User Guide

Obviously, there isn’t much point in providing a fin and rud-
der big enough to keep the airplane straight at speeds below the
stall, since at that point it won’t be flying anymore; instead, the
speed that’s set is called V
MC
, or minimum control speed. It’s
defined by the FAA as the speed at which the airplane can be con-
trolled (its heading held constant) with one engine (the “critical”
one, which we’ll discuss in a moment) windmilling, the other one
at maximum power, and the airplane in takeoff configuration.
They don’t necessarily say it has to be easy to hold, either - in fact,
they assume maximum rudder deflection, and allow an
untrimmed rudder pedal force of up to 150 lbs!
This speed is so important that it’s marked,
on the airspeed indicator of multi-engine air-
planes, with a big red radial line. The warning is
simple: if you’re flying below V
MC
, and an
engine quits, you will not be able to control the
heading of the airplane unless you reduce power
on the operating engine, give up some altitude to gain
more flying speed, or both. Obviously, if this happens only a few feet
above the ground on takeoff, your options are quite limited!
Bear in mind, too, that losing 50% of your power will cost
you a lot more than 50% of your performance. Flying on one
engine, the airplane requires big, draggy control surface deflections
to keep in control; and even then, the fuselage is still getting
dragged along perceptibly sideways. It’s not very efficient. The pub-
lished figures for single-engine ceiling rate of climb for light and
medium piston twins assume that the dead engine has been feath-
ered, gear and flaps retracted, and the failed-engine wing raised up
to five degrees to get a little help from the bank angle - and even
then they’re pretty underwhelming. Yes, the old pilot’s joke that
“the remaining engine is just enough to get you to the scene of the
crash” is an exaggeration…but not all that much of one!
194
195
LET’S GET CRITICAL
But wait - it gets worse!
You’ll recall, from our earlier discussion on P-factor, that at
low airspeeds and high power settings, such as in a climb, the
propeller’s center of effort moves out from the center along the
downgoing blade. (Class? Class?! Why do I always see the same
hands up?)
Now consider the same situation in a twin. If it has conven-
tional engines (turning clockwise as seen from behind), this dis-
placement of thrust is inward, toward the fuselage (and hence the
center of gravity, as well as the rudder) on the left engine; but out-
ward, even further from the fuselage, on the right engine. Thus, if
the left engine quits, the airplane will try harder to turn to the left
than it will to the right if the right engine quits. Losing the left engine
puts you in more trouble than losing the right one - so the left engine
is the “critical” one. On British and other European twins, with
motors that run the other way, the right engine is critical.
BACKWARDS IS GOOD
“In that case, why not just install engines and propellers that
turn in opposite directions?” I hear you cry. Why not indeed? In
fact, that’s just what Piper did on the Chieftain, although it took
some persuading to get Lycoming and the prop manufacturers to
build them. The Chieftain doesn’t have a critical engine - its sin-
gle-engine performance, such as it is, will be the same regardless
of which engine has failed. There’s another benefit, too: assuming
you have the rudder trim centered, you’ve lined up correctly with
the runway centerline, and both engines are performing properly
and equally, you can take off and fly around all day with your feet
flat on the floor!
Flight Instruction
Flight Instruction
The engines
pull to the
right.