AOPA Pilot Magazine - October 2019 - 86

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P&E

WX WATCH

FROM 'TRACE' TO 'SEVERE'
A look at the effect of ice-accretion levels
The current trace-light-moderate-severe icing definitions have been around
for decades. But in the past, several other scales, based on specific performance degradations, have been advanced. In 1998, an FAA workshop came
up with this proposal. The "Aircraft Effect" levels roughly correspond to
trace, light, moderate, and severe icing conditions in ascending order.

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AIRCRAFT
EFFECT

SPEED

POWER

CLIMB

CONTROL

VIBRATION

LEVEL 1

Less than 10
knots loss

Less than
10 percent
increase
required

No effect,
or less
than 10
percent
loss

No effect

LEVEL 2

10-19 knots
loss

10
percent-19
percent
increase
required

10
percent-19
percent
loss rate of
climb

No effect

LEVEL 3

20-39 knots
loss

20
percent-39
percent
increase
required

20 percent
or more
loss rate of
climb

Usually slow
or sensitive
response
from control
input

Controls
may have
slight
vibration

LEVEL 4

40 or more
knot loss

Not able to
maintain
speed

Not able
to climb

Little or no
response to
control input

May have
intense
buffet and/
or vibration

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a half-inch-of ice to accumulate before
inflating the boots. But the latest guidance urges inflating the boots at the first
sign of ice for the best results. Another
issue is that some pilots believe that they
can fly in ice no matter its intensity or
duration-as long as their airplane is
equipped with leading edge de-ice boots
and other de-ice equipment. Not true.
Having ice protection gear isn't enough
for all conditions. Some ice conditions-
such as SLD icing-can be so severe that
ice protection can't keep up. Even FIKIequipped airplanes are not certified for
flight into severe icing. So think of ice
protection equipment as giving you the
time to safely escape icing, not as permission to fly indefinitely.
Landing. Hopefully, any icing encounter ends with a landing at the nearest
suitable airport. During the descent to
land it's important to keep your airspeed
86 | AOPA PILOT October 2019

up and refrain from using flaps. Higher
than normal final approach speeds can offset the effects of any ice-induced drag, but
flap deflections can cause tailplane stalls.
That's because the increased downwash
of the air from the flaps causes increased
airloads and higher angles of attack on the
horizontal stabilizer-so high that a stall
occurs. The result is an abrupt loss of lift.
Remember that the tailplane surfaces generate downward lift. When a tailplane
stalls, it rises and there's a sudden, radical nose-down pitch-something that
experience and flight test studies have
confirmed.
Does contemplating any of this make
you uncomfortable? Then all the more
reason to get complete preflight briefings
and make well-planned routes during the
colder months.
AOPA
EMAIL tom.horne@aopa.orgg

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AOPA Pilot Magazine - October 2019

Table of Contents for the Digital Edition of AOPA Pilot Magazine - October 2019