It's July 3rd, 2021, and two pilots are preparing to depart Aspen,
Colorado in a Beechcraft Bonanza G36. The time is approximately 6:20 pm Mountain Daylight Time.
Their trip began in California early this morning. One of the pilots has recently purchased the
Bonanza, N36JJ, and intends to fly it home to New York. The other passenger, also from New York,
has joined the flight as a safety pilot. After a five-hour and 20-minute flight from Napa County
Airport, the two pilots landed in Aspen around 2pm for lunch and fuel. Their next stop is Des Moines,
Iowa, and they've waited a little over four hours in Aspen before returning to the airplane.
Perhaps the long delay included time to wait for cooler temperatures and a more favorable density
altitude. But the temperature now at 6:20pm is 77 degrees Fahrenheit, and the density altitude is
close to 10,500 feet - not much of an improvement from earlier this afternoon. Although the Bonanza
G36 is an excellent platform for a trip across the country - sturdy and spacious, with a Garmin G1000
and an IO-550 with 300 horsepower at sea level - the engine is not turbocharged.
The Aspen airport sits at 7,838 feet MSL - an elevation already challenging by most standards.
Given the current density altitude of 10,500 feet, the normally-aspirated IO-550 can achieve no
more than about 200 horsepower under the best of circumstances - and the G36 is a heavy airplane.
Density altitude can pose significant challenges in Colorado, which 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.
As the Bonanza waits to depart, the ground controller issues an IFR clearance.
It's 6:24pm when the Bonanza departs. The pilot advises that they will make a right turn following
takeoff and requests permission to climb up to altitude while circling over the airport.
Perhaps the pilots cannot accept the IFR clearance of 16,000
feet because they do not have supplemental oxygen on board,
or maybe they understand just how difficult it will be to coax the airplane up to that altitude.
The pilot's reference to the southeast departure is possibly referring to Independence Pass. Their
plan is to eventually cross the imposing Sawatch mountain range to the east,
with peaks topping 14,000 feet.
About 10 minutes later, N36JJ has completed two ascending circuits over the Aspen airport,
and has reached 10,000 feet MSL. The pilots decide they have enough distance to climb on route
and clear the mountain range to the east. They are perhaps unaware of the recommended mountain flying
technique of climbing to ridge-crossing height before turning toward the ridge.
It's unclear if the pilots had originally intended to take Independence Pass. They are now heading
toward an adjacent pass known as Hunter Creek Valley. This valley will ultimately lead them
to Midway Pass, which is approximately 11,800 feet. The Bonanza makes its way up the valley.
Its rate of climb is minimal, averaging about 200 feet per minute - about half the climb rate
predicted by the Pilot's Operating Handbook under these conditions. Still, the high mountain ridges
in the distance are miles away - surely far enough to continue climbing and eventually clear them.
The valley curves to the southeast, and the Bonanza follows its path.
The valley floor begins to creep higher, but the airplane maintains its sluggish rate of climb.
The Bonanza is approaching a semi-circular mountain ridgeline, with tops over 13,000 feet.
Midway Pass provides a route through this ridgeline at 11,800 feet, but at this point, the
Bonanza's altitude is only 10,700 feet. To make matters worse, the canyon is becoming narrower.
The pilot has positioned the Bonanza in the center of the valley, where the terrain elevation is
lower, but in doing so he has not allowed the airplane sufficient room for a 180-degree turn.
Best practice for canyon flying is to favor one side of a canyon based on winds or traffic,
and to allow for maximum turning room in the event a turnaround is necessary.
Now, time is running out. It's clear that the Bonanza won't clear the ridgeline in the distance,
and the valley floor creeps higher still. The pilots make the decision
to abort - to turn around and escape back down the canyon. But it's too late. With
insufficient room to make a full turn back, the Bonanza impacts terrain at 11,000 feet.
It's difficult to comprehend the impact that density altitude has on aircraft performance
without experiencing it firsthand, especially for those who are used to
flying over lower and flatter terrain - as was the case for these pilots from New York.
The safety pilot held an ATP rating and was highly experienced,
but we don't know to what extent he had experience in flying a normally-aspirated piston airplane in
high density altitude conditions among some of the highest mountains on the continent. The effects
of high density altitude were evident in the Bonanza's poor climb performance, and continued
to worsen in the minutes leading up to the crash as the pilots attempted to gain more altitude.
Precise mixture management is especially crucial at high altitudes in order to obtain the maximum
performance from an engine. We don't know for sure if the mixture was properly leaned at takeoff,
and if the pilots continued to lean for optimum engine power during the flight,
but it could have significantly affected the climb rate. The ADS-B data indicates that it took three
minutes to climb from 8,000 to 9,000 feet and four minutes to climb from 9,000 to 10,000 feet.
Given the 10-mile distance to Midway Pass, and the altitude they would need to clear it,
the pilots would have needed three to four additional minutes of climbing
in their orbit to have a chance of making it. The pilot's assessment that after a couple of
circuits they had enough aircraft performance to fly up and over the unknown canyon, was at best,
a guess. It's also difficult to recognize how fast benign-looking 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
catches pilots by surprise. Paying more attention to 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 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 in a turn faster at high
density altitudes. A coordinated turn in such circumstances is essential, yet the pilots were
likely contending with the urge to pull the nose around quickly to avoid the approaching terrain.
Finally, there are almost always up or down drafts 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 leeward side, the pilots would likely have
been dealing with a slight downdraft - further eroding their already poor climb performance.
The combined result of these factors was the impact with terrain at 6:38pm - fourteen
minutes after takeoff. 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
ground speed at ground impact - and a more destructive crash. Neither pilot survived.
Density altitude affects every airplane. We are all taught about it, but it's an abstract concept
until we see it for ourselves and experience the degraded aircraft performance in context.
Out of their element, the flatland pilots in this tragedy
linked several critical decisions together. They took off after 6pm,
likely believing at that time of day the density altitude would have been manageable. They guessed
that after two circuits they had climbed high enough to handle the mountain ridges ahead.
They didn't anticipate the potential effects of sinkers, which are the downdrafts coming over
the mountains in front of them. Finally, they waited too long in their decision to turn back.
Density altitude is a stealthy killer. You don't realize it has you trapped until it's too late.
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