8/18/2019 Understanding Engine Guages
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UNDERSTANDING ENGINE GUAGES
What those dials and lights are telling you
By Joe Castanza
You're en route to Lincoln Park Airport for that jui
cy
hundred-dollar hamburger. You check the engine
instruments. Wait—something is horribly amiss. You
have normal oil pressure, but high oil temperature.
What does this m ean? Or p erhaps you see i ndications
of low oil pressure with high oil temperature. Would
that make any difference?
If you'v e even noticed these indications, you're
already ahead of the game. The ugly truth is that many
pilots do not pay
adequate attention to engine instruments, especially these days,
flying brand-new airplanes with numerous electronic
distractions. But flying new airplanes does not preclude a
problem; airplanes and people are not infallible. If they were,
airplanes would not come with warranties, and the term pilot
error wouldn't be in the FAA's lexicon.
Even for the seasoned pilot, engine indications can be
confusing. But they don't have to be. Some indications
simply
mean that everything is OK, and those are the ones with a lot of
green on both sides of every needle. Other indicationsare signs of
trouble, how much trouble to expect, and how soon to expect it. The
key word here is expect, and it makes
the difference between your finding trouble in time to address
it, and trouble finding you after it's too late to do
anything about it—in other words, the difference between a
precautionary landing and a forced landing.
"You don't fly the airplane," Chuck Yeager once said. "The
airplane flies itself, you just tell it where to go." How true.
When you i nteract with the fli ght controls, you aren't
"flying" the airplane, you're "communicating" wi th it. And just
as
you communicate your needs to your airplane, it uses its
own language to communicate what it needs from you. It tells
you when i t needs m ore rudder, m ore airspeed, and more
power; you tell it what to do. As long as both parties li sten
to
each other, things go smoothly. In addition to the way an
airplane sounds and feels to the pilot, the airplane tells you
about its health through the engine instruments.
Here are a few things airplanes say: "I'm fine"; "I need a
richer mixture"; "I'm going to be sick"; "I'm dying." What is
your airplane telling you?
Two of the most important engine instruments are the oil
temperature and oil pressure gauges. They are the
vocabulary
—the "airplaneese"—that your airplane uses to describe its
health. Here's a quick lesson in the language:
High oil temperature with normal oil pressure
This is a tough one, and it presents a mountain of
possibilities. There is really no way to tell what is going on
without
further investigation. The best policy is to head for the
nearest airport—just in case. You can always try to work out
the
problem en route. This way, the worst-case scenario is that you
erred on the side of safety. High oil temperatures in
flight may indicate something as innocuous as a hot day or poor
cooling during a long climb to altitude. Some airplanes
just run a little hotter than others.
Something may be wrong with the electrical system. It sounds
strange, but in some older aircraft, an electrical short
may cause the oil temperature gauge to behave like an
ammeter—the higher the electrical load, the higher the
indicated
oil temperature. Try turning off unnecessary electrical
equipment, one item at a time, and remember to advise ATC,
if
appropriate. If this doesn't work, check the circuit breakers
and recycle the master switch. On the other hand, there
could be a serious engine problem. Ironically, indications of
high oil temperature with normal oil pressure are common
—and they are commonly the result of pilot error.
Maybe you missed something during the preflight inspection, such
as failing to remove a cowl plug, or you made some
sort of in-flight mistake, or something went awry during
planning. When you performed the preflight inspection, was
there oil leaking from the bottom of the cowling? There
shouldn't have been. When you checked the oil, was there an
adequate amount of oil indicated on the dipstick? There should
have been. Did you even remember to check the oil? You better
have, because i n most reciprocating aircraft e ngines, high oil
tem peratures usually result from an inadequate
oil supply. Engines rely on lubricating oil for controlling
internal heat, and without it you're in trouble.
The airplane might be telling you that you have been using too
much power with the mixture too lean—not enough fuel
in the fuel—air mixture. That's also pilot error. Normally, you
shouldn't lean the mixture when using more than 75-
percent brake horsepower (BHP). This can cause detonation: an
explosive ignition of the fuel-air mixture, and
eventually, preignition. That's when parts inside the engine
cylinder heat to an incandescent state, causing the fuel to
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ignite prior to normal ignition and forcing the cylinders to
fire out of sequence. These conditions put a serious strain on
the engine, will cause a loss of power, and may cause engine
damage or even failure. Check in the performance section
of your POH to find out what constitutes 75-percent BHP for your
airplane and notice that this figure changes with
pressure altitude and air temperature.
There are a variety of engine instruments that are useful in the
mixture-leaning process. Every airplane has a
tachometer. To use it for leaning an airplane engine turning a
fixed-pitch propeller, set the POH-specified cruise power
setting, lean the mixture until the rpm peaks, and then lean it
just a bit more until they drop slightly—15 to 50 rpm,
depending on airplane make and model. If available, exhaust gas
temperature (EGT) and fuel flow gauges make leaning
any airplane a breeze. Generally, to use the EGT gauge you would
lean the mixture until the temperature peaks, then
enrich it until the temperature is 25 to 75 degrees Fahrenheit
lower. Fuel flow gauges are a little bit easier to use. You
simply set the power as specified in the POH for your pressure
altitude and air temperature and then lean the mixture
until you have the POH-specified fuel flow indication. If you
have a cylinder head temperature (CHT) gauge on board,
it will make a valuable reference instrument; you'll see higher
indications with leaner mixtures, and that's fine. In
addition, CHT is great for confirming oil temperature
indications; if both oil temperature and CHT indications are
excessively high, the airplane is saying "Hey, I'm not
kidding—I'm hot."
Use the proper leaning procedure for your airplane, as specified
in section four of your airplane's POH. Just remember
that, regardless of the leaning procedure specified or used, if
the engine begins to run hot or rough, you've over-leaned
the mixture. Here's a way to determine whether you are running
your engine too lean: If your aircraft has a fixed-pitch
prop and is equipped with carburetor heat, set your power and
lean the mixture until you believe it's set properly. Then
activate the carburetor heat and note the indication on the
tachometer. If the rpm increased, you've over-leaned the
mixture. The rpm increased because you have enriched the
mixture—increased the amount of fuel relative to the
amount of air—by heating the air entering the carburetor. When
the air heated, it became less dense, resulting in less
air for the same amount of fuel. This enriched the mixture to a
point closer to the optimum mixture for that particular
power setting. So, here the airplane is saying, "enrich the
mixture." Try this technique on the ground first to get a feel
for how it works.
Another common cause for high oil temperature indications
wi th normal oil pressure i s the use of too high a power
setting at too slow an airspeed. This can happen during an
extended performance climb at VX, best angle of climb
airspeed. You are overworking the engine, the airspeed is too
slow to provide adequate airflow through the engine
cowling for cooling, and the oil is trying to compensate. The
airplane is saying, "I need a break." If an engine runs toohot for
too long, the oil pressure will eventually decrease. This happens
because of thermal breakdown—when oil
overheats, it begins to break down and lose its lubricating
capacity. Now you've really got a problem: high engine
temperatures can cause a loss of power and eventually engine
damage or failure.
Treat any engine that is overheating, detonating, or preigniting
in exactly the same fashion: Smoothly reduce the power,
gradually enrich the mixture to full rich, and lower the nose
slightly to increase the airspeed. Reducing power will
reduce the amount of heat being generated. Enriching the mixture
adds additional fuel that will absorb some of the
heat. Lowering the nose and increasing the airspeed will allow
more air to flow through the cowling for cooling. If there
are cowl flaps, open them. Hold level flight—don't descend—for
two reasons: First, you don't want to shock-cool the
engine; that is, cool it too quickly by diving. This may cause
the cylinder heads or engine casing to crack. Second,
conserve your altitude just in case you actually do have a
problem, because you will come to miss altitude very
quickly
when you really need it and don't have it.
Normal oil temperature with low oil pressure
This one is a littler easier to deal with. The airplane is
saying, "I'm going to be sick." Low oil pressure is the problem
here; it's a bad sign, a warning of the onset of a few possible
problems, the least of which is a gauge malfunction. There
may be an obstruction in the oil line. But here's the really bad
news: An oil line, gasket, or seal may have blown, and theengine
may be losing oil. This will invariably lead to a high
temperature/low pressure issue, and that's the worst. Either
way, there's an oil circulation issue and wi thout that
oil, engine failure is i mmi nent. Here's the way to treat this
one:
Assume that your gauges are right and head for the nearest
airport; i f one gauge i s wrong, the worst that will happen is
that you will make a safe precautionary landing. If you see the
oil temperature begin to rise further, or if this is
accompanied by decreasing oil pressure, don't panic. But do
expect a partial or full loss of power, because now the
airplane is saying, "I'm dying." Look for a place to land
safely, and go to it immediately.
Normal oil temperature with high oil pressure
This one is rare. It could be the result of an obstruction in
the oil line, a malfunctioning oil check or regulator valve, a
faulty gauge, or an electrical problem. The airplane is saying
"Something's wrong, and I'm not sure what." This is
probably not a life-or-death situation right now but there is
still a circulation problem, so watch the gauges and proceed
with caution to the nearest ai rport. Expect the oil
temper ature to rise.
High oil temperature with low oil pressure
Here are two gauges indicating serious trouble. The airplane is
saying, "I'm dying." Don't waste time trying to work thisone out
because both gauges can't be wrong. The engine is definitely about
to give up the ghost. Find a spot to land and
treat the engine as though it has already failed. Get the
approach right the first time—you may not have an engine for a
go-around. There is a noteworthy pattern here: Low oil pressure
or high oil temperature will eventually result in a high
oil temperature combined with low oil pressure, and that's the
really bad one. High oil pressure combined with normal
oil temperature is a rare, usually erroneous indication, but
don't bet your life or the lives of others on a gauge
malfunction.
There's a common theme here as well. In each situation, the best
advice is to land soon. There's a very good reason for
this: Any engine indications that are not normal are just
that—they're not normal. If you can address the problem, do so.
You may have m ore options with certain i ndications than
others; you may e ven have m ore time. Just don't count on it.
A stitch in time may save you a lot of stitches when it's
over. Remember to listen to your airplane—after all, it's bee n
listening to you. If something doesn't look right, expect a
problem, and you won't ever be caught off guard. Don't panic;
you won't ever j ust fall out of the sky. If the engine i
s still running, then you still have an airp lane. If not, then you
at
least have a glider. Continue to fly either one for as long as
it will fly.
Meanwhile, remember to look outside. And remember those three
little words that are even more important than "I
love you": Aviate, navigate, and communicate. In that order.
Check into any problems inside the cockpit, but your
primary responsibility is for what is going on outside. Make
sure that you are still flying the airplane. Your
primary responsibility to yourself, passengers, loved ones,
and strangers on the ground is to keep it in the air until
otherwise
necessary. Don't get caught in the same trap as the airliner
crew that crashed into the Everglades in 1972 when the
pilots were distracted by a panel problem. The gear was down and
locked, but one hundred people perished because a
$10 landing gear position light burned out.
Learn your airplane's language, and converse with it. A smart
pilot on the ground is a safe pilot in the air, and a pilot
who listens to his or her airplane wi ll be a smart pilot
on the ground again.
