Flight Training - December 2019 - 53

Why the temperature redline for the oil
system? O'Hare says it's a matter of science:
At high temperatures "the molecules
separate so far from each other that the oil
no longer provides protection."
compared with room temperature," and
drops further "quite rapidly" as temperatures exceed 400 degrees. At the same
time, the strength of the junction also
drops with increases in temperature. That
means small increases in temperature
above the redline mean big trouble.
Busch says, "The CHT redlines set by
engine manufacturers should be treated
as emergency temperatures," and says that
these redlines have "little or no wiggle
room in my opinion. If the pilot waits for
CHT to reach redline, it's too late to save
the cylinder." He reports that, "I've seen
at least one case of a Continental IO-550
cylinder separating catastrophically after
CHT exceeded the 460-degree redline by
just a few degrees for less than one minute.
One should never get anywhere close to
CHT redline."
Right. CHT is an engine's VNE.

DOUBLE WHAMMY

Lastly, to the one instrument that has two redlines:
the oil pressure gauge. We all know that if your
engine oil pressure gets too low-generally from a
loss of oil, but now we know that excessively hot
oil may also cause pressure to drop-the engine
seizes and you must land within your glide range.
But what happens if the oil pressure goes too
high? Lycoming says the excessive oil pressure can
"cause damage to oil lines and other parts of the
engine such as seals and gaskets, air/oil coolers,
and hydro-mechanical parts such as wastegates."
What on Earth would cause high oil pressure? The Lycoming engineering team says an
improperly adjusted oil pressure relief valve, or
something as simple as using the wrong grade of
oil for the time of year, or throttling up before the
oil is properly warmed.
But luckily-if such a thing could be said about
blown engine gaskets-redline would be exceeded
on the apron, not in the air. Says Busch, "Both of
these limits refer to oil pressure during ground operHOT OIL MIGHT BE FINE FOR A MASSAGE,
ations. The low redline pressure is the minimum
BUT NOT FOR YOUR ENGINE
necessary for engine lubrication at idle with hot oil,
OK. Hot metal: bad. What about hot oil?
and the high redline pressure is the maximum that
Why the temperature redline for the oil
can be tolerated by the engine's gaskets, O-rings, and
system? O'Hare says it's a matter of sciother oil seals" at startup with cold oil.
ence: At high temperatures "the molecules
In flight, oil pressure should be in the green
separate so far from each other that the oil arc. As to seeing red, "Oil pressure redlines are
no longer provides protection."
extremely wide, so it's very difficult to exceed
But if that's the case, shouldn't all oil
these redlines-either high or low-unless the
redlines be the same? Actually, they're
engine is operated with extremely low oil level, or
close, with the typical Continental oil
started in extremely frigid temperatures without
temperature redline at 240 degrees F,
a preheat," Busch says, adding, "Neither of these
and the typical Lycoming at 245 degrees
events should ever happen."
F; although Busch again says that the
redlines are at the extreme end of the
SEEING RED YET?
optimum. "The sweet spot for oil temSo there you have it: Most redlines come to us
perature is 180 to 200 degrees," he says.
from a blend of physics, materials analysis, testing,
"Much hotter than that reduces the life of and regulation. And even though they contain
the oil and necessitates more frequent oil
varying degrees of safety margins, they are toyed
changes; much cooler than that makes it
with at risk to your airframe, your engine, and-
difficult for the engine to boil off moisture frankly-your neck.
from the oil sump, and encourages interI guess it's not without good reason that Mr.
nal engine corrosion."
Scott in Star Trek wore a red shirt. FT
The Lycoming engineers also point
out that when oil is excessively hot
WILLIAM E. DUBOIS is an aviation writer, world speed
there's not only a reduction of viscosity,
record holder, and two-time National Champion air
but also of oil pressure.
racer. He teaches Rusty Pilots seminars for AOPA and
And speaking of pressure....
blogs his personal flying adventures at PlaneTales.net.

MISSION CONTROL
Being a pilot is more than being
an operator
There are pilots, and there are operators. Operators manage systems
clinically; pilots use their knowledge
and insight to avoid problems and
keep things running smoothly. Now
that you know why the redlines exist,
how do you avoid exceeding them?
Speed
Managing engine rpm and airspeed
are inseparable, at least in a fixedpitch propeller airplane. Expand your
scan the next time you're maneuvering, and you'll see that engine rpm
increases in a descent and decreases
in a climb. The same is true of
airspeed. If you want to descend and
you decide only to push forward on
the yoke, your airspeed will build.
The harder you push, the faster the
speed builds and the higher the
rpm goes. In a training airplane this
happens gradually, but in a slick
speedster you can exceed VNE and
engine rpm limits quickly.
The easy way to avoid problems
on both is to reduce power for a
descent. That may be only 200 rpm
for a 500-foot-per-minute descent,
or more for a slick airplane and a
faster rate. It takes practice and
trial and error to know the proper
settings. If you do exceed either red
line, reduce power and monitor the
gauges. Abnormal engine indications
may be cause to land at the nearest
airport and get things checked by a
mechanic.
Temperature and pressure
Avoiding oil and cylinder head
temperature and pressure limits is
a bit trickier. Making sure you have
enough oil and the right type goes
a long way to keeping things happy.
If the oil pressure starts climbing or
dropping in flight, your options are
limited. You can reduce power to try
and keep the pressure down. With
oil temperature you may be able to
reduce power or lower the nose in a
climb to keep it within the top range.
In cases of major oil pressure and
temperature changes I'd be looking
for a place to land-and fast. Big
fluctuations may call for a trip to the
nearest airport.
With cylinder head temperatures
the pilot is more in the loop. Lower
the nose on a climb to increase
cooling, open cowl flaps, or enrich
the mixture. One or all of these is
likely to work.
Any oil or CHT problems should
be sent to your favorite airplane doctor. The problem could be a probe,
a leak, bad baffling, or something
much more serious. -Ian J. Twombly
AOPA.ORG/FT 53

http://www.PlaneTales.net http://www.AOPA.ORG/FT

Flight Training - December 2019

Table of Contents for the Digital Edition of Flight Training - December 2019