut off the auxiliary pump and the fuel selector valve and get ready for a dead-stick landing. Otherwise, there is a high possibility of fire.

A shortage of fuel to the engine-driven fuel pump shows up as fluctuations on the fuel pressure gauge. A fuel tank may have run dry or an obstruction appeared in the line from the tank to the pump. Conversely, an obstruction in the line from the pump to the carburetor appears as a sudden rise in fuel pressure and a loss of power.

A loss of power and a sudden drop in fuel-pressure usually indicate a leak between the engine-driven pump and the carburetor. If you have these symptoms, look for a gradual drop in oil pressure. This is almost always caused by a ruptured diaphragm in the engine-driven fuel pump. This dilutes the oil by letting fuel into the crankcase.

If engine roughness occurs at the same time as the loss of power and drop in fuel pressure, try leaning the mixture. The float valve in the carburetor may have somehow gotten stuck.

During cold weather operation, sometimes a gradual drop in fuel pressure occurs, followed by a loss of power. If turning on the auxiliary pump doesnt correct the problem, you can be almost certain that the fuel lines or tanks are contaminated by water, which is freezing somewhere between the tanks and the pumps. Either get to a warmer altitude or be ready for a quiet engine.

In fuel-injected engines, the fuel pressure gauge presents different information. Its primary use is to monitor fuel flow at various power settings.

The use of the fuel pressure gauge gets even more complicated for supercharged or turbocharged engines. To use the fuel pressure gauge to troubleshoot problems requires a detailed knowledge of the engine installation. Talk to your mechanic for guidance and review the POH for emergency procedures.

Fuel-injected engines may also be equipped with a fuel flow meter that is another form of pressure gauge, calibrated in gallons per hour. The fuel flow gauge connects to the fuel manifold, or the flow divider. This is where the metered fuel is evenly divided among the four, six or eight fuel lines, leading to the cylinders. The fuel flow meter measures the pressure inside the flow divider, which is the fuel flow to the engine. The fuel flow gauge actually measures metered fuel pressure, rather than flow. It is not accurate enough to use for precise leaning even though it has percent power and altitude markings on the dial. A fuel totalizer, however, is a flow transducer and can be used for accurate leaning.

Manifold Pressure
The MAP gauge measures the absolute pressure inside the intake manifold of the engine, downstream of the throttle butterfly valve. The gauge is actually an aneroid barometer, connected into the air intake side of the manifold. The gauge directly reads how much air is flowing into the engine.

If the throttle is closed, airflow into the engine is blocked and the piston suction can drop the manifold pressure to low levels – perhaps to as low as 10 to 12 inches of mercury instead of the standard sea level pressure of nearly 30. When the throttle is opened, air is sucked in without restriction as the pistons drop on the intake stroke and the pressure in the intake manifold remains close to that of the ambient air.

Boosting the incoming air pressure with a turbocharger can increase the manifold pressure, but no engine malfunction in a normally aspirated engine will account for a MAP gauge that reads higher than the ambient air temperature.

If the manifold line breaks – and they sometimes do – the manifold pressure gauge indicates whatever ambient pressure is (say 30 inches) at all power settings. Other symptoms of a line break, which is essentially a large induction air leak, are an engine that idles poorly, runs erratically or runs excessively lean. This is cause for grounding the airplane.

Exhaust Gas Temperature
The EGT gauge was originally borrowed from jet engine use and has provided piston-engine operators with one of the best tools for combustion troubleshooting and fuel economy. The EGT system consists of a thermocouple probe or probes that are mounted in one or more exhaust stacks. The probes are wired to a galvanometer needle that is movable on a scale of Fahrenheit temperatures.

A system with a probe for each cylinder can detect which one is leaning first and how even the fuel flow is among all the cylinders. It can spot a fouled plug or a stuck exhaust valve. Some systems can even keep track of trends and identify operational errors.

The EGT gauges in most light aircraft use a scale of 25 degrees per mark, but there is no redline temperature mark. This isnt a problem with small engines, but big turbocharged engines can be damaged by trial and error leaning. The best all-around way to approach an EGT system in an airplane that you are not familiar with is to read the book and put lots of questions to an A&P mechanic. What is the redline temperature of the gauge? What does the POH say about operating at redline EGT? What are the time limits? Where is the single EGT probe located – is it on the same cylinder as the cylinder head temperature probe?

A turbine inlet temperature gauge is essentially the same as an EGT gauge, only installed at the inlet or turbocharger turbine. The gauge reads the temperature of gases going into the turbocharger, allowing you to make sure you dont send gases hot enough to cook the turbines bearings. The main thing to note with the turbine inlet temperature gauge is that it does have a redline temperature, close to 1,650 degrees F. If you want to keep your turbocharger intact, dont exceed that TIT redline reading for any reason.

Engine Instrument Clusters
In many airplanes, the instruments are clustered together; in some the instruments may be powered by the airplanes electrical bus and wired to a common circuit breaker. A malfunction of one 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. Resetting the breaker may help, but you may have to continue the flight without the tools at your disposal.

Regulations require that the fuel quantity, oil pressure, oil temperature and tachometer must be in working condition. If you are flying a supercharged or turbocharged aircraft, you are required to have a manifold pressure gauge that is operational. The cylinder head temperature gauge is not required. The airplane is grounded if these instruments are not operating normally, until repairs are made.

While you are moving across the sky, carefully cross-checking your flight instruments, dont forget there are other instruments there trying to tell you if all is well with the plane.

Also With This Article
Click here to view “Beyond the Engine Instruments.”

-by Raymond Leis

Raymond Leis is a CFII and ATP with more than 23,000 hours.


  1. It’s good to know that tachometers need to be in working condition due to regulatory requirements. That helps me plan for the future. It also means that I can always get a working one when needed.


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