Premium on Safety - Issue 44, 2022 - 8

ASI MESSAGE
Density Altitude-A Stealthy Killer
BY RICHARD MCSPADDEN
On July 3, 2021, two pilots
joined forces to fly a
Beechcraft Bonanza G36 from
California to their home state
of New York. One of them had
purchased the airplane and
was flying it home.
It's a trip many general aviation pilots dream of: flying general aviation
coast-to-coast, and these pilots planned to do it almost literally sea to
shining sea. The Bonanza G36 is one of the best airplanes around for
such a trip. Sturdy, spacious, luxurious seating, a Garmin G1000 for
navigation, and an IO-550 with 300 horsepower to pull them along at
175 KTAS. It's an exciting adventure. The gap in their planning was that
both pilots were flatlanders. The gap would prove fatal over the high
terrain in Colorado.
The owner was based out of New York, where terrain tops
out at 3,000 feet. The pilots probably took comfort in the
300-horsepower engine to get them up and over the Rocky
Mountains at some four times that altitude. They picked the
airplane up in California and flew to Aspen, Colorado, uneventfully.
The performance in California was likely what they were used to
" feeling " in New York: strong engine output producing climbs at
greater than 1,000 fpm, and responsive flight controls.
Around 2 p.m. they landed in Aspen (ASE) where altitudes became
a significant factor. Colorado has the highest average elevation of
any state and the highest average airport elevation. Every year,
the state is among the top in the nation in density altitude related
aviation accidents.
The pilots ate, fueled, and waited. Perhaps the long delay included
time to wait for cooler temperatures and more favorable density
altitude, but the temperature at 6 p.m. was still 25 degrees Celsius,
77 degrees Fahrenheit, and density altitude was near 10,000 feet.
After realizing they couldn't accept the 16,000-foot clearance (the
minimum en route altitude) on the LINZ 9 departure, they decided
to fly VFR and climb while circling over Aspen until they could make
the crossing above the imposing Sawatch Range to the east, with
peaks topping 14,000 feet. They flew several ascending circuits,
then at around 10,000 feet msl assessed that they had enough
distance to climb en route and clear the more than 13,000-foot
ridge to the east. A critical misjudgment.
It's difficult to comprehend the impact of density altitude on aircraft
performance. It's also difficult to understand how fast benignlooking
terrain is rising. Pilots focus on the big ridges ahead, but
it's seldom those that kill. It's the gradually rising valley floor that
squeezes pilots between a paltry maximum-performance climb and
deceptively rising terrain. Focusing on the high ridges in front, pilots
often wait too long to make the turn back. The floor has risen, taking
away any vertical turning room. The canyon has narrowed, limiting
horizontal turning room. At high density altitudes, ground speed is
8
greater, making for a larger turn radius. Pilots need more room for a
180-degree turn than they are used to at sea level. Exacerbating the
problem, airspeed bleeds off faster when a pilot uses normal control
pressures to roll into a turn at high density altitudes. Finally, there are
almost always up- or downdrafts to contend with in the mountains.
In this case, the winds were from the east at approximately 15 knots.
Approaching the ridge from the west, the lee side, the pilots would
likely have been dealing with a slight downdraft, slight enough that
they may not have identified it, but they may have been alarmed by
the aircraft's eroding climb performance.
As the pilots continued east, terrain below rising, imposing
terrain ahead getting closer, and aircraft performance decreasing,
they made a late decision to abort the route and turn around to
escape back down the canyon. But it was too late. From here it's
conjecture, but the cockpit was steadily increasing in stress and
anxiety. At the decision point, the friends knew they were in trouble.
They would have been on full alert, perhaps suppressing panic.
Smooth, delicate control inputs are essential when turning at high
density altitude. The turn would seem to be grinding, plodding, with
a turn rate so slow, the side of the canyon drawing nearer, and no
vertical room to use below them. It takes a good cross check and
good discipline to avoid the subconscious tendency to push bottom
rudder and pull the nose around more quickly. A coordinated turn in
such circumstances is essential.
Somewhere in the turn it all became too much. The pilots lost control
and impacted terrain. Force of impact increases by the square of the
speed. Increased density altitude results in higher ground speed for a
given indicated airspeed. The final density altitude consequence levied
on the two friends would be a higher speed at ground impact and a
more destructive crash. Neither pilot survived.
Density altitude affects every airplane. We all are taught about
it, but it's an abstract concept until we see it for ourselves and
experience the degraded aircraft performance in context. The
flatland pilots in this tragedy linked several critical decisions
together. They took off at 6 p.m., likely believing at that time of
day density altitude is manageable. They guessed that after several
circuits they had enough aircraft performance to fly up and over
an unknown canyon. They didn't anticipate the potential effects of
" sinkers, " downdrafts coming over the mountains in front of them,
from atypical winds at altitude. Finally, they waited too long in their
decision to turn back.
Density altitude is a stealthy killer. You don't realize it's got you
trapped until it's too late.
Richard G. McSpadden, Jr.
Senior Vice President, AOPA Air
Safety Institute
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