April 13, 2022. It’s a cold spring morning at Salt Lake City International Airport in Utah.
A cargo pilot is departing Runway 16 Left in a Cessna 208 Caravan, November 928 Juliet Papa.
Her destination is Burley, Idaho, 133 nautical miles to the northwest.
The forecast for Burley calls for VFR conditions, but spring weather in Idaho
is often unpredictable. This year, the weather has seemed especially volatile,
and the pilot knows not to put too much faith in the forecast conditions. The day before,
she attempted the same run and was forced to divert. Earlier this morning, she received a
full weather briefing, and consulted with other staff pilots to help inform her launch decision.
Deeming the conditions suitable, the pilot decided to proceed. Her IFR flight plan will take her from
Salt Lake City to Burley Municipal Airport, using the GPS approach to Runway 20 at Burley.
The majority of the flight passes uneventfully. At 7:53am, about 22 nautical miles from Burley,
928 Juliet Papa contacts Twin Falls Approach. Two minutes later, the pilot reports that she
is 10 miles from the waypoint MALTT, and Twin Falls Approach clears her for
the RNAV runway 20 approach via the MALTT transition at Burley Municipal Airport.
Upon reaching the waypoint, the pilot checks in.
The pilot reports her arrival at the waypoint MUTOE, and a short time later,
at 8:07am, reports passing the waypoint IREME, the initial approach fix.
The pilot begins the approach to runway 20, with precise course tracking,
adherence to step-down altitudes, and a steady, though slightly steep, descent. From the final
approach fix to the minimum descent altitude, she descends about 1,000 feet per minute.
The pilot is fast on the approach, but close to her operation’s procedures, which call for
120 knots on approach in icing conditions until visual contact with the runway is established.
With 1 mile visibility in mist and light snow, the VFR conditions forecast earlier now seem a mirage.
At the minimum descent altitude, which is 4,560 feet MSL, or about 400 feet AGL,
the pilot executes the missed approach procedure,
continuing directly over the runway before making a climbing left turn.
The Caravan climbs to 7,000 feet, heading back to the initial approach fix for another attempt.
The pilot flies outbound from the initial approach fix on the procedure turn,
then turns inbound to intercept the final approach course.
After crossing IREME, the pilot checks in. It’s now 8:26am.
The pilot begins a controlled descent, not quite as aggressive as her first approach.
She is slower this time, by roughly 20 knots.
Half a mile from Runway 20, the Caravan emerges below the clouds in a wings-level,
nose-high descent. Directly below and ahead of the airplane is a large potato processing plant,
with a series of stacks producing a large plume of steam directly on final approach to the runway.
According to an eyewitness, almost immediately after emerging below the clouds, the Caravan
flies into the plume of steam. The pilot powers up, lifts the nose, strikes a stack,
and crashes onto the roof of the plant. Sadly, she does not survive the impact.
We know what happened. The pilot was too low, too early, and hit the steam stacks.
The troubling question is why? How did she go quickly from an approach that looked under
control to disaster in a half-mile span? The NTSB, now in the midst of its investigation,
may not be able to fully determine why the flight unraveled. Small clues may offer insight.
We may learn that several small items, not by themselves hazardous, collectively proved fatal.
Perhaps culture played a role. The day prior to the accident, scheduled on the same route,
the pilot diverted. Did that indicate she was impervious to mission pressure? Or did
the notion of missing a destination two days in a row put added pressure on her to make it in?
Delivering freight on time under tough conditions has long been a proud heritage for freight pilots.
Visibility certainly played a role. The pilot may not have recognized just how low the visibility
dropped at Burley before she started the first approach. As she cruised en route, the METAR
reported 6 statute miles in light snow and a broken ceiling at 2,600 feet, technically VFR.
By the time she reached the initial approach fix and began her approach,
visibility had dropped to a mile and ceiling to 2,100 feet. A 2,100-foot ceiling is no help when
visibility is only a mile. She handled it fine on the first approach, but curiously, not the second.
ADS-B speeds are difficult to interpret and can be imprecise, but they are a potential clue.
Did the speed variances between the two approaches indicate a pilot struggling
for precision in demanding conditions? On her last approach, from near the waypoint JAMID,
her speed dropped rapidly by 25 knots. If accurate, perhaps the slowing speed indicates
she thought she spotted the runway and began slowing to landing speed. Perhaps the slower
speeds throughout the second approach set her up for trouble in icing conditions. In a Caravan,
the tail will ice first—a treacherous, insidious condition that can cause loss of control.
The steam stacks at Burley at a half mile final were at least a distraction and at worst
causal. They wouldn’t normally be a factor; a nuisance, yes, but not treacherous. Dozens
of pilots had flown hundreds of approaches over those stacks and in poor weather.
On this day, however, conditions were perfect for ruinous impact.
She approached them almost directly downwind from the plumes, which would
have been substantial on such a cold day. The FAA’s guidance in the Aeronautical
Information Manual warns against flying through exhaust plumes.
The FAA specifically uses the term “smokestack” here, but that term can be potentially misleading.
In the case of Burley, it was the emission of heated steam rather than smoke.
When flying through exhaust plumes of any kind, the AIM warns of turbulence,
reduced visibility, engine problems, icing, and flight control deficiency. Any of these
ill effects could have occurred at Burley and would have been potentially serious in
ugly weather so close to the ground. Furthermore, the effect of flying through
a plume of heated air in a surrounding cold environment could have mimicked the effects
of high density altitude. A perilous loss of lift could have occurred when entering the
less dense air within the plume, which may explain why the Caravan needed to power up.
The RNAV 20 approach into Burley has a high minimum descent altitude and a steep,
3.75-degree vertical descent angle because of the steam stacks so close to the runway threshold,
directly on the runway centerline. The steep vertical descent angle requires an aggressive
descent from the minimum descent altitude at the missed approach point to the runway.
Did the first missed approach, and steep descent angle, nudge the pilot to get low, early,
on the second approach? If so, had she factored in the close proximity to the steam stacks?
The stacks aren’t specifically mentioned on the approach chart, but obstructions
are shown at 4,179 and 4,304 feet MSL. There is also a reference in the Chart Supplement,
but it offers little detail. Another subtle clue can be found
in the approach chart. On RNAV approaches,
a light gray line (called a stipple) can be seen in cases where there is a clear,
unobstructed path from the minimum descent altitude to the runway.
In cases where an obstacle is present, and thus does not allow for a continual descent on a
single glide path down to the runway surface, the gray stipple is removed from the approach chart.
For Runway 20 at Burley, the glide slope does not show the gray stipple. It’s a hint that obstacles
are close and pilots need to see and avoid objects between the approach minimums and the runway.
The issue of the steam stacks at Burley has been a topic of discussion for several years. In 2017,
the FAA performed an aeronautical study on the steam stacks at the potato processing plant.
Despite finding that some stacks penetrated the Runway 20 approach surface area by up to 61 feet,
the FAA reported an overall determination that there was no hazard to air navigation.
However, due to numerous other deficiencies in airport design standards at Burley,
the FAA pulled federal funding from the airport in 2018.
Due to safety concerns associated with these issues, there are talks of relocating the
airport in order to obtain more room for expansion and needed improvements,
and ultimately, to meet FAA standards. A new NOTAM has been issued for Burley
since the accident occurred, alerting pilots to the stacks in the vicinity of the
airport, as well as the potentially dangerous steam plumes they can emit.
Our lesson learned from this tragic event may be the perils of risk-stacking.
Single pilot, in winter conditions, on a demanding approach, with nonstandard obstacle clearance,
nonstandard vertical descent angle, directly over stacks emitting high temperature steam,
and downwind of a debilitating plume. Clearly all the factors proved too much. One risk element
by itself may not be dangerous, but when we stack several of them together, the synergy
is a negative accumulation that even the most talented and motivated pilots cannot overcome.
It’s easy for us to identify the risks and stack them in hindsight. One takeaway from
this tragedy must be that we learn strategies to identify our accumulating risks in the moment,
and develop methods to recognize when the stack gets too precarious and it’s time to exit.